Waimakariri River

The Waimakariri (or the Waimak as it is colloquially known) is a major braided river which originates in the heart of the Southern Alps mountains. It is one of the largest rivers in North Canterbury and flows for 151 kms southeast from the Alps through the Canterbury Plains into the Pacific Ocean. It has a catchment of 2,500 sq kms and over 90% of its water originates above the Waimakariri Gorge, about 60km from the sea. It is also among the rivers responsible for transporting vast amounts of sediment from the rising Southern Alps during glacial and interglacial periods to make the Canterbury Plains. The Waimakariri is an aquifer recharge system for Christchurch city water supply.

The Waimakariri is also a significant potential flood hazard to Christchurch, leading to the development of extensive stop bank systems in the north of the city. The Waimakariri turns over two thirds of its available floodplain annually. Low flow is often around 180 cumecs, made up of many branching channels, while flood flow can be as much as 3600 cumecs and filling up the entire 2 km river bed width with water. Between 1932 and 1969, the bed of the main channel rose by 2 metres. Currently, the Waimakariri River flows into Pegasus Bay at Brooklands Lagoon, however, the river has moved it's channel many times over the last few thousand years and once used to flow out into Te Waihora/Lake Ellesmere to the south of the city, and even through Christchurch City itself.

What is a braided river? Braided rivers arise from very high sediment load eroding from the mountains. There is also a relatively big difference in the carrying capacity of the river between flood and normal stream flow. During floods the individual streams carry coarse sediment, but during normal flow they can’t so the sediment drops out and chokes the channels. Because the gravel sediment can’t stick together, the stream cannot cut a single channel with steep banks, leading to collapse of the channel walls.

Braided rivers are fairly unusual around the world and a very striking part of our New Zealand landscape. Why do you think we more often have braided rivers and streams instead of the typical meandering (curvy, one channel) type rivers? Why do we have such a high amount of sediment eroding from our mountains?

Have a closer look at the river bed. Can you get a sense of which way the water is flowing just by looking at the gravel rocks rather than the water? This is something geologists need to be able to do when we look at ancient rivers preserved in the rock record. What do you notice about the arrangement and shape of the rocks in the river bed? Any patterns to how they are laid out? Check a side-on view if you can find a cutting through the gravel

Before the Silurian, around 418-443 million years ago, it is thought that all streams on plains were braided. Since then, many streams have instead evolved as meandering streams. For meander curves to develop, the substrate of a stream must be strong enough for cut banks to hold up. What do you think happened during this time to cause this change?