Effects of experimental flooding on riverine morphology, structure and riparian vegetation: The River Spol, Swiss National Park

After the construction of two relatively large reservoirs in the late 1960s on the River Spol, discharge was greatly reduced and regulated at a relatively constant flow. Following flow regulation in 1970, natural floods occurred only in the lower Spol where the river is joined by a large tributary, the River Cluozza. Flow competence of the residual flow in the regulated river section was too low to transport downstream the input of inorganic and organic sediments from tributaries and side-valley scree (talus) slopes. Consequently, sediments accumulated on the riverbed, and alluvial fans from scree slopes extended into the river channel. The lack of flood disturbance also allowed woody vegetation to develop on previously exposed gravel banks, and the porous river bottom became clogged with fines. After the experimental floods in 2000, most alluvial fans in the channel were scoured downstream and bed sediments became less embedded due to the reduction in fines. The initial floods caused a rather broad accumulation of coarse sediments in wider reaches of the river, whereas the later floods mobilized/scoured these sediments and increased the variation in channel depth.