Riparian Buffers De-mystified!

Establishing riparian buffers can be one of the single most effective ways to protect the health of our aquatic resources.  Riparian buffers are small strips of permanently vegetated land adjacent to streams, lakes, or wetlands providing a transition zone between the water resource and human land use.  They offer multiple benefits including improved water quality, flood control, erosion reduction, increased bank stabilization, and add valuable habitat for both aquatic and terrestrial wildlife.

Riparian buffers are typically categorized as either forested riparian buffers which are primarily vegetated with tree and shrub species, or vegetated filter strips that are seeded with grasses and other herbaceous vegetation.  Both types of buffers can be implemented independently of one another or in some combination of the two.

Riparian buffers act as biological filters that slow stormwater runoff and disperse flow over the land.  They improve water quality by effectively trapping and removing excess nutrients and sediments from surface and ground water before they have a chance to make it into our streams and lakes.  These nutrients can cause excessive algal growth and dense mats of vegetation that cloud the water and decrease the amount of available light for native submerged aquatic plants.  These submerged plants provide important food and habitat for a variety of aquatic organisms. Excessive algae also decrease water quality.  As algae dies, it sinks to the bottom and is consumed by bacteria during the decomposition process, a process that consumes oxygen.   This can lead to a depletion of dissolved oxygen levels in the water column causing stress to the aquatic organisms and sometimes even fish kills.

The two most important nutrients affecting aquatic systems and plant growth are Nitrogen (N) and Phosphorous (P).  As nitrogen and phosphorus are transported into the riparian buffer either by surface runoff or through subsurface flow, they are trapped in the sediments, taken up by the vegetation, or removed by some other process.  Phosphorous typically binds to the sediment and is transported by surface water runoff.  Riparian buffers remove phosphorus by trapping the phosphorus containing sediments within the buffer (Wenger 1999).

The removal of nitrogen is a slightly more complicated process.  Nitrogen is primarily transported in its dissolved form through groundwater flow.  It is either taken up by the vegetation and stored, or permanently removed from the system through denitrification.   Denitrification is the process by which microorganisms found in the soil changes the nitrate-N to its gaseous form which then escapes into the atmosphere.  Studies suggest that denitrification is the more significant mechanism for nitrogen removal (Peterjohn and Correll, 1984); however, plant uptake can be effective at reducing nitrogen loads of subsurface flow in areas where the water table is close the surface (Lowrance et al., 1995).  Though both types of buffers are effective at removing nitrogen and phosphorous from surface waters, forested buffers are more effective at removing nitrogen from ground waters (Osborne and Kovacic, 1993).

Pollution is not only in the form of excess nutrients.  Sediment pollution is also a major problem that affects our water resources.   Increased sedimentation and siltation can affect the geomorphology or the structure and natural design of a stream channel.  It also greatly affects the ecology of water bodies by filling in fish spawning beds and habitats utilized by a variety of aquatic organisms.  Riparian buffers filter out and remove sediments from surface runoff before they enter the receiving waters.  Forested and herbaceous riparian buffers are both capable of trapping sediments.   The deep root structures from forested buffers help stabilize soils and promote further sedimentation while herbaceous or grass buffers reduce the velocity of surface runoff and trap sediments.   Although both types of buffers can effectively filter out sediments, grass buffers may not be the best choice for some situations.   In environments with high sedimentation rates, grass buffers can become covered or saturated with sediment, reducing their effectiveness.

Forested buffers not only filter out excess nutrients and pollution, but they shade streams and help stabilize stream temperatures for fish and other aquatic species.  Since water temperature also influences how much dissolved oxygen the water is able to hold, riparian buffers also stabilize dissolved oxygen levels within the stream.  This is important since many aquatic species have narrow temperature and dissolved oxygen requirements.   A change of only a few degrees can have a major effect on an organism’s chance for survival.  Forested riparian buffers also add woody debris to streams for fish and aquatic organisms to use as habitat, and they supply leaf liter and other organic debris that feeds the aquatic food chain.  Unfortunately, herbaceous riparian buffers provide little benefit to physical in-stream conditions.

Both forested and herbaceous riparian buffers also provide important riparian corridors for a variety of wildlife species.  Even narrow riparian corridors have been found to improve terrestrial species abundance and diversity.  Although narrow corridors are beneficial, wider corridors are necessary for many interior forest species.  A study conducted in Maryland which examined the use of riparian forests by neotropical migrants found that as the width of the forest buffer increased so did the abundance and diversity of bird species (Keller et al. 1993).

In addition to environmental benefits, riparian buffers can also save riparian land owners’ money.  Even healthy aquatic ecosystems undergo periodic flooding as part of their natural cycle; however, human disturbance increases both the periodicity and duration of flooding.  Riparian buffers help to slow the velocity of runoff allowing it to naturally recharge the ground water supply and enter the stream or river over time at a much slower rate.  Less flooding means less economic impact to the surrounding land owners that are susceptible to flood damage.  A study by Wenger (1999) suggests that damages from flooding could be minimized by protecting the entire 100 year flood plain from disturbance.  Riparian buffers zones can also increase property values for land owners.   A national study found that when the entire floodplain was surrounded by riparian buffers, adjacent land values increased on average by $10,427 per acre (Burby, 1988).

How effective buffers are at achieving the benefits discussed above depend both on the width and type of buffer used.  Probably the most important factor that affects how well a buffer will function is width.  According to the Maryland Cooperative Extension, there is no ideal buffer width for all applications.   The width of buffer required depends on the desired function of the buffer and the contributing area. Included in (Table 1) is a summary of some recommended buffer widths for several common buffer functions.

Table 1: Recommended riparian buffer width based on buffer function
(Tjaden and Weber, FS 725)

.Buffer Function	Recommended Buffer Width
Nutrient Removal	25-125 feet
Sediment Control	40-150 feet
Water Quality and Habitat Maintenance	35-100 feet
Streambank Stabilization	10-50 feet
Water Temperature Moderation	10-75 feet
Wildlife Habitat	40-300 feet
Flood Control	75-200 feet

How much nutrient and sediment reduction can be expected from riparian buffers also depends on the type of buffer.  A summary of potential nutrient load reductions based on multiple studies conducted in Maryland and the coastal plains of North Carolina are shown in (Table 2).  Although these results are from Maryland and North Carolina, similar reductions can be expected here in Delaware.  Both buffer types are effective at reducing excess nutrients and filtering out sediments; however, combining forested buffers and vegetated filter strips can increase reductions even more.

Table 2: Estimated reduction of nutrient loads from implementation of riparian buffers (Palace, 1998; Lowrance et al., 1995; Franti, T.G., (1997); Parsons et al. (1994); Gilliam et al. (1997); Osmond et al., (2000)

Multiple studies have shown that riparian buffers improve water quality, provide habitat, and offer flood protection.  How effective riparian buffers are at achieving these reductions and habitat improvements depends on the width and type of buffer.  Although no single recommendation exists for how wide a buffer should be, a good general rule of thumb to follow is the wider the better.

References

Burby, R. 1988. Cities Under Water: A comparative Evaluation of Ten Cities’ Efforts to manage Floodplain Land Use. Institute of Behavioral Science #6. Boulder, CO. 250pp.

Franti, T.G. 1997. Vegetative Filter Strips for Agriculture. Nebraska Cooperative Extension NF 97-352. University of Nebraska, Lincoln.

Gilliam, J.W., D.L. Osmond, and R.O. Evans. 1997. Selected Agriculture Best Management Practices to Control Nitrogen in the Neuse River Basin. North Carolina Agriculture Res. Bull. 311, North Carolina State University, Raleigh.

Lowrance, R., L.S. Altier, J.D. Newbold, R.R. Schnabel, P.M. Groffman, J.M. Denver, D.L. Correll, J.W. Gilliam, J.L. Robinson, R.B. Brinsfield, K.W. Staver, W. Lucas, and A.H. Todd. 1995. Water Quality Functions of Riparian Forest Buffer Systems in the Chesapeake Bay Watershed. USEPA 903-R-95-004/CBP/TRS 134/95. Washington DC.

Keller, C.E., C.S. Robbins, and J.S. Hatfield. 1993. Avian communities in riparian forests of different widths in Maryland and Delaware. Wetlands 13(2): 137-144.

Osborne, L.L. and D.A. Kovacic. 1993. Riparian vegetated buffer strips in water-quality restoration and stream management. Freshwater Biology 29: 243-258.

Osmond, D.L., N.N. Ranells, S.C. Hodges, R. Hansard, L. Xu, T.E. Jones, and S.H. Pratt. 2000. Tracking Nitrogen loading reductions from agriculture sources: NLEW. North Carolina State University, Raleigh.

Palace, M.W., J.E. Hannawald, L.C. Linker, G.W. Shenk, J.M. Storrick, and M.L. Clipper. 1998. Tracking Best Management Practice Nutrient Reductions in the Chesapeake Bay Program. Chesapeake Bay Watershed Model Application and Calculation of Nutrient and Sediment Loadings. Appendix H. In Chesapeake Bay Program Modeling Subcommittee (ed.) Chesapeake Bay Watershed Model Application and Calculation of Nutrient and Sediment Loadings. USEPA, Washington DC.

Parsons, J.E., R.B. Daniels, J.W. Gilliam, and T.A. Dillaha. 1994. Reduction in sediment and chemical load agriculture field runoff by vegetative filter strips. Water Resources Res. Rep. 286. North Carolina State University, Raleigh.

Peterjohn, W.T. and D.L. Correll. 1984. Nutrient Dynamics in an Agricultural Watershed: Observations on the Role of a Riparian Forest. Ecology 65(5): 1466-1475.

Tjaden, R.L., and G.M.Weber. Fact Sheet 725. Riparian Buffer Management: Riparian Forest Buffer Design, Establishment and Maintenance. Maryland Cooperative Extension. University of Maryland Wye Research Center, College Park.

Wenger, S. 1999. A Review of the Scientific Literature on Riparian Buffer Width, Extent and Vegetation. Office of Public Service & Outreach, Institute of Ecology, University of Georgia. 59pp.