Low oxygen levels responsible for fish kill below American Falls Dam

Representatives from the Idaho Department of Fish and Game (IDFG), the Idaho Department of Environmental Quality (DEQ), and Idaho Power have concluded that the cause of a recent fish kill in the Snake River immediately below American Falls Dam was a lack of sufficient oxygen in the water.

Over the weekend, calls from the public came in asking about dead fish, including trout and sturgeon, observed below the American Falls Dam extending to the Vista boat ramp and Pipeline area.

Oxygen levels have since returned to normal, and reports from anglers on August 6 have been good for both trout and sturgeon. The water itself and any fish caught in that section of river over the weekend present no health risks to humans as a result of the low oxygen levels.

So what happened?

On Saturday, August 4, an IDFG officer responded to calls about dead sturgeon and other fish floating in the Snake River near Vista boat ramp. Some of the reports also referenced an odor similar to the “sulfur smells like those found in Yellowstone National Park.”

Based on observations by DEQ and IDFG, it appeared that hundreds of fish, including Rainbow Trout, Smallmouth Bass, and Yellow Perch, died. IDFG also recovered 8 dead White Sturgeon. It is likely that some delayed mortality will occur over the next few days.

Both IDFG and DEQ consulted with Idaho Power and were able to piece together the likely cause of the fish kill.

The power house at the dam receives water from the bottom of American Falls Reservoir. As the water enters the penstocks, it is measured for temperature and dissolved oxygen. On August 2, during a few short hours, oxygen levels entering the dam dropped from over 5 mg/l to 0 mg/l. Trout need about 3.5 mg/l at a minimum for survival. The rapid decline in oxygen occurred between 10 p.m. and midnight. Low oxygen levels are normal for this time of year, but abrupt changes like that experienced on August 2 are not common.

Aren’t there safety mechanisms in place to protect fish?

There are, and they worked as they were supposed to. Idaho Power’s operations complied with requirements of its license to operate the hydroelectric power plant at American Falls and with State of Idaho water quality standards. Rapid implementation of procedures designed to maintain oxygen levels are the reason why the levels were restored so quickly and why only hundreds of fish died instead of thousands. Here’s how that works.

Below American Falls dam, there is a water monitoring site that uses equipment to measure water characteristics like temperature and dissolved oxygen levels. This monitoring site is located about 150 yards downstream of the dam where the spill way meets the tailrace and records data every 10 minutes.

When dissolved oxygen drops below a certain threshold, a blower in the dam kicks on to increase oxygen into the water going through the dam. Then 10 minutes later, the dissolved oxygen levels are measured again at the monitoring site. If the oxygen levels are not at the approved threshold, then the second blower kicks on. If the two blowers fail to bring up the oxygen levels, an alarm sounds which warns operators at the dam to release water down the spillway. The extra turbulent water churns down the spillway, bringing with it additional dissolved oxygen.

During the early morning hours on Friday, August 3, both blowers and the spillway were fully engaged, sending dissolved oxygen to the area of the Snake River below the dam. However, dead and dying fish were already floating in the water, even floating downstream— later prompting the weekend phone calls from the public to IDFG and the Power County Sheriff’s office.

What does this mean for the fishery?

IDFG personnel spent most of the day on Monday below American Falls dam, further investigating the issue, collecting some of the dead fish, measuring water temperatures and oxygen levels, talking with anglers, and checking fishing creels. Though this was an unfortunate event, it appears many fish survived. Anglers are reporting active fish and good catch rates. Though the coordinated effort among IDFG, DEQ, and Idaho Power was executed well, this incident also provides an opportunity to evaluate what went well with the safety protocols and what could be improved.


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Dead fish were observed at the Snake River Vista boat ramp below American Falls Dam over the weekend. The fish kill was the result of low oxygen levels in the river immediately below the dam. Oxygen levels have since restored to normal. Photo courtesy IDFG.