In a year with average rainfall, about 1 million tons of sediment – soil and organic matter such as algae and leaf fragments — floats down the Minnesota River.
All that sediment joins another 200,000 or so tons from the upper Mississippi and St. Croix rivers and eventually flows into Lake Pepin below Red Wing.
Cores taken from the bottom of Lake Pepin have shown that the volume of sediment flowing into the wide spot in the Mississippi is about 10 times greater than it was prior to European settlement.
In 2012, the Minnesota Pollution Control Agency proposed Total Maximum Daily Load plans calling for an 80 percent to 90 percent reduction in sediment in the Minnesota River basin and a 50 percent to 60 percent reduction in the Mississippi in the south metro.
The MPCA is still finalizing the TMDLs, and neither has yet been approved by the U.S. Environmental Protection Agency.
On Monday, March 17, the MPCA began taking public comment on a Sediment Reduction Strategy setting much smaller interim goals to be achieved over the next 16 years. The phased strategy suggests a short-term goal of reducing sediment in both rivers by 25 percent by 2020 and 50 percent by 2030.
Most of the sediment comes from collapsing stream banks and bluffs, ravines and gullies and erosion from farm fields. The new strategy blames significantly increased flow volumes in the Minnesota River in recent years for causing much of the sediment.
But the strategy document sidesteps the controversial question of whether a significantly wetter climate or in recent years or a big increase in agricultural drainage is the major cause of the increased flows. It blames both climate and drainage for the higher water in the river.
The strategy suggests local governments in tributary watersheds begin working to meet the 25 and 50 percent goals by devising ways to store water on the land during heavy rains to reduce peak flows in the rivers.
Read the 53-page strategy document.
The MPCA is accepting public comments on the strategy through April 17. Questions and comments can be submitted to:
Larry Gunderson, Minnesota Pollution Control Agency, 520 Lafayette Rd. N., St. Paul, MN, 55155. Email him at: larry.gunderson@state.mn.us. Phone 651-757-2400 or 800-657-3864.