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periods rendered it difficult to predict passibility of these <br />structures and created continuity and coordination problems. <br />The San Juan River exhibited highly variable physical <br />attributes during the study period. This was due mostly to <br />variation in discharge and because of man-made modifications and <br />natural changes in stream morphometry. Upstream of Shiprock, the <br />river was constrained by levees, and most of the flow was <br />confined to a single channel. Where braiding occurred, the river <br />generally did not split into more than two channels. The <br />gradient in this reach was 2.34 m/km and current-velocity was <br />relatively high. In May, high flows submerged most riffles and <br />cobble bars, and habitat was homogeneous, swift runs. During the <br />August and October survey, the river channel averaged 30-40 m _ <br />wide and was composed primarily of run habitat 1.0-1.5 m deep, <br />although short riffles occurred regularly throughout this reach. <br />Pool habitat > 2 m deep was present., but only where there were <br />large boulders or rocky cliff faces. Cobble-sized substrate <br />predominated throughout this reach, and extensive bars were <br />exposed at lower flows. Water temperature was generally cooler <br />upstream in a given sample period, and daytime variation in <br />October ranged from 10-170 C. Backwater habitats were relatively <br />uncommon in this reach, presumably because of higher current <br />velocity (and gradient) and less extensive channel braiding. <br />The morphometry of the San Juan River changed a short <br />distance below Shiprock. The river braided into as many as four <br />channels, all of which were passable by raft during the May <br />survey. During August and October surveys, most channels still <br />12