Minnesota River - Downtream Impacts

Downstream Impacts
Sediment, Phosphorus & Lake Pepin

Lake Pepin is filling in
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.

Lake Pepin is filling in with sediment at about 10 times its natural rate. At this rate, it will be completely filled with sediment within 340 years. 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.

Elevated Phosphorus Levels
Phosphorus is accumulating in the sediment at 15 times the natural rate. Phosphorus loading to the lake 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. Eutrophic means waters rich in mineral and organic nutrients promote a proliferation of plant life, especially algae (see photo below), which reduces the dissolved oxygen content and can cause fish kills.

Nitrates & The Dead Zone
A substantial contributor of excess nitrate

The Minnesota River and the Dead Zone
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 nitrate to the Mississippi River and the Gulf Region.

What is the Dead Zone?
In recent years, this problem has been particularly severe in the Gulf of Mexico where development of a hypoxic zone (hypoxia means “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.

Size of the Gulf of Mexico Dead Zone
The size of the Gulf of Mexico Dead Zone is increasing. The average size of the Dead Zone over the past 5 years has been 6,600 square miles. The long term average is 5,300 square miles (NOAA, 2008). In 2008, the Dead Zone in the Gulf of Mexico stretched 7,988 square miles measuring second largest since measurements began in 1985. (Source: NOAA, 2008).

This map (pending) shows the average flow-weighted mean concentrations of Nitrate-Nitrogen across the Minnesota River Basin 2000-2005. Elevated Nitrate levels can stimulate excessive levels of algal growth in streams.
 

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