Strath terraces of the Le Sueur River, south central Minnesota, preserve the record
of river incision. A combination of airborne LiDAR (light detection and ranging) and
terrace dating through optically-stimulated luminescence (OSL) and radiocarbon methods
were used to construct a conceptual model of valley excavation during the Holocene and
late Pleistocene from lateral and vertical incision. The river is responding to
approximately 70 meters of base level fall that occurred 13,400 years ago (11,500
radiocarbon years before present), when glacial River Warren carved the Minnesota
River valley. The carving of the Minnesota River valley led to widespread incision on
Minnesota River tributaries as knickpoints propagated upstream from the main stem
As the knickpoint moved up the Le Sueur River, hundreds of terrace surfaces
were formed. These terraces are strath terraces carved into glacial tills, with alluvial deposits overlying planed-off till surfaces. Observations from dating terrace alluvium
indicate that the river underwent relatively continuous incision, which is ongoing today.
The incision model derived from terraces ages was coupled with valley geometry
measured from LiDAR data to determine how valley excavation rates have changed
through time. Results from this conceptual model indicate that valley excavation has
been relatively constant through time. When this background valley excavation rate was
compared with the modern sediment load, it was determined that the modern sediment
load is 4-5 times greater than the average Holocene sediment load. This demonstrates that the post-settlement load is greater than the pre-settlement load and should guide the
management of this basin to focus on the anthropogenic changes to the basin.
University of Minnesota M.S. thesis. May 2012. Major: Geological sciences. Advisor: Dr. Karen Gran. 1 computer file (PDF); vii, 54 pages.
Johnson, Andrea Lynne.
Timing and pattern of valley excavation, Le Sueur River, south-central Minnesota, USA.
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