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Downstream Impacts

Lower Minnesota River

Lake Pepin

Lake Pepin

Lake Pepin is filling in

Lake Pepin lies downstream of the confluence of the Minnesota and Mississippi Rivers. It is a naturally occurring lake, and part of the Mississippi River on the border between Minnesota and Wisconsin.


As the Minnesota River flows into the Mississippi, it carries excess sediment and nutrients. Three rivers contribute sediment to Lake Pepin: The Minnesota, St. Croix, and Mississippi Rivers. Scientists have studied sources of sediment into the lake and determined that the Minnesota River contributes approximately 85 percent of the sediment load.

Elevated Phosphorus levels

in Lake Pepin, Phosphorus is accumulating in the sediment at 15 times the natural rate. Phosphorus loading to Lake Pepin appears to have increased by about seven times (or more) above natural rates. Lake water Total Phosphorus concentrations have increased from about 50 ppb (parts per billion) to 200 ppb, making Lake Pepin highly eutrophic.

A eutrophic body of water, commonly a lake or pond, that has high primary productivity caused by excessive nutrients and is subject to algal blooms resulting in poor water quality. The bottom waters of such bodies are commonly deficient in dissolved oxygen which can be detrimental to aquatic organisms.

For more information on initiatives to protect and restore Lake Pepin, visit Lake Pepin Legacy Alliance website. 

Lake Pepin

Source: Metropolitan Council

Lake Pepin algae bloom

Gulf of Mexico

Gulf of Mexico

As the Minnesota River flows into the Mississippi River, it carries excess sediment and nutrients which impact downstream receiving waters. The Minnesota River has been identified as a substantial contributor of excess nitrates to the Mississippi River and the Gulf Region.

In recent years, this problem has been particularly severe in the Gulf of Mexico where development of a hypoxic zone (low oxygen) has been linked to elevated nitrate levels carried to the Gulf by the Mississippi River. Reduced oxygen levels in the hypoxic zone, brought on by decomposition of algae, have damaged the shellfish industry and continue to threaten the
aquatic ecosystem of the Gulf Region.

The Minnesota River and the Dead Zone

This image shows the hypoxic zone (sometimes referred to as the dead zone) in the Gulf of Mexico. Reds and oranges indicate areas of low oxygen concentration. In July 2008, the hypoxic zone was mapped at 7,988 square miles—the second largest on record since measurements began in 1985. This is larger than the land area of the state of Massachusetts.

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