Loss of riparian vegetation
Plants and trees along the water margins and banks are called riparian vegetation.
A riparian buffer zone is a vegetated strip of land along the margins of a waterway. Riparian vegetation provides a barrier (buffer) between the water (river, streams, and estuaries) and the land. When surface water (runoff) from the surrounding catchment runs through the riparian zone, contaminants (sediments, nutrients) contained in the runoff are trapped in the roots of any riparian vegetation, allowing the silty or contaminated water to infiltrate the soils.
Healthy native forest riparian vegetation usually consists of a canopy of large trees accompanied by a thick undergrowth of shrubs and grasses. The thick undergrowth acts as a filter for surface runoff, while canopy trees above a stream can intercept airborne material, such as pesticide or fertiliser sprays, and provide shade that maintains stream water temperatures. Large canopy trees also have extensive root structures that stabilise stream banks and intercept nutrients in water flowing underground towards the stream.
Potential impacts of reducing or removing riparian vegetation on water quality and mahinga kai
- Increased bank erosion - the loss of roots decreases the stability of the bank, increasing its vulnerability at times of flooding.
- Increased water temperature - loss of shading from trees or overhanging streamside vegetation means waterways become more exposed and are more liable to fluctuate in temperature. (New Zealand native fish generally cannot tolerate temperatures over 25ºC and trout need temperatures to be less than 19ºC for growth.)
- Decreased dissolved oxygen through increased aquatic plant growth - plants and weeds growing within the waterway are more likely to thrive in unshaded waterways, potentially clogging and stemming flow, which can decrease oxygen levels.
- Modified channel form - erosion through loss of vegetation can lead to scouring and breakdown of stream and river banks, eventually changing the form of the channel.
- Loss of species habitat - many mahinga kai species need the protection and habitat provided by riparian vegetation growing around streams and rivers. (Trees provide wood and roots to the stream that are habitat for fish and kōura, and loss of cover can result in loss of breeding and feeding habitat.)
- Decreased water clarity - erosion and increased sediment from bank erosion may contribute to decreased water clarity and reduced visibility for fish to find food.
- Increased nutrients in streams - riparian vegetation filters contaminants and sediment from the land. (Loss of riparian vegetation may also be associated with changes in land use (e.g., farming, forestry) that increase the amount of contaminants that are present in surface water runoff.)