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elevations range from 1609 m at Farmington to 1423 m where the <br />river exits the state near Four Corners. Upland vegetation in <br />the vicinity of the river is composed primarily of grama <br />(Bouteloua spp.) and galleta (Hilaria spp.) (Morain et al. 1977). <br />Riparian vegetation is typically composed of native cottonwood <br />(Populus fremontii) and willows (Salix spp). Dense stands of <br />introduced salt cedar (Tamarix chinensis) and Russian olive <br />(Elea gnus anaustifolia) line the banks of the river in much of <br />the study area. The floodplain from Farmington to Shiprock is <br />largely privately owned and has been modified for agricultural <br />purposes. The land in the area is heavily irrigated, as rainfall <br />only averages 25-30 cm annually. Uplands to the north are mixed <br />federal, state, and private land while property south of the <br />r <br />river is part of the Navajo Indian Reservation. Downstream of <br />Shiprock, the river bottom and uplands are completely within the <br />Navajo Indian Reservation. Grazing is the primary land use in <br />this area. <br />The drainage area of the San Juan River in New Mexico is <br />24,702 km2 (U.S.Geological Survey 1985a). Discharge in the San <br />Juan River from Navajo Reservoir downstream to Farmington is <br />controlled by Navajo Dam. Although the San Juan River flows for <br />only 192 km in New Mexico, it is the largest river, by discharge, <br />in the state. Mean annual discharge in the San Juan River at <br />Farmington (1913-1985, gauge # 09365000) is 67.8 m3/s; high and <br />low flow values of 1943 m3/s and 0.4 m3/s, respectively, have <br />been recorded at this gauge (U.S.Geological Survey 1985). <br />9