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The bed of the Colorado River is composed of cobble- and gravel-sized particles, except for a short bedrock section in Ruby-Horsethief Canyon known as Black Rocks. The banks and adjacent floodplam are composed of silt and sand covered with thickets of the nonnative tamarisk (Tamarisk chinensis) and russian olive (Elaeagnus angustifolia), and the native sandbar willow (Salix exigua) and cottonwood (Populus deltoides). In many places in the Grand Valley the banks have been artificially modified by levees and rip-rap. Otherwise the river is "alluvial", meaning it is free to adjust its width and depth. Average gradients of the 15-mile, 18-mile and Ruby-Horsethief Canyon Reaches are 0.00175, 0.0013, and 0.0010, respectively. In the Grand Valley, the Colorado River maintains a "wandering" channel pattern formed by single-thread and multi-thread reaches. In multi-thread reaches the channel can split into a series of islands, side channels and backwaters. These reaches contain more diverse and heterogeneous habitats, which may explain the association between channel complexity and squawfish numbers (Osmundson and Kaeding, 1991). Compared to more incised reaches further downstream, side- channels and backwaters are relatively common in the Grand Valley area. Figure 3 shows an aerial photograph of a prominent island and backwater in the 15-mile reach. At high discharge water enters the side channel from upstream at a point not seen in this photograph, and flows out the mouth. As the discharge drops, water no longer enters the side channel, and instead ponds up into the area downstream, forming a backwater. Other features seen in this photograph include an island with traces of the former channel, an active gravel bar, and portions of two runs. :.?-? C Also included in this report are descriptions of geomorphic changes and existing conditions along an 85-km reach of the Gunnison River between Delta, CO, and Grand Junction (Fig. 2). The Redlands Diversion dam, located 4 km upstream from the Colorado River confluence, has historically blocked fish from migrating very far up the Gunnison River. However, the recent completion of a fish passage structure at the diversion dam now allows fish to access reaches of the Gunnison River above Grand Junction. The Gunnison River is similar to the Colorado River in many respects, the main difference being that the Gunnison River is more incised than the Colorado, and the channel is less complex overall (see also Milhous, 1998). 6 Figure 3. Aerial photograph of a segment of the Colorado River near RM 175.