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

Abstract

After the construction of two relatively large reservoirs in the late 1960s on the River Spöl, discharge was greatly reduced and regulated at a relatively constant flow. Following flow regulation in 1970, natural floods occurred only in the lower Spöl 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 Aquatic Sciencesinto 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.

Linking geomorphic character, behaviour and condition to fluvial biodiversity: implications for river management

“…The frequencies of such flows are often altered downstream of large impoundments, with major and interlinked consequences for river geomorphology and biology (e.g. Petts, 1980;Ligon et al, 1995;Power et al, 1996;Mu¨rle et al, 2003). However, biological condition may also be influenced substantially by artificial hydrological alterations that are unlikely to induce geomorphic change } for example altered seasonal timing of flows and abstraction of water at times of low flow (Bunn and Arthington, 2002).…”

Crenic habitats, hotspots for freshwater biodiversity conservation: toward an understanding of their ecology

“…Under natural flow regimes, fine sediment is removed by high-flow events involving bed load movements, thereby resetting the colmation process (Brunke & Gonser 1997). Impoundment of the River Spol in Switzerland resulted in decreased discharge and a flow regime unable to effect bed mobilisation, leading to clogging of the bed interstices (Murle et al 2003). A similar experiment conducted in the River Rhone flood plain revealed the importance of high-flow events for maintaining connectivity between surface waters and groundwaters (Claret et al 1999).…”

An experimental assessment of the effectiveness of gravel cleaning operations in improving hyporheic water quality in potential salmonid spawning areas

“…Infiltration of fine sediments into gravels in the course of high flow velocities has been demonstrated experimentally (e.g. Beschta and Jackson, 1979), but clogging due to proliferated microbial growth in sediments which were not frequently scoured could be a much more severe problem for potential salmonid spawning streams (Battin and Sengschmitt, 1999;Mürle et al, 2003). The importance of the hydrography on the DO concentrations in the hyporheic zone was demonstrated by Groves and Chandler (2005) in a 2-year study with different discharge patterns during the subsequent years.…”

“…Frequently occurring flood events can winnow out fine sediments and prevent an extensive microbial growth within the interstices of the gravel bed even if no bedload transport occurs (Zeh and Dönni, 1994). This results in an exchange of oxygen-rich surface water with the pore water of the hyporheic interstices, stressing the significance of a natural discharge regime for maintaining high instream-dynamics (Sear, 1993;Gibson, 2002;Mürle et al, 2003).…”

“…Focusing on the hydrological aspects, floods and discharge variability are considered to be important environmental factors during the spawning season (Sear, 1993;Wu, 2000;Mürle et al, 2003;Groves and Chandler, 2005). Infiltration of fine sediments into gravels in the course of high flow velocities has been demonstrated experimentally (e.g.…”