Abstract
Sediment-laden rivers and streams pose substantial
environmental and economic challenges. Excessive sediment
transport in rivers causes problems for flood control,
soil conservation, irrigation, aquatic health, and navigation,
and transports harmful contaminants like organic chemicals
and eutrophication-causing nutrients. In Minnesota, more
than 5,800 miles of streams are identified as impaired by the
Minnesota Pollution Control Agency (MPCA) due to elevated
levels of suspended sediment.
The U.S. Geological Survey, in cooperation with the
MPCA, established a sediment monitoring network in 2007
and began systematic sampling of suspended-sediment
concentrations (SSC), total suspended solids (TSS), and
turbidity in rivers across Minnesota to improve the understanding
of fluvial sediment transport relations. Suspended-sediment
samples collected from 14 sites from 2007 through
2011 indicated that the Zumbro River at Kellogg in the driftless
region of southeast Minnesota had the highest mean SSC
of 226 milligrams per liter (mg/L) followed by the Minnesota
River at Mankato with a mean SSC of 193 mg/L. During the
2011 spring runoff, the single highest SSC of 1,250 mg/L
was measured at the Zumbro River. The lowest mean SSC of
21 mg/L was measured at Rice Creek in the northern Minneapolis-
St. Paul metropolitan area.
Total suspended solids (TSS) have been used as a
measure of fluvial sediment by the MPCA since the early
1970s; however, TSS concentrations have been determined to
underrepresent the amount of suspended sediment. Because of
this, the MPCA was interested in quantifying the differences
between SSC and TSS in different parts of the State. Comparisons
between concurrently sampled SSC and TSS indicated
significant differences at every site, with SSC on average two
times larger than TSS concentrations. The largest percent
difference between SSC and TSS was measured at the South
Branch Buffalo River at Sabin, and the smallest difference was
observed at the Des Moines River at Jackson.
Regression analysis indicated that 7 out of 14 sites had
poor or no relation between SSC and streamflow. Only two
sites, the Knife River and the Wild Rice River at Twin Valley,
had strong correlations between SSC and streamflow, with
coefficient of determination (R2) values of 0.82 and 0.80,
respectively. In contrast, turbidity had moderate to strong relations
with SSC at 10 of 14 sites and was superior to streamflow
for estimating SSC at all sites. These results indicate that
turbidity may be beneficial as a surrogate for SSC in many of
Minnesota’s rivers.
Suspended-sediment loads and annual basin yields indicated
that the Minnesota River had the largest average annual
sediment load of 1.8 million tons per year and the largest mean
annual sediment basin yield of 120 tons of sediment per year
per square mile. Annual TSS loads were considerably lower
than suspended-sediment loads. Overall, the largest suspended-sediment
and TSS loads were transported during spring snowmelt
runoff, although loads during the fall and summer seasons
occasionally exceeded spring runoff at some sites.
This study provided data from which to characterize
suspended sediment across Minnesota’s diverse geographical
settings. The data analysis improves understanding of sediment
transport relations, provides information for improving
sediment budgets, and documents baseline data to aid in
understanding the effects of future land use/land cover on
water quality. Additionally, the data provides insight from
which to evaluate the effectiveness and efficiency of best management
practices at the watershed scale.
U.S. Department of the Interior |
U.S. Geological Survey