Salt wedge

A salt wedge is a layer of saltwater below a layer of freshwater, which is pushed into an estuary by the tides.

Saltwater is denser than freshwater which enables it to move up the estuary below the less dense freshwater, which in turn creates a wedge shape layer of salt water that mixes very little with the freshwater.

The structure of the Derwent Estuary is that of a drowned river valley forming around 6,500- 13,000 years ago, extending for a distance of 52 kilometres and covering an area of 198 square kilometres.

The river behaves as a salt wedge estuary with the marine flow moving upstream in the lower waters and a freshwater flow heading downstream.

The head of the salt wedge is located near Bridgewater under low flow conditions and is pushed further downstream under high flow conditions.