[1] The river has its source in the Southern Alps and flows east for 80 kilometres (50 mi) before emptying into Lake Ellesmere / Te Waihora, south of Banks Peninsula.

Terrace cliffs above the river's upper reaches gave the town of Whitecliffs its name.

This frequently occurs where State Highway 1 crosses the river at the settlement of Selwyn, about 20 kilometres (12 mi) upstream from its outflow into Lake Ellesmere.

About 15 kilometres (9 mi) upstream from Lake Ellesmere shallow groundwater rises back to the surface, and the Selwyn becomes permanent again.

The dry central reaches of the Selwyn River also constitute a significant barrier for dispersal of invertebrates and for fish migrating between Lake Ellesmere and the headwaters.

[9] The pollution at Coes Ford is thought to come from Silverstream creek, which used surrounded by many of the dairy farms in the area.

[10] Chamberlain's Ford which crosses the Selwyn RIver had an algal bloom health warning issued in December 2020.