Laserfiche WebLink
<br /> 0 <br />.' e-;. <br /> N <br /> . , <br /> .:'. <br /> r-..:> <br /> IV <br /> <br />.) <br /> <br />].24 to 2.41 mIkm. Between the confluence of the San Juan River with Lake Powell and the <br />confluence with Chinle Creek about 20 km downstream of Bluff, Utah, the river is canyon-bound <br />and restricted to a single channeL Upstream of Chinle Creek, the river is multi-channeled to <br />varying degrees with the highest density of secondary channels between the Hogback Diversion <br />about 13 km east of Ship rock and Bluff, Utah, The reach of river between Navajo Dam and <br />Farmington, New Mexico is relatively stable, with predominantly embedded cobble substrate <br />and few secondary channels. Below the confluence with the Animas River, the channel is less <br />stable and more subject to floods from the unregulated Animas River. Between Farmington and <br />Shiprock, cobble substrate still dominates, although it is less embedded. Between Shiprock and <br />Bluff, the cobble substrate becomes mixed with sand to an increasing degree with distance <br />downstream, resulting in decreasing channel stability, <br /> <br />Except in canyon-boW1d reaches, the river is bordered by non-native salt cedar (Tamarix <br />chinensis) and Russian olive (Eleagnus angustifolia ) and native cottonwood (Populus fremonti) <br />and willow (Sali]: sp.). Non-native woody plants are most abundant, with cottonwood and <br />willow accounting for less than 15% of the riparian vegetation, and common only on islands free <br />of livestock grazing. <br /> <br />The discharge pattern of the San Juan River is typical of rivers in the American Southwest. The <br />characteristic annual pattern is one of large flows during spring snowmelt, followed by low <br />summer, autumn, and winter base flows. Base flows are frequently pW1ctuated by convective <br />storm-induced flow spikes during summer and early autumn. Prior to closure of Navajo Dam, <br />about 73% of the total annual discharge (based on USGS Bluff, Utah gage) of the drainage <br />occurred during spring runoff (1 March through 3 I July). The median daily peak discharge <br />during spring runoff was 10,400 cfs (range = 3,810 to 33,800 cfs). Although flows resulting <br />from summer and autumn storms contributed a comparatively small volwne to total annual <br />discharge in the basin, the magnitude of storm-induced flows exceeded the peak snowmelt <br />discharge about 30% ofthe years, occasionally exceeding 40,000 cfs (mean daily discharge). <br />Both magnitude and frequency of these storm induced flow spikes are greater than those seen in <br />the Green or Colorado rivers, <br /> <br />Closure of Navajo Dam altered the annual discharge pattern of the San Juan River. The natural <br />flows of the Animas River ameliorated some aspects of regulated discharge by augmenting <br />spring discharge, Regulation resulted in reduced magnitude and increased duration spring runoff <br />in wet years and seriously reduced magnitude and duration spring flows during dry years, <br />Overall, flow regulation by operation of Navajo Dam has resulted in post-dam peak spring <br />discharge averaging about 54% of pre-dam values. After dam closure, base flows were increased <br />substantiaI1y over pre-dam base flows. <br /> <br />Since 1992, Navajo Dam has been operated to mimic a "natural" hydrograph with the volume of <br />release during spring linked to the amoW1t of precipitation during the preceding winter. Thus in <br />years with high spring snowmelt reservoir releases were "large" and "small" in low runoff years. <br />Base flows since] 992 were typically greater than during pre-dam years, but less than post-dam <br />years. <br /> <br />San Juan River Non-native Species tnteractions, Final Report, I February 2000 <br /> <br />7 <br />