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Table 1. Location and attributes of study sites. ite Location (RM) Length (mile) Number of Transects CH BA pring Known Use Summer inter 15-mile Reach 1 178.0-178.5 0.5 2 0 CS CS - 2 175.4-175.9 Oe5 1 3 CS CS CS 3 175.0-175.3 0.3 1 3 CS,RZ CS ° 4 174.1-174.8 0.7 2 0 CS,RZ CS CS 18-mile Reach 5 170.2-170.8 0.6 1 2 CS CS CS 6 168.2-168.6 0.4 2 2 CS,RZ CS,RZ CS,RZ 7 162.6-162.8 0.2 1 2 - CS, LV,YOY - 8 157.8-158.7 0.9 2 2 CS LV,YOY - CS adult Colorado squawfish RZ adult razorback sucker LV larval Colorado squawfish YOY young-of-year Colorado squawfish CH channel BA backwater RM river mile In the upper Colorado River, unpublished USFWS capture data show that the bulk of the adult Colorado squawfish population occurs in the upper 60 miles of river (between the top of West- water Canyon and the Grand Valley Diversion dam), while most juveniles occur in the lower 112 miles of river. As juveniles mature, many move to the upper reaches where physical habitat and food supplies are ostensibly more favorable. Within the upper 60 miles of river, the 15-mile reach produced 43% of the fish captured in a systematic netting and electrofishing survey conducted during 1991-1994, though the reach constituted only 23% of the area length. Within the 15-mile reach, study sites that we selected were 0.3-0.7 miles in length, averaging 0.5 mile, and together (2.0 MI) comprised 14.1 % of the reach (14.2 mi). In contrast, 66.8% of all squawfish radiotelemetry locations (1986-1988) in the 15-mile reach (n=377) occurred within the four study sites, and 76.5% of all 15-mile reach, razorback sucker locations (n=17) occurred there. Thus, from past and concurrent studies, we have identified the hierarchial habitat selection process of adult squawfish within the Colorado River. Adults first concentrate in the upper portion of the 8