The Denmark River basin is 2651 km2.
Mean annual rainfall varies from 1050 mm in coastal areas to 650 mm around the headwaters.
The area is predominantly drained by the Denmark and Hay Rivers which extend 50 km and 80 km inland respectively. Other tributaries include Scotsdale Brook, Sunny Glen Creek, Sleeman River, Cuppup Creek and Little River. The Wilson Inlet is the receiving environment for these systems and is a seasonally open estuary (artificially opened based on inlet water levels).
The Wilson Inlet catchment is approximately 47% cleared of native vegetation and is predominantly occupied by livestock grazing (cattle and sheep), plantations, horticulture and residential land-uses. Native jarrah forests and wetlands become increasingly cleared for farming from west to east, although there are still large areas of set aside for conservation throughout the middle of the basin.
There have been considerable modifications to the drainage of the catchment. Both the Denmark and Quickup rivers are dammed for potable water supply and there are many private farm dams throughout the catchment. Artificial drainage is present throughout agricultural areas of the catchment. Portions of Cuppup Creek and Sleeman River have been artificially modified to improve localised drainage.
Similar to the Warren River, the Denmark River had signs of salinisation. Clearing native vegetation in the upper part of the Denmark River catchment resulted in stream salinity exceeding potable levels ( i.e. greater than 500 mg/L total dissolved solids) and peaking at an annual salinity at mean flow of 700 mg/L (1980–95) at the Mt Lindesay gauging station. In 1996, under the Salinity Action Plan, it was made a recovery catchment (Government of Western Australia 1996).
As a result of a range of management interventions (private tree plantations, revegetation and fencing sensitive areas) in the upper catchment salinity levels are within Australian Drinking Water Guidelines for drinking water quality. Denmark River is the only river in Australia to recover from rising salinity. For more information click here
Government of Western Australia 1996, Salinity Action Plan, Perth.