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construction in the upper basin increased dramatically in the 1950's and 1960's with the Colorado River Storage Project (CRSP). Regulated flows from high dams t and diversion structures upstream of historic bottomland areas remove water from the river for municipal, industrial, and agricultural uses. These activities have affected the main channel and, in some instances, eliminated off-channel habitats such as backwaters, embayments, and bottomlands as nursery areas for some of the endangered fishes. The most noticeable change has been the reduction in historic peak flows. The Gunnison River is influenced greatly by upstream water diversions, operations of the Aspinall Unit, and other projects that were constructed at about the same time as the Aspinall Unit. The greatest change caused by the ' Aspinall Unit is the reduction of spring runoff. Peak discharge has steadily declined since continuous flow records began in the early 1900's. McAda and Kaeding (1991) analyzed and compared stream flow data on the Gunnison River for ' three water development periods: pre-development (1914-1938), middle development (1939-1965), and post-Aspinall development (1966-1988), and identified several changes in magnitude, timing, and duration of streamflow. Mean maximum annual discharge (average of the mean-daily flow for the highest day of the year) for ' the Gunnison River declined about 47% between the early development and post- Aspinall periods. Mean monthly flows have also changed over the three development periods. In general, spring and early summer flows have declined, and fall and winter flows have increased (Figure 2). Mean-monthly flows in May and June, the peak months of runoff, have declined 43 and 47%, respectively, in the Gunnison River. ' The mean total annual flow for the pre-Aspinall water development period (prior to 1965) and the post-Aspinall water development are very similar (1.8 million acre feet). However, the hydrograph has dramatically changed. McAda and t Kaeding (1991) estimated the effect of the Aspinall Unit on runoff by comparing the change in streamflow from the middle-development period to the post-Aspinall period. Mean-monthly streamflow declined about 1.4% in April, 25.1% in May, and ' 32.2% in June. Streamflow during the remainder of the year increased, particularly during the winter when average flows were more than 100% greater than the pre-Aspinall flows. Gunnison River mean-monthly flows have declined an average of 1,864 cfs in May and 2,346 cfs in June. Therefore, the loss of peak flows during runoff (Figure 3) has reduced the area and frequency of flooding in lowland areas (Appendix 1: Photos 4 and 5). Cooper and Severn (1994c) compared the mean number of days that flows exceeded 10,000, 15,000, and 20,000 cfs for two periods of water development (1897-1965: pre-Aspinall and 1966-1993: post-Aspinall). They estimated flows ' during post-Aspinall development exceeded 10,000 cfs on the average of only 6 days annually as compared to 22 days for the pre-Aspinall development period. The pre-Aspinall period also had, on average, a period of approximately 7 days with flows greater than 15,000 cfs, while the post-Aspinall period had only one ' day per year with these flows. The frequency of days with flows greater than 20,000 cfs is about 10 times greater (2 days vs. 0.2 days) in the pre- vs. post- Aspinall period of record. 6