Sediment and forestry
How may forestry activities increase sediments in waterways?
Forests increase soil stability because their roots bind the soil together. When large areas of land are cleared and harvested of trees, the soil becomes vulnerable to erosion as tree roots die and as the land is exposed to direct rainfall. Trees increase the soil's ability to take up water, and as a result there is usually a significant increase in the amount of surface water runoff from the surrounding catchment when trees are felled.
Removal of riparian vegetation directly next to a waterway is likely to cause banks to become unstable and prone to slips, thereby increasing erosion, especially during floods. Other impacts that can produce sediment include earthworks for roads and landing sites, and hauling logs across bare slopes.
More information on loss of riparian vegetation
Potential impacts of sediments on water quality and mahinga kai include:
- Decreased water clarity - increased sediment loading into a stream will decrease water clarity and reduce visibility for fish seeking food and places to live.
- Damage to fish gills and filter feeding apparatus of invertebrates.
- Changes to the benthic (bottom) structure of the stream/river bed - coarse substrates such as gravels and boulders are replaced/smothered by sand and silt.
- Decreased numbers of invertebrate species from smothering of habitat - invertebrates are a food source to some mahinga kai (e.g., kōura and fish) and diverse invertebrate communities are also an indicator of healthy stream systems.
- Decreased algal food supply at base of food chain - sediments can scour algae from rocks, make algae unpalatable, or reduce light to levels where algae cannot grow, because plants need light to photosynthesise.
- Increased contaminants from surrounding land - sediments can transport attached pollutants such as nutrients, bacteria, and toxic chemicals into our streams.
