Natural and man-made water bodies are generally density-stratified. The variations in density can be due to temperature, salinity, and particulate matter in suspension. The thermal stratification of lakes and reservoirs is an example (e.g. Harleman 1982). In the presence of the earth’s gravity, the fluid of non-uniform density would move towards a state of stable stratification—the heavier fluid tends to stay at the bottom while fluid of smaller density is generally found at a higher elevation. The stable stratification is disturbed when momentum and buoyancy sources, in the form of jets, plumes, puffs and thermals, are present to produce the vertical motions. The rise of buoyant fluid draws fluid from a lower elevation and discharges it into higher elevation. The fall of dense fluid would produce the opposite effect. Small density differences can have a marked effect on the vertical exchange. A variety of significant mixing processes observed in the atmosphere, rivers and lakes, estuaries and the ocean can be modeled as the interaction of jets/plumes/puffs/thermals (JPPT) with a stratified fluid.