Watershed Primer Part 3: Streams and Riparian Buffers

How Streams Work
Streams and rivers are integral parts of the landscape that carry water and sediment from high elevations to downstream lakes, estuaries, and oceans. Healthy streams migrate laterally over time and maintain a "dynamic equilibrium" with their contributing watershed. When that "dynamic equilibrium" is upset, such as by rapid changes in development patterns, streams cannot respond quickly enough to adjust the other physical processes to accommodate increased runoff or sediment loads. Instead they experience extensive - and undesirable - erosion and sedimentation that sets off a sequence of undesired consequences. Thus, it is important to understand how streams work so that watershed management and land use decisions can support the natural characteristics of the receiving streams and maintain their "dynamic equilibrium".

When rain falls in a watershed, it either runs off the land surface into streams or lakes, infiltrates into the soil or evaporates. As surface runoff moves downslope, it concentrates in low areas and forms small stream channels. These are referred to as "ephemeral" channels that only carry water during rainfall runoff. Downstream from ephemeral channels are "intermittent" streams, which carry water during wet periods of the year. These streams are partially supplied by ground water rising to the surface as stream baseflow. Intermittent streams dry up periodically as ground water levels decline in drier seasons. Further downstream where baseflow is large enough to sustain stream flow throughout the year, "perennial" streams are formed.

The size and flow of a stream are directly related to its watershed area and geology. Other factors which affect channel size and stream flow are land use, soil types, topography, and climate. The "geomorphology" (or size and shape) of the channel reflects all of these factors.

One way of categorizing stream size is by "stream order". "First order" streams are the uppermost perennial tributaries in the watershed and have not yet intersected another perennial stream. When two "first order streams" intersect, they form a "second order" stream; when two "second order" streams intersect, they form a third order stream and so on. The first order figure (below) illustrates how the streams that make up a small watershed would be categorized by stream order. In the study area, 54% of the land area drains to first order streams, and 53% of the total stream miles are comprised of first order streams. First order streams are very small in size and in flow volume and, therefore, are much more vulnerable to impacts on water quality and quantity than larger streams.

First Order Streams
First Order streams Illustration of first order streams and the stream order concept. (Schueler , 1995(a))

While streams and rivers vary greatly in size, shape, slope, and bed material, all streams have common characteristics. Streams have left and right streambanks (looking downstream) and streambeds consist of mixtures of bedrock, boulders, cobble, gravel, sand, or silt and clay. Other physical characteristics of streams include pools, riffles, steps, point bars, meanders, floodplains, and terraces. All of these characteristics are related to the interactions among climate, geology, topography, vegetation and land use of the watershed. The study of these interactions and the resulting streams and rivers is called "stream (or fluvial) geomorphology."

Stable streams migrate across the landscape slowly over long periods of geologic time while maintaining their form and function. Naturally stable streams must be able to transport the sediment load supplied by the watershed. Stream instability occurs when increased runoff and scouring causes the channel to incise (downcut or degrade) or when increased sediment load and excessive deposition causes the channel bed to rise (aggrade). The product of sediment load and sediment size is proportional to the product of stream slope and discharge (or stream power). A change in any one of these variables causes a rapid - and usually undesirable - physical adjustment in the stream channel.

The most important stream process in defining channel form is the "bankfull discharge". Bankfull discharge is the flow that transports the majority of the stream’s sediment load over time and thereby forms the channel. The bankfull stage, during bankfull flow, is the point at which flooding may begin to escape the stream channel and enter the floodplain. On average, bankfull discharge occurs approximately every 1.5 years. In other words, each year there is about a 67 percent chance of having a bankfull streamflow event. Because of its important role in maintaining the form and function of stable streams, it is important to manage watersheds to maintain a stable bankfull streamflow. In affected channels, flooding may occur before bankfull stage, if the channel is aggraded, and may not occur at bankfull stage is the channel is degraded.

Stream width generally increases in the downstream direction in proportion to the square root of discharge. Stream width is a function of discharge, sediment transport, bed and bank material, and vegetation along the riparian edge of the stream. Vegetation along the stream corridor provides resistance to erosion, food for aquatic species, shade to moderate stream water temperature, and filtering of sediments and pollutants prior to reaching the stream.

Natural streams have sequences of riffles and pools or steps and pools that maintain the channel slope and stability (see figure). The riffle is a bed feature with gravel or larger size rocks. The water depth is relatively shallow and the slope is steeper than the average slope of the channel. At low flows, water moves faster over riffles, which provides oxygen to the stream for fish and aquatic insects. Riffles are found entering and exiting meanders and control the bottom elevation of the stream. Pools are located on the outside bends of meanders between riffles. The pool has a flat slope and is much deeper than the average depth of the stream.

A stream and its floodplain comprise a dynamic environment where the floodplain, channel and bedforms evolve through natural processes that erode, transport, sort and deposit alluvial materials. The result is a "dynamic equilibrium" where the stream maintains its natural dimension pattern and profile over time, neither "downcutting" or "aggrading". Land use changes in the watershed and channelization can dramatically upset this balance. A new equilibrium may eventually result, but not before large adjustments occur in the channel form, such as extreme bank erosion or channel downcutting. By understanding and applying natural stream processes to stream and watershed management, land uses within the watershed can be accommodated while maximizing the stream’s natural flow-carrying ability and its biological potential. (adapted from NC Cooperative Extension Service 1999)
The Role of Riparian Buffers

Forested riparian (meaning "along the water") buffers are as essential to watersheds as ground water and rainfall. An inter-connected network of forested riparian buffer is essential for a healthy and thriving ecosystem. The benefits of the forested buffer cannot be mimicked by any other BMP or management practice. Forested buffers along stream banks protect stream waters from direct sunlight and resulting high temperatures, provide detritus in the stream that serves as food and shelter for aquatic species, and stabilizes stream banks, stream channels and floodplains from the erosion and scour of high velocity flood flows. These forested buffers also serve as the link between terrestrial wildlife and their source of water, food and cover. The roots absorb and "take up" nutrients and other pollutants from ground water as it migrates through the root zone. Plant stems and leaves filter sediment and pollutants from overland flow of stormwater through passing through the buffer.

A substantial body of scientific research documents the need and functions of forested riparian buffers. As yet, no model is readily available to determine optimum riparian buffer width for site-specific conditions (such as size of the contributing runoff area, upland slope, stream size, soil and bedrock characteristics, etc.). However, the following illustration presented in published literature, summarizes the functions of forested buffers and the range of minimum widths that have been shown through research to be needed to achieve them.

The functions needed to protect and restore the streams of the study area include water temperature moderation (shading), streambank stabilization, margin for stream movement and meandering, source of aquatic food and shelter, and nutrient removal and sediment removal. The figure on buffer width and function (below) supports the 100 foot total riparian buffer width (slightly more than 30 meters) that is recommended for use in this Technical Report and that has been used for many years in the Chesapeake Bay watershed communities and elsewhere. A minimum width of 100 feet is consistent with guidance presented in the "Chesapeake Bay Riparian Handbook" (CBP 1999), "PA Stream Releaf" guidelines (PADEP 1998), PA Association of Conservation Districts BMP Manual (PACD 1999), USDA "Riparian Forest Buffers" guidance document (USDA 1991), and is supported by documented research in numerous publications of scientific literature (Todd 2000, Castelle and Johnson 2000, among others). Much of this buffer width is included in floodplains, wetlands and steep slopes.

Buffer Function
Summary of Minimum Buffer Width for Desired Buffer Functions (after Todd, A.H., 2000)

Research has confirmed that in addition to width and vegetation, the length and inter-connectedness are also very important in achieving these functions. Thus, extending and connecting buffers to the maximum extent possible along water features to create a "network" of forested riparian buffers is a critical element of watershed management.

The benefit of riparian buffers in removing non-point source pollutants from farmland is also well documented in the scientific literature. The following summarizes results from many research studies (Dosskey, 2000):

Table 1: Summary of Research Results of Pollutant Reduction from Vegetated Riparian Buffers (Dosskey, 2000)
Function Pollutant Type % Reduction in Amount
Filter surface runoff Sediment Sediment-bound Dissolved Microbes range 40 to 100 range 27 to 96

range 10 to 100 range 43 to 91
Filter ground water runoff Nitrate range 32 to 99

Research continues to strive to develop methods of calculating riparian buffer widths based on specific site conditions. The size of the stream may be less important than the size of the land area draining to the buffer and the type and quantity of pollutants in that drainage area. This is because the buffers purpose is to infiltrate, slow and cleanse the overland flow and shallow ground water draining from upland areas.

The consensus of the various guidance documents that are available recommend that the minimum 100 foot width include three zones:

  • A stream-side zone that is an "undisturbed forest" zone immediately adjacent to the stream with natural vegetation consisting of predominantly trees with shrubs and undergrowth. This zone provides tree and other vegetation to stabilize stream banks, provide shading to the stream, provide leaves, limbs and other organic matter that provide food and shelter for aquatic living resources, infiltration of overland runoff, removal of sediments and nutrients through filtering and uptake by the vegetation, and a margin of protected land area for movement and meandering of the stream channel.
  • A "managed forest" zone that is adjacent to the undisturbed forest zone, with native vegetation consisting of trees with undergrowth, grasses, etc. This zone provides infiltration of overland runoff, removal of nutrients and sediments by the filtering of overland runoff through the vegetated ground cover, removal of nutrients from infiltrated runoff and shallow ground water by the roots of trees and plants. This zone must be "managed" or maintained to exclude invasive species and to periodically prune the trees and shrubs to continue vigorous growth that results in continued uptake of nutrients.
  • A "filter zone", forming the upland side of the buffer and immediately adjacent to the managed forest zone consists of grasses, forbs and dispersion features. This zone provides for surface runoff to be dispersed to shallow sheet flow prior to entering the forested zone to enhance the infiltration and reduce erosion. The vegetation and dispersion features (such as level spreaders) remove sediments from the runoff and slow the velocity of the runoff to reduce erosion and enhance infiltration through the forested zone. The vegetation also removes nutrients through uptake by the roots. This zone must also be "managed" by occasional (i.e., annual) mowing (to encourage continued plant growth and nutrient uptake) and to maintain the dispersion features.

Trees are the primary performer among the vegetation of forested buffers as they absorb and "take up" more nutrients than shrubs and grasses. Their leaf litter and detritus on the ground helps slow down and remove sediments from overland flow. Their canopies provide shade for the stream and drop material that provides shelter for instream habitats.

The meadow grasses of the filter stream are also important as they serve to disperse the incoming overland flow before it enters the forested zones, thus allowing more infiltration and less erosion to occur. They also filter out sediments and take up nutrients as the water passes through the filter zone.

Lawn grasses and other maintained landscape areas generally provide no buffering benefit. In fact they can contribute to impairing streams by the overuse of fertilizer and pesticide chemicals. Planting lawn grasses to the stream’s edge creates a root mass that does not allow stream channels to meander and migrate as they should, and results in excessive instream erosion.

Table 2 summarizes some of the many benefits that forested riparian buffers provide for watershed resources as well as for watershed communities.

Table 2: Benefits of Forested Riparian Buffers (Center for Watershed Stewardship 1996)

  1. Reduces watershed imperviousness by 5% (average 100 ft width buffer)
  2. Distances areas of impervious cover from the stream
  3. Improves septic system effectiveness prior to effluent seeping to stream
  4. Reduces small drainage problems and complaints regarding standing water, backyard flooding, and bank erosion
  5. Stream "right of way" allows for lateral migration of stream meandering and widening while protecting property and structures
  6. Effective flood conveyance
  7. Protection of streambanks from erosion
  8. Increased property values
  9. Increased pollutant removal in ground water
  10. Increased pollutant removal from surface runoff
  11. Foundation for present or future greenways
  12. Provides food and habitat for instream aquatic resources (fish, insects, benthic organisms, etc.)
  13. Moderates stream temperatures by reducing extreme warm temperatures and increasing extreme cold temperatures to provide necessary aquatic habitat conditions
  14. Protection of associated wetlands
  15. Prevent disturbance of steep slopes and prevent severe runoff and erosion rates from those slopes
  16. Preserves important terrestrial habitat and transition zones (1 mile of stream corridor provides 25 to 40 acres of habitat)
  17. Migration corridors for wildlife conservation
  18. Essential habitat for amphibians that require both aquatic and terrestrial habitats and depend on riparian environments to complete their life cycle
  19. Fewer barriers to fish migration
  20. Protect headwater streams from extensive modification from storm drain nclosures/channel hardening
  21. Provides space for other stormwater treatments and BMPs
  22. Allows space and access for future stream restoration, bank stabilization, or reforestation.

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