NITROGEN AND SEDIMENT LOADING TO THE UPPER MISSISSIPPI RIVER: ASSESSMENTS OF 25 
WATERSHEDS IN MINNESOTA AND WISCONSIN

Erik Mundahl1 and Neal Mundahl2

1Michigan Technological University, Houghton, MI 49931. 

2Winona State University, Winona, MN 55987

      This study was designed to test two hypotheses: 1) that Southeastern 
Minnesota and West Central Wisconsin tributaries are contributing 
disproportionately more (compared to drainages further upstream) sediments and 
nutrients to pools 5-8 of the Upper Mississippi River during summer 2003; and 2) 
that these pools would be retaining sediments and exporting nutrients. Turbidity 
and nitrates were measured monthly near the mouths of each of 25 tributaries to 
pools 5-8, and at the tailwaters of Lock and Dams 4-8.  Discharges were measured 
at 21 tributaries, whereas discharges at four tributaries and the Lock and Dams 
were obtained online.  Turbidities were converted to suspended sediment loads 
using a standard conversion developed for this region.  GIS watershed and land 
use data were used to determine percentage row crops in each watershed, and then 
compared to sediment and nitrate loads to determine if row crop agriculture 
increased sediment and nitrate stream loads.  Tributary drainages comprised 
11.8% of the watershed area upstream of Lock and Dam 8, but contributed 22.1% of 
suspended sediments and 12.4% of nitrates delivered to Lock and Dam 8. When 
percentages of row crops per watershed were compared to sediment and nitrate 
loads, no significant (P>0.40) correlations were found either on a monthly or 
total summer basis.  Pools 5-8 were exporting more sediments than were being 
delivered during this period of below normal precipitation, with output (107 
metric kilotons) exceeding inputs (100 metric kilotons).  Nitrates also were 
exported, but total summer outputs (17.9 metric kilotons) just equaled inputs 
(17.9 metric kilotons).  Disproportionate contributions of sediments and 
nitrates from the Southeastern Minnesota and West Central Wisconsin tributaries 
to pools 5-8 of the Mississippi River are degrading the river environment by 
increasing sediment load and contributing additional nutrients to the Gulf of 
Mexico Dead Zone.

Keywords: nitrogen loading, sediment loading, Mississippi River, turbidity, 
tributaries