Chapman River - Hydrology

Hydrology

Most of the Chapman River's 1,160 km² catchment consists of cleared agricultural land. This land is heavily fertilised and subject to soil erosion, so the river carries high concentrations of nitrogen, phosphorus and chlorophyll a. This, together with the encroachment of urban areas into the river's riparian zone, has resulted in a degraded river system with eutrophic lower reaches.

The river generally does not flow in summer, but always flows in winter. From the mouth to about 1.5 km upriver, the river is estuarine. This stretch always contains some water. The mouth itself is usually closed by a sandbar, but this opens at the onset of heavy flow, and closes after flow has receded. When the sandbar is open, sea water enters the estuary, mildly affecting its salinity.

During periods of very high rainfall, soils are washed into the river and carried out to sea. Following heavy rains in June and July 1986, it is estimated that: the Chapman River washed 39,000 tons of suspended sediment into the ocean; that at peak flow a ton of sediment entered the ocean every seven seconds; and that this loss of sediment represented the loss of some A$73,000 of soil nitrogen.