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fine sediment at the mouths of backwaters in the Grand Valley. The deposited sediment was not displaced during several consecutive years of low flow and backwater mouths progressively filled until fish access was blocked. When vegetation encroaches into the channel during periods of low flow, deposited sediments become stabilized. It then becomes increasingly difficult to scour sediments from these habitats (Pitlick and Van Steeter 1998). Over time, this process leads to a loss of channel complexity and a concomitant loss of the preferred habitats associated with complex sites. Comparing historic with recent aerial photographs of the river, Van Steeter and Pitlick (1998) calculated that 25% of the historic side channel and backwater area in the Grand Valley was lost from this channelization process during the preceding 50 years. Similar analyses by Pitlick and Cress (2000) indicate a 31 % loss of side channel and backwater area in the 45- km De Beque-to-Rifle reach. After the channel is shaped by the high flows of spring, the quantity (total area) of preferred mesohabitats is affected by river stage. In the 15-mile reach, weighted area of mesohabitats preferred by adult Colorado pikeminnow in summer (eddies, pools and backwaters) was 291/o higher at a discharge of 1,630 cfs than it was at 1,240 cfs, and 42% higher than at 2,870 cfs (Osmundson et al. 1995). In 1983, a year of very high spring flows, Carter et al. (1985) mapped Colorado River habitats near Parachute, Colorado at flows ranging from 1,710 to 28,300 cfs (measured at the USGS gage near De Beque). Their 3.2-km (2-mile) study area, judged to be representative of the De Beque-to-Rifle reach, was mapped once in March and then weekly from 18 June to 3 September (12 mapping dates). Wetted area was broken into 12 categories, or mesohabitats. Results revealed that at flows less than 10,000 cfs total area of both pools and backwaters was highest at 1,710 cfs, the lowest flow studied (Fig. 4 and 5). Eddies, however, did not appear until mapping occurred at the next higher discharge, 3,840 cfs. To determine which of these two flow levels provided the greatest area of preferred habitat for adult Colorado pikeminnow, I multiplied the preference rating for each of the three preferred habitat types (derived from Osmundson et al. 1995) by the total area of the corresponding habitat type and summed the values to provide weighted area of preferred habitat at each flow level (see Osmundson et al. 1995 for methods). At 1,710 cfs, the 23