Long-term ecological responses of the River Spöl to experimental floods

The river Spöl flows from Livigno reservoir on the Swiss–Italian border in the central Alps. Flow regulation since 1970 has decreased and stabilized the mean annual discharge of 8.6 to 1.0 m³/s (winter: 0.55 m³/s, summer: 2.5 m³/s). Thirty experimental floods were released between 2000 and 2016 to improve the ecology of the river. A 100-m reach ∼2.3 km downstream of the dam was used to monitor changes in water physicochemistry, periphyton, transported and benthic organic matter, and macroinvertebrates. Fish redds (native Brown Trout) were counted each autumn along the same stretch (∼2.6 km) of river. Most physicochemical variables displayed strong seasonal cycles unrelated to the flow program, but N, P, and water temperature increased significantly over the 18-y study. Seston decreased after each high flow, but was seasonal and increased over the study. Periphyton biomass decreased after each high flow to low levels and increased between high flows. Benthic organic matter decreased 2× over the study but increased between floods. Macroinvertebrate densities decreased over the study. High flows reduced macroinvertebrate densities to 200 by 2003. Redd counts decreased to