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<br />.
<br />
<br />boundary of the Southern Ute Reservation, At Vallecito Reservoir, the river drains a 270-square-mile
<br />watershed, Completed in 1941 by the U, S Bureau of Reclamation (USBR), the earth-fill dam
<br />measures 162 feet in height and has a crest length of 4,010 feet. The reservoir has a storage capacity
<br />of 129,700 acre-feet.
<br />
<br />The USBR constructed the dam to provide irrigation water to the Pine River Project and is
<br />part ofthe Upper Colorado River Storage Project. The USBR's Operation and Maintenance Branch,
<br />Water and Land Division, Durango Area Office, is responsible for the safe and proper operation and
<br />maintenance of the dam and reservoir. The Pine River Irrigation District is under contract with the
<br />USBR to operate the dam under criteria developed by the USBR. The U.S, Army Corps of
<br />Engineers is responsible for providing and approving the operating criteria for flood control subject to
<br />the provisions of Section 7 of the 22 December 1944 Flood Control Act (58 Stat 890), The USBRis
<br />responsible for the administration of flood control operations,
<br />
<br />Normal releases from Vallecito Dam are made through the outlet works that have a maximum
<br />capacity of3, 100 cfs, The spillway is controlled by three 37-by-19-foot radial gates; the spillway has
<br />a peak discharge capacity of30,000 cfs. Operating criteria are designed to minimize the occurrence
<br />of releases greater than 2,500 cfs, which is considered for operating purposes to be the downstream
<br />channel capacity, Below Vallecito Dam, the Los Pinos River flows south through La Plata County
<br />for approximately 35 river miles until it reaches the ColoradolNew Mexico State line, The Los Pmos
<br />River continues to flow south until it outlets into the Navajo Reservoir on the San Juan River in
<br />northwestern New Mexico,
<br />
<br />. HYDROLOGIC ANALYSIS
<br />
<br />Three U.S, Geological Survey (USGS) stream gages exist near the watershed under study,
<br />and the gages are listed in Table 1, Flow recorded at the Los Pifios River at La Boca stream gage is
<br />regulated by Vallecito Reservoir 34 miles upstream, The flow also reflects diversions for irrigation of
<br />about 33,000 acres upstream of the gage station, Flow recorded at the Spring Creek at La Boca
<br />stream gage is affected by regulation because nearly all of the irrigation in this basin is water diverted
<br />from Los Pinos River that causes a considerable change in the annual pattern and natural flow,
<br />
<br />The Albuquerque District investigated discharge-frequency relationships for the stream gages
<br />on the Los Pinos River between Vallecito Reservoir and La Boca, Because the stream gages do not
<br />record natural and unregulated flow, a hydrologic analysis supported completely by frequency-
<br />discharge relationships computed for the gages proved unsatisfactory, Consequently, the
<br />Albuquerque District developed a computer model of the watershed using the Flood Hydrograph
<br />Package Computer Program HEC-I, developed by the Hydraulic Engineering Center (HEe) in Davis,
<br />California, to determine the 0.2%-, 1.0%-, 2,0%-, 0.4%-, and lO,O%-chance (500-, 100-, 50-,25-,
<br />and I O-year) flood peak discharges at selected locations, The synthetic discharge-frequency peaks for
<br />the watershed model were produced using the unit hydro graph, rainfall, infiltration, and routing data
<br />described in the following sections. To ensure that the hydrologic analysis is consistent with
<br />established hydrology for the area, the analysis was presented to, and reviewed and accepted by, the
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