In the Missouri River main stem reservoirs in Montana, North Dakota, and South Dakota, hydrodynamic processes have decreased the lengths of shorelines and changed their configuration during the first 20 to 25 years of impoundment. Shale shores of Lakes Fort Peck, Oahe, Sharpe, and Francis Case were rapidly eroded by wave action and water level fluctuation. Banks slumped, embayments filled, and stumps and debris were covered by sediment. Shale shores developed mean slopes of 5-7% in sections of reservoirs with 10- to 25-km fetches and more than 10% in sections with 3- to 7-km fetches. Glacial till tended to armor the shorelines and deter their degradation in all reservoirs. Bank-cutting was particularly severe up to 1973 in the readily erodible Fort Union sediments along Lake Sakakawea. Water level fluctuation retarded the development of stable shores. Some aquatic vegetation developed along shorelines and in shallow sections of reservoirs in which water levels fluctuated little. Substrate suitable for terrestrial vegetation did not develop along the shores of fluctuating reservoirs, except where alluvial deposits were present. Physical changes of the shore probably influenced fish abundance and species composition primarily by changing the quality and quantity of spawning and nursery habitat. Species that appeared to be adversely affected by shore changes required protected embayments or flooded vegetation for reporduction: white crappie, Pomoxis annularis: black crappie, P. nigromaculatus: yellow perch, Perca flavescens: northern pike, Esox lucius: bigmouth buffalo, Ictiobus cyprinellus: small mouth buffalo, I. bubalus: and carp, Cyprinus carpio. Fish that spawn in tributaries or on rocky shores were not greatly influenced by shore changes (although the growth rate of several species were slowed): sauger, Sander canadensis: channel catfish, Ictalurus punctatus: white bass, Morone chrysops: goldeye, Hiodon alosoides: and river carpsucker, Carpiodes carpio. One species, the walleye, Stizostedion v. vitreum, appeared to be benefited from the shore changes. Physical changes along shores have not been adequately addressed in reservoir ecological planning. Fish population data collected before reservoir shores have reached a reasonable degree of stability do not provide a reliable estimate of the ultimate species composition in a reservoir.