How can sediment be a pollutant?
“It’s a natural substance and it’s not poisonous.”
This may be true but sediment is a serious environmental pollutant. In the Northland region, tens of thousands of tonnes of sediment enters our streams, lakes, estuaries and harbours every year!
But it can’t be as serious as industrial pollution, can it?
Worldwide, sediment is by volume the biggest single water pollutant! In the Northland region sediment is the biggest cause of shellfish losses in estuaries - not chemical spills.
Sediment effects waterways in two main ways:
- there are physical effects: smothering, reduced light penetration, scouring and
abrasion; and
- sediment also provides particles for other pollutants to attach to, carrying them into our waterways.
Nobody likes swimming, fishing or boating in dirty water.
The effects of sediment pollution are not always obvious, but the effects are often felt far beyond the stream or beach where it ends up.
Uncontrolled sediment run-off has a devastating impact on freshwater streams. It destroys the sheltered estuarine areas of our upper harbour inlets. It affects our offshore fisheries, which depend on the estuaries. It spoils people’s enjoyment of beaches and streams. It clogs up stormwater drains and stream beds, causing increased flooding.
No-one uses that grotty little stream, anyway.
People ask “what’s the point in keeping sediment out of agrotty little drain? It’s already polluted anyway. A bit of sediment can’t do any more harm. Nothing lives there.” Nothing could be further from the truth!
It is amazing how much life exists in our small rural and urban streams - they are home to plants (algae), insects and their larvae; invertebrates like shrimps, snails and native freshwater crayfish , (koura); native fish such as eels, bullies and kokopu (often referred to as native trout); and birdlife.