Altered water flow and hydro
How do hydro-dams potentially influence water flows?
Hydro-dams alter and permanently modify waterways. The addition of a hydro-dam changes the natural characteristics of a waterway by permanently affecting water levels and flows along a lake or river, thereby obstructing and redirecting the natural movement of water and the associated mahinga kai, invertebrates, and plants along its length . Large dams prevent the upstream and downstream migration of mahinga kai, such as tuna.
Potential impacts of altered water flows on water quality and mahinga kai include:
- Modified flow regime - alteration of flow may lead to changes in water velocity and the benthic (bottom) structure of the stream/river bed, e.g., coarse substrates such as gravels and boulders may be covered by sand and silt, which affects the fish and invertebrates that live there.
- Altered species composition - fish and invertebrates are adapted for different flows and habitats. For example, some mahinga kai species can rely on short burst swimming to get past high velocity areas, but most species are unable to negotiate sustained high velocity water flow.
- Modified channel form - variability in flow may lead to scouring and breakdown of stream and river banks, eventually changing the form of the channel or the existing floodplains and associated bank side habitats.
- Increased bank erosion - increased flows and flash floods threaten the stability of the river bank, increasing its vulnerability at times of flooding and damaging cover habitat for mahinga kai.
- Increased water temperatures - 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. Trout need temperatures to be less than 19ºC for growth.
- Loss of species habitat - many mahinga kai species need the protection and habitat provided by riparian vegetation growing around waterways. Damage to banks and floodplains from an increase in flow can result in loss of breeding and feeding habitats.
- Decreased water clarity - erosion and increased sediment loading into a stream 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 the waterway.