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system operations. Ditch company personnel estimate overall system losses generally in <br />the range of 20 percent to 30 percent. <br />Return Flow Locations: The Left Hand Ditch Company system has historically benefited from <br />return flow from all of the ditches within the Left Hand Creek basin. The initial reach of <br />the ditch from the South Fork St. Vrain diversion does not intercept intervening <br />tributaries en route to James Creek, tributary to Left Hand Creek. Return flows below the <br />lowest ditch company diversion (Williamson Ditch) constitute most of the flow in Left <br />Hand Creek during the late summer to winter months. These flows are measured at the <br />Left Hand Creek at Mouth at Longmont stream gage (06725000). <br />Irrigated Crops: 7,500 acres are listed in the HydroBase database under the Left Hand Ditch <br />structure. Based on information from the references reviewed for this memorandum, the <br />irrigated acreage under the system started with about 20,000 acres in the late 1800s; was <br />reduced to about 16,000 acres by 1970; and reduced to about 12,000 acres in the mid- <br />1990s. The Water Commissioner believes that 12,000 to 16,000 acres currently irrigated <br />by the Left Hand Ditch Company is an appropriate estimate. Ditch company <br />representatives did not provide a more accurate estimate of the amount of irrigated <br />acreage under the ditch system. <br />Crops irrigated under the Left Hand Ditch system are primarily alfalfa, grass hay, and <br />corn. Corn accounts for about 10 percent of the total irrigated acreage under the ditch <br />system but is only grown east of 63rd Avenue (road on east side of Boulder Reservoir), <br />where it accounts for about 25 percent of the irrigated crops. <br />HydroBase Data: The monthly data available in HydroBase over the 1950-2002 period of <br />record pertaining to the Left Hand Ditch are sporadic, as summarized below: <br />• Diversions from the St. Vrain Creek are recorded for about 11 years under a gage located <br />on the ditch (LEFTHDCO, 1992-2002, approximately 13,000 ac-ft per year, on average) <br />and as a release from the Left Hand Diversion Point (0502000, 1993-2002, approximately <br />15,000 ac-ft per year, on average). The monthly release data are generally the same as the <br />gage data except more diversions are listed during the shoulder seasons (May and <br />October) under the release data. <br />• The Left Hand Ditch diversion structure (0500603) has about 5 years of monthly <br />diversions listed over 1955-1992. <br />• C-BT deliveries to the Left Hand Ditch Company ditches are recorded under the <br />BFCLHCOO gage. These data are available over 1955-1999 and average about 1,200 ac- <br />ft/yr. <br />2) Left Hand Ditch Reservoir System <br />The ditch company uses five reservoirs to store and regulate water supplies for ditch company <br />operations. These units have a total active storage capacity of approximately 4,800 ac-ft and are <br />used in an aggregate fashion to maximize deliveries to shareholders. These five reservoirs <br />include two lower units in the Left Hand Creek basin (Left Hand Valley Reservoir and Allens <br />Lake) and three upper units -two in the Left Hand Creek basin (Gold Lake and Left Hand Park <br />Left Hand Ditch Operating Memorandum.doc 6 of 17 January 5, 2005 <br />