The Creation of Sink Holes


Minnesota Karst Bedrock
Sink holes are created when carbonate rocks (mostly limestone and dolomite) near the surface are dissolved by rain water. Fillmore County in southeastern Minnesota has the greatest area of active karst topography in the State. The diagram at the left taken from the Caves in Minnesota document from the Minnesota Geological Survey shows a typical distribution of the bedrock layers.

In karst areas surface streams can disappear underground into a sink hole. A good example is the South branch of the Root River. In times of low water flow the entire river can disappear underground between Mystery Cave in Forestville State Park only to appear above ground again at Seven Springs 1.5 miles away. When doing this the river bypasses about 5 miles of river meander (bends or curves in the river).
The map below shows the location of the Karst areas in the driftless area in southeast Minnesota. The two counties with the greatest amounts of karst areas are Olmsted (west of Winona) and Fillmore.
Minnesota Karst Map
 
The geology of karst areas has a great affect on the water sources. The opportunity for water pollution because of the sink holes causes government groups like the Minnesota Pollution Control Agency (MPCA) to pay special attention to this geologic feature. The MPCA has a web site that provides additional information on karst features and definitions. Some of the structures of karst features in Mower county and the groundwater drainage systems in Fillmore county are available in these pdf documents from the Minnesota DNR.

Looking at the surface, a sink hole does little to reveal the structure of the ground underneath.  In this photo from The Minnesota Geological Survey shows a sink hole early in the spring before vegetation grows up to hide all the features.
sink hole
sink hole

In the photos above, each clump of trees surrounds a sink hole. Before the geology of sink holes was known a number of practices cause environmental problems. Farmers would often try to fill the hole so that the area could be farmed. The new soil would hold for a short time until the next major rain fall or snow melt and then the soil would be carried away in the sink hole. When farmers realized that the efforts to kept the sink hole filled were useless they often became the location for garbage dumps. The trees and tall grass would help to hide the garbage and the garbage would often disappear into the ground. Three sets of techniques are being used at this location to help prevent pollution.

1)  Management practices: Manage the amount of nutrients (fertilizer) and pesticides to reduce the amounts that are able to be washed down the sinkhole.

2)  Vegetative and tillage practices: Conservation tillage, contour farming, strip cropping, filter strips, cover crops, crop rotation and pasture management all are combined to reduce the amount of soil and other particles from going down the sinkhole with the water.

3)  Structural Practices: Waste management systems, run-off management systems, livestock exclusion (fencing out livestock), wetland protection and restoration, grassed waterways, cased wells, sealing abandoned wells, upgrading septic systems, removal of underground storage tanks.


sink hole diagram
The diagram at the left (from the display at the DNR sinkhole demonstration site just southeast of the city of Fountain, MN) outlines some of the measures taken ot prevent pollution of ground water at the sinkhole site. A earth dike is put around the sink hole to prevent direct water run off into the bedrock opening. An overflow pipe takes water from the outside basin to the opening after the soil and other particles have settled out. Rocks near the opening along with the grass and trees help to prevent further washing away of the soil. The grass and trees also help to serve as a mechanical filter for the water and as a way to remove some of the chemical nutrients in the water.

If you'd like to visit it yourself, the sinkhole is alongside the Root River Bike Trail that starts in Fountain.


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