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Twenty -one of the 49 factors analyzed were found to be most useful in identifying differences among <br />reaches and included parameters such as valley width, sinuosity, adjacent irrigated land, channel <br />slope, low flow total water area, and high flow total water area. A more complete description of <br />factors and analyses used to identify geomorphic subreaches is presented by Bliesner and Lamarra <br />(1993, 1994, 1995) and summarized by Holden and Masslich (1995). <br />Ongoing studies by Bliesner and Lamarra (1996) provided additional data on various physical and <br />biological attributes of the system that should further enhance descriptions of the eight geomorphic <br />subreaches. Some of the parameters continuing to be evaluated based on geomorphic reach <br />designations include substrate characteristics, plant biomass, macroinvertebrate biomass, detritus <br />biomass, and backwater (areas surrounded by land on three sides with an outlet to the main river) <br />productivity. Using information presented by Bliesner and Lamarra (1995, 1996), a generalized <br />description of the eight geomorphic reaches was developed and is presented below. <br />Reach 1 IRM 0 -161 Lake Powell Confluence to near Slickhorn Canyon. <br />Reach 1 is a narrow, canyon -bound reach. Channel morphology in the reach has been strongly <br />influenced by Lake Powell, with influence increasing in a downstream direction and varying <br />depending on lake level. Reduced water velocities associated with the backwater effect of the lake <br />have resulted in a large deposit of sediment in this reach and a channel gradient that is lowest of all <br />reaches (0.00105 foot per foot [ft/ft]). Substrates are primarily sand and silt with cobble substrates <br />occurring rarely and in isolated areas. Sinuosity of the channel is low in this reach, and shallow <br />sandy runs and shoals are the most prevalent habitats. Except for some road access points, direct <br />effects of human activity adjacent to the river in the reach are minimal. <br />Related Biological Information: For the period of 1987 -1996, roughly 80 percent of young-of-the- <br />year (YOY) (i.e., fish 20 -100 mm TL) and early juvenile (i.e., fish 100 -400 mm TL) Colorado <br />squawfish captured in the San Juan River have been found in this reach or just downstream in the <br />inflow of Lake Powell, indicating its importance as a nursery area in the system. The presence of <br />these fish has been associated with periodic availability of high quality backwaters and other low <br />velocity habitats in the inflow area of Lake Powell. It appears that both the quality and quantity of <br />nursery habitat in this reach are highly variable and affected by the level of Lake Powell and <br />antecedent flow conditions in the San Juan River. The relatively high numbers of YOY Colorado <br />squawfish in this reach are probably also a result of the overall decrease in water velocity and <br />ponding associated with the inflow area that results in a natural collecting area for young fish <br />engaged in active or passive downstream dispersal within the system. <br />This reach contains the highest diversity of predaceous fish due to the presence of reservoir species <br />that move into the San Juan River from Lake Powell. The presence of an ephemeral waterfall <br />located near RM 0.0 created an effective barrier to fish movement for approximately 6 years (1989 - <br />1995) and appeared to significantly reduce the diversity and abundance of predatory species such as <br />striped bass, largemouth bass, and walleye that would typically exhibit seasonal movements into the <br />lower San Juan River. Inundation of this feature by rising lake levels during the spring of 1995 <br />eliminated this barrier. Dramatic increases in catch rates of these three predacious species in the <br />lower San Juan River during the summer of 1995 reflected the biological significance of this feature <br />(Ryden and Ahlm 1996). In addition to restricting the access of predators, the waterfall may have <br />also prevented access to the system by adult Colorado squawfish and razorback sucker, thereby <br />San Juan River Recovery Implementation Program December 1997 <br />Summary Report 6 PR -602 -1 <br />