T54 Geological Mapping: Key to Successful Management of Water and Land Resources II

Sunday, 18 October 2009
1:30 PM - 5:30 PM
B116

MAPPING TO INFORM MODELING OF TURBIDITY IN AGRICULTURAL WATERSHEDS OF THE MINNESOTA BASIN

Paper number: T54-11
Program: Technical Sessions
Time: 4:00 PM – 4:15 PM

Authors

Abstract

The Minnesota River is the major sediment source to a large, riverine lake on the Mississippi River, Lake Pepin. Both the Minnesota River and Lake Pepin are impaired for turbidity and the focus of Clean-Water-Act-mandated restoration. Work funded in part by the Minnesota Pollution Control Agency (MPCA) includes mapping in combination with geochemical sampling, stream gaging and modeling to: locate and quantify sediment sources and sinks; develop an understanding of Holocene sediment dynamics; and document the response of the system to recent land-use and hydrologic change.

The soil in the watershed is developed in Late-Wisconsin-age, fine-grained, glacigenic parent material. High organic content and moisture retention of the soils make them ideal for farming if drained; at least 30% of the area was originally wetland. The most turbid and productive watersheds have glacial lake sediment at the surface.

Results from atmospheric- and fallout-radionuclide tracing of surface sediment show that surface sediment, although a modern (and not pre-settlement) contributor to the load, is not the dominant cause of the increased load to Lake Pepin. Cosmogenic isotopes are being used to refine the role of the re-entrainment of post-settlement alluvium—surface sediment that was dislodged early in the agricultural history of the watershed but had until recently been stored.

The channel now occupied by the Minnesota River was created suddenly 11,500 radiocarbon years ago by glacial Lake Agassiz discharge. Incision ranged from 45 m at the lake outlet to 70 meters 300 km downstream. Knickpoints that were created at all tributary mouths have migrated upstream exposing bluffs of glacigenic sediment, typically with >60% fines. Stream gages installed above and below the major knick zones show dramatic increases in sediment loading from the bluffs and ravines that expose the glacial section. This is consistent with the geochemical results, direct measurements of bluff retreat, and estimates from an earlier, site-specific study.

Key to restoration efforts planned by the MPCA is accurately targeting sediment sources as the Minnesota River watershed continues to respond to late-glacial and post-settlement events that have exposed deep sources of fine-grained glacial sediment and altered the hydrology.