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<br />Dam in 1936. Dam construction began in the upper Colorado River shortly after <br />that (i.e. Green Mountain Dam in 1942). The first dam of the Aspinall Unit <br />(Blue Mesa) closed in 1965. Therefore, to accommodate the changes in water <br />usage during the period of record, we divided the available streamflow data <br />into three periods: early development (1914-1936), middle development (1937- <br />1965), and post-Aspinall development (1966-1989). We compared post-Aspinall <br />streamflow with early- and middle-development streamflows to determine the <br />effect of the Aspinall Unit on streamflow in the Gunnison and Colorado rivers. <br />We also used synthesized data (HDR Engineering, Inc. 1989) to estimate <br />what Gunnison River flows would have been during the middle-development and <br />post-Aspinall periods without any water development in the basin. The <br />synthesized data were only available for 1952 through 1983, so we compared it <br />with actual streamflow for those years to estimate the effect of Aspinall on <br />Gunnison River flows. These estimates served to validate changes estimated <br />with the previously described method and to provide an estimate of the effect <br />of early irrigation diversions on streamflow. <br />EFFECT OF THE ASPINALL UNIT ON THE GUNNISON AND COLORADO RIVERS <br />Water Temperature <br />The synthesized water temperature data showed a large difference between <br />pre- and post-Aspinall temperatures at Crystal Dam. However, the water warmed <br />rapidly as it moved downstream--water temperatures differed by a maximum of 2 <br />°C at river mile 20 (Figure 2, Table A1). River mile 20 is midway through the <br />reach of Gunnison River currently inhabited by Colorado squawfish (Valdez et <br />al. 1982a). The observed difference was less at the mouth of the Gunnison <br />River (Figure 2). Water temperature in the Colorado River is essentially <br />12 <br />