A new diagnosis of two creeks in Carver County says that too much water draining from farm fields has intensified erosion and violated water clarity standards.
Two creeks that meander through Carver County have lost much of their clarity and allure, mainly because too much runoff from farm fields contributes silt and extra water that makes the streams cloudy and smelly.
A pair of new reports about Bevens and Carver Creeks shows that the amount of dirt running into them must be reduced by more than 80 percent during high flow periods to meet state standards for water clarity.
Both creeks flow east and south across the county, moving through wetlands and farmland as they make their way from high ground to the floodplains and waters of the Minnesota River. Some segments of the streams are channeled through narrow farm ditches, and other portions are 8 to 10 feet wide and run through wooded, scenic areas.
The streams are among more than 1,000 lakes and rivers that state officials have found to be "impaired" due to one or more problems that include too much bacteria, nutrients and excess sediment.
Carver County water analyst Tim Sundby said too much sediment hurts the aesthetics of the streams and makes them less attractive for recreation. The scientific term is "turbidity," he said, a cloudiness in the water caused by suspended clay, silt and other fine materials.
Cloudy water over several years will also upset the ecology of a stream and the wildlife that depends upon it, Sundby said.
Because sunlight cannot penetrate as far into the muddy water, he said, healthy plant growth on stream bottoms slows down, and fish and insects cannot find food. Some of the fine particles settle on the stream bottom, creating a layer of muck that covers gravel essential for many fish to lay eggs and reproduce.
It's natural for some dirt to flow into streams from fields and gullies, especially during heavy rains. However, state pollution officials said that farming methods, storm water from cities and other conditions have worsened the runoff problem.
"It's the human-induced acceleration that we're worried about," said Chris Zadak, project manager for the Minnesota Pollution Control Agency. "We're getting streams more frequently with higher flows that have more erosive power," he said.
According to the Bevens and Carver creek analyses, most of the two watersheds are devoted to agriculture. They estimate that 30 percent of the sediment comes from corn and soybean fields, and most of the remainder comes from erosion of gullies and stream banks.
Possible solutions include planting grass buffer strips along the stream banks to slow down the rate of runoff, changing farm tillage practices to reduce erosion, and building or restoring wetlands and ponds to hold more water instead of draining most of it into streams.
Sundby said that after federal officials approve the new study -- which is essentially a diagnosis of the problem -- he and others will spend a year on another study to map out priorities and strategies to begin implementing solutions. Early cost estimates range from $3.1 million to $9.7 million for Bevens Creek, and from $6.8 to $15.5 million for Carver Creek, depending on what solutions are pursued.
Not the only problem
Bevens, Carver and nearby Silver Creek also violate water quality standards for fecal coliform, an indicator of other bacteria in water that can be unhealthy for humans.
According to a separate state study, about 90 percent of the bacteria come from manure in feedlots that washes into the creeks, or that is spread on farm fields as fertilizer and runs off into the creeks. About 10 percent of the bacteria come from non-functioning or poorly designed septic systems at individual homes.
County officials say efforts are underway under a state-approved 2007 plan to work with farmers and homeowners to manage manure properly and to update septic systems.
"The big treatment plants in the metro are pretty tightly controlled as far as pollutants in the water," said Joe Mulcahy, environmental scientist for Metropolitan Council Environmental Services. "Now it's the runoff that's a lot harder to address because it's everywhere."
Tom Meersman • 612-673-7388