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<br />o <br />:"..J <br />l- <br /> <br />Colonel John N Reese <br /> <br />DRAFT <br /> <br />25 <br /> <br />...~. : <br /> <br />Physical Habitat <br /> <br />1-" <br />CD <br /> <br />The Service believes that peak spring flows are very important for maintaining <br />channel geomorphology, creating and providing access to off-channel habitats, <br />stimu1atjng spawning migrations, and preserving suitable spawning substrates. <br />The Project's potential reduction of San Juan River spring peak flows on the <br />order of one percent, while small when stated in terms of percent reduction in <br />flows, could significantly affect endangered fishes. Over time, the potential <br />small reduction in spring peak flows could measurably affect the important <br />functions provided by these flows (maintenance of channel geomorphology, <br />creation of off-channel habitats and access to those habitats, stimulation of <br />spawning migrations, and preservation of suitable spawning substrates) if the <br />overall flows are reduced below the threshold flows necessary to maintain <br />these functions. The precise spring peak flows necessary to maintain these <br />functions have not been determined. However, as evidenced by past water <br />development and population declines of the endangered fishes, it is reasonable <br />to assume that spring peak flows, over a number of years, have been reduced <br />below, to, or near to threshold levels for some or all of these functions. <br /> <br />Seasonally flooded habitats, such as vegetated shorelines, side channels, <br />mouths of ephemeral washes and tributaries, have been identified as important <br />during runoff as staging areas for the Colorado squawfish and razorback sucker <br />(Ryden and Pfeifer 1995a, 1995b, Miller 1994). B1eisner and Lamarra (1994) <br />found that overbank flooding occurs in the San Juan River at approximately <br />5,000 cfs and the area of inundation increases with higher flows. The extent <br />and frequency of reduction of flooded habitats would be approximately <br />proportional to the extent to and frequency at which the Project would affect <br />Navajo Dam operations and San Juan River peak flows. <br /> <br />BioloQica1 Environment <br /> <br />Data collected by Osmundson and Kaeding (1991) indicated that during low water <br />years nonnative minnows capable of preying on or competing with larval <br />endangered fishes greatly increased in numbers. No specific data is available <br />relating nonnative fishes with flow regimes on the San Juan River, however, <br />the Bureau concluded in its biological assessment for the Animas-La Plata <br />Project that reduced spring peak flows resulting from that project may <br />contribute to enhancement of nonnative species. Any enhancement of nonnative <br />species resulting from the Red Mesa Reservoir Enlargement Project would likely <br />be proportional to the Project's reduction of flows. <br /> <br />Summary <br /> <br />The proposed Project would reduce the amount of water delivered to critical <br />habitat, alter the hydrologic regime, and degrade water quality by redUcing <br />dilution. Without actions taken to offset impacts, further flow reductions in <br />the San Juan River are likely to jeopardize the continued existence of the <br />Colorado Squawfish and razorback sucker and adversely modify or destroy their <br />critical habitat. The San Juan River is an essential component of the habitat <br />of the Colorado squawfish and razorback sucker in the Colorado River Basin and <br />is particularly important in ensuring survival of these species in the event <br />