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The Minnesota River carries more suspended sediment than most of the state’s rivers.The average sediment load in the Minnesota River at Mankato is equal to 2,700 tons per day or a 10-ton dump truck load every five and a half minutes.
Animation courtesy of www.Louswheels.netfirms.com
What are Total Suspended Solids (TSS)?
Sediment
is a natural part of the ecosystem and the
transport of sediment is a natural function of rivers. Streams
transport sediment washed in from the watershed and deposit it on natural
bars or into the larger rivers and lakes that they empty into. Modification
of the landscape has accelerated the rate of erosion of soil into waterways.
Problems
occur when the protective vegetative covering from soils is removed. Loose
sediments are then free to wash into the streams with surface water runoff
during rain storms. Increased
runoff has resulted in stream bank erosion.
Why
is TSS important?
Elevated sediment (suspended soil particles) in
waterways has many impacts. It makes rivers look muddy (increases
turbidity), affecting aesthetics and swimming.
Excess
sediment degrades the river system, filling reservoirs, destroying aquatic
habitats, altering biotic communities, increasing water treatment costs
and reducing the river’s aesthetic qualities.
Additonally, sediment carries nutrients, pesticides, and other chemicals
into the river that may impact fish and wildlife species. Sedimentation
can restrict the areas where fish spawn, limit biological diversity, and
keep river water cloudy, reducing the potential for growth of beneficial
plant species. Find out more about where the
sediment comes from.
What
is the standard for TSS?
Minnesota does not currently have a water quality standard
for total suspended solids (TSS) but does have a turbidity
standard of 25 nephelometric turbidity units (NTUs). NTUs are the units
used to measure turbidity. The term Nephelometric refers to the way the
instrument estimates how light is scattered by suspended particulate material
in the water. Learn more about turbidity on water on the
web.
Minnesota Pollution Control Agency studies have shown a strong relationship
between TSS and turbidty and can use TSS for a surrogate to assess water
quality conditions required by the Clean Water Act. In the Western Corn
Belt Plains and Northern Glaciated Plains ecoregions,
which encompass the majority of the Basin, surrogate TSS thresholds of
58 and 66 milligrams
per liter (mg/L),
respectively, can now be used too assess waters when sufficient turbidity
data are lacking.
What is the status of TSS in the Minnesota River Basin?
Although sediment is a part of the natural environment, human activities
sometimes increase the amount that ends up in our streams. Total
suspended solids are a major water quality concern in the Minnesota River.
Soils in the Basin have a high silt and clay content. Eighty-six percent
of the suspended sediment in the Basin is characterized by fine particles
of silt and clay that are easily transported in water.
The Minnesota River carries more suspended sediment than most of the state’s
rivers. The United States Geological
Survey measured sediment in the river at Mankato for 13 years and found
a median concentration of 92 milligrams per liter, a load equal to 2,700
tons per day or a 10-ton dump truck load every five and a half minutes
(Basin Information Document).
The
map and chart at right illustrates the Total Suspended Solids levels in
2002 for selected Minnesota River Basin watersheds. The
illustrations show flow-weighted mean concentrations of TSS. This is equivalent
to routing all of the flow that passed a monitoring site during a specific
timeframe into a big, well-mixed pool, and collecting and analyzing one
sample from the pool to give the average concentration.
For the major tributaries, substantial differences
in TSS flow-weighted mean concentrations are apparent across the Minnesota
River Basin with concentrations seldom exceeding 100 mg/L in major tributaries
in the upper part of the Basin. In contrast, concentrations in major tributaries
in the lower part of the Basin frequently are much greater than 100 mg/L.
Learn more about turbidity
References:
State
of the Minnesota River: 2002 Surface Water Quality Monitoring.
Minnesota River: Basin Information Document. Minnesota Pollution
Control Agency. November, 1997.
Duluth Streams website - http://www.duluthstreams.org/
MPCA Glossary website - http://www.pca.state.mn.us/gloss/index.shtml
MPCA Ecoregion website - http://www.pca.state.mn.us/data/eda/wqguide.html
Water on the Web website
- http://waterontheweb.org/under/waterquality/
For more information about TSS in the Minnesota River Basin, see publications.
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This page was
last updated 9/15/04
Minnesota River Basin Data Center | Minnesota State University, Mankato
184 Trafton Science Center S, Mankato, MN 56001 | Phone: (507)389-5492
| FAX: (507)389-5493 | Email: mrbdc@mnsu.edu