SPDSS_Task5_KeyStructure_NorthSterling

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North Sterling Reservoir is impounded by Point of Rocks Dam. Point of Rocks Dam consists of a single embankment approximately 5,000 feet in length. The embankment was originally constructed as an earthen clay dam with a concrete face on the upstream side. In the 1920's the concrete face was removed and the embankment was covered with riprap using rock from the surrounding area. A stability study was conducted in 2003 and concluded that the dam was in satisfactory condition. The dam has never failed. The spillway consists of two grass lined sections on the east side of the reservoir just south of the dam. The original spillway was approximately 100 feet in length, and an additional 530 feet was added to in 1982. The capacity of the current spillway system is approximately 152,000 cfs, although the reservoir has never spilled. The capacity of North Sterling Outlet Canal is 800 cfs with a length of approximately 65 miles. Capacity: Total Storage: Active Storage: Dead Storage: 80,590 acre-feet 74,590 acre-feet 6,000 acre-feet Area/Capacity Data: Unknown date of survey. Reservoir gage height, area, and cumulative capacity information are provided in Table 1. The cumulative capacity numbers include the 6,000 acre-foot dead pool. Table 1 North Sterling Reservoir Elevation-Area-Capacity Gage Height (ft) Area (acres) Cumulative Capacity (acre-feet) 41.0 516.48 6,000 45.0 676.26 8,369 50.0 876.87 12,279 55.0 1,098.66 17,213 60.0 1,335.77 23,303 65.0 1,573.42 30,464 70.0 1,840.16 38,997 75.0 2,105.57 48,903 80.0 2,416.89 60,162 85.0 2,739.18 72,984 87.7 (Spillway) 2,880.42 80,590 Since original construction, sedimentation in the upstream area of the reservoir has reportedly reduced the active pool by approximately 4,000 acre-feet. Historical measurements of reservoir contents (discussed below) reflect the original elevation- capacity relationship shown in Table 1. North Sterling Irrigation District.doc Page 6 of 11