Deposition and Erosion

East side erosion in ravine 2013Even a casual walk through the park, especially from the east entrance, reveals tremendous amounts of active erosion in the floor of the valley which people often find disturbing.  What isn’t so obvious is that the black soil that is being eroded is not actually native to the valley bottom – it was deposited there between about 1850 and World War II following the introduction of intensive European-style farming to the upland fields above the valley.

Many of the deep stream valleys in the Upper Mississippi Valley region exhibit similar soil deposits.  Since about the 1930s these soil deposits have shifted from being in depositional mode to being eroded away as farmers became aware of their soil erosion and implemented conservation practices, dramatically reducing the amount of soil loss from their fields and the attendant soil deposition in the valleys.  Today’s erosion on the valley bottom is a natural process which is moving the soil further downstream and eventually will return the valley to its approximate configuration before European settlement. 

A second type of erosion and deposition active especially on the eastern side of the park is what geologists call “mass movement,” more commonly known as slumps.  Three Carleton College students (Dave Ratner, Bonnie Rohr and Judith Friedman) mapped the landslides in the park in 1980 and many of these small landslides are still visible if you know where to look (their paper and map is available in the Publications tab of this website under “Academic And Professional Papers”).  This down-slope movement results from the soft nature of the bedrock and is a natural and normal type of weathering in this sort of environment.

A slump overrode the boardwalk in 2014.
A small landslide overrode the boardwalk in 2014.

In recent years extreme rain events have accelerated both types of erosion and deposition, especially along east-side trails, necessitating relocation of some sections of the trails.  The FCRWA board has been exploring measures to alleviate some of the more manageable  points of runoff from the upland fields in order to restrain some of the erosion.

References:  See the “Academic And Professional Papers” under the Publications tab.