Almendinger, J. E.. St. Croix Watershed Research Station, dinger@smm.org
Engstrom, D. E.. St. Croix Watershed Research Station, dre@tc.umn.edu

 
TWO CENTURIES OF HUMAN IMPACT ON SEDIMENT AND NUTRIENT LOADING TO THE UPPER MISSISSIPPI RIVER: THE LEGACY OF LAKE PEPIN
 
Long-term changes in sediment and phosphorus loading to the upper Mississippi River were quantified from an array of 25 sediment cores from Lake Pepin, a large natural impoundment downstream of the Minneapolis/St. Paul metropolitan area. Cores were dated and stratigraphically correlated using lead-210, cesium-137, radiocarbon, magnetic susceptibility, pollen, and loss-on-ignition data. All cores showed a dramatic increase in sediment accumulation and phosphorus concentration during European settlement beginning in 1830. Lake-water total-phosphorus concentrations, estimated from fossil diatoms, increased from 50 to 200 g/L during this period. Sediment loading to Lake Pepin from the Mississippi River has increased by an order of magnitude since 1830. Modern fluxes are about 900,000 t/yr and are more than 80% detrital mineral matter. About 17% of the lake's volume in 1830 has been replaced by sediment, and at current accumulation rates the remainder will be filled in another 340 years. Phosphorus accumulation in Pepin sediments has increased 15-fold since 1830, rising from 60 to 900 t/yr. This rise represents a seven-fold increase in phosphorus loading from the Mississippi coupled with more efficient retention by sediments. Although gradual increases in nutrient and sediment inputs began with European settlement, the most dramatic changes have occurred since 1940.
 
Day: Tuesday, Feb. 2
Time: 09:15 - 09:30am
Location: Sweeney Center
 
Code: SS19TU0915S