High water temps, low flows, and ammonia contribute to Big Sioux fish kill
PIERRE, S.D. – The South Dakota Department of Environment and Natural Resources (DENR) investigation indicates low flows, high water temperatures, and elevated ammonia levels contributed to a fish kill that occurred recently in the Big Sioux River.
The investigation measured ammonia levels in the river that met water quality standards and would not normally be toxic to fish. But with the fish already stressed by low flows and high water temperatures even these levels became detrimental to the fish.
Big Sioux River water temperatures were 90 degrees last Friday, 72-78 degrees after a Saturday morning rain, and back up to 90 degrees by Tuesday. River flows, which normally average 250 cubic feet per second (cfs), had dropped to as low as 28 cfs last Friday and parts of the river had only a few inches of water.
After the fish kill was reported on July 27, DENR had water quality staff sampling the river from above the diversion canal to Brandon on both Saturday and Sunday. More on-site sampling was done on Monday and Tuesday.
DENR investigators reviewed permitted discharge records from John Morrell because it is located immediately upstream from the area of the fish kill. Although ammonia levels in John Morrell's discharge rose last week due to the warm temperatures impacting its wastewater treatment plant, ammonia levels in the treated wastewater were still within the water quality standards specified in the plant's surface water discharge permit. No violations were noted. Wastewater staff from John Morrell cooperated fully with the investigation.
"We want to thank those who promptly reported this fish kill as that allowed DENR to get on-site quickly to document water quality conditions," said DENR Secretary Steve Pirner. "Because South Dakota is a prairie state and subject to drought, aquatic systems become stressed when river flows become low and water temperatures are high. Other factors such as water quality can compound those stress levels, so DENR must remain diligent to ensure water pollution controls remain in compliance."
Staff from the City of Sioux Falls and the Department of Game, Fish and Parks, and SDSU have also aided with the investigation.