*2.1. Study Site*

The hydrogeomorphic impacts of floods were studied on a small tributary of the Bˇelá River in the foreland of the Eastern Sudetes (NE part of Czechia, Central Europe; 50◦17 N, 17◦17 E; Figure 1). The area comprises a Proterozoic and Palaeozoic basement composed of orthogneiss, which is overlaid by glacifluvial deposits of the Saalian and Elsterian glaciations with different grain-size distributions of till sediments. These deposits are the result of deglaciation phases in proglacial areas on the ice margins and were recently covered by the Holocene sandy loam colluvium [40,41]. The study site is characterised by a temperate climate, with a mean annual precipitation between 850 and 900 mm. Most precipitation falls during the spring and summer months [42], with an occasional occurrence of extreme daily rainfall (more than 50 mm per day). Documentary evidence provides information about several flood events (e.g., 1903, 1921, 1971, 1977, 2007, 2009, and 2014), relating to both advective rains and short-term intense rainfall events that are currently responsible for intense gully incision and damage to infrastructure within the studied region [38,43].

**Figure 1.** Location of the study site: position within Central Europe (**a**), within the Eastern Sudetes (**b**), and (**c**) orthophoto image of the studied catchment (1—study channel reach, 2—main road, 3—railway and 4—cycle path).

We focused on an approximate 700-m-long reach of an unnamed first-order stream draining a catchment area of 1.8 km<sup>2</sup> with a mean elevation of 417.5 m a.s.l. Fields and dwellings dominate the upper and middle catchment area, while the lower part (i.e., the study reach) is predominantly covered by a mixed forest (Figure 1c). The mean stream gradient of the study reach is 0.05 m/m, with a maximum up to 0.17 m/m. The channel cuts into the Pleistocene and Holocene deposits, with typical

alternations of stepped-bed morphology and bedrock outcrops (Figure 2a) accompanied by poorly sorted bed sediments (Figure 2b,c). Fresh slope/bank failures (up to 50 m long and 10 m high) and generally unstable banks have caused channel widening during recent floods, resulting in the frequent occurrence of exposed tree root systems and flood scars on riparian vegetation along the entire reach (Figure 2b–d). In contrast, there is no evidence of deep channel incision due to the resistant gneiss bedrock and large interlocked boulders at several reaches within the stream. Slopes and floodplains surrounding the channel are overgrown by a mixed forest composed predominantly of *Pinus sylvestris* L., *Picea abies* (L.) Karst., *Alnus glutinosa* (L.) Gaertn, and *Tilia cordata* Mill. At 0.5 river km (r. km), the stream drains to a 50-m-long concrete trough within a viaduct under a railway. Moreover, culverts are presented under a cycle path in the lower part and under a road through a village in the middle part of the catchment.

**Figure 2.** Morphological features of the study channel reach: (**a**) channel bottom composed of gneiss bedrock; (**b**) channel widening evidenced by the system of exposed tree roots in boulder-step reach; (**c**) step-pool reach with the presence of channel widening and fresh, shallow slope failure; (**d**) flood scars (higher older, and lower younger) on the tree stem of *A. glutinosa* in the direction of flow (note the remnants of concrete parts of centring transported by past floods).
