Trevallyn Dam and Lake Trevallyn is a concrete gravity dam built on Dolerite bedrock. The spillway height is 26.8 meters and the dam wall is 177 meters long. The lake has a surface area of 1.48 km and storage capacity of 12.33 million m3. Height of the dam wall is 33 metres and the total volume of the wall 61,000 cubic metres. It various at times depends on who did the measuring.