Riparian vegetation and urbanisation
How can urbanisation impact riparian vegetation around a waterway?
Urbanisation has changed the appearance and functioning of many waterways, which have been modified predominantly through channelisation and removal of riparian vegetation. Construction and the development of the surrounding catchments leads to an increase in sediments and associated chemical contaminants entering the waterway through surface and stormwater runoff.
Streams and rivers are often piped or channelised, with artificial bank structures and very small riparian areas so that limited vegetation can grow. Many streams in parks have their edges sprayed for weeds, leaving them bare and reducing the ability of any riparian vegetation to intercept sediments or contaminants. Vegetated riparian buffers can help to reduce inputs of contaminants from surface runoff in urban areas, but the amount of impervious area in the catchment and of stormwater that enters the streams in pipes can have an overriding effect on stream health.
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. (Our native fish fauna 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 habitats.)
- Decreased water clarity - erosion and increased sediment from bank erosion may contribute to decreased water clarity and reduced visibility for fish seeking 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.
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