Hundreds of dead fish discovered in American Falls Reservoir

Below is a press release and photos from the Idaho Department of Fish and Game.

AMERICAN FALLS – On the morning of July 26, Idaho Department of Fish and Game fisheries biologists, along with Idaho Department of Environmental Quality (DEQ) personnel, responded to calls from the public regarding a possible “fish kill” in the American Falls Reservoir in Power County.

Fish and Game Fisheries biologists report that approximately 250 dead rainbow trout have been observed along the shoreline, which extends south from the boat dock to the dam. However, it is difficult to estimate the total number of dead fish in the reservoir.

Based on water quality tests conducted by the DEQ, low oxygen levels were most likely the cause of the fish deaths. On Tuesday, July 23, oxygen levels in American Falls Reservoir were about 9 mg/L at the surface and had dropped to 4 mg/L by Friday morning. The DEQ state standard for dissolved oxygen is 6 mg/L.

Why the sudden drop in oxygen levels? Strong winds (with some gusts of 50 to 60 mph) from a storm Thursday night caused the water in the reservoir to “tip over,” causing water to rise from the bottom of the reservoir to the top. Large bodies of water like the American Falls Reservoir are stratified, meaning there are different layers of water with different temperatures and oxygen levels. In the summer, the cold bottom layer of a large body of water contains little oxygen, and when a sudden tipping of the water layers occurs, it can be fatal to fish.

“The winds acted like a giant spoon, mixing the reservoir, so to speak,” says Ryan Hillyard, fisheries biologist for Fish and Game Regional. “And that mixing resulted in distressingly low levels of dissolved oxygen in some parts of the reservoir where fish live.”

Most of the fish affected by wind-induced upheaval are rainbow trout, which is no surprise to biologists.

“Trout are more sensitive to low oxygen levels than other fish species such as bass or perch,” says Hillyard. “Although we saw one dead bass at the reservoir, most of the dead fish were trout.”

At 56,000 acres, the American Falls Reservoir is Idaho’s largest reservoir when full. Events like this have happened before in large bodies of water like this, and while they can be distressing to anglers, the fish kills that usually result are really just a small drop in a very large bucket. It’s likely that most of the fish were able to find refuge in other areas of this large reservoir, and the impact on the fishery is unlikely to be significant.