The hydrologic, geomorphic, and ecologic effects of reservoir operations are thought to be key factors in the decline of native fishes in the upper Colorado River basin. The present paper examines the extent to which changes in streamflow and sediment loads have affected alluvial reaches of the Colorado River near Grand Junction, Colorado. The analysis shows that since 1950, annual peak discharges of the Colorado River and its major tributary, the Gunnison River, have decreased by 29--38%. The total volume of runoff delivered to the study area has not changed significantly over the period of record, but the annual hydrograph has been modified greatly by reductions in peak flows and augmentation of base flows. Annual suspended sediment loads of the Colorado River and Gunnison River have likewise decreased. This was particularly apparent during the period from 1964 to 1978, when annual sediment loads were 40--65% less than the long-term average. Analysis of aerial photographs indicates that between 1937 and 1993 the main channel of the Colorado River has narrowed by an average of 20 m and about 1/4 of the area formed by side channels and backwaters has been lost. ¿ 1998 American Geophysical Union