Many of the factors adversely affecting Minnesota’s rivers and streams have a common theme: they increase the rate at which water drains off the land and increase the volume of the drainage. Left to its own devices, nature has a functioning “plumbing system” that filters water and leaves soil intact. But land uses, urban development, and other activities have modified nature’s plumbing, which can lead to problems.
Altering nature's "plumbing"
The use of drain tile and ditches in agriculture, hard surfaces in urban areas, the increased number of intense rain storms, and stream beds straightened to accommodate row crops all quicken water movement across land and into waterways. This quick movement, along with greater volume, increases streambank erosion and the flow of harmful pollutants into rivers and streams.
For instance, recent research shows that about half of Minnesota’s 83,000 stream miles have been physically altered by humans. Channelizing, ditching, and damming projects have changed the natural course of streams and their drainage areas. Altered stream channels can result in higher flows, higher levels of pollutants entering waterways, and degraded habitat.
The map below shows where streams in Minnesota have been altered and where they have retained their natural course. Streams that have been dammed are marked in purple. To see more detail, click on the map for a larger PDF version.
Erosion and runoff
Some agricultural practices — such as installing drain tile, cultivating next to bodies of water with no buffer between water and plowed earth, and allowing cattle to graze in and around streams — have accelerated soil erosion and increased sediment and pollutants in rivers and streams. Extreme weather events, particularly high rainfall amounts in short-burst storms, also contribute to the problem.
Too many impervious surfaces
Cities and towns have abundant hard surfaces, such as roofs, streets, parking lots, and sidewalks. Rain washes across these “hardscapes” rather than soaking into the ground, and carries contaminants into storm drains and on to rivers and streams.
Key factor: Agricultural
Drain tile is plastic pipe installed under farmland to remove excess water from subsoils, creating optimum moisture conditions for crops. In tiled cropland, most of the rainwater that ends up in surface water (ditches, streams) flows through tile drainage and carries excess nutrients along with it. The use of drain tile in Minnesota is increasing.
Key factor: Land use
The way land is used, as well as its natural vegetation and landforms, have a major influence on water quality in the state.