The Kaihu River flows southward through Northland, approximately 30 kilometres north of Dargaville on State Highway 12, eventually joining the Wairoa River just downstream of Dargaville. The river runs approximately 38 kilometres in total length. Its character changes considerably from upper to lower reaches: the upper section flows over boulders and cascades through a narrow, deep bush-clad gorge that is particularly scenic, before opening onto farmland where the river slows and meanders between pine and willows. The lower reaches near the confluence with the Wairoa are swampy and drain-like in character.

For paddlers, the most practical and commonly used section runs from the Whatoro Bridge near Kaihu township downstream. Canoes and kayaks are well-suited to this stretch, and the Northland Canoe Club has historically used the river both for its annual Marathon Canoe Race — part of a nationwide series attracting entrants from across New Zealand — and as an introductory trip for novice paddlers. The race typically starts at the Jordan Valley Road Bridge. The river is generally considered easy, rated Grade 1, with one rapid of Grade 1+ that leads into a pool above a falls. This makes it accessible to beginners while still offering some minor technical interest.

Rafts can be used on approximately 14.5 kilometres from the Whatoro Bridge to Mamaranui, though the river sees relatively little use below that point. The upper gorge section above the Old Sawmill is generally considered too small for most recreational craft. The lower swampy reaches near Dargaville are not suitable for paddling due to shallow and boggy conditions.

The river tends to run clear most of the time and is described as picturesque, particularly in the upper reaches where the bush-clad gorge provides a visually distinct environment. The Mangatu River joins the Kaihu approximately 5 kilometres upstream of the motor camp along Oputeke Road, offering an additional tributary for those exploring the area. Fly fishing for trout is also practiced on the river, so paddlers may encounter anglers, particularly upstream from Kaihu township.