The Arahura River rises in the Southern Alps at Browning Pass and flows west, then north, then west again across the Westland lowland before reaching the sea approximately 5 miles north of Hokitika on New Zealand's West Coast. The river is roughly 35 miles in length, with major mountain tributaries including the Harman River, Newton Creek, Olderog Creek, and Wainihinihi River.

The river offers two very different paddling experiences depending on which section is attempted. The lower Milltown Gorge section is accessible by road at both ends, with a put-in anywhere along Milltown Road from Lake Kaniere and a take-out at Arahura Valley Road. This section begins with easy shallow braids and Grade II shingle chutes before the valley closes in at the Milltown Gorge, where bedrock banks and large boulders create occasional Grade III features, though the run is generally rated Grade II overall. Deep clear pools, well-spaced rapids, and native forest above high banks characterize this stretch. It is popular with kayakers and rafters and is suitable for intermediate paddlers, though not recommended as a first-time run. The run takes approximately four hours if both the Milltown Gorge and lower sections are combined.

The upper wilderness sections are a different proposition entirely. Access requires a helicopter, and the standard run is approximately 8.3 miles at Class IV to V difficulty, with a gradient reaching up to 212 feet per mile through the Cesspool Gorge. The crux is Cesspool, a waterfall into a heavily aerated pool with dangerous sieves below, which is typically portaged. The canyon below Cesspool is considered one of the more striking features on the run. The upper sections demand the ability to catch eddies quickly and make rapid decisions, as scouting is difficult in certain spots. Flows between roughly 500 and 1,500 cfs at the take-out are considered runnable, and the season is year-round following rain, with an early start recommended in winter. Anyone considering the upper run is strongly advised to go with someone who has prior knowledge of the access and the rapids.

The Arahura holds significant cultural importance to Ngāi Tahu as a source of pounamu (greenstone), and portions of the upper river fall within the Waitaiki Historic Reserve, managed jointly by a Ngāi Tahu commercial entity and the Department of Conservation. Access for kayakers to certain upper gorge sections has historically been restricted, and paddlers should verify current access permissions before planning a trip.