The Mangakahia River flows approximately 48 kilometres through the Northland Region of New Zealand's North Island, originating in the Mataraua Forest and eventually joining the Wairua River to form the Wairoa River, roughly halfway between Whangārei and Dargaville. The name Mangakāhia translates from Māori as "stream of the New Zealand passionfruit vine."

The river's character varies considerably along its length. The middle section, spanning around 10 kilometres at an average gradient of 6.9 metres per kilometre (1:146), offers limited paddling opportunity. This stretch is navigable for approximately 4 kilometres when water levels are above normal, after which rapids become too shallow to run. Shingle shoals are a recurring feature and may require portaging. Access to the water is a noted challenge in this section, as the banks are frequently obstructed by fallen willows, and getting a craft down to water level can be difficult unless the lower river is also in a boatable state.

The lower section, approximately 15 kilometres in length, runs at a much gentler gradient of around 2.2 metres per kilometre (1:457) and includes flat water above the dam at Wairua Falls. While this section is large enough for paddling use, access below the falls is described as difficult.

Jet boats make limited use of the middle Mangakahia above Twin Bridges when flows are elevated. The river is generally too shallow and small for motor launches or drift boats under normal conditions, though drift boats could navigate the middle section with some portaging of shallower areas. Paddlers should be aware that water levels significantly affect what is runnable, and the river's accessibility is closely tied to current flow conditions.