Aquatic biota in the unchannelized Missouri River (South Dakota) was studied prior to construction of bank-stabilizing structures. Cattail-dominated marsh habitat adjacent to the main channel was important to the production of benthos, aufwuchs, drift, fishes and, to a lesser extent, zooplankton. The importance of this habitat to the integrity and stability of the biota of this large lotic ecosystem was emphasized. Colonization rates and taxonomic diversity on artificial samplers by aufwuchs macro-invertebrates were faster and larger and the standing crop was 71% greater in the unchannelized river than in the bank-stabilized river. The drift was dominated by aufwuch taxa, principally caddisflies (Hydropsyche spp.). Fishes from 35 species utilized marsh habitat; primarily as a nursery while immature. (Modified author abstract)