Excess sediment is a top cause of impairment in U.S. rivers and streams. A number of streams
on the north shore of Lake Superior’s western arm are on the Minnesota Pollution Control
Agency’s impaired waters list due to turbidity problems. The underlying geology of the north
shore, in addition to the steep slopes of the Lake Superior escarpment, forms a stream base
vulnerable to erosion and excessive sediment deposition in streams. This vulnerability is
created, at least in part, by an area of clay loam soil that many north shore stream channels
intersect as they come down the escarpment to the shore of Lake Superior. The steep slopes
cause high stream velocities which, combined with the high erodability of this soil layer, create
high erosion potentials, particularly on outside channel bends.
The increased fine sediments traveling through and accumulating in stream substrates
potentially presents several problems for aquatic biota. Excess sediment deposits reduce
habitat space for aquatic macroinvertebrates, which are vital components of the food web. In
addition to potentially decreasing food sources for fish, the excess sediment deposits can bury
fish spawning habitats. Even if the fish can clean off nesting areas, they will expend extra
energy doing so.
There are many stream condition indicators using stream fish or macroinvertebrates, but none
address excess sediment specifically. In many areas of the country there are any number of
human‐caused stressors affecting stream condition, including agricultural runoff, high
stormwater discharges, loss of stream shoreline habitat, deforestation, development, and
industrial discharges. When there are many stressors impacting streams, it is hard to
differentiate among them to determine which stressors are creating which problems for stream
biota. While some north shore streams have non‐turbidity impairments, there are considerably
fewer than in other parts of the country. The dominance of erosion‐based impairments
provided the opportunity to develop an indicator diagnostic of excessive sediment deposition in
stream substrate as the cause of biotic impairment in north shore streams.
We selected stream macroinvertebrates for indicator development for several reasons. They
are less mobile than fish, meaning that they have limited ability to escape from disturbance,
and even more limited ability to return after a disturbance ceases (at least until the next
generation begins). Macroinvertebrates are easy to collect, are present in relatively high
abundances, and have high morphological diversity. For all of these reasons,
macroinvertebrates are commonly used in stream condition assessments, and their use is
ubiquitous across the US and across agencies. Because most agencies collect stream
macroinvertebrate information already, their use to create a diagnostic indicator could allow
agencies and managers to get more information out of data they already have, without the
need for additional sampling. The goal of this project was to develop a suite of stream macroinvertebrate metrics diagnostic
of invertebrate community impairment caused by excessive fine sediment deposition in stream
substrate; in other words, burial or partial burial of streambed rocks by sand, silt, and clay. Such
a diagnostic tool would aid managers in their stream assessment work. While similar projects have been previously attempted (and failed) in other parts of the country, most have been in
areas suffering from a number of stressors, making development of an indicator diagnostic of
just sediment impairment more difficult. Our hope in attempting such work using north shore
streams was that the relative lack of other stressors in northeastern Minnesota would make the
development of such an indicator more possible. Having such an indicator should help agencies
make a stronger connection between the Total Maximum Daily Load (TMDL) turbidity
measurements and sediment deposition presumed to be causing harm to stream biota.
Minnesota’s Lake Superior Coastal Program; Project No. 306-11-12; Contract No. B46828
Brady, Valerie; Herrera, Larissa.
Developing a Diagnostic Tool for Assessing Excessive Sediment Harm to Stream Communities.
University of Minnesota Duluth.
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