Instream barriers and altered water flow
Instream barriers and diversions alter the natural flow of rivers, streams, and lakes.
Instream barriers may include culverts, fords, dams, weirs, and pipes that are used in infrastructure such as bridges and road building, town water supply, and for stormwater discharge into waterways. These alter the natural flow of rivers, by taking, diverting, or damming water, which in turn alters the habitat that species rely on to live, migrate, and breed. Altered water flow can also lead to erosion of river banks and disruption of river bed habitats.
A high proportion of New Zealand’s indigenous fish fauna are diadromous (18 species of the total 35 indigenous freshwater fish migrate as a part of their natural life cycle) requiring connection between high quality adult habitat and marine or lake environments. Incorrectly installed or maintained instream structures can prevent or restrict upstream and downstream migration of fish as well as modify the natural hydrology of a waterway.
Some species, such as tuna, are able to use the wetted margins of waterfalls, rapids, and spillways to bypass obstacles. Other species rely on short burst of swimming to get past high velocity areas. However, many species are unable to negotiate instream barriers that are not designed for fish passage, e.g., culverts that are perched, undercut, have sustained high velocity water flow, or lack wetted margins.
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.
- 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 affects 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.