Geoprocessing - Desktop GIS: Mapping the Planet with Open Source Tools - page 146

Figure 10.1: Results of line-of-sight analysis in GRASS
on a DEM to a given level. You just specify the start point and the water
level. You don't have to be real picky about the start point because
GRASS analyzes the DEM and fills it such that the deepest point will
be equal to the depth you chose. This means you can actually pick what
will become a very shallow area (in other words, a higher elevation), and
the lake will be created properly. After all, water does flow downhill.
First let's look at the usage for r.lake :
GRASS 6.2.2 (albers_c6):~ > r.lake help
Description:
Fills lake from seed at given level
Keywords:
raster