The Arthur River flows through Fiordland National Park on New Zealand's South Island, draining from Lake Ada into Milford Sound/Piopiotahi. The river runs through an ancient glacial valley, its course shaped by resistant rock formations, shifting sediment, and notably by a large earthquake that triggered landslides creating the boulder-choked rapids that define the whitewater sections. The water is described as remarkably clear and fast-flowing, with a slight natural tint from vegetation and fine glacial silt.

Paddling the Arthur River is subject to strict regulations. Access is only permitted as far as Lake Ada, and paddlers must hold a permit issued by the Fiordland National Park Visitor Centre. The put-in is reached by crossing Deepwater Basin, accessed via the boat ramp off Deepwater Basin Road in Milford Sound. Because Deepwater Basin is partially fed by the Cleddau River, which is already infected with didymo (Didymosphenia geminata), all paddlers must thoroughly clean their vessels before entering the Arthur River to prevent the spread of this invasive algae. Permits are required, and the didymo contamination risk means strict biosecurity protocols apply.

The whitewater character is defined by two major rapids, both formed by earthquake-triggered landslides. The entrances to these rapids are tight and technical, cluttered with boulders that back up the water. Between the rapids there is a notable stretch of slack water. The style does not fit neatly into a single category — the boulder-strewn nature of the drops creates demanding, technical lines. Lake Ada sits at the top of the navigable section, formed by the same boulder debris that creates the upper rapid.

The surrounding environment is quintessential Fiordland: dense beech forest, towering peaks, and abundant waterfalls. The river valley also forms part of the Milford Track corridor, one of New Zealand's most famous walking routes, and features historic rock cuttings blasted by hand when the track was first constructed in the late 1800s. Giant Gate Falls, a 30-metre waterfall that plunges into pools before draining toward the Arthur River, is a notable landmark in the lower valley. Sandflies are extremely numerous in the area and should be anticipated throughout any visit. This section of track and river can flood during heavy rain, which may cause delays or closures.