River flow refers to the quantity of water passing a point in the river over a certain amount of time.
Different rivers have different flow patterns, such as sharp peak flows following rain with low flows in between, or high spring flows from snow melt.
These flow characteristics affect how much water is available for irrigation, drinking water, hydro–electric power generation, and recreational activities such as fishing and boating. River flows are very important for maintaining the health and form of a waterway.
Changing the natural flows of water means altering natural processes.
The natural flow of water is changed when we modify waterways:
- for urban and rural development
- for flood control
- by taking water for drinking, irrigation and electricity generation.
Impacts of reduced and less variable river flows on freshwater habitats and ecosystems include the following.
- Low river flows reduce the quantity of habitat for freshwater fish, invertebrates (like snails and kōura), and other species.
- Reduced river flows may increase the concentration of nutrients and other pollutants in a waterway.
- Reduced and less variable river flows can increase water temperatures, especially in streams without shade, which can affect freshwater species.
Many of our native fish move significant distances up and downstream to feed, reproduce and hide.
Altered river channels and flows can make it difficult or impossible for fish to make these journeys. Culverts for example, are often narrower than natural channels and can have faster and more uniform water flows.
The ability of fish to move through fast-flowing water depends on a species’ swimming ability and the distance to travel as well as the presence of slower-flowing water for resting along the way.