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c- c') ... ....... CJ -..; Platania, Dudley, and Maruca. 2000. Dnft. of Fishes in the San Juan River 19CJ l-1997. FlNAt. complex series of islands. Immediately after flowing through the upstream series of islands and associated secondary channels, the San Juan River formed a single channel. About 0.75 miles downstream of that point, the river channel abutted a canyon wall, followed a sharp (ca. 850) curve, and traveled about 0.25 miles before reaching the next set of islands and secondary channels. In this one mile reach between islands, the river was confined to a single channel about 30-60 m wide with moderate gradient and cobble dominated substrate. The Four Comers drift-net sampling site was about 0.20 miles downstream of the river's bend in the segment confined to a single channel. The closest and largest point source of inflow and sediment to this drift-net station was Chaco Wash which was located about 4 miles upstream of the study site. The Four Comers site was selected, in part, to provide a sampling location in close proximity to a putative Colorado pikeminnow spawlling bar. The original Four Comers site, RM 123, was sampled 1991-1993, while the current site (RM 128) has been sampled from 1992 to present (1999). Except where otherwise indicated, the two Four Comers sites were combined (1992-1993) for all analyses. The Mexican Hat site was bonnded by at least 16 river miles upstream and 53 river miles downstream of canyon confined reaches. This drift-net sampling site was located about 0.5 miles downstream from the downstream end of a 1800 river bend and in one of the few open segments of river downstream of Sand Island. Aquatic habitat at this site was characterized by high gradient main channel runs and riffles over a substrate of primarily bedrock. The channel was abont 50-75 m wide and generally over 2 m deep. Point sources of water and sediment in closest proximity to this site included Chinle Creek, and Comb, Butler, and Montezuma washes. This site also received the cumulative sediment and drift from sources upstream of the aforementioned tributaries. The Mexican Hat site was sampled every year during the study period and the study site did not changed. This site was chosen because it was the most downstream sampling location that could be easily accessed and readily sampled. METHODS Dara Collec/ion Conical plankton nets with an 0.5 m diameter mouth were fitted on rectangular frames (30.5 x 43.5 cm during 199/-1994; 36 x 47 em during 1995-1997), eqnipped with removable collection buckets, and nsed to collect drifting larval fishes (=drift-nets). Nets were 4 m long, had a 560 ~m mesh (bar measnre) and an open-mesh to net-mouth ratio of 8: I. Tranter and Smith (1968) reported that filtration efficiency approached 100% when open-mesh area was more than three times the area of the net mouth. Rings (6-cm diameter) welded to the comers of the drift-net frame allowed the nets to be attached to steel fence-posts (T -posts) placed in the river. Between one and three nets were used during each set. Nets were placed about 5-10 cm below the water's surface. For most years, the volume of water filtered by each net was measured by mechanical flow-meters suspended in tile center of the nets. However, there were years when flow meters were not used with every net at every site and there were also years when flow meters were never deployed. Values of mean daily discharge for the San Juan River were obtained from the United States Geological Survey gauging stations at Four Comers, New Mexico and Bluff, Utah. Passive drift-netting on the San Juan River at Mexican Hat was conducted by the Utah Division of Wildlife Resources (UDWR) dnring 1991-1994, samples at Four Corners were taken by New Mexico Department of Game and Fish (NMGF) during 1991-1994, and both sites were sanlpled by personnel at the Museum of Southwestern Biology, Division of Fishes at the University of New Mexico (UNM) during 1995-1997. Passive drift-net sampling was conducted daily from late June or early Jnly through mid to late August at all sites for all years (Table I). This sampling period was selected because it encompassed the reported reproductive season of Colorado pikeminnow (Haynes et aI., 1984; Nesler 7