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.