Thursday, July 29, 2021

IDWR warns maintaining lake level, river flow standard may not be possible with summer drought

Staff Writer | July 13, 2021 1:00 AM

River flows near Priest River are the lowest they've been in the 71 years of record, according to Idaho Department of Water Resources data.

Without substantial rainfall, this summer’s drought could threaten to reduce inflows to Priest River to less than 60 cubic feet per second — a standard that’s been in place as the minimum inflow for the river since the 1970s, wrote Douglas Jones, northern regional manager for IDWR in a press release.

Despite a long and cool spring, lower-than-average precipitation in the winter meant dam gates had to be operated in order to reach the minimum three-foot lake level for recreation by Memorial Day weekend, Jones wrote.

Early June water supply predictions were less than 50% of normal, Jones wrote. Currently, the lake is holding at roughly four inches above the minimum lake level. Based on current calculations, IDWR staff are predicting that storage will decrease by roughly 0.01 feet every two days for 60 days before reaching the minimum level.

As it stands, if — and it is a big if — the basin gets minimal rain and gate operations can be maintained appropriately, it may be possible to meet lake level and river flow objectives through Labor Day weekend, Jones wrote.

However, he added, that level of gate management has “never been needed or attempted.” Operators have limited options, he said, especially if the basin doesn’t see significant rain.

The problem was not wholly unanticipated. In 2015, IDWR had conducted a study to improve management and ensure the lake stayed at the three-feet minimum level during the recreational season following a particularly bad drought year, Jones wrote. That resulted in ongoing construction to increase storage capacity at the dam by six inches to supplement lake levels during dry seasons.

However, although construction had begun this season to increase storage at the dam, only six out of eleven gates have had storage added, with the rest scheduled for completion this fall and winter.

That incomplete work on the dam gates means operators will be unable to use the additional storage in one of the years it’s needed most, Jones wrote.

“Unless meaningful rain falls in the basin, flows in Priest River may decrease below 60 CFS,” he wrote.