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<br />o <br />C:""l <br />('J <br /> <br />11 <br /> <br />;. .~ <br /> <br />to <br />c...,) <br /> <br />(km) above each of five reaches of the San Juan River (parenthetical references are the <br />geomorphic reaches encompassing the locations): Hogback (#5 - Mixer to Hogback). Mixer <br />(#4 - Aneth to Mixer). Montezuma Creek (#3 - Chinle to Aneth). John's Canyon (#2 -Canyon). <br />and Grand Gulch (#1 - Lake Powell influence). <br /> <br />All low-velocity habitats throughout the five geomorphic reaches will be sampled one week <br />following the release of Age-O Colorado sQuawfish. This initial sampling will determine if low- <br />velocity habitats within each reach are retaining fish or if fish are selecting these habitats. <br />After the first sampling, each reach will be sampled bi-weekly until deemed unnecessary. The <br />later sampling will determine the duration that fish are retained within low-velocity habitats <br />in each reach. This intensive sampling schedule will allow the monitoring of dispersal, habitat <br />selection, and movement of fish among habitats and reaches over time. <br /> <br />III. AFFECTED ENVIRONMENT <br /> <br />Based on the perceived range of environmental impacts resulting from the proposed <br />implementation of the preferred alternative, this EA incorporates as its analysis area the <br />immediate environs of the San Juan River and its permanent and ephemeral tributaries from <br />Navajo Dam downstream to Lake Powell. Baseline information concerning the delineated <br />affected environment is provided in the following discussions. The No Action Alternative sets <br />the environmental baseline (i.e., the affected environment) for comparison of the effects of <br />the preferred alternative. The environmental effects (changes from present baseline <br />conditions) reflect the identified major issues and other key elements of the environment. <br /> <br />A. CLIMATE, AIR QUALITY, GEOLOGY, AND SOILS <br /> <br />The San Juan Basin is typical basin and range topography with deep canyons, dry washes, <br />upland mesas, and hogback ridges with igneous dikes. Soils are derived from sandstones, <br />clays, and barren shales containing little organic matter. This area is classified as the Navajo <br />section of the Colorado Plateau physiographic province (Fenneman 1931). Elevation within <br />the basin varies from 1460 to 2100 meters (4790-6890 feet). <br /> <br />The climate and vegetation of the San Juan Basin is characteristic of the Great Basin, a cold- <br />temperature desertland. The Great Basin has cold, harsh winters, low precipitation scattered <br />throughout the year, with great extremes in both daily and seasonal temperatures (Brown <br />1982). Mean annual precipitation at Aztec, New Mexico is 242 mm or 9.53 inches. Winter <br />precipitation is dominant (more than 50 percent of the total precipitation falls during winter <br />months). although the Chihuahuan desert monsoon provides warm weather moisture during <br />summer months (Cully et al. 1987). <br /> <br />B. WATER <br /> <br />Flows in the San Juan River are dependent upon a number of factors. Climatic conditions, <br />water use demandsfor municipal; domestic, industrial, and agricultural activities, and release <br />of flows from Navajo Dam in response to the first two factors all influence the flow regime <br />of the river. These effects are attenuated as distance from the dam increase-s; the river <br />reflects more short term fluctuations from unregulated tributaries intersected en route to Lake <br />