Author
 |
Kelley, David |
|
NATER, EDWARD - UNIVERSITY OF MINNESOTA |
|
Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 10/11/1999 Publication Date: N/A Citation: N/A Interpretive Summary: Humans tailor the landscape around them to suit different purposes. In Minnesota, much of the landscape has been altered to accommodate the modern cultivation of row crops and animals. As a result, much of the original prairie and wetlands no longer exist. Now, areas that had limited erosion before alterations are susceptible to higher rates of erosion, and the eroded soil is carried much more easily off the land and into rivers as suspended sediment. The quantity of suspended sediment in rivers such as the Minnesota River and the Upper-Mississippi River have risen almost continuously since European settlement of the Upper Midwest around 1830. As a result of this study, it was found that the bulk of these increases have come from the Minnesota River basin, whose sediment load has increased 12- fold over the last 160 years. One result is that Lake Pepin, a lake in the middle of the Mississippi River near the Minnesota-Iowa border, has already ylost nearly 17% of its volume since 1830 and will lose the upper one-third of its basin in another century. It is estimated that at the present rate of sediment delivery, the lake will be completely filled within 340 years, well before its nearly 4,000-year estimated lifetime. Another result is that excess nutrients and contaminants attached to these sediments are being transported further downstream in the Mississippi river, causing problems for fish, wildlife, and humans along the way and eventually in the Gulf of Mexico. Because we now know which river system is responsible for most of the sediments reaching the Mississippi River, scientists can focus their efforts on reducing suspended sediments in the Minnesota River and put their energy and money into the place where it will do the most good. Technical Abstract: This contribution presents an assessment of the modern and historic fluxes of sediments exiting the Mississippi, St. Croix, and Minnesota watersheds and entering Lake Pepin, a natural riverine lake on the Upper Mississippi River. Through a novel provenance methodology using a Chemical Mass Balance receptor model and elemental signatures for sediments in Lake Pepin and the erivers draining its three principal catchment areas, sediments in this depositional basin were apportioned to their watersheds based on date and depth. Using the apportionment data, estimated trapping efficiency of the lake, and watershed basin areas, sediment fluxes and erosion rates were calculated for each basin on a decadal basis. The relative apportionment of sediments from the Minnesota River watershed increased since European- settlement of the region circa 1830 AD from 83.43% to 87.06% for the upper, 82.89% to 89.95% for the middle, and 77.88% to 86.72% for the lower reaches sof the lake. Whole-sediment loading to the lake shows a 12-fold increase from historic levels in the mass of Minnesota River-derived sediments. The amount of sediment currently supplied by this river is more than seven times the amount supplied by the headwater-Mississippi and St. Croix Rivers combined. Sediment contributions from these two combined watersheds have risen seven-fold since settlement. The causes of these increases are attributable to intensive agricultural production within the Minnesota River basin. Watershed alterations have resulted in a decrease in wetlands, riparian zones, and native prairie, and an increase in acreage artificially drained by sub-surface tile lines and surface ditching. |
