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.