Impacts of hydro
Impacts of hydro-electric activities on water quality and mahinga kai.
When the natural flow and seasonal variations of a waterway are interrupted by hydro-dams extreme care must be taken to maintain the amount of water needed to support healthy ecosystems. The amount of water needed is called environmental flow, which considers maximum and minimum flow levels to support a healthy ecosystem. Failure to provide an environmental flow can have serious consequences for water quality and mahinga kai.
Potential impacts of hydro-dams on water quality and mahinga kai include:
- Altered fish migration - barriers may prevent native fish that move from sea to freshwater as part of their life cycle, such as tuna, 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, 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.
- Damage to banks and floodplains - varying flows and flash floods threaten the stability of a river bank, increasing its vulnerability at times of flooding and damaging breeding and feeding habitat for mahinga kai.
- Increased water temperature - flow effects temperature. Loss of flow means waterways can fluctuate in temperature, and if unshaded, water can reach high temperatures unsuitable for mahinga kai. Fish generally cannot tolerate temperatures over 25ºC.
- Decreased water clarity - erosion and increased sediment loading into a river due to changes in flow will decrease water clarity and reduce visibility and the ability of fish to find food.
- Increased nutrients - a decrease in flow may increase the concentration of nutrients within a river.