Sediments and hydro

How may hydro-dams influence sediments in waterways?

Dams impact on the amount of sediment that moves downstream. Sediments are trapped behind a hydro-dam’s reservoir (hydro-lake) and build up. This reduces the reservoir's storage capacity. The rate at which this happens varies greatly among hydro-dams. Dams also reduce the amount of sediments deposited downstream. This increases erosion and scouring and decreases the amount of sediments available to replenish downstream habitats. This is especially important to mahinga kai that rely on sediment deposits for habitat and spawning grounds.

Potential impacts of sediments on water quality and mahinga kai

• Decreased water clarity - increased sediment loading in 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.
• Decreased habitat available for spawning.

Find out more about the potential environmental impacts of sediments in waterways