The Waiau River is the largest river in the Southland region of New Zealand, flowing from Lake Te Anau into Lake Manapouri before continuing south to Te Waewae Bay on Foveaux Strait. The name translates to "River of Swirling Currents." The upper section, between Lake Te Anau and Lake Manapouri, is the stretch most commonly paddled and offers a range of trip options that can be combined with overnight stays at Shallow Bay or Moturau Hut.

Access points on the upper Waiau include Yerex Reach, Queens Reach, Rainbow Reach, and Boulder Reach. The river passes through Fiordland National Park, and the surrounding landscape includes towering mountains and native flora and fauna. The river also passes through locations used as film sites for Lord of the Rings productions, where it stood in for the River Anduin.

Paddlers should be aware of several hazards on the upper section. Fallen trees can create snags, strainers, and trap points, and eddies can affect boat control. Jetboats operate in the area, so paddlers should stay clear of them. Didymo (Didymosphenia geminata), an invasive algae, is present in this river and requires paddlers to follow standard biosecurity cleaning procedures for their equipment before and after use.

On the lower Waiau, water levels can rise rapidly. Meridian Energy periodically releases flushing flows down the river to manage didymo buildup on the riverbed, which can cause sudden and significant changes in water level. Paddlers planning trips on the lower river should monitor Meridian Energy and Southland Fish & Game communications for advance notice of these releases. The lower river also has limited access points, meaning those who do reach it will generally find solitude.

The river connects with Lake Manapouri, making it possible to combine a river paddle with time on the lake.