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Article

The Impact of Spontaneous and Induced Restoration on the Hydromorphological Conditions and Macrophytes, Example of Flinta River

by
Stanisław Zaborowski
1,*,
Tomasz Kałuża
1 and
Szymon Jusik
2
1
Department of Hydraulic and Sanitary Engineering, Poznań University of Life Sciences, Piątkowska 94, 60-649 Poznań, Poland
2
Department of Ecology and Environmental Protection, Poznań University of Life Sciences, Piątkowska 94, 60-649 Poznań, Poland
*
Author to whom correspondence should be addressed.
Sustainability 2023, 15(5), 4302; https://doi.org/10.3390/su15054302
Submission received: 3 February 2023 / Revised: 24 February 2023 / Accepted: 26 February 2023 / Published: 28 February 2023
(This article belongs to the Special Issue Sustainable Development of Fluid Mechanics and Hydraulic Engineering)
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Abstract

:
Highly modified riverbeds are not able to spontaneously reproduce natural processes. The restoration of natural river systems is an important challenge to modern river engineering. Various procedures and solutions, both technical and non-technical, are applied in this process. This involves looking for simple solutions that are close to nature and that interfere with river ecosystems to a minimal extent. One of these solutions is deflectors, which constitute a type of simplified spur. This study presents the results of the research on the transformations of hydromorphology and macrophytes on selected sections of the Flinta River, which represents the most common type of river in the Central European Lowlands (a small river with a sandy substrate). Two neighbouring sections of the watercourse were selected. The first one has not been subject to any regulatory measures for over 30 years and is undergoing spontaneous restoration, while the second one was significantly altered (straightened, cleared of hydrophytes, and desilted) ten years ago. Three deflectors were introduced in this section in the years 2017–2018. Research conducted on both sections enabled the determination of the possibility of initiating renaturalisation processes by way of implementing simple solutions in the form of low-cost wooden deflectors. It also provided the basis for the assessment of the impact the measures taken had on the hydromorphological status of the watercourse and on macrophytes. Based on the studies conducted, it was possible to determine the size, dynamic, and scope of the changes taking place in the river under various conditions of its transformation, including those resulting from anthropopressure.

1. Introduction

In order to achieve at least a good ecological status of surface waters, which is the main objective of the Water Framework Directive [1], water administrators should work to improve the quality of degraded or significantly transformed sections of rivers [2]. In the case of lowland watercourses, their ecological status depends on the flow conditions and on biodiversity in the area of the bed of the watercourse [3,4,5,6]. Highly modified riverbeds, subjected to regular maintenance treatments (desilting, straightening, stabilising of banks, hoeing and mowing of plants), which are the most anthropogenically altered, are not able to spontaneously reproduce natural processes. This situation also strongly affects the condition of the macroinvertebrates [7], macrophytes [8], and river ichthyofauna [9]. In such cases, achieving the effect of the improvement of the ecological status of the watercourse requires engineering and restoration measures, which would initiate and support natural morphodynamic processes that improve the ecological quality of rivers [10,11]. The restoration of natural river systems is therefore an important challenge to modern river engineering [12,13]. It also provides the basis for interdisciplinary research that encompasses collaboration among hydrologists, biologists, ecologists, and geomorphologists, aiming to improve the ecological status of rivers [14,15,16]. The planned measures should intensify the transformation of the riverbed layout, enhancing its heterogeneity in order to ensure appropriate conditions for the development of macrophytes, as well as fish and macroinvertebrates [7,17]. This means creating favourable conditions for river biota, which will result in an overall increase in biodiversity. Properly planned restoration measures have a significant impact on the hydrodynamics of the river and on the functioning of the ecosystem. An important element of this is the beneficial impact on the development of macrophytes, which absorb the dissolved biogens and speed up the sedimentation of organic matter, positively affecting water quality [18].
The biological and anthropogenic measures aiming to improve the ecological condition of rivers often cause significant hydromorphological changes [19,20]. Only extensive, long-term research allows for the assessment of the effectiveness and applicability of such measures. The restoration of the original status is difficult and, in many cases, impossible to achieve [21,22]. Obtaining an ecosystem that would be resilient to disturbances and sustainable over time as a result of such measures is not always obvious or realistic [23]. Various procedures and solutions, both technical and non-technical, are applied in this process [24]. This involves looking for simple solutions that are close to nature and that interfere with river ecosystems to a minimal extent [25,26,27]. One of these solutions is deflectors, which constitute a type of simplified spur. They can be made of stone, gabions, wood, or fascine [28]. They direct the flows and often, in consideration of the fish, are made in such a way as to form a bump [29]. Lenar-Matyas et al. [28] believe that deflectors enhance the roughness of the bottom of riverbeds and, if properly positioned, force the flow of the current, improve oxygenation of the water, and contribute to the self-purification of the watercourse [30]. Deflectors used in river restoration measures are described as a series of low structures in the riverbed, which irregularly narrow the riverbed and cause variation in the velocity of the flow [31]. They deflect the current and so also contribute to the erosion of the banks of the watercourse, initiating the meandering of the riverbed and supplying material for the formation of streambed sediments such as point bars and mid-channel bars. One of the first examples of the use of deflectors to improve river habitats in Poland involved applying them to restore good spawning conditions for Samo trutta morpha lacustris in the Wda and Trzebiocha rivers in the mid-1990s. It involved placing branched tree trunks in the riverbeds at an angle of 45° to the river stream. It was assumed that the deflectors would initiate meandering by eroding the opposite bank and creating shallows, offering favourable conditions for sea trout fry and smolts [32]. Since then, this method of river restoration has been applied a few more times in Poland, for example, in the Kwacza River, which is a tributary of the Słupia River, in the years 2007–2008 [33]. However, scientific research on its impact on hydromorphological conditions and organisms has not been conducted on a larger scale.
This study presents the results of the research on the transformations of hydromorphology and macrophytes on selected sections of the Flinta River, which represents the most common type of river in the Central European Lowlands (a small river with a sandy substrate). Two neighbouring sections of the watercourse were selected. The first one has not been subject to any regulatory measures for over 30 years and is undergoing spontaneous restoration, while the second one was significantly altered (straightened, cleared of hydrophytes, and desilted) ten years ago. Three deflectors were introduced in this section in the years 2017–2018. Research conducted on both sections enabled the determination of the possibility of initiating renaturalisation processes by way of implementing simple solutions in the form of low-cost wooden deflectors. It also provided the basis for the assessment of the impact the measures taken had on the hydromorphological status of the watercourse and on macrophytes.

2. Study Area

The research was conducted in the years 2005–2022 on two 0.5 km-long fragments of the Flinta River, in its estuarine section, in the vicinity of Rożnowice village, in Rogoźno municipality (Wielkopolskie province, Poland) (Figure 1). The reference section (A) was located between 1 + 150 and 1 + 650 km (starting from the mouth, 1 + 150 means 1.150 km) of the course of the river. It was distinguished by the absence of maintenance work and natural processes occurring along its entire length. Compared to section B, for the entire duration of the analysis, it was distinguished by a relatively high sinuosity ratio and the presence of numerous natural morphological elements. Research section (B) was located closer to the mouth of the Wełna River, between 0 + 050 and 0 + 550 km of the course of the Flinta River. It was subject to periodic maintenance work, such as desilting, the deepening of the bed, the reinforcement of banks with fascine fences, mowing of the slopes, and removing macrophytes from the bottom. This resulted in a low level of naturalness of the riverbed and homogeneity of the local conditions. At the beginning of the research, the section was almost completely straight (Figure 2a). Maintenance work was performed for the last time in 2011; once it ended, previously unobserved streambed sediments and signs of spontaneous restoration appeared. In the years 2017–2018, three wooden deflectors were installed within this section (at the cross sections of 0 + 080, 0 + 095, and 0 + 110 km of the course of the river), which significantly accelerated the process of restoration.
The Flinta River is the right-bank tributary of the Wełna River and an element of the Wełna–Warta–Odra river basin. The length of the watercourse amounts to 27 km, while the total catchment area covers 345.47 km2 [34]. In terms of abiotic typology, it represents the most common type in Poland; namely, a sandy lowland stream. The features of the researched sections were the width of the riverbed of 4–5 m and a longitudinal slope of 1.2–1.3‰. Ryczywół municipality, through which the longest section of the Flinta River flows, covers several areas of high natural value [35]. Apart from the eastern part of the Notecka Forest, it includes two Natura 2000 sites: Bagno Chlebowo (PLH300016) and Dolina Wełny (PLH300043), as well as the Dolina Wełny Protected Landscape Area [34].
There is one water gauge station on the Flinta River, located in the town of Ryczywół (km 14 + 355). A meteorological station is also located there. Both stations belong to the Measurement and Observation Network of the Institute of Meteorology and Water Management—National Research Institute (IMGW-PIB). Publicly accessible hydrological data cover the period since 1951. Flows characteristic of the Ryczywół cross section are summarised in Table 1.
Like most watercourses in Poland, the Flinta River has been strongly transformed as a result of regulation and reclamation measures carried out since the 19th century. Regulation works have significantly reduced the length of the river. In some sections, its course has been shortened by up to half, which has drastically changed the character of the watercourse, leading to ecological degradation. With time, the increase in the slopes resulted in the lowering of the elevation of the riverbed and significantly accelerated the drainage from the catchment area, which exacerbates water shortages in the face of the progressing climate change. Due to its high potential for renaturalisation, the Flinta River has been selected for the pilot National Surface Water Restoration Programme (KPRWP) and a preliminary restoration plan has been developed for it. The programme constitutes one of the measures included in the updated water management plans in Poland (PGW) and fulfils the requirements of the WFD [1], while at the same time responding to the identified hydromorphological pressures and the urgent need to improve the condition of surface waters [36].

3. Materials and Methods

3.1. Construction of Deflectors

In the research of the B river section, three lightweight openwork deflectors made of woven wicker were introduced in the years 2017–2018 [37]. The location where the deflectors were installed was not accidental. The place that was selected was known and researched with respect to restoration processes. Research into plant baskets has been carried out in the estuarine section of the Flinta River [38]. Due to the shape of the valley, the geological structure of the bed (fine sands, medium sands, and gravel), and intensive sediment transport, the selected section is susceptible to transformation. The wicker deflectors were made according to recommendations in the literature [39]. Their width was selected so that they would cover a third of the riverbed, with the spacing of the deflectors amounting to five times the width of the riverbed. The structure of the deflectors consisted of wooden logs with a diameter of 0.04 m. They were driven into the riverbed to a depth of 0.50–0.70 m, spaced at approx. 0.40 m, with wicker branches woven between them, which produced an openwork structure (Figure 3). The structures were positioned at a 70–90° angle to the streamline, which was advantageous, as it ensured a more effective deformation of the structure and therefore a more effective initiation of restoration processes.

3.2. Geodetic Surveying

The geometric measurements of the riverbed layout constituted the primary research that allowed for the assessment of changes in hydromorphological forms in the riverbed. For this reason, a thorough measurement of the original layout of the riverbed, banks, and a section of the river valley was made before the deflectors were implemented. Once the deflectors were constructed in the riverbed, 15 cross-sections were delineated. For this purpose, a pair of wooden poles were driven into the ground and levelled for each of the cross-sections. In order to record the ongoing phenomena more precisely, the cross-sections were densified and their number was increased to 28 (for example cross-sections, see Figure 4). The measurements were made with the GPS RTK geodetic set and optical level Nicon AX-2s. Each measurement made with the automatic level was initiated from a point with a known ordinate, which made it possible to obtain a measurement accuracy of 0.002 m. The measurements were further verified against the known ordinates of individual pairs of poles, between which cross-sections were previously made. A tape measure was used in the measurements of the distances, which made it possible to achieve the accuracy of approx. 0.02 m in linear measurements.

3.3. Changes in Sediment Size

In the course of the geodetic surveying conducted on the riverbed, samples of the river sediment were collected to assess the impact that the deflectors had on the changes in the granularity of the river sediment. The first samples were taken before the installation of the deflectors in July 2018. The next samples were taken after one year (in July 2019) and, since then, not less than 2 times per year during the measurements carried out. The last sampling was conducted in June 2022. Samples were taken at strictly planned and repeatable points. These were located, respectively, at the cross-section of each deflector at the water current line, below the deflector where sedimentation occurred (in the “shadow” of the deflector), and approximately 2 m downstream of the deflector at the current line. Moreover, for each deflector, an additional sediment sampling point was located about 2 m upstream of the deflector (a total of 12 points directly associated with the deflectors). In addition, debris samples were collected for 3 additional points, having a control and reference character. Two were located midway between the deflectors (one at deflectors 1–2 and the other at deflectors 2–3) and the last point numbered 15 located 15 m upstream of the last deflector. There were 15 points in total. Samples were taken from the bottom layer from 0 to 5 cm. The points were located in the current line and behind and in front of the deflectors, so as to capture all the changes associated with the phenomenon of the erosion and accumulation of river sediment as a result of restoration activities. The second stage of the research consisted of an indoor analysis, which was carried out in accordance with the (PKN-CEN ISO/TS 17892-4) standard. The collected sediment was prepared for testing (shaking, separation, and drying), and then sifted on a set of standardised sieves at the Water Laboratory at the University of Natural Sciences in Poznań. The results were tabulated and analysed.

3.4. Hydromorphological Surveys

Hydromorphological evaluation was conducted at each site according to the Hydromorphological Index for Rivers (HIR) method in the years 2006, 2012, 2017, 2021, and 2022 [40,41]. The HIR data were collected from 500 m stretches of rivers. The HIR surveys were performed in ten profiles (spot checks) distributed at 50 m intervals. The macrophyte survey section was located inside each HIR site, always between the 6th and 8th spot check. Two numerical metrics based on the HIR protocol were produced: Hydromorphological Diversity Score—HDS, and Habitat Modification Score—HMS. High HDS values indicate an extensive presence of a number of natural river features and high landscape diversity along the river. High HMS values indicate extensive anthropogenic alteration such as bank and channel re-sectioning and reinforcement or other river engineering constructions. To assess the hydromorphological status of the river, the calculated values of the HIR index were referenced to the current standards [42].

3.5. Macrophyte Surveys

Macrophyte surveys were carried out during the intensive growth of aquatic plants (from mid-June to mid-September) in the years 2005, 2006, 2007, 2008, 2012, 2017, 2021, and 2022. Field surveys were conducted using the Macrophyte Method for River Assessment, which is the official Polish method of macrophyte monitoring [43]. The macrophyte assessment was based on the presence of algae, mosses, horsetails, liverworts, and monocotyledonous and dicotyledonous plant species that are biological indicators of water quality [44]. All submerged, free-floating, and emergent plants were considered. The assessment also included macrophytes attached to or rooted in parts of the river bank substrate where they were likely submerged for most of the year. Aquatic plants were surveyed along 100 m-long river stretches. The survey included a list of species and estimated ground cover of plants. The presence of each species was recorded with their percentage cover using the following nine-point scale: <0.1%, 0.1–1%, 1–2.5%, 2.5–5%, 5–10%, 10–25%, 25–50%, 50–75%, and >75%. Based on the macrophyte database, four macrophyte metrics were calculated: species number, percentage of coverage, Shannon–Wiener index [45], and the Macrophyte Index for Rivers—MIR [43]. To assess the ecological status of the river, the calculated values of the MIR index were referenced to the current standards [42].

4. Results

The geodetic surveys of the geometry of the riverbed performed in the years 2018–2022 revealed changes in the lateral layout of the bed for lateral cross-sections in the vicinity of the deflectors (Figure 5). The maximum bottom erosion in the cross-section of the deflector reached up to 40 cm (Figure 5b). Meanwhile, the lateral cross-section was widened by up to 100 cm, which indicates a strong bank erosion. In cross-section 4 (Figure 5a), situated approx. 15 m downstream of the last deflector, no directional changes in the layout of the riverbed were observed. The layout of the cross-section remained stable and the maximum differences in elevations of the riverbed amounted to 20 cm. Meanwhile, upstream of deflector 3, in cross-section 27, spontaneous morphological changes were observed. They were caused by, i.a., an abundant growth of macrophytes and by the impact of the group of deflectors situated downstream of this cross-section (Figure 5c). Within cross-section 27, we can observe a strong erosion of the right bank (50 cm on average) and an accumulation of material on the left bank (elevation of the bottom by approx. 60 cm), which leads to a shift of almost 100 cm in the river’s streamline. The bottom erosion shown in Figure 5c amounts to approx. 40 cm.

4.1. Changes in Sediment Size

Changes in riverbed morphology occurring in rivers during restoration are accompanied by changes in the granularity of the bottom sediment. The impact of the deflector, causing the narrowing of the watercourse and the deformation of the streamline, translates into an increase in flow velocity and into lateral as well as vertical erosion. This is linked with a change in granularity both in the profile of the deflector and downstream of it, in the section impacted by the deflector. One of the things observed in the course of the research was an increase in granularity d50% downstream of the deflector from 0.31 mm to 3.9 mm (Figure 6a). Such a significant change resulted from the washing out of finer fractions and from the armouring of the streambed at a length of approx. 2–3 m in the streamline downstream of the deflector. Meanwhile, the slowed flow of water downstream of the deflector caused an accumulation of finer-diameter material. The emergence of sediment deposition in the form of bottom blowdowns was associated with the reduction in the sediment diameter and with the reduction in the d50% value from 2.4 mm to 0.18 mm over the distance of approx. 5–6 m downstream of the deflector (Figure 6b). It is worth adding that diameter changes between deflectors were insignificant and not durable (Figure 6c). Changes in the granularity of the material on the bottom of the riverbed are highly significant from the point of view of the macroinvertebrates and ichthyofauna.

4.2. Hydromorphological Surveys

The hydromorphological surveys were conducted in the period preceding the desilting of the riverbed (2006), one year after it was performed (2012), after the installation of the first deflector (2017), and after the installation of all deflectors (2021, 2022) (Figure 7). Earlier, before the year 2006, the section was subjected to regular maintenance work in a multi-year cycle. This resulted in a low heterogeneity of natural morphological elements in the riverbed (Figure 7a), a small number of erosive (cliffs) and accumulative (bars) forms (Figure 7b), the profiling of the banks of the riverbed (Figure 7c), and, as a result, a moderate hydromorphological status (Figure 7d). The negative impact of the desilting of the riverbed on the hydromorphological status of the Flinta River was especially visible in 2012, one year after the work was completed. No erosive or accumulative forms were observed during the field survey (Figure 7a), the HDS index fell to 15 (Figure 7b), the HMS index increased to 28 (Figure 7c), while the overall hydromorphological status deteriorated to poor (Figure 7d). After 2011, maintenance work was discontinued. Until the installation of deflectors in the years 2017–2018, the hydromorphological status improved as a result of a spontaneous self-induced restoration (Figure 7d). The heterogeneity of the riverbed increased (Figure 7b), while the traces of profiling ceased to be visible (Figure 7c). However, the intensification of this process was observed only after the installation of deflectors. This is particularly evident in the case of erosive and accumulative forms, the total number of which increased from three in 2017 to fourteen in 2022 (Figure 7a). Chronologically, bank undercuts began to appear first in 2017; then, in 2021, bars appeared, and their number increased in 2022. Eventually, a very good hydromorphological status, similar to that observed in the reference section (A), was achieved in 2022 (Figure 7d and Figure 8c).
Apart from the field survey of the section containing deflectors, a similar survey was conducted on the reference section, situated approximately 600 m upstream, in the years 2012, 2017, and 2022. No maintenance work had been conducted on that section in the past (Figure 8). The very good hydromorphological condition of that section can be regarded as relatively stable in time. HIR values there range between 0.797 and 0.844 (Figure 8c). The number of erosive and accumulative forms within that section changed over time; however, it was constantly maintained at a high level. Moreover, mid-channel bars, not present in the section with deflectors, appeared there (Figure 8a). It is noteworthy that, in the years 2012 and 2017, there was a relative balance between unstable forms (eroding cliffs, unvegetated bars) and vegetated forms (stable cliffs, vegetated bars), while in 2022, stable forms clearly dominated. This indicates that during at least two growing seasons preceding the field survey, there were no high tides that would trigger the erosion and transportation of the bed load, which made it possible for plants to grow on the surface. Comparing the two sections of the river, it has to be concluded that a gradual improvement of hydromorphological conditions has been observed within the section with deflectors; it was spontaneous at first and then induced by the impact of deflectors since 2017. Meanwhile, the comparative section has been relatively stable in hydromorphological terms over time (Figure 8).

4.3. Macrophyte Surveys

Aquatic vegetation surveys were conducted in the same years as the hydromorphology surveys. Additionally, in the period preceding the desilting of the riverbed between 2005 and 2008, surveys were conducted every year (Figure 9). Before 2011, the ecological status was relatively stable, with the number of macrophyte taxa ranging between 22 and 25 (Figure 9a), the overall coverage by macrophytes varying between 62% and 75% (Figure 9b), and the MIR index oscillating between 37.6 and 39.2 (Figure 9d). The desilting of the riverbed carried out in 2011 had a highly destructive impact on macrophytes, mostly because it caused their mechanical removal. Field surveys performed one year after that work was carried out showed a significant reduction in the number of macrophyte species, which dropped to nine (Figure 9a); their overall coverage, which fell to approximately 2.5% (Figure 9b); and biodiversity (Figure 9c) as well as a deterioration in the ecological status to the lower range of moderate status (Figure 9d). Until the installation of deflectors in the years 2017–2018, the ecological status of macrophytes improved slightly as a result of spontaneous restoration (Figure 9d). The number of macrophyte taxa increased to 15 (Figure 9a), and their overall coverage increased to approximately 25% (Figure 9b); however, biodiversity, measured with the Shannon–Wiener index, remained at a similar level (Figure 9c). The intensification of the process of macrophyte regeneration occurred only after the installation of deflectors. In 2022, nearly all of the analysed indexes returned to the status they had before the desilting of the riverbed (in the years 2005–2008), or even exceeded it (Figure 9a,c,d). Only the overall coverage by macrophytes (the degree of vegetation overgrowth in the riverbed) remained at a lower level of 35–45% (Figure 9b). Eventually, in the year 2022, the ecological status was at a good level, comparable to that observed in the reference section (A) (Figure 9d and Figure 10d).
As in the case of hydromorphology, apart from field surveys carried out in the section with deflectors, macrophyte surveys were carried out in the years 2012, 2017, and 2022 on the reference section, which was not affected by maintenance work (Figure 10). The good ecological status of that section can be regarded as very stable over time. The MIR index values there ranged between 37.0 and 37.9 (Figure 10d). The other parameters that were analysed were also subject to minor fluctuations. The total number of species ranged between 25 and 29 (Figure 10a), and the overall coverage by macrophytes oscillated between 37.5 and 61.5% (Figure 10b). Only with regard to biodiversity, a significant drop in the value of the Shannon–Wiener index, from 1.10 to 0.78, was observed between the years 2012 and 2017 (Figure 10c). The cause of this phenomenon is not clear. Comparing the two sections of the river, it has to be concluded that, like in the case of hydromorphological conditions, a gradual improvement of the ecological status has been observed in the section with deflectors; it was spontaneous, initially, and then induced by the impact of deflectors since 2017. Meanwhile, as far as macrophytes are concerned, the reference section has been stable over time (Figure 10).
The macrophyte species identified in the two analysed sections were similar and typical of small lowland rivers with sandy bottom material (Figure 11). Vascular plants representing the helophyte group dominated. There were also occasional occurrences of bryophytes (Fontinalis antipyretica, Leptodictyum riparium) and macroalgae (Lyngbya sp., Vaucheria sp.). Species such as Berula erecta, Elodea canadensis, Glyceria fluitans, G. maxima, Phalaris arundinacea, Rorippa amphibia, and Veronica anagallis-aquatica occurred in the largest quantities. The share of specific taxa changed over time (Figure 11). PCA ordination, which takes into account all species identified at least three times during the entire duration of the research, shows that the taxonomic composition in the section with deflectors in the years 2005–2008 was very similar. In the year 2012 (one year after desilting) there was a drastic change in the structure of macrophytes, which stabilised again after the installation of deflectors in the years 2017–2022. However, the species composition at that time was different than in the period preceding the maintenance work, as it resembled more closely the reference section (A), where the macrophyte structure was relatively stable (Figure 11).
The maintenance work in the year 2011 had the greatest impact on the population of Elodea canadensis, which had formerly covered 40–60% of the bottom of the researched section (Figure 12). The species belongs to the group of elodeids, which are macrophytes rooted at the bottom and completely submerged. The desilting resulted in its complete extermination due to mechanical destruction. It reappeared in 2021; however, it no longer grows on a mass scale as it did before (the coverage amounts to 3.75%). It is not a major loss for the aquatic environment, as Elodea canadensis is an alien species, originating from North America, which, however, does not currently show invasive characteristics. The other analysed species responded to desilting in a similar manner; however, the population of emergent species, helophytes, was recreated much faster and to a greater extent (e.g., Glyceria fluitans, G. maxima) (Figure 11).

5. Discussion

The analyses conducted for the two sections of the Flinta River made it possible to assess the impact of spontaneous and induced restoration processes on organisms and the ecological status of a small lowland river. This research fits into the wider context of the discussion on the purposefulness of such measures and on the assessment of the pace of the spontaneous restoration of watercourses that had previously been subjected to regulation.
Intensive agricultural land use imposes multiple pressures on streams. This applies to the artificial maintenance of the course of a regulated riverbed (which is often a straight line), the removal of vegetation, and the disruption of the processes of sedimentation and erosion [46]. This includes the loading of streams with nutrient-enriched soil from surrounding crop fields, which may cause a deterioration in the sediment quality [47]. The extent of the river restoration process currently plays a significant role in the protection of the aquatic environment, especially in those riverbeds that had been modified in the past [48]. The feasibility of restoration may be the result of natural processes occurring in sections of rivers where the anthropogenic pressure (including that related to agriculture) is low [49], or where, due to the poor ecological status (coexisting with high natural potential), technical measures are taken with a view to initiating such processes [31,50].
In recent decades, there has been an evolution in the approach to restoration ecology, moving away from the efforts to reconstruct pristine and reference sites [51,52] and towards a goal-oriented strategy [53]. The purpose is to maximise ecological functions and services. This shift in emphasis has been due to the effective lack of undisturbed reference sites [53] and the fact that ecological restoration projects must take values and socio-economic concerns involved in renaturalisation processes into account [54,55]. At the same time, restoration measures have moved from targeting relatively small sections to comprehensive measures on a catchment-wide scale [56]. For example, before the year 2000, nearly 75% of restoration projects carried out in the European Union addressed only narrow issues, focusing on the riverbed, banks, or valleys. Meanwhile, after 2015, the share of such projects decreased to <50%, and more comprehensive projects started to dominate [57]. This approach entails the challenge of coordinating restoration measures in various habitats, at different levels of degradation, and with different needs when it comes to recultivation and the accessibility of resources [58].
Even attempts to implement large-scale (catchment-wide) solutions require referring to the possibility of applying different techniques and methods to initiate natural restoration processes [52]. Techniques to control the riverbed, stabilise the bottom, protect stream banks, and restore natural habitats can be an important part of river restoration projects [39]. The research conducted on the Flinta River, such as the research by Choi [54] allows for the profiling of the purpose and scope of such measures in relation to macrophytes, the sinuosity of the bed of the watercourse, and morphodynamic changes. Deflectors were a simple solution that was taken into account in the research conducted on the Flinta River. On the one hand, they locally restrict the width of the canal in order to speed up regular flows through the narrowed part of the cross-section, while on the other hand, they deform the stream, contributing to the initiation of riverbed meandering processes [39]. The structures of deflectors, whether single or working in groups in meandering streams with low gradients, direct the flows towards the centre of the canal and, under some conditions, increase the depth and velocity of the flow, thereby creating mesohabitats such as pools and fluvial deposits, improving fish habitats and opportunities for macrophyte development [29]. In the case of the Flinta River, such riverbed features were achieved within three years following the introduction of deflectors. Meanwhile, a condition corresponding to the conditions observed in the reference section was achieved within seven years. Some categories of maintenance work (including, i.a., the desilting of the riverbed) have an especially destructive impact on aquatic organisms, causing their mechanical removal and a change in abiotic conditions [17,59]. The subsequent regeneration of groups of organisms takes a long time and, in the case of the cyclic repetition of maintenance work, is usually incomplete, which may lead to a progressive deterioration of the ecological status of the waters [60]. Research conducted by Hachoł and Bondar-Nowakowska [61] showed that even after ten years, macrophytes did not recover to the condition seen before the regulation. In such cases, the pace of spontaneous restoration may be insufficient and may require initiation and intensification through planned restoration [19].
A significant feature from the point of view of macrophyte development was the fact that the researched sections of the Flinta River were characterised by the absence of tall vegetation (trees and shrubs) growing on the banks of the riverbed. This constituted conditions allowing for the impact of the shading of the riverbed on macrophyte development to be ruled out [62]. Research conducted by Jusik and Staniszewski [63] has shown that increased shading constitutes a natural factor limiting biodiversity. However, its impact on macrophyte indicators (MIR, RMNI, IBMR) that describe the ecological status is small. Research conducted by Jahadi et al. [64] and Guo et al. [65] has shown that interaction between riverbed vegetation and the structure of the flow has a key significance for river engineering and hydromorphological processes associated with the transformations of the lateral and vertical layout of the riverbed in restoration processes. Macrophytes influence fluvial processes as well as respond to them. Their overground biomass modifies the flow of water and traps sediments, while their underground biomass impacts the features of the bottom material, and, consequently, its susceptibility to erosion [66].
The flow regime and its variability constitute fundamental parts of river ecosystem management [67]. It is generally believed that the flow regime is the main factor influencing biodiversity in aquatic ecosystems [68,69], while variable flow conditions support natural restoration processes. The research also involved looking for the reason for such significant and fast hydromorphological changes shortly after the introduction of the deflectors (which was confirmed by the measurements made). The research took into account flow hydrographs from the years 2016–2021 (Figure 13). In the case of the Flinta River, the success of the far-reaching transformations of the riverbed was the result of, i.a., the emergence of flows that significantly exceeded average values in the first year following the introduction of deflectors (average flow of 0.66 m3·s−1, Figure 13). Generally speaking, the whole year 2017 was a wet year, and so was the first half of 2018. Given the higher values of flow velocity, the installed deflectors initiated morphodynamic processes, which provided the basis for further restoration processes. A similar connection between the flow regime and transformations of the riverbed was pointed out by, i.a., Palmer and Ruhi [70], who indicated that the assessment of the relative impact of flow conditions on abiotic and biotic factors is currently a challenge to ecology. It is also important for the precise design and prediction of the outcomes of restoration. The most significant changes in riverbed morphology occur during major surges, especially during overbank flows [71]. In the near future, the process will be under the strong influence of the intensifying global climate change, associated with rising temperatures [72] and changes in precipitation patterns [73]. In the catchment of the Flinta River, a temperature rise of 1.8–2.0 °C was observed in the years 1961–2018, which translates into an average increase of 0.33 °C per 10 years [74], while in the years 1951–2013, a minor increase in the total precipitation was observed, though it occurred only in the winter season [75]. This resulted in an increase in average flows between January and March by about 10% and in a significant drop in average flows between April and September (reaching 50% in June and July). Currently, channel-forming flows occur only in the winter or early spring. At the same time, average monthly flows (SSQ) remained at a relatively stable level and amounted to 0.64 m3·s−1 in the years 1951–1970: 0.73 m3·s−1 in 1976–1995 and 0.61 m3·s−1 in 2001–2020 (Figure 14).

6. Conclusions

Environmental research into natural processes taking place within river valleys often requires long time series of observations, which allow for the capturing of significant changes and trends in transformations taking place in the aquatic environment. Continuous research conducted in multi-year time horizons is highly significant and valuable; however, due to numerous limitations (for example, financial limitations), it is undertaken rarely and often it is abandoned once a research project is completed. The series of studies and measurements carried out in the analysed section of the Flinta River in the years 2005–2022 covered various aspects of the assessment of the ecological status of the river and of hydromorphological transformations of the watercourse. Based on the studies conducted, it was possible to determine the size, dynamic, and scope of the changes taking place in the river under various conditions of its transformation, including those resulting from anthropopressure. The basic conclusions following from the conducted research can be applied to the following issues:
  • Regular maintenance work that was carried out on the Flinta River (section B) resulted in a reduced diversity of macrophytes and various other hydromorphological elements of the riverbed. The work has also contributed to the significant degradation of the river in terms of its ecological status (poor status was identified shortly after the desilting and removal of vegetation).
  • The seven years of spontaneous restoration and the introduction of deflectors in 2018 enabled a significant improvement in the quality of the watercourse in section B. An increase in the heterogeneity of the riverbed was observed, along with an increase in the amount of erosive and accumulative forms in the researched section, as well as an overall improvement in the hydromorphological status. The assessment conducted in 2022 showed that the hydromorphological status was very good.
  • Deflectors introduced into a lowland river cause a deformation of the streamline, an increase in the velocity of the flow, and an initiation of the processes of lateral and vertical erosion downstream of the deflector, as well as the accumulation of sediment on the convex bank. They are also associated with changes in granularity, which can be significant for macroinvertebrates and ichthyofauna.
  • Studies of macrophytes showed that their ecological status was relatively stable. The desilting carried out in 2011 had a high impact on the quality of the environment, causing the mechanical removal of about 60% of species occurring in it and leading to a drastic drop in their coverage (to approximately 2.5%). The introduction of deflectors contributed to the increase in the biodiversity of macrophyte species. A continual improvement in the ecological status was also observed until a good status was achieved in 2022.
  • The research carried out in the reference section (A) situated approx. 600 m upstream of the research section (B), on which no regulation work has been carried out, showed that it displayed hydrodynamic balance. Regardless of the continual natural changes taking place in the riverbed, it was observed that the dynamic of these changes was significantly smaller than in the section with deflectors. The good ecological status of this section can be regarded as stable over time. The values of the MIR index in that section ranged between 37.0 and 37.9.

Author Contributions

Conceptualisation. S.Z., T.K. and S.J.; methodology. S.Z., T.K. and S.J.; software. S.Z., T.K. and S.J.; validation. S.Z., T.K. and S.J.; formal analysis. S.Z., T.K. and S.J.; investigation. S.Z., T.K. and S.J.; writing—original draft preparation. S.Z., T.K. and S.J.; writing—review and editing. S.Z., T.K. and S.J.; visualisation. S.Z., T.K. and S.J. All authors have read and agreed to the published version of the manuscript.

Funding

The publication was co-financed within the framework of the Ministry of Science and Higher Education program “Regional Initiative Excellence” in the years 2019–2022, Project No. 005/RID/2018/19.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data is unavailable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. The Flinta River catchment with the research section marked: 1—rivers; 2—Flinta catchment; 3—gauge station; A—reference section; B—research section (with deflectors).
Figure 1. The Flinta River catchment with the research section marked: 1—rivers; 2—Flinta catchment; 3—gauge station; A—reference section; B—research section (with deflectors).
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Figure 2. Transformations of section B of the Flinta river (2013—before the introduction of deflectors; 2018—one year after the introduction of the first deflector and one week before the addition of other deflectors; 2022—four years after the introduction of all deflectors).
Figure 2. Transformations of section B of the Flinta river (2013—before the introduction of deflectors; 2018—one year after the introduction of the first deflector and one week before the addition of other deflectors; 2022—four years after the introduction of all deflectors).
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Figure 3. The deflector layout scheme and placement along the studied section of the Flinta River: (a) completed deflector in the river; (b) scheme of the layout of deflectors in the river, b—width of the river, which in the study section varied slightly and ranged from 3 to 4 m. RB—right bank; LB—left bank; α—70–90°.
Figure 3. The deflector layout scheme and placement along the studied section of the Flinta River: (a) completed deflector in the river; (b) scheme of the layout of deflectors in the river, b—width of the river, which in the study section varied slightly and ranged from 3 to 4 m. RB—right bank; LB—left bank; α—70–90°.
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Figure 4. Orthophotomap of the Flinta research study section with deflectors (section B) in 2021, (a) location of cross-sections; (b) enlargement location of deflectors and cross-sections used; S2, S3, S15—samples of the river sediment.
Figure 4. Orthophotomap of the Flinta research study section with deflectors (section B) in 2021, (a) location of cross-sections; (b) enlargement location of deflectors and cross-sections used; S2, S3, S15—samples of the river sediment.
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Figure 5. Changes in cross-sections between 2018 and 2022, for cross-sections (a) no. 4, (b) no. 8 and (c) no. 27.
Figure 5. Changes in cross-sections between 2018 and 2022, for cross-sections (a) no. 4, (b) no. 8 and (c) no. 27.
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Figure 6. Changes in sediment size: (a) point no. S2—downstream of the deflector cross-section (cross-section no. 6—2 m downstream of the deflector in the water current); (b) point no. S3—cross-section downstream of the deflector (cross-section no. 7—1 m downstream of deflector “in shadow” of deflector); (c) reference point no. S15—in cross-section no. 27.
Figure 6. Changes in sediment size: (a) point no. S2—downstream of the deflector cross-section (cross-section no. 6—2 m downstream of the deflector in the water current); (b) point no. S3—cross-section downstream of the deflector (cross-section no. 7—1 m downstream of deflector “in shadow” of deflector); (c) reference point no. S15—in cross-section no. 27.
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Figure 7. Time variability of selected hydromorphological parameters within the research section with deflectors: (a) number of erosive and accumulative forms; (b) HDS index; (c) HMS index; (d) HIR index; h.s.—hydromorphological status.
Figure 7. Time variability of selected hydromorphological parameters within the research section with deflectors: (a) number of erosive and accumulative forms; (b) HDS index; (c) HMS index; (d) HIR index; h.s.—hydromorphological status.
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Figure 8. Comparison of the variability over time of selected hydromorphological parameters between the section with the deflectors and the comparative one: (a) number of erosive and accumulative forms (A—reference section, B—section with deflectors); (b) HDS index; (c) HIR index.
Figure 8. Comparison of the variability over time of selected hydromorphological parameters between the section with the deflectors and the comparative one: (a) number of erosive and accumulative forms (A—reference section, B—section with deflectors); (b) HDS index; (c) HIR index.
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Figure 9. Time variability of selected macrophytes parameters within the research section with deflectors: (a) species number; (b) percentage of coverage; (c) Shannon–Wiener index; (d) MIR index.
Figure 9. Time variability of selected macrophytes parameters within the research section with deflectors: (a) species number; (b) percentage of coverage; (c) Shannon–Wiener index; (d) MIR index.
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Figure 10. Comparison of the variability over time of selected macrophyte parameters between the section with the deflectors and the comparative one: (a) species number; (b) percentage of coverage; (c) Shannon–Wiener index; (d) MIR index.
Figure 10. Comparison of the variability over time of selected macrophyte parameters between the section with the deflectors and the comparative one: (a) species number; (b) percentage of coverage; (c) Shannon–Wiener index; (d) MIR index.
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Figure 11. Changes in the coverage of the most important species of macrophytes in the section with deflectors.
Figure 11. Changes in the coverage of the most important species of macrophytes in the section with deflectors.
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Figure 12. PCA ordination diagram of changes in the taxonomic structure of macrophytes in the years 2005–2022. (a) Scatter plot—sites: diamonds—section with deflectors; circles—comparative section. (b) Scatter plot—species: Agrsto—Agrostis stolonifera; Alipla—Alisma plantago aquatica; Berere—Berula erecta; Calcop—Callitriche cophocarpa; Caracu—Carex acutiformis; Carrip—Carex riparia; Elocan—Elodea canadensis; Epihir—Epilobium hirsutum; Equpal—Equisetum palustre Eupcan—Euphatorium cannabinum; Fonant—Fontinalis antipyretica; Glyflu—Glyceria fluitans; Glymax—Glyceria maxima; Iripse—Iris pseudacorus; Lemmin—Lemna minor; Leprip—Leptodictyum riuparium; Lyceur—Lycopus europaeus; Lynsp_—Lyngbya sp.; Lytsal—Lythrum salicaria; Menaqu—Mentha aquatica; Myopal—Myosotis palustris; Phaaru—Phalaris arundinaceae; Phraus—Phragmites australis; Potcri—Potamogeton crispus; Ranrep—Ranunculus repens; Ransce—Ranunculus sceleratus; Roramp—Rorippa amphibia; Rumhyd—Rumex hydrolapathum; Scisil—Scirpus silvaticus; Scrumb—Scrophularia umbrosa; Siulat—Sium latifolium; Soldul—Solanum dulcamara; Spaeme—Sparganium emersum; Spaere—Sparganium erectum; Spipol—Spirodela polyrhiza; Stapal—Stachys palustris; Verana—Veronica anagallis-aquatica; Verbec—Veronica beccabunga.
Figure 12. PCA ordination diagram of changes in the taxonomic structure of macrophytes in the years 2005–2022. (a) Scatter plot—sites: diamonds—section with deflectors; circles—comparative section. (b) Scatter plot—species: Agrsto—Agrostis stolonifera; Alipla—Alisma plantago aquatica; Berere—Berula erecta; Calcop—Callitriche cophocarpa; Caracu—Carex acutiformis; Carrip—Carex riparia; Elocan—Elodea canadensis; Epihir—Epilobium hirsutum; Equpal—Equisetum palustre Eupcan—Euphatorium cannabinum; Fonant—Fontinalis antipyretica; Glyflu—Glyceria fluitans; Glymax—Glyceria maxima; Iripse—Iris pseudacorus; Lemmin—Lemna minor; Leprip—Leptodictyum riuparium; Lyceur—Lycopus europaeus; Lynsp_—Lyngbya sp.; Lytsal—Lythrum salicaria; Menaqu—Mentha aquatica; Myopal—Myosotis palustris; Phaaru—Phalaris arundinaceae; Phraus—Phragmites australis; Potcri—Potamogeton crispus; Ranrep—Ranunculus repens; Ransce—Ranunculus sceleratus; Roramp—Rorippa amphibia; Rumhyd—Rumex hydrolapathum; Scisil—Scirpus silvaticus; Scrumb—Scrophularia umbrosa; Siulat—Sium latifolium; Soldul—Solanum dulcamara; Spaeme—Sparganium emersum; Spaere—Sparganium erectum; Spipol—Spirodela polyrhiza; Stapal—Stachys palustris; Verana—Veronica anagallis-aquatica; Verbec—Veronica beccabunga.
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Figure 13. Hydrogram for the period November 2016–October 2021.
Figure 13. Hydrogram for the period November 2016–October 2021.
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Figure 14. The variability of monthly average water flows in the Flinta River since the mid-20th century.
Figure 14. The variability of monthly average water flows in the Flinta River since the mid-20th century.
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Table
Table 1. Flows characteristic of the Ryczywół water gauge in the years 1951–2014. Characteristic discharges: NNQ—lowest of the annual low; SNQ—average of the annual low; WNQ—highest of the annual low; NSQ—lowest of the mean annual; SSQ—average of the mean annual; WSQ—highest of the mean annual; NWQ—lowest of the annual high; SWQ—average of the annual high; WWQ—highest annual high.
Table 1. Flows characteristic of the Ryczywół water gauge in the years 1951–2014. Characteristic discharges: NNQ—lowest of the annual low; SNQ—average of the annual low; WNQ—highest of the annual low; NSQ—lowest of the mean annual; SSQ—average of the mean annual; WSQ—highest of the mean annual; NWQ—lowest of the annual high; SWQ—average of the annual high; WWQ—highest annual high.
Characteristic Flows [m3·s−1]
NNQSNQWNQNSQSSQWSQNWQSWQWWQ
0.010.100.410.240.661.720.773.267.28
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MDPI and ACS Style

Zaborowski, S.; Kałuża, T.; Jusik, S. The Impact of Spontaneous and Induced Restoration on the Hydromorphological Conditions and Macrophytes, Example of Flinta River. Sustainability 2023, 15, 4302. https://doi.org/10.3390/su15054302

AMA Style

Zaborowski S, Kałuża T, Jusik S. The Impact of Spontaneous and Induced Restoration on the Hydromorphological Conditions and Macrophytes, Example of Flinta River. Sustainability. 2023; 15(5):4302. https://doi.org/10.3390/su15054302

Chicago/Turabian Style

Zaborowski, Stanisław, Tomasz Kałuża, and Szymon Jusik. 2023. "The Impact of Spontaneous and Induced Restoration on the Hydromorphological Conditions and Macrophytes, Example of Flinta River" Sustainability 15, no. 5: 4302. https://doi.org/10.3390/su15054302

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