The Mawheraiti River, also known as the Little Grey River, is a tributary of the Grey River on New Zealand's West Coast, joining the Grey near Ikamatua, a settlement located between Greymouth and Reefton. The river rises near the Inangahua River north of Reefton and flows southward for approximately 33 kilometres before reaching its confluence with the Grey. Its average gradient is slight, suggesting generally low-gradient, calm water rather than whitewater conditions.

The river is characteristically tea-coloured, a common trait of West Coast streams influenced by tannins from surrounding vegetation. It is described as a smallish stream, which paddlers should factor into trip planning, particularly regarding navigability during lower water levels. The West Coast region is subject to heavy flooding, and water levels can vary considerably depending on recent rainfall.

Access to the river is available from the Atarau Road Bridge and where State Highway 7 crosses the Mawheraiti. Where the river does not run alongside the road, access requires permission from adjacent landowners. Paddlers should be aware of this land access requirement when planning put-in and take-out points.

The river's scenic and recreational values are rated as exceptional in recreational river assessments, suggesting the surrounding landscape offers worthwhile natural character. The area reflects the broader West Coast environment, with evidence of historical mining operations present in parts of the wider Grey River catchment.

Given the West Coast's rainfall patterns and the river's susceptibility to flooding, water levels can change rapidly. Paddlers planning trips should monitor conditions carefully and be prepared for the river to run high and fast following rain events. The slight average gradient and small size of the river mean it may be best suited to paddling during moderate flow conditions rather than during or immediately after significant rainfall events.