The entire Nelson Lakes region is a complex system of freshwater streams and lakes. Since the park lies at the beginning of the Southern Alps, there is a general decrease in altitude leading northwards. That is the reason why water accumulates in the valleys of Rotoroa and Rotoiti, which have been carved out by the latest glaciation.
Past the lakes, Mount Murchison, and its smaller neighbouring peaks, the altitude drops below 1000 m above sea level. The massive Buller River is a remnant of the last ice age and had once found its way through this descent. It is fed by almost every major stream (tributaries) within the park which give it enough power to reach the west coast in 170 km. Its origin, however, remains at the north side of Mount Travers within the Saint Arnaud Range.