Of the pollutants in our streams, the most
expensive to deal with and most damaging to
stream life is sediment. It plugs spaces
between gravel and rocks (interstices) in the
streambed, preventing their use as living
spaces for life forms that live on the stream
bottom. Sediment can smother eggs of fish,
especially trout and salmon, whose eggs rest
in the interstices of well-oxygenated gravel
beds. An excess of sediment in streams spells
the end of natural reproduction in trout.
In most "natural" systems, the input of
sediment from the surrounding basin is
modest. What enters is shifted and moved
throughout the river channel. It is the
sudden influx, usually from human activities,
that creates problems.
Erosion and sedimentation occur over broad
areas and result from many activities. An
excess of sediment often results from the
- Construction and use of
logging roads and skid trails, especially
in steep topography.
- Road construction, home
and commercial construction.
- Off-road vehicle use.
- Plowing and other
agricultural activities that remove plant
cover and loosen topsoil.
- Overgrazing and continual
trampling of streambanks.
activities are responsible for three times
more sediment pollution than the next
greatest source (U.S.
EPA, 1990). Damage from sediment is
exacerbated by adhesion of nutrients and
pollutants washed in with soil. Bare fields,
an absence of vegetation, and spring plowing
often coincide with spring rains, when cold
temperatures increase waters ability to erode
and carry sediment (see Temperature).