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Reservoir Construction <br />Beginning in the early 1900s, several reservoirs were constructed on the Rio Grande, Conejos and <br />other tributaries. With the exception of Platoro Reservoir on the upper Conejos River, most <br />storage projects were completed prior to 1930. Numerous small storage projects have occurred <br />since the 1930s (e.g. stock ponds). Six of the seven largest storage projects total approximately <br />250,000 acre-feet of storage capacity, of which about 122,000 acre-feet is on the Rio Grande <br />mainstem. Platoro Reservoir, which was completed in 1953 with a capacity of approximately <br />53,000 acre-feet, is a "post-Compact" reservoir and has special administrative conditions limiting <br />its use. <br />It is important to consider historical reservoir operations in selecting the study period because of <br />the effects they have both on gaged stream flows and on historical diversion records. Reservoirs <br />on the Rio Grande, for example, are routinely used to store water during the spring months and <br />make releases to several of the larger canals during the later summer. In recent years, storage in <br />reservoirs on the Rio Grande and Conejos has occurred under direct flow water rights. In <br />addition, reservoir releases have been made during the later fall and winter to downstream canals <br />for the purpose of recharging the shallow aquifer. These operations should be considered when <br />evaluating historical reservoir contents and direct flow diversion records. <br />Groundwater Use <br />Since the late 1950s there has been a large increase in the use of groundwater for irrigation <br />supplies in the San Luis Valley. Since the early 1970s, approximately 2,000 new center pivot <br />sprinkler systems have been installed, many of which rely on groundwater. New well <br />construction was so dramatic during this period that the State put a moratorium on new well <br />construction because of concerns over excessive groundwater mining. <br />It is important to consider historical trends in the use of groundwater in the study period selection <br />because of the effect on actual crop consumptive use and because of the potential effects on <br />recorded use of other water supplies. Many of the farms in the San Luis Valley are water short <br />without groundwater supplies, with surface rights providing a reliable supply through only part of <br />the growing season. The supplemental supplies provided by groundwater have significantly <br />increased agricultural productivity. The use of groundwater, however, has in some cases changed <br />the pattern in which surface waters are used, with implications on historical diversion and stream <br />flow records. <br />Other Court Decisions <br />Over the years there have been numerous legal actions and cases, agreements and resolutions <br />affecting water use in the San Luis Valley. Several of these, such as the so-called Rules and Regs <br />Case allocating Compact delivery obligations between the Rio Grande and Conejos River, are <br />important to consider because of their effect on historical water availability and water use <br />practices. Many of the more significant actions have occurred within the past 30 years. <br />Conclusion Regarding Key Events <br />The water resource development events shown in Table 7 are those that were identified as <br />potentially affecting selection of the RGDSS study period. As previously described, most <br />C:Acdss\Task2-3.doc Identify Study Period April 16, 1999 Page 13 of 18 <br />