The Oldfield River is an ephemeral

Ephemeral

Ephemeral things are transitory, existing only briefly. Typically the term is used to describe objects found in nature, although it can describe a wide range of things....

 river

River

A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...

 in the Goldfields-Esperance region of Western Australia

Western Australia

Western Australia is a state of Australia, occupying the entire western third of the Australian continent. It is bounded by the Indian Ocean to the north and west, the Great Australian Bight and Indian Ocean to the south, the Northern Territory to the north-east and South Australia to the south-east...

 that rises 95 kilometres (59 mi) inland from the South Coast at the edge of the Yilgarn plateau. The river starts at 300 metres (984 ft) above sea level then flows in a south easterly direction crossing the South Coast Highway

South Coast Highway

South Coast Highway is a Western Australia highway. It is a part of the Highway 1 network.With a length of , it runs from Esperance to Walpole roughly in parallel to the Western Australia's south coast...

 near Munglinup

Munglinup, Western Australia

Munglinup is a small town located in the Goldfields-Esperance region of Western Australia.The town is located along the South Coast Highway between Ravensthorpe and Esperance and close to the Munglinup River....

.

The river gently undulates through sandstone

Sandstone

Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...

 forming gentle valleys with many granite

Granite

Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...

 outcrops, the river then carves deeper valleys through the siltstone

Siltstone

Siltstone is a sedimentary rock which has a grain size in the silt range, finer than sandstone and coarser than claystones.- Description :As its name implies, it is primarily composed of silt sized particles, defined as grains 1/16 - 1/256 mm or 4 to 8 on the Krumbein phi scale...

 before entering the coastal plain.
The river is then joined by its tributary

Tributary

A tributary or affluent is a stream or river that flows into a main stem river or a lake. A tributary does not flow directly into a sea or ocean...

, the Munglinup River

Munglinup River

The Munglinup River is an ephemeral river in the Goldfields-Esperance region of Western Australia.The headwaters of the river rise near Cheadanup Nature Reserve and flows in a southerly direction through cleared farmland then crossing the South Coast Highway near the town of Munglinup before...

, before flowing into the Oldfield estuary which discharges into the Southern Ocean

Southern Ocean

The Southern Ocean comprises the southernmost waters of the World Ocean, generally taken to be south of 60°S latitude and encircling Antarctica. It is usually regarded as the fourth-largest of the five principal oceanic divisions...

.

The only other tributary of the Oldfield River is Coujinup Creek.

The river is regarded as saline

Saline water

Saline water is a general term for water that contains a significant concentration of dissolved salts . The concentration is usually expressed in parts per million of salt....

with high nutrient levels, moderate sedimentation, moderate fringing vegetation and has a low flood risk.