How Sediment Destroys Fisheries

North Rakaia River below Bain’s Bridge

[The content in square brackets below is cut and pasted from an AI generated google search.

My own contribution will continue beneath;

Excessive sediment-soil, sand, and silt entering waterways-is considered one of the leading causes of river degradation and, in severe cases”kills” them by suffocating aquatic life, destroying habitats, and altering water quality.

While a natural part of aquatic ecosystems, human activities such as deforestation, agriculture, and construction have accelerated erosion, causing over 10 times more sediment to enter waterways than in pre-human times. 

Here is how sediment kills rivers:

1. Smothering Aquatic Life and Habitats

• Destroys Benthic Habitats: Fine sediment settles on the riverbed, filling the essential spaces between stones and gravel, known as interstitial spaces. This habitat loss kills invertebrates, such as nymphs and larvae, which are a primary food source for fish.
• Suffocates Fish Eggs: Sediment smothers gravel beds, which are necessary for fish spawning. The sediment layer acts as a blanket, restricting water flow to buried eggs, which prevents oxygen from reaching them and causes asphyxiation.
• Kills Organisms: When in high concentrations, sediment acts as an abrasive, damaging the gill structures of fish, reducing their ability to breathe, and damaging their tissues. 

2. Disruption of the Food Web and Ecosystem

• Reduces Light and Photosynthesis: Suspended sediment increases turbidity (cloudiness), preventing sunlight from reaching aquatic plants, which inhibits photosynthesis and reduces food for herbivores.
• Impairs Predation: High turbidity restricts visibility for fish that rely on sight to hunt, making it difficult for them to find food.
• Carries Toxic Contaminants: Sediment particles act as carriers, transporting contaminants like pesticides, heavy metals, phosphorus, and nitrogen from the surrounding land into the water. These pollutants can lead to long-term toxicity, bioaccumulating in top predators.
• Causes Algal Blooms: Excess nutrients attached to sediment can trigger rapid algae growth (algal blooms), which, upon dying and decomposing, consume the dissolved oxygen, resulting in fish kills. 

3. Physical Destruction of the Channel

• Alters Flow and Structure: Excessive sediment deposits can change the depth, flow, and shape of a river, often making it wider, shallower, and slower.
• Shallowing and Infilling: The sediment builds up on the riverbed, which reduces the overall capacity of the channel and can lead to increased flooding.
• Dams and Infrastructure: Sediment can choke up infrastructure like pipes and drains, and it also fills up dams and reservoirs, reducing their storage capacity and causing “hungry water” (a lack of sediment) downstream, which causes further erosion. 

4. Direct Impacts on Species

• Fish and Invertebrates: In addition to the above, sedentary species and filter feeders (e.g., mussels) can be buried.
• Invasive Species: Sediment-filled, low-oxygen waters often benefit invasive species like carp, which thrive in these conditions and further degrade the environment. 

In summary, sediment turns healthy, vibrant rivers into shallow, low-oxygen environments that are unable to support their original biodiversity, effectively killing the aquatic ecosystem. ]

Words of the author;

The AI summary above “sets the mats” for my explanation of how irrigation has altered natural flow patterns following; the abstraction, “diversion”, and impounding water from New Zealand rivers to, (in the words of irrigation proponents & planners); “create ‘new water”.

Consent applications are replete with deceitful content such as “we are only taking Alpine Water” when it is now accepted that river catchments must be managed as a whole.

In the rivers I am referring to, the sediment is endogenous to the catchments, and is typically not the result of forestry or agriculture. Toxic contaminants are not an issue.

The essential concept to understand that regulated flow regimes are a key driver of poor ecological health as they cause excess sediment to drop out of the water.

For those able to read and take in a really solid piece of science, I recommend Doug Rankin’s paper;

Journal of Hydrology (NZ) 64 (2): 197-214
© New Zealand Hydrological Society (2025)

Impact of the Opuha dam on the turbidity and health of the Opuha and Opihi Rivers

Douglas A. Rankin1* and Isabelle Barrett2
“Ramifications for irrigation storage schemes and their impacts on rivers

Analysis of the impacts of the dam on the Opuha River has revealed a notable and perhaps largely unrecognised consequence

Fine sediment routinely released from the dam is likely to be causing significant damage to the ecological health of rivers downstream.

Such potential impacts have been recognised elsewhere (Young et al., 2004) but until recently appear to have not been considered in the Opuha, Ōpihi and Kakahu Rivers. If other schemes are constructed in a similar fashion, then there will be a real risk that they too will release turbid water that could compromise the ecological health of rivers downstream.

The recently commissioned 52-metre-high Waimea Dam on the upper Lee River near Nelson, with a 13 Mm3 storage lake designed for water augmentation for irrigation, residual river flow, and community water supply on the Waimea Plains (Tonkin and Taylor, 2019), may also release turbid water to the Lee River and Waimea River downstream.

The dam has no facility to discharge sediment from the lake bottom at the foot of the dam, or to bypass or flush sediment at any appreciable rate (Tonkin and Taylor, 2019), and so, like the Opuha dam, may result in negative physical and ecological impacts in the lake and rivers downstream. A proposed 83-metre-high dam on the Makaroro River, to create a 93 Mm3 water storage reservoir (formerly the Ruataniwha Water Storage Scheme) to enable regional irrigation water security and minimum flows in the Tukituki River in Hawkes Bay (Ministry for the Environment, 2024), may also lead to similar negative ecological impacts for the Tukituki River downstream, unless the dam is appropriately designed.”

Until this insight becomes part of the Assessment of Environmental Effects (AEEs) for new projects, Angler Oral Histories will continue to record notable fisheries lost to water abstraction and sedimentation.

(Along with a range of other water quality parameters).

I apologise to the authors for not undertaking a more rigorous review of this critically important paper.

Dr Peter Trolove

Executive member

NZFFA

A healthy un-modified river in South Westland