2010-11-22_REVISION - M2009023 (7)

Home Browse Search

inhabited or potentially habitable for use in spawning and feeding, as a nursery, or serve as corridors between these areas. In addition, oxbows, backwaters, and other areas in the 100-year floodplain, when inundated, provide access to spawning, feeding, and nursery habitats. Water depletions reduce the ability of the river to create and maintain these important habitats. Food supply, predation, and competition are important elements of the biological environment. Food supply is a function of nutrient supply and productivity, which could be limited by reduction of high spring flows brought about by water depletions. Predation and competition from nonnative fish species has been identified as a factor in the decline of the endangered fishes. Water depletions contribute to alterations in flow regimes that favor nonnative fishes. Water Quantity In the San Juan River, the magnitude of spring flows has declined by 45 percent since Navajo Dam was built. Such flow reductions negatively affect Colorado pikeminnow and razorback sucker in four ways: 1) reducing the river's ability to build and clean cobble bars for spawning; 2) reducing the dilution effect for waterborne contaminants from urban and agricultural sources that may interfere with reproductive success; 3) reducing the connectivity of main-channel and bottomland habitats needed for habitat diversity and productivity; and 4) providing a more benign environment for nonnative fish and invasive, nonnative, bank-stabilizing shrubs (salt cedar) to persist and flourish (Osmundson and Burnham 1998). In general, the existing habitat has been modified to the extent that it significantly impairs essential behavior patterns, such as breeding, feeding, and sheltering and injures the endangered fish species. Water depletions in the San Juan River Basin have been recognized as a major source of impact to endangered fish species. Continued water withdrawal has restricted the ability of the San Juan River system to produce flow conditions required by various life stages of the fishes. In 1963, the Navajo Dam was closed, and Navajo Reservoir began to fill with water from the San Juan River. Historically, flows in the San Juan River prior to the Navajo Dam were highly variable and ranged from a low of 44 cubic feet per second (cfs) in September 1956 to a high of 19,790 cfs in May 1941 (mean monthly values) at the U.S. Geological Survey Station gauge near Shiprock, New Mexico. At the U.S. Geological Survey's Bluff, Utah gauge the average lowest base flow over 33 years during the pre-dam recording period (1929-1961) was 201 cfs in September. The average peak flow during this time period at the Bluff gauge was 12,409 cfs with a maximum of 33,800 cfs (Holden 1999). Conversely, post-Navajo Dam flows in the San Juan River have ranged from a low of 185 cfs in July 1963, while the reservoir was filling, to a high of 9,508 cfs in June 1979 at the Shiprock gauge. Holden (1999) reports the average lowest base flow over 30 years post-dam (1962-1991) at the Bluff gauge was 476 cfs during August. The average peak runoff flow at the Bluff gauge during this time period was 6,749 cfs and the maximum peak runoff was 15,200 cfs. From 1992-1997 (six years) during a flow research period the average lowest base flow was 476 cfs during August, the average peak runoff was 8,772 cfs and the maximum peak runoff was 11,600 cfs (Holden 1999). Occasional rainfall events, such as in September 2002 when the San Juan River was recorded at about 17,100 cfs at the Four Corners gauge, have higher peaks than the peak runoff flows in the spring but they usually just last a few hours. Since 1963, Navajo Dam has significantly altered flow of the San Juan River by typically storing spring peak flows and releasing water in summer, fall, and winter months resulting in an average 17