Bound by the Richardson (Whakaari) Mountains[3] to the east and the Forbes Mountains[4] to the west, its snow-covered headwaters rise above 2000 m. The upper parts of the Rees River occupy a formerly glaciated valley that was fed by the Tyndall Glacier, which now drains into the adjacent Dart Valley.

Below Rees Saddle the river valley is constrained by a series of steep alluvial fans that are fed from tributary basins.

[7] However, the steep hillsides, easily erodible rock, proximity to the Alpine Fault, and intense rainfalls contribute to geohazard risks in the region.

A debris flow swept away a hiker during a river crossing in an unnamed tributary in the Upper Rees Valley near Cleft Peak in January 2002.

The lower Rees Valley, which continues to operate as a beef and sheep grazing farm, was a filming location for Mission: Impossible – Fallout,[10] and the television drama series Top of the Lake.