Aaron Burke, of Ilam School, asks :-

How far can mud travel along a river?

Graham Mackay, a hydrologist at the Freshwater Division of the National Water and Atmospheric Research Ltd, responded.

When mud is carried into a river during a flood it gets carried downstream until the flood stops. If some water spills onto the land, mud will be left there when the water has gone. Although the whole river looks muddy, most of the mud travels near the bottom. Some of the mud settles there when the flood goes down, but it travels further downstream during the next flood and so on until it reaches the sea.

The mud is then moved around by waves and currents before it settles on the sea bed. Sometimes this mud travels further still, down the slopes to the deepest parts of the seabed. It travels suddenly and quickly, like snow does in an avalanche, forming huge rivers of mud on the deep sea bed.

I suggest that you shake up some mud in water in a large softdrink bottle and time how long the smaller mud particles take to settle at the bottom. The water in a river in flood travels at about 50km each hour. You can then work out how long it takes the mud to travel the 100km from the mountains of New Zealand to the sea.