Soil erosion is a process that happens naturally. How fast it happens depends on how steep a place is, the weather, plants - or lack of them - and what’s below the soil. Changes that people make to the land can also make it erode more quickly. As the soil erodes, it finds its way into our rivers and harbours.
One of Northland's biggest water pollutants is silt or, more precisely, fine clay particles. Slow-flowing rivers, like the Northern Wairoa and the Hātea River above the Whangārei Town Basin, are among the most polluted by silt.
Our monitoring shows that water from the Northern Wairoa sometimes contains more than a kilogram of clay particles per cubic metre of water.
Because silt is natural, it might be argued that it is a less harmful polluter than sewage effluent or chemicals. However, an overload of silt can "kill" a stream or river just as effectively as a factory spill.
Silt can bury small plants and animals in streams, lakes and harbours. It can settle on the bottom and block up rivers and streams, making them more likely to flood. Silt particles clog water intakes, causing problems for people who depend on a river or stream for their water supply.
Dirty, dark water absorbs the heat of the sun more than clear water, so muddy waterways tend to get really hot in the summer, which kills plants and animals. Often the stronger pest weeds like oxygen weed survive, while the more delicate native ones die off.
Most of us want our streams and waterways as clear and clean as possible. As well as all the practical reasons, clear sparkling streams, rivers and harbours are a big part of what we love about Northland.
The problem ends up in the water, but the solution lies on the land.
Soil erosion can be prevented or reduced by improving the way the land is used, perhaps by planting trees, or by allowing native bush to grow in areas that are at risk from erosion.
The risk of erosion increases if land:
- has few plants on it;
- is steep;
- is on the bank of a river or lake;
- has earthworks;
- is cleared for planting or during harvesting;
- has erosion-prone geology (for example mudstone or pumice);
- has high stock density or machinery; or
- is in an area of high and intensive rainfall.
Common ways to reduce erosion include:
- planting trees on hills and stream banks;
- fencing gullies and waterways to prevent stock getting in;
- removing wild goats and rabbits (they eat the vegetation);
- keeping stock off steep pasture when it is wet;
- minimising the use of earthworks and using appropriate practices;
- retiring unproductive land and planting it out;
- not cultivating steep land; and
- using best practice when making roads and undertaking earthworks.
We’ve set out rules and policies that reduce erosion in the Regional Water and Soil Plan. This Plan respects the importance of streamside planting in protecting waterways from silt buildup. Farmers are encouraged to fence off and maintain a strip of land alongside streams and rivers to help "sieve out" silty water.
We require that a strip of plants is left alongside any streams, rivers or lakes when trees or scrub are cleared - for example to plant a forest or when a forest is being harvested.
The Plan also places extra controls on land disturbance activities beside water bodies and on erosion-prone slopes. Vegetation clearance, earthworks and quarrying beside rivers and streams, and land preparation on steep slopes, require resource consent.
Each year, hundreds of hectares of exotic forest are planted in Northland. As crops mature over the next 40 years, huge areas of exotic forest all over the region will be clear-felled.
If resource consent conditions are met, from planting to felling, there will be little impact on the environment. Although it can look pretty shocking for a short time when trees are clear-felled, erosion should be reduced.