This is due to the pressure applied by the object . A buffalo has more surface area than the man . Therefore , the pressure exerted by the buffalo is less than the pressure exerted by the man . The water exerts the upward force back on the buffalo . Therefore , the buffalo floats on water and not the man .
(note:man can float on water if he is in a horizontal position .this way he covers more surface area and pressure exerted is reduced)
So, a buffalo floats on the river but not the man.