“The McArthur River is the lifeblood of this part of the Gulf, a major river system flowing through the country of four main language groups before it reaches Yanyuwa country – coastal areas up to 17 km inland, the river mouth, mangroves, saline flats, seagrass beds and Sir Edward Pellew Islands.  The McArthur River is an extensive river with a myriad of channels branching from the main river.  The massive volume of the McArthur River flows centrally onto the Sir Edward Pellew Islands.  When it is in flood, this river is up to 4km wide, and fresh water can be tasted 10km out to sea at Centre Island.  Five years ago, the river flow washed cane toads out to all the islands except North Island in the big flood that year.  Whatever enters the McArthur River upstream, flows downstream out to these islands.”  (Stephen Johnson, Yanyuwa Elder).

    Geology - The origins of the McArthur River begin 1800 million years ago (mya) when the underlying basement rock (Fagan Volcanics) were formed. Successive periods of sedimentation and disposition (1780-1680, 1400-1300 mya) formed the McArthur Basin. The next ~1300 million years saw a series of geological events (faulting, uplift, erosion, marine inundation, sedimentation) that continued to shape the region in to how we see it today. The down warping of the Gulf of Carpentaria (~ 7 mya), tilted the northern portion of the region towards the Gulf, rejuvenating coastal-flowing streams and produced the McArthur River Catchment boundary, as we know it today. More recent history is marked by the rise and fall of sea levels, with the current Gulf inundation occurring some 6500-8000 years ago. 

The River - The McArthur River System is one of the largest in Northern Australia, it is a complex mix of fresh, saline and hypersaline environments. The topography of the McArthur River Valley is predominantly flat, or gentle rolling alluvial floodplains with grasslands or open woodlands intersected by hilly country in four places. For most of the year the McArthur River is confined to a single channel, entrenched 10m below the surrounding landscape, this overflows to form  multiple channels during the wetter months. The last 96km of the River are susceptible to tidal influences, with the final 45km made up of flat coastal plain. 

The McArthur River has a catchment of 20 000 km2 and flows for 300km, almost centrally through the full length through the catchment, with a mean annual flow of 4200 million cubic metres. This represents 36% of water flow from the NT’s Gulf Rivers. The next largest NT River in the region is Lummen Bight with a flow of 1600 million cubic metres, less than 40% of the Macarthur’s flow. The pattern of flow is closely linked to rainfall with over 90% of runoff occurring between December and April.

A google inspired fly through of the McArthur River.

The River has a fall of 250m from its upper reaches to the Gulf of Carpentaria,  with most of the fall occurring in the first 150km and only 20m of fall over he last 150 km.   Streamflow is highly variable along the McArthur River and from year to year. For example, streamflow along the McArthur River, between the Tooganginie Creek and Kilgour Rivers (where the minesite is), can vary by a factor of 15 from its minimum to its maximum flow. When the River is not flowing, surface water is restricted to waterholes and lagoons within main streams and on floodplains, with minor spring-fed flows arising intermittently along the River course. These waterbodies form essential refugia during the dry season for both aquatic and terrestrial animals. 

Climate - The climate of the McArthur River catchment is dry monsoonal with nearly all rain falling between October and April. Rainfall at Borroloola, 50km from the coast, is 950mm, spread over 65 days, with an average intensity of 15mm per day. At Mallapunya a further 110kn from the coast. Average rainfall is 710mm, spread over 20 days.