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, 3 , Finally, a series of measurements were taken at a site in the 18 mile reach to evaluate conditions under which the coarse bed material of the Colorado River becomes mobile. , Setting The study area covers approximately 32 miles (51 kilometers) of the Colorado River near Grand Junction, Colorado (Fig. 1). This area is commonly referred to as the 15 and 18 mile reach and is important habitat for both the Colorado squawfish and razorback sucker (USFWS, 1987). The flow of this part of the Colorado River is dominated by snowmelt with peak flows occurring usually near the end of May (Fig. 2). The 1993 peak flow was 1,250 cubic meters per second (m3/ s) (44,300 ft3/ s) which has a recurrence interval of approximately 8 years. Both bank stabilizing activities and changes in the natural flow regime have affected this area. Much of the land along the 15 and 18 mile reach is privately owned. Property owners have built dikes or stabilized the river channel in attempts to stop the river from encroaching on their land. Dikes or concrete debris confine the river on at least one of its banks throughout much of the study area. As was discussed above, this decreases the river's ability to migrate and potentially decreases the channel complexity. In addition to bank stabilization, the hydrologic regime of the 15 and 18 mile reach has been altered by upstream water development. For example, Liebermann et aI. (1989) suggested that flows at the Cameo gauge, just upstream of the study site, have decreased through time due to upstream water development. The median flow has decreased by 13% when comparing the pre-intervention median (1934-49) to the post- intervention median (1950-83). , , , ~ . ~ . . ,