How may horticultural activities impact riparian vegetation around a waterway?
When large tracts of land are cleared, cultivated, and harvested, the soil becomes vulnerable to erosion and compaction, especially when heavy machinery is used. Waterways in cultivated areas are prone to high sediment and nutrient inputs from the surrounding land, particularly when the area directly adjacent to the water is cultivated.
Riparian vegetation around waterways increases soil stability, provided by the tree roots which bind the soil together. Maintaining a riparian buffer when cultivating the land can significantly decrease the amount of sediment entering streams in surface water runoff from the surrounding exposed catchment. Even 5 m wide buffers of long grass can reduce the amount of sediment entering streams.
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 species 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.)