Petersburg Lake is the unofficial title (due to Petersburg Bridge and Dam being in the same area) of a big lake located at the west of Silverton.
In both Carbon and World, this lake is featured prominently on the map even though it isn't visible from the roads nearby.
The lake does exist, but the sight of it is blocked off by mountain rocks, whilst only a tight river connects it with the small lake behind Petersburg Dam. In both Carbon and World, there are textures of the lake.