Buffer zones and river corridors
Rivers can be fantastic corridors for wildlife if there is good habitat both in the river and on the banks. The edge, or interface between land and water is called the ‘riparian zone’ and this area is crucial not only as wildlife habitat but for air and water quality, carbon sequestration and natural river function.
A healthy riparian zone will have a mixture of herbaceous plants, shrubs and trees extending back from the bank for at least 10 metres, preferably more. These areas are called ‘buffer zones’ in an agricultural context – the area between farmed crops or grazing and the river-bank.
Advantages of buffer zones
- buffers provide habitat connectivity: ‘wildlife corridors’ for otters, water voles, birds and bats to move around the countryside.
- tree roots strengthen stream banks and provide shelter for trout, otters, dippers etc.
- the tree canopies provides shade to keep the river cool during hot summers.
- trees act as a barrier to prevent wind-blown agricultural pesticide and herbicide sprays from reaching the river.
- woody material that falls in the river has multiple benefits: scour pools, trapping sediment, food and habitat for invertebrates, increased channel roughness to slow the flow.
- leaf litter input to the river from bankside trees and shrubs is an important food source for invertebrates.
- buffers improve water quality by creating a physical barrier of rough vegetation that slows the flow of overland runoff and increases infiltration into the soil, helping to trap and retain pollutants before they reach the watercourse.
- increased infiltration of water through the diverse root systems within the buffer zone reduces the amount and rate at which heavy rain reaches the river.
- the increased roughness of river-banks with wooded or shrubby buffer strips helps to slow the flow of flood waters, reducing erosion rates and flood peaks.
- buffer zones should allow a natural connection between a river and its floodplain, allowing flood water to spill out of the channel and reduce the intensity of flooding downstream.
- adequate buffer zones allow the river space to move — to erode, deposit, meander in a natural way.
Buffer zones are not the answer to all the issues that our rivers and their wildlife face. It is important to deal with problems at a catchment scale and to think of buffers as the last line of defence before sediment, pollutants and flood water reaches the river.
Buffer zones are very frequently recommended by us as part of our Advisory Visit service. Pictured below left is an all-too-common situation where livestock have broken down the banks of the river, increasing the amount of sediment and creating a shallow, over wide channel with no shade. The water warms up quickly in summer creating a hostile environment for trout, and sediment choked gravels result in lower survival rates of trout eggs and poor quality habitat for invertebrates.
Fencing livestock out of the rivers allows vegetation to grow, providing shade and essential habitat for riverflies and other wildlife. The river will be protected from sediment and pollutants and is able to create a more natural channel form. (Photo right — ideally the buffer zone on the left would be wider!)
For a good example of large scale buffer zone creation in a sheep farming area (River Skirfare in the Yorkshire Dales), see this blog by WTT Research and Conservation Officer, Jonny Grey.
Buffer zone challenges
A buffer zone generally represents a loss of productive land for the farmer, although a high level of ‘public goods’. In England, the change in farm subsidy system from one based largely on land area to one based on ‘public goods’ (ELMS) should encourage the creation of more, bigger and better buffer strips.
An initiative has been instigated by the Beaver Trust to create a 100,000ha nature recovery network along 25,000km of England’s rivers by 2030. Beavers and buffers are natural allies. Beavers need trees and plants to eat and space to create beaver ponds and there can be conflicts where that space is not available – see our Beaver Hub for more information.
The species composition of buffer zones, whether they are fenced to prevent livestock grazing and how the zone is managed are all questions to which there is no ‘one size fits all’ answer. In some cases, tree planting of native species is appropriate, in others, natural regeneration will take place if livestock is excluded. Buffer zones may benefit from management if nutrient control is a priority, and some carefully managed grazing may help control invasive species such as giant hogweed and Himalayan balsam. It is important to understand the details of the particular situation before creating buffer zone, or simply allowing natural processes to create them.