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These results indicate that the principal historical change in the Rifle-DeBeque reach has been for the channel to become less complex as a result of side channels filling in. The process by which this occurs is envisioned as follows. Side channels are characterized by lower flow depths and lower flow velocities than the main channel, thus even under natural conditions their sediment transport capacity is less than the main channel. If, as we have indicated in earlier work (Pitlick et al., 1999), the amount of sediment delivered to these reaches has not changed appreciably, but the river has lost some of its ability to carry this sediment, then whatever it cannot carry will be stored somewhere in the channel. Side channels are the likely sites of storage because they have a lower sediment transport capacity. It is also true that flows through side channels are more ephemeral. Side channels are topographically higher than the main channel, and so are not inundated as often- some side channels may experience flow every year, while others may not experience flow for several years, and then perhaps for only a few days. This allows sediment to build up on the bed, and increases the chance that vegetation will colonize the deposits and stabilize them. Vegetation promotes further deposition until, eventually, the side channel fills to the level of the floodplain. It is hard to say whether a 20-40% reduction in side-channel habitats is significant as far as the native fishes are concerned, or whether this would limit their reintroduction, but it seems clear that these features have been lost over time and the channel is less complex now than it was in the past. In previous studies, Pitlick et al. (1999) found that between 1937 and 1993 the total area of the main channel of the Colorado River decreased by 15%, and the total area of side channels and backwaters decreased by 26% (Table 4). When proportioned over a total reach length of 84 km, the changes in main-channel and side-channel area equated to decreases in average width of about 20 m and 7 in, respectively. The present study yields similar results for the Rifle-DeBeque reach, although the change in main channel width appears to be negligible. In comparing the two reaches (Tables 3 and 4), we note that the total area of side channels remaining in the Rifle-DeBeque reach (-70 ha) is about the same as the total area of side channels lost in the Grand Junction reach (-60 ha). This result has important management implications because it suggests that a considerable amount of potential habitat for the Colorado pikeminnow is available upstream, and that access to this additional habitat can be provided by outfitting the diversion dams in DeBeque Canyon with fish- passage structures. However, Osmundson's (1999) analysis of water temperatures in these upper reaches suggests that conditions favoring growth of Colorado pikeminnow may be sub-optimal much of the time. It is not really clear, therefore, that the additional habitat in the Rifle-DeBeque reach is as suitable in a biological sense as the reaches downstream. This question probably warrants further study, but our results suggest that adult pikeminnow may benefit from having access to the reach, particularly in low-water years when water temperatures are higher. Table 4. Summary of previous analyses of geomorphic changes within the 15-mile, 18-mile and Ruby-Horsethief Canyon reaches of the Colorado River (from Pitlick et al., 1999). -------------------------------------- 1937 ---------------- 1993 Total Area Total Area (ha) ----------------------------- - - (ha) - - ----- Main Channel 1125 ---------------- 958 Islands 460 419 Side Channels 225 -------------------------------------- 167 ---------------- Change in Change per Change Total Area Unit Length' in Area (ha) (m) -------- - (%) ------------- ----------------- -167 ------------ - -20 -15 -41 -5 -9 -58 -7 -26 1. The change in area per unit length is computed on the basis of a total reach length of 84 km. Islands and side channels are not continuous over this length. 21