SEATTLE - With a rainstorm storm expected to add widespread modest to heavy rainfall starting early this morning in the Kootenai River Basin, which is already exceeding precipitation records for the month of June, the U.S. Army Corps of Engineers is operating Libby Dam to its fullest flexibility in an effort to reduce flood damage.
The storm is forecast to add 0.5-1.5 inches of precipitation to the basin over a 24-hour period starting today before drying out later in the week. The additional rainfall is on top of record precipitation upstream of Libby Dam which is approaching 200 percent of average for June.
Current inflow at Libby Dam is approximately 70,000 cubic feet per second and outflow is 40,000 cfs. Elevation at Koocanusa Reservoir is 2,452.9 feet and it has been rising 1-1.5 feet per day during the past five days. The reservoir is expected to continue rising by about one foot per day through the end of the month and could reach 2,458 feet, or within one foot of full pool, by June 30. Total inflow volume at the western Montana dam is approximately 3.3 million acre feet; the third highest volume in the past 50 years.
The Corps is operating Libby Dam to reduce flood damage, modifying outflows to keep the reservoir refill rate manageable while simultaneously maintaining elevation at Bonners Ferry, Idaho, as low as possible. The river at Bonners Ferry will remain above the flood stage of 1,764 feet as a result of high tributary inflows, precipitation and dam outflows. The current elevation at Bonners Ferry is 1,764.30 feet.
Intense June rainfall in the Kootenai Basin has also resulted in a high water level on Kootenay Lake, downstream of Libby Dam. The current elevation at Queens Bay is 1,753.0 feet, the highest level since 1974. Kootenay Lake is projected to rise to 1,753.8 feet by June 29 and possibly higher due to very high runoff conditions being experienced in the basin from snowpack runoff and record June rainfall. Libby Dam operations are being managed to provide flood risk reduction for both the Canadian and U.S. portions of the Kootenai Basin.
Outflows from Libby Dam were increased from 38,000 cfs to 40,000 cfs Monday at 1 p.m. MDT. The Corps intends to hold that outflow rate steady and target 1,765 feet elevation at Bonners Ferry through Wednesday.
The amount of actual precipitation on Tuesday will determine the next steps for flood risk mitigation operations at Libby Dam.
The Corps’ emergency operations center, reservoir control center and Kootenai River flood team members are continually monitoring conditions. Additional volunteer flood fight team members and reservoir managers at the Corps’ Seattle District remain on alert and poised to respond as necessary to areas affected by flooding.
Current flow information for many of the river basins in the Northwest may be found here: http://www.nwd-wc.usace.army.mil/nws/hh/index-j.html