**1. Introduction**

Present-day requirements imposed on river and canal maintenance are closely related to the assurance of good ecological water quality and adequate condition of the semiaquatic ecosystem comprising the river and its valley [1,2]. The ecological water quality of rivers and streams is affected both by the hydromorphological conditions of the watercourse channel and by biological factors, related to the flora and fauna characteristic of its specific area [3–5].

In turn, the condition of a semiaquatic ecosystem is connected with the spatial system of the watercourse and with the biological equilibrium within the valley [6]. This should be reflected in the approach to river maintenance, particularly the adopted flood control system [7]. Presently binding guidelines for the maintenance of rivers classified as natural (or semi-natural) not only

accept vegetation within their channel cross-section, but also assign them numerous protective and environmental functions [8–10]. This relates to the comprehensive, modern understanding of water management, which for planned engineering measures requires an environmental impact analysis following the principles of sustainable development [1].

The legal framework for works in rivers is outlined, among others, in the Water Framework Directive. Recently, the European Union has enacted the Water Framework Directive with the aim of securing and, where possible, improving the ecological status of watercourses throughout all Member States. The fundamental legal issues are set out in the Acts of the European Parliament, which define the rights and responsibilities of all parties with respect to rivers and streams [1–3]. In addition, in many countries separate measures are being implemented to improve the natural condition of rivers (e.g. in the USA, Canada, Norway) [6,9].

The above-mentioned conditions need to be reconciled with economic concerns imposed by the analysis of efficiency for water management investments. These include primarily flood control measures, limiting the highest water stages and flows, as well as actions ensuring good ecological status of surface waters at the lowest water stages and flows [1]. Thus, these requirements jointly result in the need to precisely estimate hydraulic parameters of flow in rivers and canals [11]. It is the accuracy of assessment that determines the decision to adopt a specific solution, as well as the technology required to implement it. Referring this aspect to river hydraulics, it may be treated as an evaluation of river channel capacity under specific geometric parameters, river regulation infrastructure, and vegetation growth [12]. This pertains particularly to overgrown river channels and conditions found in rivers with floodplains.

Growth of aquatic vegetation is the main factor in determining flow conditions in small rivers and canals [9]. Aquatic vegetation in channel sections causes an increase in flow resistance. It is a potential source of flood risk and resulting losses [13], when heavy rains in the vegetation period may lead to flash floods in small watercourses, whose flow capacity is reduced as a result of vegetation overgrowth [12]. This in turn leads to a decrease in channel flow capacity. Growth of aquatic vegetation is the primary biological process affecting flow conditions. This growth is modified by hydraulic and geometric parameters of the channel, as well as other physical factors (light, temperature, changes in water levels) along with chemical, edaphic, and biotic factors [14,15]. However, the main factor initiating the whole cascade of resulting phenomena is connected with sunlight, which, in association with biogenic compounds availability, limits productivity of aquatic ecosystems [16–18].

Growth of aquatic plants (macrophytes) depends on the action of various ecological factors. The most important of these include light and heat, water quality [19], pH, water hardness, water current intensity [20], and bed substrate [21]. The response of aquatic plants to ecological factors is very strong, which has contributed to their utilization in the evaluation of the environmental condition. In most European countries, systems based on macrophytes are extensively applied to assess water status [22]. The value of macrophytes as bioindicators is related to their capacity to characterize more permanent changes in habitat characteristics [19].

Growth of aquatic plants is strongly dependent on light availability [23]; thus, all changes in water transparency lead to changes in the structure of plant communities, vegetation density, and depth of plant growth [24,25]. The maximum increase in biomass of aquatic vegetation is observed under particularly advantageous growth conditions, including insolation [26,27]. The light factor determines not only biomass of aquatic plants, but also their structure, as an increase in insolation results in the development of plant organization. Additionally, the light factor in the water affects growing plants through modification of water temperatures [27,28].

Numerous studies indicate that an important factor differentiating lighting conditions in watercourses is the vegetation-growing process on their banks, particularly forest and shrub communities [29]. In the case of small and medium-sized rivers flowing under the forest tree canopy, the amount of light reaching the water surface is several to several dozen percent lower than in rivers flowing through open areas. Such a situation may result in almost complete elimination of aquatic plants [6,30,31]. In other countries, the close dependence of aquatic plants on light conditions has already been used in practice to reduce excessive river channel vegetation overgrowth [18]. It has been suggested to increase shading of rivers in order to limit growth of common species of submerged plants [25]. This method has also been proposed to control excessive spread of invasive species in watercourses [24].

Trees and shrubs are natural landscape components in river valleys [32]. Factors affecting the richness of vegetation surrounding the channels and floodplains of lowland rivers include much slower water flow velocity, a greater width of the floodplain valley as well as river meandering and oxbow lake cutoff. The character of vegetation in the immediate vicinity of a watercourse channel and floodplains depends on the type of land use, groundwater levels, and frequency of flooding [12]. Typically, the area not utilized agriculturally is overgrown with floodplain forests. The areas with non-forest vegetation are usually various types of meadows and pastures, while in more moist locations, they are sedge rushes or fens [33]. Vegetation growing on riverbanks and floodplains has a considerable modifying effect on flow conditions. Apart from the obvious effect of direct factors on flow resistance during high flows (e.g. debris, bottom deposits, local obstacles) an essential role is also played by indirect factors, e.g. shading of the river channel and inhibition of aquatic vegetation growth. These changes are also reflected in river channel hydromorphology and the distribution of flow velocities [13].

When modeling flows in natural river channels, it is necessary to consider the effect of vegetation on flow resistance [34]. A problem typically found in vegetation-covered areas is related to the different flow dynamics compared to the dynamics of a stream free from vegetation. Velocities and depth gradients are then much smaller. This is naturally reflected in the dynamics of debris transport and hydromorphological changes in the watercourse channel [5,34,35].

In order to determine the shading area, the location of trees within the riverbank zone needs to be described. Energy reaching the ground in a given spot depends on the position of the sun, insolation time, and cloud cover. Due to the continuously changing cloud cover, the input energy fluctuates considerably, and the changes may be detected only through precise measurements. The weather service reports mean monthly diurnal total solar radiation for various weather stations. This facilitates the development of an average annual model for the area covered by a given station. Based on the available data, Rickert [36] proposed a procedure to calculate the effect of shading on macrophyte growth. Data include photosynthetically active radiation (PAR) reaching the water surface and the rest of the radiation, which may be expected in the river channel at various turbidity levels based on total radiation in % or expressed in W/m2. This method is relatively labor-intensive and includes the need for analyses of individual hourly sequences. At a tree height of, e.g., 10 m, this is equivalent to shade range of 8 m in length. Solar radiation reaching the water surface is partly reflected and partly intercepted. The percentage of solar light reflection depends on factors such as angle of incidence (which then depends on latitude end land relief), wavelength, and land structure. The smaller the angle of incidence is, the greater is the amount of light which is reflected.

Variation in solar radiation is a fundamental to control most processes on the Earth. The solar radiation reaching the water surface is controlled by location (latitude, longitude elevation), season, atmospheric conditions, topography, and vegetation. Shading is a key parameter due to the control on the amount of direct radiation reaching the water surface. The factors influencing the fraction of solar radiation reaching the water surface can be taken into account by using tools and models working in a GIS environment. Nowadays calculations and analyses are carried out in different scales on the basis of data with different spatial resolution and type. Increasingly often high-resolution LIDAR data and GIS tools are used in shading analyses or assessments of incoming radiation reaching surface water bodies [37–40].

LIDAR data provide information on riparian attributes related to elevation, biomass overhanging the river, and vertical tree structure. In addition, high-resolution products generated by LIDAR data processing, such as the digital surface model (DSM) and digital elevation model (DEM), are used [41]. Bachiller-Jareno et al. [42] presented a methodology to estimate tree height and canopy extent, and

a model that simulates the position of the sun across the sky for hourly or sub-hourly intervals to calculate the daily shade over the river surface. In addition, Loicq et al. [40] described the use of a spatially explicit method applying LIDAR-derived data to compute riparian shading based on direct and diffuse solar radiation. Karrasch and Hunger [41] demonstrated that during shading modeling based on DSM and DEM the illumination of the water body is underestimated, while shading is overestimated. In the United Kingdom, the Environment Agency (EA) has developed catchment 'shade maps' for every catchment managed under the Water Framework Directive in England and Wales [43].

The aim of this study was to determine the effect of vegetation growing on the Wełna riverbanks on changes in flow conditions within a relatively short river reach. Analyses were conducted from July to August.
