Function and Design for Protection and Enhancement of Water Resources
Streamside forests are complex ecosystems vital to the protection of our streams and rivers
Streamside forests are crucial to the protection and enhancement of the water resources of the Eastern
United States. They are extremely complex ecosystems that help provide optimum food and habitat for
stream communities as well as being useful in mitigating or controlling nonpoint source pollution (NPS).
Used as a component of an integrated management system including nutrient management and sediment
and erosion control practices, streamside forests can produce a number of beneficial effects on the quality
of water resources. Streamside forests can be effective in removing excess nutrients and sediment from
surface runoff and shallow groundwater and shading streams to optimize light and temperature conditions
for aquatic plants and animals. Streamside forests also ameliorate the effects of some pesticides and
directly provide dissolved and particulate organic food needed to maintain high biological productivity
and diversity in the adjoining stream.
The removal of streamside forests has adversely affected the vitality of our water resources
In natural conditions, streamside forests protected most of the rivers and streams of our nation, but
deforestation associated with agricultural and urban expansion has drastically reduced the extent of
streambank protected by forest. The result has been an adverse effect
on the quality of water and aquatic
habitats. In many of our streams and estuaries, water is unfit for human consumption, industrial use or
recreation. Shellfish and finfish production is also reduced. These problems are linked, in part, to
contamination from nutrients, sediment, animal waste, and other pollutants associated with agricultural
and urban runoff.
Continued strengthening of the Clean Water Act reflects the public’s concern for clean water
The Water Pollution Control Act of 1948 or "Clean Water Act" and its subsequent amendments through
1987 demonstrate strong congressional determination to improve the quality of our water resources.
laws have done much to clean up point source contaminants by requiring states to establish and enforce
water quality standards, by requiring specifications and licensing for the discharge of effluents, and by
funding the installation of municipal sewage treatment plants. As a result of the cleanup of concentrated
pollution from specific sites, nonpoint source pollution
from specific sites, nonpoint source pollutants,
which are typically dispersed in origin, have increased in relative importance and now account for more
than 50% of the pollution in our nation’s waters. Nonpoint source pollutants include sediment, nutrients,
pesticides, animal wastes and other substances which enter our water supply as components of runoff and
Streamside forests remove pollutants in several ways
Recent research has shown that streamside forests can:
- improve the quality of water resources by removing or ameliorating the effects of pollutants in runoff
- increase the biological diversity and productivity of stream communities by improving habitat and adding
to the organic food base.
Streamside forests function, often simultaneously, as filters, transformers, sinks and sources.
The streamside forest removes sediment and sediment-attached phosphorus by filtration
The streamside forest functions as a filter by removing sediment and other suspended solids from surface
runoff. Sediment is probably the most common and most easily recognized of the nonpoint source
- Cropland erosion accounts for about 38% of the approximately 1.5 billion tons of sediment that
reach the nation’s waters each year. Pasture and range erosion accounts for another 26%.
- Sediment suspended in the water can reduce or block the penetration of sunlight, adversely affecting the
growth and reproduction of beneficial aquatic plants.
- Sediment deposited on the stream bottom can interfere with the feeding and reproduction of bottom dwelling fish and aquatic insects, weakening the food chain. Large
deposits of sediment can overfill stream channels and floodplains, greatly increasing the potential for
- Several mechanisms of sediment removal are at work in the streamside forest. Some sediment settles out
as the speed of the flow is reduced by the many obstructions encountered in the forest litter. Additional
sediment is filtered out by the porous soil structure, vegetation and organic litter as the runoff flows over
and into the floor or the streamside forest.
- Phosphorus is also reduced by the filtering action of the streamside forest because about 85% of available
phosphorus is bonded to the small soil particles comprising the sediment. Approximately 4% of the
phosphorus is attached to soil particles too small to be filtered by these processes resulting in a removal of
about 80% of phosphorus by the riparian forest filter. The minor amount of ammonium which is bound to
sediment can be filtered out in the same way.
- However, dissolved phosphorus and nitrate must be removed by either microbial or biochemical
The streamside forest transforms nitrate to nitrogen gas
The streamside forest functions as a transformer when chemical and biological processes occurring within
it change the chemical composition of compounds. For example, under well oxygenated soil conditions,
bacteria and fungi in the streamside forest convert nitrogen in runoff and decaying organic debris into
mineral forms NO3
. These forms can then be synthesized into proteins by plants or bacteria. When soil
moisture is high enough to create anaerobic conditions in the litter and surface soil layers, denitrifying
bacteria convert dissolved nitrogen into various nitrogen gasses, returning it to the atmosphere. Studies
that the amount of nitrogen in runoff and shallow groundwater can be reduced by as much as
80% after passing through a streamside forest.
The streamside forest can also function as a transformer
when toxic chemicals such as pesticides are converted to non-toxic forms. Because of continued
improvements in the formulation and management of pesticides, only very small amounts manage to leave
the area of application. These residues, borne by runoff, are converted to non-toxic compounds by
microbial decomposition, oxidation, reduction, hydrolysis, solar radiation and other biodegrading forces at
work in the soil and litter of the streamside forest.
While scientists have long understood the biological processes at work in the streamside forest, additional
data are necessary to fully quantify their importance with respect to pesticide degradation.
The streamside forest acts as a sink by storing nutrients for extended periods of time
The streamside forest
can function as a sink when nutrients are taken up by plants and sequestered in
plant tissue. Some estimates indicate that 25% of the nitrogen removed by the streamside forest is
assimilated in tree growth which may be stored for extended periods of time in woody tissue and possibly
removed as logs or other forest products. Nitrogen and other nutrients may also be passed up the food
chain when plant tissues are consumed by animals and converted to animal tissues. In wetter areas,
nutrients in leaf litter can be stored for longer periods as peat. Sediments filtered out by the streamside
forest remain to become incorporated into the forest soil.
The streamside forest provides a source of energy for aquatic life
The streamside forest functions as a source when it provides energy to streams in the form of dissolved
carbon compounds and participate organic detritus. These materials are critical to processes within the
stream itself, helping to restore and maintain nature’s equilibrium. In small, well-shaded upland streams,
as much as 75% of the organic food base may be supplied by dissolved organic compounds or detritus
such as fruit, limbs, leaves and insects that fall from the forest canopy. Benthic detritivores (the stream
bottom bacteria, fungi and invertebrates that feed on the detritus) form the basis of the aquatic food chain.
They pass on this energy when they are, in turn, consumed by larger benthic fauna and eventually by fish.
Thus the streamside forest functions as an important energy source for the entire aquatic food chain
from headwaters to estuary.
Simple removal of nonpoint pollutants is not enough to improve the quality of water resources. A
balanced, integrated, adaptive community of riparian and aquatic organisms comparable to the natural
systems of the region with stability and capacity for self repair must be reestablished. The
restoration of a healthy aquatic ecosystem from the headwaters to the estuaries to the oceans requires the
reestablishment of significant amounts of riparian forest.
Control of point source pollutants was a start; control of nonpoint pollutants and repair of the aquatic
ecosystem through reestablishment of the streamside forest is a logical next step in improving the quality
of our water resources.
Specifications for such a statewide forest should consider the following:
- Streamside forests should be used in conjunction with sound land management systems that include
nutrient management and sediment and erosion control.
- Sediment removal - The streamside forest must be wide enough to filter sediment from surface runoff.
Maximal effectiveness depends on uniform shallow overland flow. Percent removal of total suspended
solids is a good indicator of effectiveness.
- Nutrient removal - Periodic flooding and the presence of forest litter contribute to conversion of nitrate to
gaseous nitrogen by denitrification. Plant uptake also accounts for significant removal of nitrogen. Trees
must be removed periodically to remove nutrient sequestered in woody biomass and to maintain system
- Periodic minor ground shaping may be necessary to encourage dispersed flow and prevent concentrated
- A portion of the riparian forest immediately adjacent to the stream should be managed to maintain a
stable streamside ecosystem and to provide detritus and large stable debris to the stream.
- Crown cover should be managed to minimize fluctuations in stream temperatures within the range
necessary for instream aquatic habitat.
- Instream slash and debris removal practices should be revised to conserve existing large stable debris by
retaining useful stable portions of jams whenever possible. Unstable tops and smaller debris with potential
to form problem jams should be removed a sufficient distance to prevent re-entry during storm events.
For additional information on specifications/Riparian Forest Buffers please contact the Maryland Department of Natural Resources Forest Service.
United States Department of Agriculture Forest Service