Littoral beach systems are used to illustrate "mass conservation" in an Open Mass System. These beaches are linked to both river systems and to fault-produced submarine canyons. Specifically, the Oceanside Littoral Cell is used to illustrate how urbanization of a watershed in a semi-desert climate, with the attendant damming of rivers to produce reservoirs, has contributed mightily to the loss of once wide, sandy, long littoral beaches in San Diego and greater Southern California.