About sediment

Sediment is eroded soil and rock that is washed off the land by rainwater and flooding. It travels down waterways and stormwater to arrive at the coast. Very fine sediment is classed as mud (and feels slippery in your fingers). Coarser sediment is sandy.

Sediment is called ‘suspended’ when it is floating in the water column, and ‘deposited’ when it settles onto the bottom of a river, stream or marine environment.

Too much sediment in a waterway or marine environment is regarded as a pollutant.

Sources of marine sediment

Sedimentation is a natural process but many human activities on land make it worse. Activities that churn up the soil or remove or change the vegetation, like forestry, earthworks and farming can speed up the process and the amount of sediment significantly. Some sediment in the marine environment today was generated by activities, such as large forest clearance, that happened decades ago.

Some types of rock (like sandstone) are particularly erodible and create more sediment than others.

Marine sediment comes from the marine environment as well as from land. Particles that have already reached the coast and settled on the seafloor can be resuspended by wind and waves.

The quantity of sediment in New Zealand

In New Zealand, the equivalent of about 7 million dump trucks of sediment is added to the ocean every year. By weight, this is about 75 times more than the amount of waste that goes to landfill annually.

Studies at estuaries in Waikato estimate that sedimentation rates are 200 times higher than they were before the land was converted to farming, forestry and urban use.

Sedimentation is a serious threat to ocean life

Too much sediment in the marine environment is harmful. Smaller sized, fine sediment is more harmful than coarse-grained sediment. Suspended and deposited sediment both cause harm to plants and animals.  

Plants need sunlight for photosynthesis but sediment cuts down the amount of light that reaches plants underwater. Reduced light can stunt or kill seaweed like seagrass and kelp, which have important roles in marine ecosystems.

Sediment reduces underwater visibility – many animals need to be able to see to hunt their prey. Seals and penguins, including hoiho, can’t find and catch their prey if the sea is cloudy. Murky water may be a factor in the population declines observed for some of these species. 

Sediment smothers plants and animals and can eventually bury them. This clogs up the filter-feeding apparatus of animals like cockles and mussels. In a healthy ecosystem, filter-feeders clean the water by stripping sediment out and binding it to the seafloor.

Sediments also carry pollutants like phosphorus, toxins, bacteria, pharmaceuticals and metals into freshwater and the marine environment.

Effects of marine sediment on people

Sediment can change the health of a whole ecosystem. This affects mahinga kai (food gathering) and our ability to collect shellfish, catch fish and protect threatened species. Murky water also affects our ability to go diving for fun and enjoy recreation near the coast.

Sediment is particularly problematic in some places because it affects the things we value. These include Kaipara Harbour, where snapper spawn; Porirua Harbour, a once-treasured seafood-collecting site; and Otago, where hoiho live. 

While sediment has a negative influence in the sea, the same particles were once fertile topsoil on land. Topsoil is essential for growing food and forests, so keeping sediment on land makes good sense for both environments.

You can help reduce marine sedimentation

Reducing sediment in the marine environment starts by simple actions on land. These include:  

• Fencing waterways to stop farm animals trampling stream banks.
• Planting and maintaining vegetation beside streams and rivers.
• Creating a 5–10 m buffer zone of long grass beside streams to trap sediment from floods before it reaches a waterway.
• Using traps, ponds, mesh fences and other devices to keep sediment out of waterways during earthworks.