Fish kill results from 'perfect storm' at Corps lake
October 18, 2012
Iron deposits, low dissolved oxygen, fish kill raise concerns at Youghiogheny River Lake
A fish kill was reported in Youghiogheny River Lake, Confluence, Pa., on Sept. 7, and continued to kill fish for about two weeks afterwards. Around the same time, unusual deposits of iron and a strong sulfur odor were reported in the Youghiogheny River, just downstream of the dam. Both of these incidents were related and were attributed to the temperature of the lake, which was more stratified than it had ever been before.
During the summer months most lakes exhibit some thermal stratification. Surface water becomes warmer as it receives more sunlight. Deeper waters, which are not mixed by wind or short term temperature changes, become cooler, denser and contain less oxygen over time. Anoxia, or depletion of dissolved oxygen, eventually develops on the lake bottom. Under this condition, a reducing environment (prevention of oxidation) is created, resulting in elevated levels of minerals and chemicals, including iron, manganese, ammonia and hydrogen sulfide.
Project staff and a Pennsylvania Fish and Boat Commission conservation officer investigated and tracked the fish kill. They estimated about 200 fish died, involving four different species: white suckers, northern pike, walleye, and rock bass. Several days before the fish kill started, Corps of Engineers Water Quality Biologist Rose Reilly and her team conducted an annual water quality survey at the lake. The survey results indicated concentrations of most of the tested parameters were within normal ranges, but the lake was strongly stratified. The lake's surface waters were aerated but much warmer than normal, and the lake bottom was more anoxic than normal.
After the fish kill was reported, the lake staff immediately started to conduct visual surveys and conductivity tests throughout the lake in the areas with the highest number of dead fish and other areas of concern. The results of these tests and data from lake tributary monitors, which are maintained in partnership with the Youghiogheny Riverkeeper (public advocate for the watershed), confirmed the fish kill was not caused by a human source of pollution.
After ruling out pollution, it was determined the cause of the fish kill was natural and related to the extreme drought, warmer air temperatures, a shallower lake, longer lake retention times; and, as a consequence, stronger and earlier lake stratification. Leading up to this, the lake experienced a "perfect storm" of natural conditions including a spring with no snow pack and limited runoff, causing early stratification resulting in no dissolved oxygen below 30 feet. Additionally, little spring rainfall allowed the 30 feet of surface water that had oxygen to warm to the point of stressing cold water species and causing a fish kill during the hottest times. In essence, the cool and cold water fish species were squeezed between too warm lake surface waters with oxygen and cool lake bottom waters without oxygen.
In response to the report of iron floc, or flakes, in the Youghiogheny outflow by project staff, the Water Quality Team conducted another lake water quality survey on Sept. 28. Youghiogheny Dam is a bottom withdraw/release reservoir. Because it was the end of the summer stratification season, all of the water being released was the low quality, anoxic, water from the lake bottom. It contained high levels of sulfur and dissolved iron. When this anoxic, metal laden water was released below the dam and aerated, the iron precipitated out and the sulfur was released. As a result, the beautiful Youghiogheny River with its high quality cold water fishery took on the look and smell of a river affected by acid mine drainage, complete with orange rocks and a rotten-egg odor. Fortunately, it was a natural, short lived event, and the lake is now back to releasing the high quality water normally expected in the Youghiogheny River.