The river flows through the South Downs National Park, and is a designated Site of Nature Conservation Importance, in recognition of its value for wildlife. It supports a wide range of fish, and its upper reaches are the only location in Sussex where native white clawed crayfish can be found. The quality of the water is generally good, and the river is measured at four gauging stations, three on the main channel, and one of the River Lod, just before its junction with the Rother. Water from the underlying Lower Greensand aquifer and the adjacent chalk aquifer helps to maintain the flows during the summer months, despite the fact that large volumes are abstracted from both the aquifers and the river for the public water supply.