The Zengwen reservoir receives its water from an extremely steep and rugged catchment basin of in the Alishan Range of south-central Taiwan. The elevation of the land ranges from on the shore of the reservoir to over in the Alishan National Scenic Area near the summit of the range. About 77 percent of the watershed is forest, 13 percent is agriculture, 9 percent other uses and less than 1 percent is urbanized. The reservoir has suffered severe siltation (sedimentation) from the watershed, which has caused a large reduction in its capacity.