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. Turbidity currents on the ocean floor can form when storms or earthquakes stir up the sediment from the ocean bottom. The sediment mixes with the water near it, making a denser mass of turbid water. If the place where this happens is on a slope, then the turbid water flows down along the bottom into deeper water.