The Engelberger Aa (or Engelbergeraa) is a river in Switzerland . It rises west of the Surenenpass in the Canton of Uri , flows through the valley of Engelberg (Obwalden ) and Nidwalden . At Buochs it flows into Lake Lucerne and is therefore a tributary of the Reuss, which drains the lake.

The source of the river is at an altitude of approximately 2,200 m (7,200 ft), a few hundred metres to the west of the Surenenpass ridge. From here it flows westwards, descending rapidly and collecting many mountain streams. It roughly follows the route of the Surenenpass hiking trail to Blackenalp (1,760 m (5,770 ft)), the first farm dwelling along the route.