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

Fish and Game Fisheries biologists report that approximately 250 dead rainbow trout could be observed from the shoreline extending south from the boat dock to the dam, though it is difficult to estimate how many total dead fish are present in the reservoir.

Based on water quality tests conducted by the DEQ, low dissolved oxygen levels most likely caused the fish deaths.  On Tuesday, July 23, dissolved oxygen levels in American Falls Reservoir measured approximately 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 dissolved oxygen levels? High winds (with a few gusts of 50 to 60 mph) from a Thursday evening storm caused the water in the reservoir to “turn over”, bringing the water from the reservoir’s bottom to the top.  Large bodies of water like American Falls Reservoir are stratified, meaning that there are distinct water layers varying in temperature and dissolved oxygen levels.  During the summer, the cold bottom layer of a large water body contains low levels of dissolved oxygen, and if there is a sudden turnover of the water layers, that can be deadly for fish.

“The winds acted like a giant spoon, mixing the reservoir so to speak,” says Fish and Game Regional Fisheries Biologist Ryan Hillyard. “And, that mixing action brought stressfully-low levels of dissolved oxygen to some parts of the reservoir occupied by fish.”

Most of the fish affected by the wind-generated turnover are rainbow trout, which is not surprising to biologists.

“Trout are more sensitive to low dissolved oxygen levels than are other fish species like bass or perch,” says Hillyard.  “Though we did see a dead bass while at the reservoir, the majority of dead fish were trout.”

At 56,000 acres, American Falls Reservoir is Idaho’s largest reservoir when full.  Events like this have happened before at large water bodies like this one and though they can be alarming to anglers, the fish deaths that typically result are truly just a small drop in a very large bucket.  It is likely that most fish were able to find refuge in other areas of this large reservoir, and impacts to the fishery are likely not going to be significant.