Complex sentence: The removal of this mass, unbalances the forces that stabilize the dam against its reservoir as the mass of water still impounded behind the dam presses against the lightened mass of the embankment, (made lighter by surface erosion).
Split into simpler sentences: The removal of this mass, unbalances the forces that stabilize the dam against its impoundment. The mass of water still impounded behind the dam presses against the lightened mass of the embankment, (made lighter by surface erosion).