|Major cities||Queentown, Adelaide|
The Matt River is the most significant water body in the Republic of Matthewopia. it is approximately 3.42 km long, of which 22 m runs through Matthewopian territory. The river collects in Australian territory in a small pond, which later flows into Dry Creek in Australia. The flow is very variable, as it relies on rainwater to fill it. A constructed dam and one holding reservoir contain peak flow.
The water from the river is used to supply irrigation to the plants and animals in the area and water for Royal Brand Spring Water.
Only 22 metres of the river runs through Matthewopian territory. It flows from its source in Yatala Vale in Mount Lofty Ranges, South Australia, passes briefly through Matthewopia, then empties into the Barker Inlet. The Matt River is mostly dry in Summer.