The influence of floods on benthic insect populations in a Swiss mountain stream and their strategies of damage prevention

This paper describes flood induced changes of stream bed morphology and the effects of spates on the populations of dominant benthic insect taxa of a Swiss mountain stream (Kalte Sense, CH). The typical spates showed a steep increase of discharge, due to the topography of the catchment area. Within 6 hours the discharge may rise from 0.5 up to 20 m(3)/s or more. During the 475 day study 45 floods were recorded of which 10 had discharges ranging f rom 15 to 25 m3/s. The floods which exceeded 15 m3/s caused large scale bed sediment movements and severe reductions in macroinvertebrate abundances. The effects of four flood situations on five insect taxa (Heptageniidae, Ba etidae, Leuctridae, Chironomidae, Simuliidae) were recorded and analysed. I n general. young larvae were affected much more than the older stages. Ther efore, the severity of the floods and the amount of bed sediment transport correlated strongest with the reduction in abundance of young larvae. Indiv idual severe flood events reduced the insect fauna to 60% or even 27% of th e preflood value, a series of severe floods in November 1992 resulted in a reduction to 8%. However, during the period from July to December, the inse ct populations usually recovered within 3 weeks and losses were compensated by young larvae which had just hatched.