SPATIAL-DISTRIBUTION PATTERNS OF CRUSTACEA IN THE FLOODPLAIN AQUIFER OF AN ALLUVIAL RIVER

Interstitial crustaceans were collected from a grid of shallow samplin g wells penetrating the alluvial floodplain aquifer (ca. 10 m x 5 km x 10 km) of the Flathead River, Montana, USA. Eighteen taxa were identi fied, which collectively encompassed a range of hypogean-epigean affin ities. The subterranean amphipod Stygobromus spp., the most common cru stacean, occurred in all wells but was rare in the channel well. When well data were pooled into 'habitat types' (channel, bank, near-, cent ral-, and far-floodplain), distinct faunal patterns were apparent. Cru staceans constituted an increasing percentage of the total interstitia l fauna from the channel to the near-floodplain, then maintained simil ar relative abundance levels with increasing distance from the river. Stygobionts attained maximum values at near- and central-floodplain ha bitats where copepods and ostracods dropped to the lowest levels. Dist ribution and abundance patterns of Crustacea at the floodplain scale a re structured by hydrogeologic and geomorphic processes reflected only in part by distance from the river channel. The flood plain appears t o contain a latticework of alluvial-filled paleochannels of high hydra ulic conductivity that induce spatial discontinuities within the aquif er and that may play an important role in determining crustacean distr ibution patterns.