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I I 1 I I I I I I I I I I I I I I I I 1 INTRODUCTION Problem Statement It has been suggested in a number of recent reports (Stanford, 1994; Tyus, 1992; Osmundson and Kaeding, 1991) that the decline in native fish populations in the Colorado River system is due primarily to habitat loss and environmental changes associated with the construction of reservoirs and water diversions. This is a reasonable suggestion given that reservoirs modify flow hydrographs and reduce sediment loads, and these effects typically cause the river to become narrower downstream (Andrews, 1986; Williams and Wolman, 1984). However, until we began examining this issue, no one had actually determined the significance of channel change on the Colorado River nor had the link between flow regulation and channel change been well established. Our purposes then in undertaking geomorphic studies in the Grand Valley area and Ruby-Horsethief Canyon were to quantify historic (1937-present) changes in river morphology, to study the response of existing habitats to the present-day flow regimen and to predict the flow magnitude which will transport coarse sediment for habitat creation and maintenance. In previous reports (Van Steeter and Pitlick, 1994; Pitlick and Van Steeter, 1994), we have indicated that there is a tendency for the river to become narrower and for potential habitat to be lost when low or moderate flows occur several years in succession. And, although the long-term trend is toward a less complex channel, we have documented the formation of new habitat in areas that were changed by very high flows in 1983 and 1984. This report describes work in progress aimed at defining historic changes in river morphology, effects of the 1993 and 1994 snowmelt flows on specific side channel/backwater habitats, and an evaluation of the threshold for transport of coarse bed material. A historic analysis of changes in channel morphology was completed by using historic and recent aerial photographs.