PRIEST LAKE LEVEL AND PRIEST RIVER FLOWS 2021 MANAGEMENT OBJECTIVES

-Douglas Jones Idaho Department of Water Resources (IDWR) Northern Regional Manager July 9, 2021 A few years ago, in response to the terrible 2015 water year, the Idaho Water Resource Board (IWRB) initiated a water management and alternative solution study. The study analyzed the hydrology of the basin and proposed options intended to improve lake and river management. The objective was to avoid violating the statutorily mandated Minimum Lake Level in the event of another drought event like 2015. It was determined that adding capability to store up to 6 inches of additional storage in the lake (used only during the driest of years) could provide supply to assure the lake is kept at least to the 3.0 ft level through the Recreation Season. Since the late 1970’s, a policy objective was implemented to provide no less than 60 cubic feet per second in the river below the dam unless the lake level was nearing the minimum specified by law. Nothing has changed with respect to this policy. The construction work at the dam this past season to added 6 inches of height to 6 of the 11 gates. The remaining gates will be modified this fall and winter. Unfortunately the normally abundant total precipitation quantity (snow and rain) did not fall in the basin this past winter. This created a much less- than-normal volume of water available for summer-long base flows entering the lake. Although the long, cool spring stretched out the water supply to some extent the gates had to be operated in May to get the lake UP TO the Minimum Lake Level before the Memorial Day weekend. Early June water supply predictions based on precipitation indicated lake inflows were going to be less than 50% of normal. Another dry and abnormally hot summer has worsened the conditions. IWRB and IDWR staff have monitored river flows and lake level daily and it has been noted that summer 2021 river flows near Priest River have been the lowest in 71 years of record. Gate operations have reflected decreases in lake inflow while the lake has been stable at about 4 inches above Minimum Lake Level. This indicates that flows are not low due to more water being stored. One implication of the incomplete work on the gates is that 6 inches of additional storage could not be realized this year, a year when it was needed the most. IDWR staff have evaluated the volume of water represented in the 4 inches of storage currently available above the Minimum Lake Level. Although it is not guaranteed, calculations estimate that the lake level could decrease an average of 0.01 feet every 2 days for approximately 60 days before the lake level reaches the Minimum Lake Level. It appears that if minimal rain falls this summer and gate operations can be managed appropriately, then all lake level and river flow objectives (60 cfs-policy) can potentially be met through the Labor Day weekend. This level of gate management has never been needed or attempted. The close tolerances and limited options available to operators mean that unless widespread, meaningful rains fall in the basin, flows in Priest River below the dam may decrease below 60 cfs.