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Flood plains and Valleys

In this image of the Hapuku River Terraces near Kaikoura, the lines of trees define the slopes (risers) and the grassy paddocks are the horizontal terraces at different levels. Image: GNS Science.

In this image of the Hapuku River Terraces near Kaikoura, the lines of trees define the slopes (risers) and the grassy paddocks are the horizontal terraces at different levels. Image: GNS Science.

River flood plains experience both depositional and erosive processes. Deposition fills up hollows in the valley floor and erosion carves out meandering river beds. River erosion can create spectacular features, such as waterfalls, canyons, caves, and pillars, as softer rock is washed away leaving the harder, erosion resistant material behind.

Braided rivers: In New Zealand, rapid erosion rates, frequent rainstorms and steep gradients allow large sediment loads to be transported by rivers. As this material gets spread across the river bed, it gives rise to a shifting network of small interweaving channels. These braided rivers are commonly seen in New Zealand in the Hawkes Bay, Marlborough and the Canterbury Plains. Large braided rivers are in fact quite rare elsewhere occurring mainly in Canada, Alaska, the Himalayas and the Andes.

Braided rivers transport eroded rock from the Southern Alps to the Canterbury Plains. Image: GNS Science.

Braided rivers transport eroded rock from the Southern Alps to the Canterbury Plains. Image: GNS Science.

Oxbow lakes: Faster flow on the outside of each bend causes lateral (sideways) erosion. Sedimentation dominates on the inside edge where the water is moving more slowly. This means that the bends keep moving outwards until neighbouring loops eventually meet up. The river can then short cut past one of its sections to leave an abandoned meander called an “oxbow lake”.

River terraces: Tectonic uplift affects rivers by increasing erosion from the source area, and accelerating the down cutting by the river to create river terraces. Older terraces are higher up, and the more recently eroded are lower down, near the river level.

Following faults: Many river valleys are eroded along fault lines where the rocks have been crushed and weakened by earthquakes.

Find out information about a field trip to see the Hutt River Terraces and the Wellington Fault.