The Serpentine River is no longer a river. It is just a series of pools. That is the effect of the Serpentine Dam, which is one of the three component dams in the flooding of the original Lake Pedder and which now help retain water in the new impoundment of Lake Pedder.
So the current Lake Pedder isbest understood as the Huon-Serpentine Impoundment. This name denotes the two major rivers dammed to create the current lake (Huon and Serpentine) and describes the technical status of the lake as an element of a hydro-electric scheme (impoundment) more accurately than the terms lake or reservoir.