Broken Stream Banks

Failure to maintain buffer zones worsens farm pollution

April 25, 2014

Broken Stream Banks: Riparian Buffers Cut Pollution

Better use of riparian buffers could do a great deal to cut the sediment and nutrients that run off cropland and cloud the Minnesota and Mississippi rivers.

The buffers work in several useful ways, including:

  • filtering and retaining sediment;
  • immobilizing, storing and inactivating nutrients and other chemicals;
  • sustaining healthy stream environments and preventing stream banks from eroding;
  • maintaining vigorous aquatic and terrestrial habitats;
  • storing water and recharging subsurface aquifers; and
  • reducing flooding or moderating its impact.17

Multiple studies have shown that buffers can reduce sediment and nutrients in surface runoff from crop fields by 12-to-90 percent.18

Moreover, the plant roots in vegetative buffers reinforce and strengthen stream banks, helping to keep them from eroding and slumping into the water. One Minnesota study found that slumping stream banks contributed 31-to-44 percent of the total sediment dissolved in the Blue Earth River.19 In Iowa’s Walnut Creek, two studies found that stream banks contributed 50-to-80 percent of the sediment load.20 21

By contrast, a long-term project in Iowa’s Bear Creek found that buffering stream banks with strips of grass and/or trees reduced erosion by 80 percent.22

Maintaining a buffer of permanent vegetation between row crops and waterways will not solve all of the pollution problems in the basin. Buffers will do little or nothing to capture nitrogen and other pollutants flowing through tile drain systems. But science and professional experience show that buffers can make an important contribution to cleaner water by trapping phosphorus and sediment running off farm fields and by cutting stream bank erosion.

Minnesota’s progressive shoreland management rule protects critical riparian buffers on agricultural land, providing a powerful tool to help meet the goals for cleaning up the state’s waterways. EWG’s assessment, however, shows that it will take more work to harvest the full benefit of the law, especially on smaller streams and ditches.