Flow refugia for benthic macroinvertebrates during flooding of a large river

Benthic macroinvertebrates shifted from deep water to shallow water of the shore zone during annual flooding of a large, gravel-bed river. The shore z one is seasonally inundated during flooding for up to 4 mo of the year, and is a geomorphic feature of many large rivers in northern latitudes with li mited floodplain habitat. We collected samples of invertebrates along with measures of several hydraulic and sedimentary variables 5 times through the flood cycle from 4 fixed water depths; stations shifted laterally over a d istance of 30 m between baseflow and peak flow. In deep water (1.5 and 3.0 m), shear velocity and substrate mobility increased as a result of flooding . At shallow depths (0.2 and 0.5 m) within the shore zone, stable substrate and low hydraulic stress remained throughout the flood cycle, providing fl ow refugia to benthic invertebrates. Invertebrate density and species richn ess were similar during months on the rising limb of the hydrograph while t he wetted area of the channel markedly increased. Persistence of the invert ebrate community appears to have been facilitated by a shift in a large pro portion of organisms from deep water to shallow depths of the shore zone du ring flooding. Total density and the density of most collector-gatherers su ch as Rhithrogena and Baetis was highest at 1.5 m prior to flooding but shi fted to depths of 0.5 and 0.2 m at peak flow The filter-feeding caddisfly H ydropsyche was most abundant at 1.5 m in all months of the study while the location of 1.5 m samples shifted laterally over a distance of 30 m through the flood cycle. The ecological importance of the shore zone as a flow ref ugium was demonstrated by the broad diversity of species with varying feedi ng behaviors and morphologies that concentrated in this zone during floodin g.