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The Rakaia valley in New Zealand has a sediment layer 100m deep. In the upper profile, the bed is wide with more sediment than water. Around 15,000 years ago after the ice age, the river was deeper than present due to meltwater from glaciers. Sediment from landslides and scree filled the river, raising its level. The present river flows on this raised bed in braided channels, resembling the mudflats normally associated with a river mouth.
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First broadcast:
21 February 2008

Classroom Ideas

Can be used to discuss where all the sediment shown comes from, and why there is a high input of sediment - this could lead to a star diagram of factors which increase the rate of sediment input into a river. Pupils could identify some factors which cause streams to braid. The clip makes the point that the river is able to cope with floods because of the sediment each side of the river - students could articulate why this allows the stream to respond to flooding, and this can lead to the debate over whether we should encourage all rivers to flow freely without human development.

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