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Il?TROD?TICH This paper is part of a Final Report submitted to the Bureau of Reclamation (BOR) in fulfillment of Contract No. 6-CS-40-04490, entitled "Winter Habitat Study of Endangered Fish - Green River." It integrates the results of a 2-year investigation (1986-87 and 1987-88) of wintertime movement and habitat of adults of the rare and endemic Colorado squawfish (Ptychocheilus lucius) and razorback sucker (Xyrauchen texanus) in the Green River. The investigation was conducted from December 1 to March 31 of each year in the 100-mile reach from Echo Park, CO to Ouray, UT. The objectives of this investigation were to: 1. Determine if adult Colorado squawfish and razorback suckers overwinter in specific river regions. 2. Describe microhabitat (water column depth, velocity, substrate, cover and temperature) used by overwintering adult Colorado squawfish and razorback suckers. 3. Evaluate fish movement and changes in habitat with changes in flow of the Green River during the winter months. This investigation was conducted on the Green River from Echo Park, CO to Ouray, UT (Figure 1). This 97-mile reach of river has three flatwater sections; Echo Park (2 miles), Island and Rainbow Parks (7 miles), and Split Mountain to Ouray (72 miles). It also has two whitewater sections; Whirlpool Canyon (9 miles) and Split Mountain Canyon (7 miles). For the purposes of this investigation, the study area was divided into upper, middle and lower regions according to stream gradient and river morphology. The upper region from Echo Park to lower Split Mountain Canyon (25.3 miles) has a relatively steep gradient with primarily a gravel-boulder-bedrock substrate. The middle region from lower Split Mountain Canyon to Jensen Bridge (18.3 miles) has a moderate gradient with a gravel-cobble substrate in the upper reaches and a sand substrate in the lower reaches, and the lower region from Jensen Bridge to Ouray Bridge (53.5 miles) is a low-gradient, meandering river with primarily a sand substrate. This investigation consisted of two phases. Phase I involved capturing 1.0 adult Colorado squawfish and 10 adult razorback suckers in mid to late October and surgically implanting each with a radiotransmitter. ' Phase II consisted of six 10-day tracking and monitoring trips over a 4-month winter period from December 1 to March 31. The same general approach was used for both years of the investigation; year 1 = 1986-87; and year 2 = 1987-88. Twenty different fish of each species were radiotagged over the 2-year period. 28