Understanding Tairawhiti’s braided rivers

Olivia Steven is an environmental scientist for the Gisborne District Council.

For this column we will focus on braided rivers, which flow in multiple and mobile channels across a gravel floodplain.

Braided river beds are typically shallow, which allows multiple channels or “braids” to form in the wider sections of the river bed. These rivers typically form in highly-erodible catchments that provide large amounts of sediment and gravel to the river system.

They are considered “hungry” as they require sediment input to thrive. Excessive extraction of gravel from them can cause them to erode their banks or bed to keep their bellies full of gravel.

Such riverbed systems are ever-changing habitats for a range of plant and animal species. They can provide suitable breeding and nesting habitat for several native bird species like the poaka/pied stilt, tōrea/South Island pied oystercatcher and two species of dotterel, the tūturiwhatu/banded dotterel and the black-fronted dotterel.

Many native fish also live and spawn in these braided rivers. For example, the Waiapu River provides important habitats for tuna kuwharuwharu/longfin eel, tuna hinahina/shortfin eel, panoko/torrentfish, redfin bully, inanga/common galaxias, pōrohe/smelt. The redfin bully spawns in the Waiapu in flowing water under rocks from July to November each year.

Native birds such as the banded dotterel/tūturiwhatu only nest on areas of bare gravel on these rivers. These birds rely on frequent floods to clear the gravel beds of weeds and debris.

A braided river system is healthy when it has multiple “braids” (separate water channels) and varying sized channels. When the river has multiple braids, it is easier for the flood waters to flow over the gravel and clear it of any vegetation.

Invasive weeds such as pampas and gorse are prevalent on many of the braided rivers of Tairāwhiti.

Areas of invasive weeds can offer habitat for predators. Many nesting bird populations that breed along these rivers are in decline due to mammalian predators, modified river habitat and human activities.

Your actions can help the special and threatened species that live in and around braided rivers.

Riverbed birds nest between July and February and need special consideration during this time.

Keep pets away from riverbeds and keep dogs on a leash at all times.

Keep clear of nesting colonies and watch out for eggs and chicks. They are hard to see and very fragile.

If you disturb birds accidentally, move on quickly so that the birds can return to their nests.

Avoid driving vehicles on riverbeds. Park on the bank and walk to your destination.

Above all, respect braided rivers and the plants and animals that need your consideration to survive.