The Kalgan River is approximately 140 kilometres long and, along with the King River, drains into Oyster Harbour. The river starts west of the Stirling Ranges with a catchment area of about 2,490 km2, just over three-quarters of which has been cleared for agriculture and has the biggest water contribution to Oyster Harbour. It is the region’s fourth-largest river in terms of average annual flow (53,400 megalitres), and has the third-largest catchment area.
The dominant land uses are cropping and mixed grazing. There are also several bluegum plantations, mostly in the lower half of the catchment. There is also a large area of conservation and native vegetation at the top of the catchment is part of the Stirling Range National Park and the smaller area near the center of the catchment is the Porongurup National Park.
Most of the upper reaches of the Kalgan River lie within the National Park. These tributaries are marginally saline to brackish, suggesting the levels of salinity are natural. The loss of catchment vegetation (66% of the catchment is cleared) has increased salinity levels downstream. The lower section of the Kalgan River is tidal; a rockbar at the Upper Kalgan bridge separates the upper section from the tidal lower section. The tributaries of the Kalgan River include Napier Creek, Boonawarrup Creek, Young River, Stony Creek, and Gaalgegup Creek. The river flows through two pools of note: Meriwarbelup Pool and Noorubup Pool.
The environmental pressures along this river are very varied and complex to resolve due to the varied land uses and land capacity.
Western pygmy perch
Western mud minnow- of conservational significance
Lamprey - of conservational significance
Cherax sp. novel (new crayfish species)
South-west glass shrimp