A shaded ‘natural’ stream will have an average water temperature of around 16°C but open, slow flowing waterways may have temperatures peaking at around 30°C in summer months.
Wetland on a farm
Research has shown that one kilometre of good stream bank shading will cool a stream by 5°C and there is greater impact on streams with less water (making them easier to cool). As water temperatures fall, dissolved oxygen increases – by around 20 per cent for the 14°C range noted above.
The combined impact of these two shading effects is of huge benefit to our native fish and other aquatic species (think caddisfly, mayfly and stonefish larvae).
Confirming contaminant reductions can be a slow process and is difficult to quantify. When riparian planting is undertaken with a focus on stream shading, the impacts can occur within a couple of seasons and keep increasing over time as plantings mature. Monitoring water temperatures and incorporating some eDNA sampling will quickly build up a picture of improvements in aquatic health. One of the favourite activities of those with good riparian plantings is taking their children or grandchildren out to look for eels.
The important numbers: stream shading needs to be about 25 per cent to be effective. A stream with 70 per cent shading is apparently the magic number – at this level there will always be some areas of the stream in shade, providing habitat for the cool-water loving native fauna. With basic flax plantings along an average width stream – say 1.5-2 metres, you may achieve 25 per cent shading.
Fenced and planted streams
Carex are often used along stream edges as well but these have more limited shading capacity and may only provide a narrow shading effect along the bank.
Planting close to waterways can be challenging, especially in hill country where high intensity rainfall events can cause flooding which swamps riparian plantings. In this situation you need to focus on trees with a narrow trunks with good root structures to reduce water resistance during floods. Ribbonwoods, lacebark and cabbage trees are all good options. They are naturally occurring riparian plants and have a good-sized canopy for their height. If you have space for bigger trees, tōtara and kahikatea will cope with wetter conditions. In fact, tōtara will thrive on the edge of rivers – they have the ability to put out roots from the trunk if they become silted up.
As with any riparian planting – remember your weeding. If you have blackberry or convolvulus issues in your area, these can swamp mature plantings in a couple of years if left unchecked.
Cows grazing
Riparian plantings are long and narrow by their very nature. This creates a lot of ‘edge’ which makes weed incursion easier. We all hope that the birds will bring in native seeds and assist with our native regeneration projects, but they are not choosy about which fruit they eat, and blackberry is a favourite – a less fortunate outcome!
Understanding the full range of positive impacts a well-planned riparian planting can have on aquatic life, alongside contaminant reduction, is important to ensure riparian projects are planned appropriately. It is important to consider the short and long term impact your mature riparian plantings will achieve for your local ecology and environment.
Dairy farm riparian planting
