Beach gravel, Waitaki Fan, South Canterbury

The Waitaki Fan, straddling the boundary between South Canterbury and North Otago, is a large alluvial fan formed at the mouth of the Waitaki River built up from gravel brought to the coast by the river. The majority of this gravel is made up of greywacke, a coarse sandstone that occurs widely in New Zealand’s mountainous areas. The gravel is also characteristically very smooth and well-rounded, having being slowly ground down in the process of being transported downriver to the coast over thousands of years.

The coastal margin of the Waitaki Fan is presently eroding rapidly and is marked by sheer cliffs that are continually retreating as waves and currents remove many thousands of tonnes of gravel every year. It is thought that the recent rapid erosion of the fan is at least partly attributable to the existence of several hydro schemes on the Waitaki River, due to the way they trap river gravels that would otherwise be carried down to the coast by the river to replace some of that which is carried away by erosion.

Now that the coastline is being starved of gravel the sea is able to attack the cliffs directly, causing the accelerated erosion that is of concern to local authorities and residents alike. Much of the gravel that makes up the beaches here is in fact ‘recycled’ from older gravel deposits that are now being eroded by the sea.