When soils erode, sediments are washed into waterways.
Sediment in a stream is natural, but if sediment levels get too high, it can disrupt ecosystems and kill mahinga kai. Excess sediments can cause damage by blocking light that allows algae (an important food source) to grow, harming fish gills, filling up important habitats, and stopping fish from seeing well enough to move around or feed.
Sediments are a natural part of a stream, lake, or river, and the type and amount found in streams are influenced by the geology of the surrounding area. Natural processes that add to sediments in waterways include instream scouring of the river bed and banks and erosion of sediments from the surrounding catchment from natural slips and any exposed soils. Sediments can enter streams from alongside a reach or from upstream via the myriad smaller interconnecting streams that form a river network within a catchment area.
While sediment movement is a natural part of a functioning freshwater ecosystem, human activities around a waterway (such as dam or road construction or land use change from native forest to pasture) can greatly increase the amount of sediment that enters the system. This can have considerable effects on water quality and the plants and animals that live there. The addition of sediment to rivers and streams above normal levels is a serious issue.
Sediments in waterways travel downstream in suspension when water velocity is high or turbulent. When there is a decrease in velocity, especially in pools and deep areas of a stream/river, sediments will eventually settle and can be seen as deposits of fine material or by the formation of sand bars on the river or stream bed.
Sediments in suspension can have a significant impact on the water quality of a waterway because sediments decrease water clarity, which reduces visibility. Water clarity is usually measured as turbidity. Turbid waters prevent the growth of aquatic plants and algae (because plants need light for photosynthesis) and decrease the ability of fish to find food or to detect predators and prey, thereby increasing stress. Sediments may smother stream invertebrates which are an important food source for fish.
Excessive sediment deposits on the river/stream bed can significantly alter and degrade habitat. Some animals are dependent on the rocky bottoms of streams, while others live in deep sandy pools or around woody debris. Sediments fill the spaces between stones that invertebrates live in, and in extreme cases can bury woody debris, stony substrates (gravels and cobbles), and root mats, and fill pools and channels. This reduces the amount of invertebrate habitat and cover and spawning grounds (a place to lay eggs) for fish. An increase in the amount of sediment deposited on the river/stream bed can also significantly change the flow and depth of rivers or streams over time and infill lakes and estuaries. Natural cleaning processes - where the water flows through the gravel bed of a stream and interacts with the microbes living on stone surfaces, removing nutrients and some pollutants - can also be short-circuited by excessive sediment deposits.
Potential impacts of sediment on water quality and mahinga kai
- 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 from agriculture and horticulture into our streams.