Native forests allow water to infiltrate naturally. Water is taken up by the trees and then released slowly, reducing the risk of flooding. The roots of trees stabilise stream banks and provide good hiding places for insects and fish. Shade from the trees keeps the water cool for animals in the stream. The insects, leaves and logs falling into the water provide food and homes for stream life. Leaves also fall on the ground and build up a litter layer, which is a good filter for water running off the land – slowing it down and settling out the sediment before it gets to the stream. The water that enters streams from native forests is generally cool, clear and safe for swimming and requires less treatment for drinking.
It is helpful to retain or restore tree cover to stream banks and headwaters to protect soil, regulate water flow, provide good habitat and buffer run-off from the land.
Land cover is an important factor impacting water quality. For example, if you have a stream that’s surrounded by forests and there’s nice trees covering the stream, that will provide shade to prevent things like periphyton or nasty algae growing, and also shade lowers water temperature, and our native fish and invertebrates really like cool water. Also planting trees – riparian planting – provides food and a carbon source for our fish and invertebrates, whereas if you have a stream with no riparian planting, it’s more susceptible to what’s on the land. For example, soil will be washed into it, and with soil comes nutrients and sediment, and that can really impact fish. With no riparian planting, the stream will heat up, and when water gets hot, it loses its oxygen and fish really need oxygen to breathe as well. So the land use cover really does impact the water quality.
Dr Eloise Ryan
Waikato Regional Council
Footage of riffles in partially shaded stream with eel, Meet the Locals: Tuna, Department of Conservation. Released under CC BY 3.0
This video has been developed in partnership with the Waikato Regional Council as part of the Rivers and Us resource.