Though the lower reaches of the river are braided, part of the upper river flows through a canyon known as the Ashley Gorge.
The river emanates from mountains in the west Lees Valley adjacent to Island Hills station and exits the hills at a gorge near Oxford township. It has tributaries of Duck Creek in this valley and is an accumulator of watershed between Lees Valley and Oxford township.
Behind Waikuku Beach is one of the largest, least modified estuaries in New Zealand. It is abundant in bird life, including the wrybill (Anarhynchus frontalis) and Black stilt (Himantopus novaezelandiae). Many migratory birds over-winter here.