Instream barriers and forestry
How do instream barriers as a result of forestry activities potentially influence water resources?
Forestry roads and bridges provide a transport network throughout large tracts of forest in Aotearoa. Instream barriers in forests consist of fords, culverts, weirs, and discharge pipes used in the construction of roads and bridges, providing access to harvesting sites and allowing trucks to transport logs out of the forest. Establishing access into and out of plantation forests, as well as cutting down trees, has significant impacts on waterways and mahinga kai.
Instream barriers alter the natural flow of water which many species rely on to migrate and breed. Altered water flow can also lead to erosion of river banks and disruption of river bed habitats.
Potential impacts of instream barriers on water quality and mahinga kai
- Altered fish migration - barriers may prevent native fish that move from 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 access to 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 when 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.