The riparian zone consists of the banks and immediate margins of rivers and streams, including adjacent wetlands and flood plains. Some definitions also include the shorelines of lakes and bays, but do not include ocean shoreline. The riparian buffer is the vegetated area along the river bank which stabilizes the soil of the bank and gives some protection to the river from polluted runoff and subsurface flow.
A forested riparian buffer usually consists of at least 2 integrated riparian buffers: trees, and grasses or shrubs. In some cases, flood-resistant trees may grow right up to the river bank, while in others, only the tree roots reach as far as the bank, while surface vegetation at the edge of the bank is low and matted.
Healthy biosphere
A healthy riparian forest is a sheltered habitat for fish and wildlife, including species which are threatened elsewhere. The large riparian zones which are associated with major rivers can provide wildlife corridors for even large mammals with extensive territorial needs, while the river itself is a corridor for dispersing plant seeds.
The vegetation in the buffer is rich in nutrients. Even the leaves which fall into the water are an important food source for small aquatic animals near the bottom of the food chain. The water is clean, with a good mixture of shaded areas and areas exposed to sunlight.
When a forested riparian zone is removed as part of forest harvesting, the temperature of the stream rises, both at the site and for a considerable distance downstream. This can be detrimental to local fish populations as well as the riparian vegetation.
Protection against erosion
A riparian buffer absorbs excess water and reduces stream energy. Rivers with less powerful currents carry less sediment and cause less erosion. Water which carries less sediment is clearer, with fewer suspended solids to make the water murky.
A region without a riparian buffer has nothing to protect it against constant erosion, especially after spring runoff and flash floods. This kind of erosion causes river banks to keep expanding until they threaten distant forests. This also increases the risk of flooding outside the established flood plains in the surrounding area.
Isolated trees and other plants have no chance against this kind of erosion. Only a strong vegetated mat can bind soil strongly enough to resist it.
Biofilter
Many pollutants reach a river almost entirely through surface runoff. Riparian buffers which are sufficiently wide and established prevent runoff from carving channels to the river or stream. These kinds of channels increase erosion and pollutant runoff into the river, so preventing them also reduces erosion and pollutant runoff.
Instead, rainfall and other runoff spreads through the riparian buffer. This action filters away excess nitrogen, phosphorus, and sediment and prevents them from contaminating the river’s water. Riparian forests can remove as much as 75% of dissolved nitrates from crop runoff. In turn, this protects downstream regions which rely on that river from the effects of agricultural runoff.
Nitrates can still enter a river below the root zone. However, the deeper the roots, the deeper the root zone will go. Forested riparian zones have deeper roots than nearly any other kind of riparian buffer, so fewer excess nitrates excape into the river.
When roots decay, they naturally move surface nutrients to deep soil layers. Some excess nutrients which are absorbed by a riparian buffer will enter the river when the surface vegetation which absorbed them decays. This can be prevented by harvesting some of that vegetation before it decays. For example, very old trees can be cut down and used for wood products, with saplings of the same species planted in their place. In an established riparian buffer, the native wildlife will do the same thing by feeding on the local vegetation.
Converting nitrates to nitrogen gas
The microbial balance of the forested riparian soil is just as important as the trees. Many nitrates in this zone are converted to inorganic nitrogen gas through microbial dentrification.
This process only occurs in anaerobic water-saturated zones with abundant organic matter, especially the organic carbon produced by decaying roots. Most areas of this kind are riparian buffers with dense, non-sandy soils.
Forest management
Maintaining a healthy riparian forest can increase logging profits over the long term. The riparian buffer keeps newly exposed soil nutrients from washing away into the river. It also moderates temperature and groundwater in its vicinity, which will enable replacement seedlings to get a good start. The same seedlings will also be protected from damaging floods by the untouched riparian forest.
