Operations
The total usable storage capacity or active capacity of Dworshak Reservoir, including flood control, is 2,016,000 acre feet (2.487 km3). The inactive capacity (the portion of the reservoir's capacity below the power generating outlets and the lower river outlet works) is 682,000 acre feet (0.841 km3), and the dead pool (below the river outlet works) corresponds to a storage of 770,000 acre feet (0.95 km3). In addition, the reservoir has a surcharge capacity (above the spillway gates) of 92,000 acre feet (0.113 km3), bringing the maximum amount of water that can be retained behind the dam to 3,560,000 acre feet (4.39 km3).
Most of the active capacity not used for flood control is utilized for power production. Because the Lenore dam on the Clearwater River was never built, Dworshak cannot be used as a peaking power facility. Thus, the power plant is operated as a base load plant, with a relatively constant release. Discharge rates from the dam only increase significantly during high water summers, when flooding requires the opening of the spillways. The power plant generates an average of 1.693 billion KWh each year. Water releases from Dworshak Dam are also controlled to optimize power generation at four downstream dams on the Snake River and four more on the Columbia River.
Each winter, the level of Dworshak Reservoir is drawn down an average of 155 feet (47 m) to prepare for the North Fork's annual freshet, which once could reach more than 100,000 cubic feet per second (2,800 m3/s) after a heavy snowmelt. The reservoir is required to maintain a minimum of 700,000 acre feet (0.86 km3) of winter flood-storage space, and dam releases are operated so that water levels reach a maximum of 1,570 feet (480 m) in July. However, annual flood control reservations vary with the amount of snowpack in the 2,440-square-mile (6,300 km2) drainage basin above the dam. The annual flood-control drawdown generally begins in September and ends on April 1 of the following year; snowmelt floods are captured in the reservoir between April and July. Although the reservoir covers more than 17,000 acres (6,900 ha) at full pool, the surface area decreases to 9,050 acres (3,660 ha) at the lowest point of the drawdown. Flood storage space in Dworshak can be interchanged with other major dams in the Columbia River system, including large Columbia mainstem dams such as Grand Coulee, depending on varying flood control requirements in the Columbia Basin.
Water releases from Dworshak are also timed to benefit Pacific salmon and steelhead migration in the Clearwater, Snake and Columbia Rivers. During late spring and summer, large volumes of cold water are released through the dam's low level outlets to help cool water in downstream rivers, creating more suitable temperatures for these fish species. These environmental or "flow augmentation" releases significantly increase water levels during the late summer, with up to 14,600 cubic feet per second (410 m3/s) being released through the dam in August and 10,400 cubic feet per second (290 m3/s) in September. The temperature of the released water generally varies from 46 to 48 °F (8 to 9 °C).
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