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<br />,....., <br />:.J <br />I'" <br />'.1 <br />1-- <br />00 <br /> <br />Colonel John N Reese <br /> <br />DRAFT <br /> <br />24 <br /> <br />Because the precise timing of additional return flows resulting from the <br />Project has not been determined and would vary from year to year, they were <br />not included in the monthly flow analysis (however, return flows were included <br />in the calculations for average annual net depletion). The addition of larger <br />return flows to the La Plata River during the lowest flow months of the summer <br />could potentially be beneficial to aquatic life there but would depend on the <br />flow of the La Plata River, the location of return flow additions, and the <br />water quality of the return flows relative to the river. Any benefits of an <br />increase in San Juan River flows resulting from these larger return flows in <br />the La Plata River during the summer would be very small and would also depend <br />on the water Quality of the return flows relative to the San Juan River. <br /> <br />Water aual itv <br /> <br />An increase in heavy metal, selenium, soil salts, PAHs, pesticides, or other <br />contaminant concentrations in Project-affected rivers and the subsequent <br />bioaccumulation in the food chain would adversely affect the predatory <br />Colorado squawfish, as well as the razorback sucker. Selenium is of <br />particular concern due to its tendency to concentrate in low velocity habitats <br />that are important habitats for Colorado squawfish and razorback suckers. <br />Increases in return flows from seleniferous soils and reduced river flows <br />typically result in increases in concentrations of selenium as well as other <br />contaminants. <br /> <br />The irrigated area served by Red Mesa Reservoir has not been identified as a <br />significant source of contaminants. However, concentrations of contaminants <br />could increase as a result of a reduction in dilution caused by increased <br />depletion. The magnitude of an increase in contaminant concentrations likely <br />would be small and in proportion to the magnitude of depletion compared with <br />flows in critical habitat in the San Juan River. <br /> <br />Very little information is available on the influence of turbidity on the <br />endangered Colorado River fishes. It is assumed, however, that turbidity is <br />important, particularly as it affects the interaction between introduced <br />fishes and the endemic Colorado River fishes. Because these endemic fishes <br />have evolved under natural conditions of high turbidity, it is concluded that <br />the retention of these highly turbid conditions is an important factor for <br />these endangered fishes. Reduction of turbidity may enable introduced species <br />to gain a competitive edge which could further contribute to the decline of <br />the endangered Colorado River fishes. The Project's influence on turbidity <br />levels has not been studied. However, relatively clear La Plata River water <br />will be diverted to storage in the winter, while relatively more turbid return <br />flows will enter the river during the irrigation season. The addition of more <br />turbid return flows during the irrigation season would generally coincide with <br />the natural addition of turbid water from summer storm events. Howevef, <br />return flows would occur continuously through this part of the year, whereas <br />storms occur sporadically. Because of distance to critical habitat and <br />relative quantities of flows in the San Juan and La Plata rivers, the effect <br />on turbidity levels in critical habitat in the San Juan River likely would be <br />very small during both the winter diversion period and the summer irrigation <br />season. <br />