The relatively dry spring helped produced excellent water quality in many streams and rivers throughout Minnesota. These conditions were good for fish, as well as fishing and other forms of river recreation. In fact, a small-mouth bass was caught in the Maple River in southern Minnesota when the water was clear.
One storm can change that. Heavy rains lead to runoff from farm fields — many of which are bare at this time of year — as well as from paved areas in cities. The rain flushes pollutants from the landscape and raises stream levels. The results are erosive flows, higher levels of pollutants, and muddy waters.
For example, rains on May 16-17 led to higher levels of sediment in the Le Sueur River near Mankato, as shown in the sample bottles at right. The results before and after:
- Total suspended solids: 32mg/l
- Nitrate: 3.4 mg/l
- Phosphorus: 0.059 mg/l
- Total suspended solids: 1,100 mg/l (34 times higher)
- Nitrate: 11 mg/l (3 times higher)
- Phosphorus: 0.904 mg/l (15 times higher)
The Le Sueur is a tributary to the Minnesota River, which carries high sediment loads to the Mississippi and downstream. The Maple River referenced above is a tributary to the Le Sueur.
Catching that flush of pollutants is the job of the Watershed Pollutant Load Monitoring Network. This network of state and local partners monitor the quality of major rivers throughout Minnesota. Their work is important for detecting whether water quality is getting better or worse, tracking the impact of restoration work, and identifying sources of pollution and ways to reduce them.
The MPCA uses the data in total maximum daily load studies and watershed restoration and protection strategies it develops with local partners. These studies determine the reductions needed in pollutants and how to restore or protect water quality. Local partners then use the data for planning and to implement the best strategies in priority locations.
Across Minnesota, scientists are tracking peaks in stream flows. For example, the Minnesota River at Mankato has doubled in flow since 1940. These peak flows, fueled by heavy storms and increased artificial drainage, lead to increases in runoff of stormwater and erosion of streambanks, ravines, and bluffs. The runoff and erosion bring more sediment and other pollutants into lakes and streams.
Climatologists predict that heavy rains are becoming more common, making water retention even more important for keeping pollutants out lakes and streams. Practices that retain water include:
- Building soil health on cropland
- Planting cover crops
- Restoring wetlands
- Building water and sediment retention basins
- Planting rain gardens
- Planting buffers of native vegetation along lakes, streams, and ditches