Nearly half of the more than 80,000 miles of streams in Minnesota have been changed by human activity, according to the results of a two-year study released Monday.
The Minnesota Pollution Control Agency and Minnesota Geospatial Information Office compared aerial photos dating back to the 1930s with modern photos to find that 49.6 percent of stream miles in the state have been altered. The agencies also analyzed data from aircraft using laser rangefinders to determine precise topographic outlines.
While it's been known for decades that many miles of rivers and creeks in the state had been altered for agriculture, dams, development and other human activities, this was the first study to quantify how many miles no longer are running their natural course.
The changes are most noticeable in the Twin Cities and farm regions, especially in central, western and northwestern Minnesota, where large numbers of straight-line ditches were dug in an effort to drain land for agriculture.
In some counties in central and northwestern Minnesota, more than 90 percent of the stream miles have been changed by people over the past 80 years.
ADVERTISEMENT
"A lot of the streams in urban areas are now subterranean. For our purposes, they are no longer considered a definable stream," said Benjamin Lundeen, who headed the project for the PCA.
Northeastern Minnesota has the least miles of altered streams in the state, with most areas well below 30 percent. Some areas of the Boundary Waters Canoe Area Wilderness have zero miles of altered streams, while much of the North Shore has less than 3 percent of stream miles altered.
Lundeen said the project also helped refine the estimate of how many miles of streams Minnesota has. That number is now believed to be about 82,000 miles, down from the old estimate of 92,000 miles.
PCA officials said having comprehensive data on the extent of stream alterations "adds a new layer of context and perspective to ongoing monitoring efforts." It's known, for example, that altered stream channels can result in higher flows, higher levels of pollutants entering waterways and decreased habitat for fish.
The PCA said the results also will help determine potential causes of pollution problems within each of the state's 81 watersheds.
"Knowing where the changes have occurred along the rivers can help show us where some of our problems might be coming from,'' said Steve Mikkelson, a PCA spokesman. "When you straighten out a river, one thing you often get is faster stream flow, which can move a lot more sediment downstream. When a stream meanders, everything happens slower."
The information will help the agency and local groups as they develop watershed restoration plans for specific streams.
The state has 81 major waterways. The project was funded by the Clean Water, Land and Legacy Amendment that receives a portion of the state's sales tax.
