Instream barriers and urbanisation
How do instream barriers influence water resources as a result of urbanisation?
Instream barriers consist of culverts, dams, weirs, and discharge pipes that are used to supply urban centre goods and services. Culverts are used to create access ways and roads, while dams and weirs provide a reservoir for water storage. Discharge pipes transport stormwater away from impervious surfaces such as roads and car parks.
Instream barriers alter the natural flow of water which many mahinga kai species rely on to migrate and breed. Altered water flow can also lead to erosion of river banks and disruption of mahinga kai habitats.
Using fish-friendly culverts and maintaining the natural slope of a stream are important for mahinga kai to successfully migrate upstream. Any instream structures should incorporated designs that facilitate fish passage (modified culverts, wetted margins, and low flow areas) to upstream habitats.
Potential impacts of instream barriers on water quality and mahinga kai
- Altered fish migration - barriers may prevent native fish that move from the sea to freshwater as part of their life cycle (such as īnanga, part of the whitebait catch) from moving upstream and downstream and accessing otherwise suitable habitat.
- Increased velocity - sustained high water velocity prevents some fish from accessing upstream habitats.
- Modified channel form - erosion from vegetation removal along banks and changes to stream flow after construction of a road crossing or similar barrier can lead to scouring and breakdown of stream and river banks, impacting on mahinga kai habitat.
- Modified flow - flow changes as stream banks are modified and realigned, which can lead to changes in the benthic (bottom) structure of the stream/river bed; coarse substrates such as gravels and boulders are replaced and covered by sand and silt.
- Loss of species habitat - many mahinga kai species need the protection and habitat provided at upstream sites inland from the sea. Barriers that make upstream habitat inaccessible to species that prefer higher elevation can result in loss of breeding and feeding sites.