Well, I've managed to download a bunch of digital elevation maps and combine them into a single raster. Then I've extracted contours. One map I've created looks like this:
The problem I face now is figuring out how to extract polygons from the contour vector layers and create the lake surfaces for the maps.
BTW, the reason that the contours only cover the eastern part of the map is that the contour extraction routine crashes for wider regions. I still need to go back and extract the western half and merge the two layers into a single contour coverage.