The river flows through the South Downs, an Area of Outstanding Natural Beauty and since 2011, a designated national park. There are two large aquifers in the area, one consisting of chalk, to the south of the river valley, and the other the Lower Greensand Group, underlying the whole of the river. The aquifers are separated by a Gault Formation, which consists of clay. At the western edge of the region, both the chalk and the gault turn to the north, creating a scarp slope. The base flow of the river consists of water from the Lower Greensand aquifer, and from springs along the bottom of the chalk scarp slope. These help to maintain the flows in the river during the summer months, although there have been significant periods where flows have been low, notably in the drought of the early 1990s, and again in 1995/96. These led to declines in the populations of the macro-invertebrates which are used to measure the health of a river, but these have subsequently recovered. In order to monitor the hydrology, the Environment Agency has a series of observation boreholes along the valley, and maintains gauging stations at Princes Marsh, close to the source, at Iping Mill, at Halfway Bridge on the River Lod just above its junction with the Rother, and at Hardham, just before the river joins the Arun.