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<br />, <br /> <br />upstream water development. Subsurface soils in the Grand Valley once were part of the bed <br />of a substantial inland sea. Irrigation return flows percolating through these so-caIled Mancos <br />shales draw out the considerable salts that are residues of this sea. The loading of salts to the <br />Colorado River from sources in the Grand Valley (not all caused by irrigation activity) is <br />estimated to be 580,000 tons per year, about seven percent of the annual average salt load <br />measured at Imperial Dam near the border with Mexico,3 <br />Construction of the Grand Valley Diversion Dam for the federal Grand Valley Project <br />totally blocked fish passage at this point of the Colorado River, Irrigation diversions in the <br />summer months caused drastic reductions in streamtlows of the Colorado River, particularly <br />in the section below the headgate of the Grand Valley Irrigation Company to the confluence <br />with the Gunnison River known as the "IS-Mile Reach." These consequences of irrigation <br />development in the Grand Valley contributed to the dramatic decline during this century of <br />two species of fish native to this part of the river - the Colorado squawfish and the <br />razorback sucker, In 1967, the Fish and Wildlife Service listed the Colorado squawfish as an <br />endangered species4 and, in 1987, the Upper Colorado River Endangered Fish Species <br />Recovery Program was initiated.' The need for increased flows through the IS-Mile Reach <br />has been identified as an objective of the recovery program,6 <br />Upstream demands on the Colorado River have increased markedly during this <br />century, Perhaps most dramatic have been the transmountain diversions taking water out of <br />the Colorado River Basin on the west side of the Continental Divide for use in the Front <br />Range of Colorado, Private irrigation interests constructed small structures moving water <br />across the mountains beginning in the late 1800s, and large scale diversions began with <br />construction of the Moffat Tunnel by the City of Denver during the 1920s and with the <br /> <br />3 . Dept. of Interior. Bureau of Reclamation, Grand Valley Unit Final Environmental Impact Statement at 5-1 <br />(1986){hcrcafter Grand Valley FEIS). <br /> <br />· 32 Fed. Register 40001 (March II. 1967). <br /> <br />, U.S. Department of the Interior, Fish and Wildlife Service. Recovery Implementation program for Endangered <br />Species in the Upper Colorado River Basin, September 1987 (hereafter "Recovery Implementation Program"). <br /> <br />· U.S. Bureau of Reclamation, Study of Alternative Water Supplies for Endangered Fishes in the I S-Mile Reach <br />"fthe Colorado River (Jan. 1992) at 2 (hereafter "Alternative Water Supplies"). <br />3 <br />