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PHYSICAL INFORMATION <br />The Boulder Larimer Ditch system is comprised of one major headgate on the Little Thompson <br />River and one off-channel reservoir, Ish Reservoir. Storage releases are made to shareholders <br />under the Old Ish Ditch and the New Ish Ditch. <br />1) Boulder Larimer Ditch (Structure ID 0400588) <br />The Boulder Larimer Ditch headgate is located on the south side of the Little Thompson River <br />approximately one and one-half miles below the mouth of the canyon. The earth-lined ditch <br />diverts direct flow and storage water and conveys Colorado-Big Thompson (C-BT) water to <br />shareholders in the Boulder Larimer Ditch and New Ish Ditch companies from the Little <br />Thompson River. C-BT water is conveyed to the Little Thompson River below the canyon <br />mouth via the St. Vrain Supply Canal from Carter Lake. <br />The ditch headgate consists of two 7-foot wide radial gates that are manually controlled to <br />correspond with the amount of river water diverted in priority and C-BT water delivered to Ish <br />Reservoir. Two sand traps located in low points of the ditch are used during spring runs to pull <br />out sediment from the high flows. <br />The Boulder Larimer Ditch originally conveyed water exclusively to the Ish Reservoir inlet <br />ditch, located on the northern portion of the west side of the reservoir. A bypass ditch off the <br />Boulder Larimer Ditch along the north side of the reservoir above the reservoir inlet ditch was <br />constructed in the 1950s to allow C-BT water to bypass Ish Reservoir and be conveyed directly <br />to New Ish Ditch shareholders. <br />Length: Approximately 6 miles in length, ending at the Ish Reservoir inlet ditch. <br />Capacity: The capacity of the Boulder Larimer Ditch at the headgate is greater than 400 cubic <br />feet per second (cfs). The capacity of the Ish Reservoir inlet ditch is similarly greater than <br />400 cfs. According to ditch company representatives, the ditch capacity is currently <br />restricted to about 300 cfs due to a misaligned culvert under Boulder County Road 23, <br />about one mile down ditch from the headgate. <br />Measurement Device: Diversions through the Boulder Larimer Ditch are measured <br />approximately 100 feet down ditch of the headgate through a concrete, 15-foot Parshall <br />flume equipped with a Stevens recorder. <br />Diversions through the bypass ditch or into the reservoir are controlled via a splitter box <br />with bypass ditch flows measured in a concrete, 8-foot Parshall flume. <br />Conveyance Efficiency: Ditch company representatives report about 10 percent shrink in <br />diversions from the river to the reservoir in an average year. Ditch losses have been on <br />the order of 20 percent or more during the recent drought. Losses in the outlet ditches <br />below the reservoir are unknown. <br />Boulder Larimer Ditch Memorandum.doc 4 of 11 October 28, 2004 <br />