The Rye is a stream rising east of Ashtead and flowing into the River Mole near Leatherhead, Surrey.

For much of its course the brook follows a straight channelled course which was dug during the Second World War in order to drain the surrounding land so that it could be used for agriculture.

The Environment Agency measures the water quality of the river systems in England.

Each is given an overall ecological status, which may be one of five levels: high, good, moderate, poor and bad.

There are several components that are used to determine this, including biological status, which looks at the quantity and varieties of invertebrates, angiosperms and fish.