The marble rolls quickly to the bottom of the bowl, and then it rolls back and forth, gradually slowing, until it stops. The marble uses up its energy of position by moving from the lip to the bottom of the bowl. It uses up its energy of motion by rubbing against the air and against the bowl, turning its energy of movement into heat by friction. If solid material is mixed into the water, then the density increases. However, if the fresh water has a lot of mud mixed into it, then it may be denser than the sea water.
Cold water is denser than hot water, because when water gets hot it expands.
The same amount of material takes up more space, so it is less dense. A density current in which the denser fluid consists of sediment mixed with water is called a turbidity current, because the water is turbid, or murky. Turbidity currents are among the most common kinds of density currents. Turbidity currents form in lakes in the springtime when heavy rains or melting snow wash mud into rivers. They are also common on the ocean floor.