A blockage can cause the bowl level to drop as well as rise. When all is well, the water level in your toilet bowl is regulated by the level in the tank and toilet's internal S-trap. When the suction that empties the bowl during a flush is broken, the bowl level rises to the height of the trap outlet. blockage can cause the bowl level to drop as well as rise. When all is well, the water level in your toilet bowl is regulated by the level in the tank and toilet's internal S-trap. When the suction that empties the bowl during a flush is broken, the bowl level rises to the height of the trap outlet.