EFFECTS OF FLOOD TIMING AND SPATIAL DISTRIBUTION ON NITRATE EXPORT FROM THE 
UPPER MISSISSIPPPI BASIN
David M. Soballe1, Dennis M. Wasley2, and Richard H. Coupe3
1Environmental Laboratory, Army Engineer Research and Development Ctr, 3909 
Halls Ferry Rd, Vicksburg, MS 39180, 2Minnesota Pollution Control Agency, 520 
Lafayette Road North, St. Paul, MN 55155-4194.  3U. S. Geological Survey, 308 
South Airport Road, Pearl, MS 39208-6649 

We examined the transport of nitrate + nitrite nitrogen (nitrate) in the 
Mississippi River during the period 1992-2001 with emphasis on two major floods 
(1993 and 2001) in the Upper Mississippi River basin (above the Missouri River 
confluence with the Mississippi River). We found significant influences of flood 
frequency (antecedent conditions), seasonal timing, and geographic distribution 
of flooding on nitrate transport. The results show that major floods dominate 
the movement of nitrate in this system and that the spatial and temporal 
distribution of flooding has substantial influence on nitrate transport. Of 
particular importance to the concentration of nitrate in flood waters is the 
level of discharge from the basin during the preceeding year. The character of 
flood waters delivered from the Upper Mississippi Basin to the Gulf of Mexico 
can also be strongly influenced by dilutional flows from other basins in the 
Mississippi drainage (e.g., the Ohio River) so that the seasonal timing of Upper 
Mississippi floods relative to flooding in other subbasins can have important 
consequences. Our results show that the size of the summer hypoxic zone in the 
Gulf of Mexico, which varies in relation to discharge from the Upper Mississippi 
River, appears particularly sensitive to May discharge from the Upper 
Mississippi.
		
keywords: nitrogen, Gulf hypoxia, antecedent conditions, loading