07-28-2009, 03:10 PM
Residents and visitors to the Pierre area have been noticing dead fish floating on the surface of the Missouri River from Oahe Dam to town, but there is no cause for concern.
The dead fish are primarily lake herring that are being drawn through the power plant intakes in Lake Oahe and being deposited downstream into Lake Sharpe. Larger fish that get close to the suction of the intakes can swim away, but lake herring and other small fish cannot.
Lake herring, which school by nature, prefer cold water and were introduced into Lake Oahe as a forage fish.
The forage fish are being drawn through Oahe Dam because of the current elevation of the lake and the intakes. The water intakes are located midway between the water surface and the lake bottom and have a top and bottom elevation of 1,534 and 1,524 feet above sea level, respectively.
At the current water elevation of 1,612 above sea level, water is removed from Lake Oahe between 78 to 88 feet down. Fish near the intakes in that deep water are susceptible to being pulled through the dam.
Lake Oahe is considered a two-story lake, containing an upper warm-water layer where walleye, catfish, and other warm fish live and a deep, cold-water layer that can support Chinook salmon, rainbow smelt, and lake herring.
The cold-water layer is currently being drawn from Lake Oahe, through the dam and into the downstream Lake Sharpe.
This year, the change from the warm-water to the cold-water layer, or thermocline, is approximately 55 feet deep. Fish in that layer of cold water are susceptible to being pulled through the dam.
Some people who've seen floating fish in the tailrace were concerned because lake herring are easily mistaken for walleyes, without close inspection.
Lake herring are abundant in Lake Oahe this year, and the suctioning of fish from Lake Oahe into Lake Sharpe is important to the Lake Sharpe fishery. Many of the fish in Lake Sharpe feed on these new fish and grow larger. Rainbow trout and Chinook salmon are again taking advantage of food provided in the tailrace area.
The recent drought, which lasted nearly nine years, resulted in low water levels and warmer water temperatures in the tailrace. Releases from Oahe Dam were relatively low and few forage fish were drawn through the dam during that period.
The current high level of the lake has reversed that scenario.
The dead fish are primarily lake herring that are being drawn through the power plant intakes in Lake Oahe and being deposited downstream into Lake Sharpe. Larger fish that get close to the suction of the intakes can swim away, but lake herring and other small fish cannot.
Lake herring, which school by nature, prefer cold water and were introduced into Lake Oahe as a forage fish.
The forage fish are being drawn through Oahe Dam because of the current elevation of the lake and the intakes. The water intakes are located midway between the water surface and the lake bottom and have a top and bottom elevation of 1,534 and 1,524 feet above sea level, respectively.
At the current water elevation of 1,612 above sea level, water is removed from Lake Oahe between 78 to 88 feet down. Fish near the intakes in that deep water are susceptible to being pulled through the dam.
Lake Oahe is considered a two-story lake, containing an upper warm-water layer where walleye, catfish, and other warm fish live and a deep, cold-water layer that can support Chinook salmon, rainbow smelt, and lake herring.
The cold-water layer is currently being drawn from Lake Oahe, through the dam and into the downstream Lake Sharpe.
This year, the change from the warm-water to the cold-water layer, or thermocline, is approximately 55 feet deep. Fish in that layer of cold water are susceptible to being pulled through the dam.
Some people who've seen floating fish in the tailrace were concerned because lake herring are easily mistaken for walleyes, without close inspection.
Lake herring are abundant in Lake Oahe this year, and the suctioning of fish from Lake Oahe into Lake Sharpe is important to the Lake Sharpe fishery. Many of the fish in Lake Sharpe feed on these new fish and grow larger. Rainbow trout and Chinook salmon are again taking advantage of food provided in the tailrace area.
The recent drought, which lasted nearly nine years, resulted in low water levels and warmer water temperatures in the tailrace. Releases from Oahe Dam were relatively low and few forage fish were drawn through the dam during that period.
The current high level of the lake has reversed that scenario.