Abstract
Heavy snow and early spring rainfall generated substantial
amounts of runoff and flooding in the upper part of the
Missouri River Basin in 2011. Spring runoff in the upper and
middle parts of the basin exceeded the storage capacity of
the Missouri River reservoirs and unprecedented amounts of
water were released into the lower parts of the basin resulting
in record floods from June through September on the
Missouri River in Iowa and Nebraska and extending into
Kansas and Missouri. Runoff from the Missouri River Basin
in April through September 2011 was 8,440,000 hectare
meters (68,400,000 acre feet) and was only exceeded during
flooding in 1993 when runoff was 11,200,000 hectare meters
(90,700,000 acre feet).
Nitrate and total phosphorus concentrations in the Missouri
River and selected tributaries in April through September,
2011 generally were within the expected range of
concentrations measured during the last 30 years. Substantial
discharge from the upper and middle parts of the Missouri
River Basin resulted in nitrate concentrations decreasing in
the lower Missouri River beginning in June. Concentrations
of nitrate in water entering the Mississippi River from the
Missouri River were less in 2011 than in 1993, but total
phosphorus concentrations entering the Mississippi River were
substantially greater in 2011 than in 1993.
The Missouri River transported an estimated
79,600 megagrams of nitrate and 38,000 megagrams of total
phosphorus to the Mississippi River from April through
September 2011. The nitrate flux in 2011 was less than
20 percent of the combined total from the Upper Mississippi
and Missouri River Basins. In contrast, the total phosphorus
flux of 38,000 megagrams from the Missouri River constituted
about 39 percent of the combined total from the Upper
Mississippi and Missouri River Basins during April through
September 2011.
Substantially more nitrate but less total phosphorus was
transported from the Missouri River Basin during the historic
1993 than during the 2011 flood. Greater runoff from the
lower part of the basin contributed to the greater nitrate transport
in 1993. In addition to the differing amounts of runoff and
the source of flood waters, changes in land use, and management
practices are additional factors that may have contributed
to the difference in nitrate and total phosphorus flux between
the 1993 and 2011 floods.
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First posted November 12, 2013
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