Density currents are caused when two fluids of different densities meet. They do not to mix at once. As a rule, the denser liquid will sink, and flow along the bottom of the less dense fluid. The denser fluid forms a density current.
Density is the amount of material in a given space. The more mass a given amount of something has, which on the surface of the earth usually means the heavier it is, the denser it is.
A square foot of wood is denser than a square foot of feathers, for example. Fluids can be of different densities too. Oil, for example, is less dense than vinegar, so if salad dressing separates, the oil floats on top of the vinegar. Many industrial processes, some desserts and some cocktails take advantage of the natural tendency of fluids of different densities to arrange themselves with the densest fluid on the bottom and the least dense on the top.
Two streams of water can also have different densities, depending on their temperature. In warm water, the molecules are father apart because warm water expands, so it is less dense than cold water. Therefore cold water may form a density current along the bottom of a warm lake or stream. Salt water is more dense than fresh, because the salt and other elements it contains add to its density. Muddy water is often more dense than pure salt water, though, because it contains more and heavier added material. Therefore muddy streams cause turbidity currents where they flow into the ocean.
Density current flow
Gravity keeps a density current in motion. On a slope, a density current will continue to flow, as it gradually mixes with the water above it. A density current caused by an inflow of mud may flow across a lake, and then be reflected back by the other side. The current will gradually slow as it flows, and eventually may slow enough that it drops its sediment, as the flow comes to a stop.
Turbidity makes water dark or hazy with visible suspended solids that cloud the water the way smog or smoke clouds the air. Turbidity currents are often visible as sunken layers or even sunken rivers in clear water. They may be caused by outflows of polluted water, slumping banks, outflows of rivers, or sediment shaken loose by earthquakes.
Scientists observe them in submarine canyons, at the edges of the continental shelf or where there is seismic instability underwater. On slopes, turbidity currents can reach speeds up to half the speed of sound, and bring extreme changes to the ocean floor.
Gravity sets density currents in motion, but friction slows them down. Density currents eventually lose their force, drop their sediment if they are carrying any, and blend with the surrounding water