Painted Rocks Reservoir expected to fill a month early due to warmer weather

Painted Rocks Reservoir is nearing full pool and expected to start spilling water within the next 7-10 days in what “appears to be an early start to the runoff season” according to Larry Schock, civil engineering specialist with the Montana Department of Natural Resources and Conservation.

And with warmer weather predicted near the end of this week the reservoir may fill a little sooner, he said in a statement released earlier this week. Full pool at Painted Rocks normally does not occur until the first or second week of May.

“Between the atmospheric rivers of rain in December, the warm stretches during the winter season, the heavy wet snow we received a couple of weeks ago, and the current warm weather there has been a lot of water deposited into the reservoir over the last several weeks,” Schock said. “Because of these unusual occurrences it appears that the snow that is at 5000 feet and below has already melted in the drainage basin above the reservoir, and the ice covering the reservoir has almost completely melted.”

Painted Rocks Reservoir is currently 8 feet below the spillway crest at an elevation of 4,717.5 feet and impounding 27,464 acre-feet (AF) of water. Full pool at Painted Rocks is 32,632 AF. Based upon the current outflows of 313 cubic feet per second (cfs), and inflows measured on April 1 of 627 cfs, it is anticipated that the reservoir will fill and spill within the next 7-10 days, according to Schock.

Schock said that while the releases from the low-level outlet are currently at 313 cfs, the flows in the West Fork of the Bitterroot River may end up closer to the 500-600 cfs range once the reservoir fills and begins to have water flowing over the spillway.