Cold dense plumes have been associated with coral killoff on tropical shelves as well as with sediment movement from banks to the deep basins adjacent to shallow banks. This paper presents evidence that plumes of dense, salty water generated over shallow banks entrain ambient water rather than descending intact to a density compensation level (as has sometimes been assumed) and that plumes can spread laterally distances of tens of kilometers from their source. In Exuma Sound, dense, salty water appears to be formed year-round by cooling or by evaporation over shallow banks. The water is forced by tidal currents through the channels between islands onto the narrow shelves of Exuma Sound where it likely moves as gravity currents. The gravity currents entrain basin water as they cross the shelf and cascade over the steep shelf edge into Exuma Sound, after which they are advected by ambient currents around the sound. A numerical streamtube model was used to explore early details of plume evolution. The model predicts that the plumes entrain 2.6--5.0 parts ambient water, particularly as they traverse the gently sloping shelf. The resulting loss of density contrast means that the model plumes reach their density level in Exuma Sound at relatively shallow depths, 50--95 m, just below the base of the mixed layer. The final depth of modeled plumes is relatively consistent with observed plume depths, which vary seasonally (~75 m in early winter to ~45 m in summer) depending on the depth and density characteristics of the seasonal mixed layer. Over the sound, observed plume thickness ranges from ~40 m in winter to ~20--30 m in summer. Final salinity values exceed 36.8 practical salinity units; final density ranges from sigma-t ~23.75 to ~24.5. Plume depth, vertical structure, and longevity are shown to depend on upper ocean mixing processes such as convection and wind mixing. ¿ 2000 American Geophysical Union