The river Adur runs north to south down the western side of the catchment. The main river is fed by two arms, the Adur East and the Adur West, both fed by many tributaries and ditches. The arms meet just west of Henfield, where the river becomes tidal and embanked for some nine miles before it reaches the estuary at Shoreham-by-Sea. The headwaters of the river sit in the High Weald and are dominated by sandtones and clays.  Further downstream the river is afftected by the chalk geology of the South Downs. This changes to impermeable clay in the lower stretches, making the main river very reactive to rainfall.