Riparian Buffers

(Published July 2002)

The word "riparian" refers to the bank adjacent to a body of water. Riparian buffers or riparian corridors are strips of land alongside waterways that intercept surface runoff and subsurface groundwater discharge to help improve water quality. Riparian buffers are an important "Best Management Practice" (BMP) which apply to all land bordering a waterway, whether or not it is used for agricultural purposes. The idea is for plants to trap sediments and consume nutrients that would otherwise enter the water. The National Research Council, an arm of the National Academy of Sciences, recently identified restoring riparian areas to be a desirable national goal (www.nas.edu).

As an example of the "worst possible case," consider an agricultural field tilled up to the waters edge. Following a heavy rain, soil from the field can wash directly into the waterway. The sediment causes the creek to shoal-up, and smothers bottom-dwelling organisms like oysters and submerged aquatic vegetation. The runoff carries dissolved substances like nitrate and phosphate from fertilizer into the water, along with chemicals that may have been applied to the field to control weeds or pests. The nitrate and phosphate fertilize the growth of tiny suspended algae in the water. The algae increase the turbidity of the water, limiting light penetration and the ability of submerged aquatic vegetation to grow. When the tiny algae die, bacteria decompose them and consume dissolved oxygen, which is necessary for animals like fish and crabs. A "bloom" of algae after a rainfall event can actually result in a "fish kill" if dissolved oxygen levels are seriously depleted. This is especially true during summer when the water is warm and already contains less dissolved oxygen than is true of cold, winter water.

A simple way to prevent these problems is to isolate the tilled field from the water by a permanently vegetated strip of land. The width of the strip is ideally between 50 and 100 feet, and it functions most efficiently if it contains deep-rooted trees as well as grass. It is important that no bare ground is present, which would permit surface runoff to carry sediment directly into the water. The reason for the deep-rooted trees is to intercept the subsurface groundwater flow. Surface runoff is an easily observed process, whereas the movement of water beneath the land surface is not obvious. But just as surface water flows "downhill" over the land, shallow groundwater flows "downhill" toward the waterway, beneath the land surface. In our soils, the rate of shallow groundwater flow is probably between about 30 and 300 feet per year. The water flows fastest in sandy soils, and in very muddy soils flow rates can be very low.

The plants in the riparian buffer act as physical filters of surface runoff, but more importantly as consumers of nutrients from the groundwater. For the farmer, there are several negative aspects of riparian buffers. Land is obviously lost to production when a buffer is established, but compensation may be available, especially under the new Farm Bill. The habitat provided by the forested buffer zone may also harbor vermin and pests that might cause crop losses.

For the homeowner, these negative aspects of riparian buffers are not important. In the case of homeowners, the "worst possible case" is a chemically maintained open lawn adjacent to the water. The shallow roots of grasses do not consume nutrients discharged to the groundwater by the septic system, and excess fertilization and chemicals easily contaminate the groundwater and the waterway. Many people object to trees blocking their view of the water. But large deep-rooted trees, pruned to remove low branches, “frame” the view of the water and, if properly placed, can also provide shade during the summer which can translate into a decreased need for air-conditioning.

Recent articles on forest buffers and the Farm Bill were published in the June 2002 issue of the Bay Journal (www.bayjournal.com).