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,-, :"1 ~ -.1 C; Platania. Dudley, and Maruca. 2000. Drift of Fishes in the San Juan River ]991-1997. FfNAL STUDY AREA w The San Juan River is a major tributary of the Colorado River and drains 99,200 km' in Colorado, New Mexico, Utah, and Arizona (Figure I). From its origins in the San Juan Mountains of southwestern Colorado at elevations exceeding 4,250 m, the river flows westward for about 570 kIn before confluencing with the Colorado River. The major perennial tributaries to the San Juan River are (from upstream to downstream) Navajo, Piedra, Los Pinos, Animas, La Plata, and Mancos rivers, and McElmo Creek. In addition there are numerous ephemeral arroyos and washes that contribute relatively little flow annually bUI input large sediment loads. Navajo Reservoir, completed in 1963, impounds and isolates the upper 124 km of the San Juan River and regulates downstream discharge. The completion of Glen Canyon Dam in 1966 and subsequent filling of Lake Powell ultimately inundaIed the lower 87 km of the San Juan River by the early 1980s. The San Juan River is now a 359 krn lotic system bounded by two reservoirs (Navajo Reservoir near its head and Lake Powell at its mouth). The San Juan River is canyon-bound and resIricIed to a single channel between its confluence with Chinle Creek (ca, 20 km downstream of Bluff, Utah) and Lake Powell. The river is predominaIely multi-channeled upsIream of Chinle Creek with the highest density of secondary channels occurring between Bluff and the Hogback Diversion (ca. 13 km upstream of Ship rock, New Mexico). There is a general downstream decline in channel stability in the section of river between Bluff and Shiprock. Below the confluence with the Animas River near Fannington, New Mexico, the channel is less stable and more subjcCI to floods from it largest and unregulated tribUlary, Ihe Animas River. Conversely, the regulated reach of river between FanningIon, New Mexico and Navajo Darn is relatively sIable with few secondary channels. From Lake Powell to Navajo Dam, the mean gradient of the San Juan River is 1.67 mIkrn, Examined in 30 km increments, river gradient ranges from 1,24 to 2.41 mIkrn but locally (i.e., <30 krn reaches) can be as high as' 3.5 mIkrn. Between Shiprock and Bluff, San Juan River substrate is primarily sand mixed among some cobble. The proportion of sand is greatesI in the downstream mas I reaches and declines along an upstream gradient. From FanningIon to Navajo Dam, Ihe San Juan River substrate is dominated by embedded cobble. Although less embedded, cobble is also the most common subsIrate between Shiprock and Fannington. Except in canyon-bound reaches, the river is bordered by nonnative salt cedar (Tamarix chinensis) and Russian olive (Elaeagnus angustifolia) and native cottonwood (Populus fremontii) and willow (Salix sp.). Nonnative woody planIS dominated nearly all sites and resulted in heavily stabilized banks. Cottonwood and willow accounIed for less than 15% of the riparian vegetation. The characterisIic annual hydrographic pattern in the San Juan River is typical of rivers in the American Southwest with large flows during spring snowmelt, followed by low summer, autumn, and winter base flows. Summer and early autumn base flows are frequently punctuated by convective stonn-induced flow spikes. Prior to closure of Navajo Dam. aboUl 73% of the total annual San Juan River drainage discharge (based on USGS Gauge # 09379500; Bluff, Utah) occurred during spring runoff (1 March through 31 July). Median daily peak discharge during spring runoff was 10,400 cfs (range = 3,810 to 33.800 cfs), Although flows resulting from summer and autumn storms contributed a comparatively small volume to total annual discharge, the magnitude of storn,- induced flows exceeded the peak snowmelt discharge about 30% of the years, occasionally exceeding 40,000 cfs (mean daily discharge). Both the magnitude and frequency ofIhese stonn induced flow spikes are greater than those recorded in the Green or Colorado rivers. Closure of Navajo Dam altered Ihe annual discharge pattern of the San Juan River. The natural flow of the Animas River ameliorated some aspects of regulated discharge by augmenting spring discharge. Regulation resulted in reduced magnitude and increased duration of spring runoff in wet years and substantially reduced magnitude and duration of spring flow during dry years. 3