Unique properties of fresh water make the bubbling system highly- 

 suitable for lakes and to a somewhat lesser extent for brackish estu- 

 aries. The system is less effective in salt water, because maximum 

 density of water with salinity in excess of 2U.7 0/00 is attained at 

 the freezing point. However, factors other than the upward transport 

 of warm water, as previously discussed, also contribute to the main- 

 enance of an ice-free area. 



In regions where upward circulation of sensible heat is not a 

 factor, maintenance of an ice-free area is predominantly dependent 

 upon speed and intensity of the induced currents. 



