How Greenways Work: A Handbook on Ecology
Greenways that follow rivers offer a terrific opportunity to protect riparian ecosystems and the variety of ecological roles they perform.
By Jonathan M. Labaree, October 1997
Chapter Four: Water Resources
By acting as a sink, a filter, a barrier, and a source, a greenway can maintain the following functions which are associated with riparian zones:
Riparian zones moderate water flow through the connected processes of physical resistance from vegetation, absorption of water into the soil, transpiration of water from plants, and groundwater discharge. When a stream floods, it overflows its banks and inundates the surrounding flat land, known as the flood plain. If that area is nothing but farmland or pavement, the river will maintain much of its velocity. If, on the other hand, the flood plain is covered with its natural vegetation, it will act as a sink, absorbing the flooding river's energy and much of its water (at least temporarily). Riparian plants will slow the flood down as stems and trunks block the water's progress. As the stream's velocity subsides, more water seeps into the flood plain's rich soils. This water is either released into the atmosphere by plants, evaporated from the soil when the flood subsides, or discharged slowly as it makes its way through the ground to the stream again.
Nutrient and sediment filtration
Riparian zones filter out nutrients and other harmful chemicals from surrounding land uses. A common problem in developed areas is increased nutrient load in rivers from industrial, residential, and agricultural areas. Riparian systems can reduce the nutrients in water coming from surrounding lands. A riparian zone's ability to filter a given nutrient depends upon such factors as its vegetation, slope, the initial concentration of the nutrient, and how the nutrient is moving. Nutrients and toxins can be attached to sediment or dissolved in surface runoff, groundwater, or soil water (water stored in the soil).
Riparian vegetation filters nutrients and toxic metals by trapping sediment particles to which they are attached. Phosphorus and nitrogen, two nutrients whose overabundance can harm river and lake systems, most frequently travel into these systems attached to sediment. Vegetation and microorganisms in a healthy riparian zone will consume many of the nutrients which are dissolved in surface runoff or in soil water. Woody riparian vegetation removes a significant amount of nitrogen from groundwater as well. Numerous studies indicate the effectiveness of riparian systems at removing nutrients and other chemicals, such as oils, insecticides, and herbicides.
One of the ways in which riparian zones regulate a stream's temperature is shade. Overhanging and near-stream vegetation lowers water temperatures by blocking solar energy. Many fish species, including trout and salmon, are adapted to the resulting cool water. Trout, for example, will not survive in streams with average summer temperatures above 72° F. Removing vegetation can seriously affect those species' chances of surviving both because they are adapted to cool temperatures and because warmer water retains less dissolved oxygen than cool water.
A riparian zone stabilizes a stream's banks primarily by providing structure through its system of roots. The deep fluvial soils left by millennia of sediment deposition allow plants to send their roots deep into the earth. The resulting tangled mass of underground vegetation traps soils and protects them from the eroding forces of running water.
Food and habitat for aquatic communities
Vegetation protects not only soil, but offers important cover to fish and other aquatic species. They can hide under fallen logs or in the shade of an overhanging tree. The lush riparian vegetation is also a source of food as leaves and other debris fall into the stream.
A riparian zone's ability to perform its functions will depend heavily on:
Status of upstream vegetation
If there is development along a stream, natural riparian zones downstream of that development will bear the brunt of stronger floods, no longer moderated by upstream vegetation. Similarly, increased amounts of sediments, nutrients, and toxins may overtax the remaining riparian zones and degrade them beyond their natural ability to restore themselves.
Land use of associated uplands
Activity in uplands also has enormous impact on riparian zones. Devegetation in these areas often increases the amount of water reaching the river system. If uplands have been converted to industrial, residential, or agricultural use, runoff from those sites can be laden with toxic and nutrient-rich waste. While riparian zones are effective filters of such waste, they can be overwhelmed.
The state of the riparian zone itself is also important. Mature forests are frequently at equilibrium, meaning that the amount of nutrients going into the system is equal to the amount coming out. Forests dominated by younger trees, however, will retain the many nutrients which the trees need to grow. In these cases, the effectiveness of riparian habitat at performing its filtering functions may be reduced compared to a riparian zone with younger trees.
How do you design a greenway to protect riparian functions? Unfortunately, there is not a set answer which tells us exactly how wide a greenway needs to be or precisely what parts of the landscape to include. Ideally, a greenway should maintain the riparian corridor in as natural a state as possible. Using that broad statement as a guide, we can develop some notion of what should be included in a greenway.
Efforts should concentrate on including the flood plain, its banks, and portions of the uplands in the greenway system on at least one side of the river and preferably both sides. It is during this journey from upland to river channel (see Figure 10 on page 23) that most of the filtering of sediments and chemicals takes place. A river's flood plain, bank system, and associated uplands generally increase in magnitude relative to the size of the river. Rather than seeking a universally applicable standard for greenway width, therefore, planners should base greenway design on local conditions. Ideally, they should undertake a study which demonstrates how much sediment is entering the riparian zone, and how wide the zone needs to be to filter out an acceptable percentage of sediment and nutrients.
For example, a study in a coastal watershed in Maryland revealed that the first 250 feet of the riparian zone filtered out most of the sediment from runoff. Nearer the stream, the amount of filtering decreased (Karr and Schlosser, 1977). A similar study from North Carolina showed that more than fifty percent of sediment in runoff from agricultural fields was filtered in the first 300 feet of riparian vegetation (Lowrance, et al., 1985). In both cases, the slope of the surrounding agricultural fields was relatively slight (0 - 7%). Riparian corridors will have to be wider where the source of runoff is large, slope is greater, human use more intense, or nothing is done in the fields to prevent erosion. These many variables make a study of the site very important.
Width should not be the only consideration -- there are specific sites within the landscape which have direct bearing on riparian functioning. Intermittent tributaries (streams which form seasonally in response to increased rainfall or snow melt), gullies, and swales through which water reaches the stream should be included in a greenway because they are important components for the filtering and flow moderation functions of the riparian zone. Keeping these areas intact and vegetated with native species is important. Areas of aquifer recharge or discharge, such as wetlands, seasonally wet soils, or springs, are also critical. Identifying recharge and discharge sites generally requires a professional hydrologist and should be part of an initial site study.
Guidelines for maintaining riparian functions in a greenway:
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Updated March 16, 2007