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sediment while the banks and floodplain are made up mostly of fine sand and silt. In many places, tamarisk, Russian olive and willow line the banks; in addition, banks in many places have been artificially modified by levees and rip-rap. Information on average channel width and depth as well as flows necessary to top the banks within the 15-mile reach is currently being prepared by Van Steeter and Pitlick (University of Colorado). Flows in the 15-mile reach are greatly reduced during the irrigation season (April-October) when two local irrigation systems withdraw large amounts of water from the river. The Grand Valley Canal diversion is immediately upstream of the 15-mile reach; the Government Mghline Canal dam and diversion is located nine miles upstream. Together, they account for a net loss to the 15-mile reach of 1200 to 1600 cfs, depending on the month. These canals have been in operation since before the turn of the century. In addition to this, large dams and transbasin diversions have been built in the headwaters of the Colorado River beginning in the mid-1930's; this has further reduced flows in the 15-mile reach, particularly during the spring runoff period. Changes in the hydrologic regime of the reach since the historic period were described by Osmundson and Kaeding (1991; see Appendix IX). Fish that share the 15-mile reach with the endangered Colorado squawfish and razorback sucker include four other native species and at least 17 non-native, introduced species (Appendix II; Table 1). Flannelmouth sucker (Catostomus latipinnis) and bluehead sucker (Catostomus discobolus), both native species, are the most common of the large fish in the reach; red shiner (Cyprinella lutrensis), fathead minnow (Pimephales promelas) and sand shiner (Notropis stramineus), all non- native species, are the most abundant of the small fishes there (Osmundson and Kaeding 1989; USFWS unpublished data). Summer and Winter Flow Needs Site Selection Eight study sites were selected (Table 1); four in the 15-mile reach (Fig. 2); four in the 18-mile reach. Known concentration areas for one or both species were selected as study sites. Concentra- tion areas for adults were identified using location data from a previous radiotelemetry study (for sites in both the 15- and 18-mile reaches); for larvae and young-of-year Colorado squawfish (18- mile reach), from previous dip-net and seine surveys (data from Osmundson and Kaeding 1989). We felt that specific areas the endangered fish have selected over other areas are most important and were the ones to focus our studies on. This is in contrast to having selected sites on the basis of how representative they are of the reach as a whole as IFIM attempts to do. Our focus on preferred habitat types within reaches selected by the fish follows the hierarchial nature of the habitat selection process described by Johnson (1980). First-order selection is the selection of physical or geographical range of a species (Colorado River basin in this case). Within that range, second-order selection determines the home range of an individual or social group (a particular reach within a particular river) . Third-order selection is the usage made of various components within the home range (mesohabitats). Fourth-order selection, not investigated in this study, is the procurement of food items from those available at the third-order selected sites. 7