Altered water flow (hydrology)
Each waterway has its own particular characteristics relating to water flow, velocity, and quantity.
Hydrology in this sense refers to the occurrence and character of water in waterways on or below the land surface. Character can include the normal variation in the amount and speed of water in a stream (flow regime), seasonal flow patterns, and the timing and frequency of floods and droughts. Each of these characteristics is influenced by climate and landscape (mountains, valleys, and flood plains). Therefore, changes to land use can have profound effects on river hydrology. Within streams, flow patterns such as riffles, pools, and runs add variety to habitat for mahinga kai. The size of the channel, water depth, and velocity influence water quality (e.g., nutrients and temperature) and how sediments are transported, both of which can affect the plants and animals that live there.
The flow regime of a waterway can alter when land use is changed and when river/stream banks are modified and realigned. Modified waterways often have steeper banks, lack riparian vegetation, and are prone to fluctuating water levels, and elevated flow. It is important that when modifications (such as construction of a dam or weir, or the addition of a culvert or pipe) are made, the natural characteristics of a water body are maintained.
Potential impacts of altered water flow 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 bankside habitats (wetlands).
- Increased bank erosion - increased flows and flash floods threaten the stability of river banks, increasing their vulnerability at times of flooding and damaging cover 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. 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 streams and rivers. Damage to banks and floodplains from an increase in flow can result in loss of breeding and feeding habitat.
- 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 stream.