In oceanology, bottom water is near the ocean floor. It has characteristics are markedly distinct from the above layer in terms of oxygen content, salinity, bulk temperature (characteristic temperature), and hence density.
The Antarctic Bottom Water is the source of most bottom water in southern parts of Pacific Ocean, Indian Ocean and Atlantic Ocean, and even in parts of the North Atlantic. Bottom waters flow very slowly, driven mainly by slope topography and differences in temperature and salinity, especially compared to wind-driven surface ocean currents.
Bottom water by an estuary of a river discharging into a saline body exhibits peculiar transport of mud. Due to fresh/saline water intermixing by the estuary, a horizontal isohale gradient is created, with lower salinity levels upstream, which generates the upstream flow of the bottom water. Mud particles carried by river begin settling down as the current and turbulence decrease. When the particles nearly reach the floor, they are carried back to the head of estuary to accumulate at the point where the salinity of the surface and bottom waters become comparable and the bottom flow decreases. This process results is a distinguished pile of mud at this point.
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Bottom water of lakes may feature lower level of oxygen, to the point of completely vanished dissolved oxygen (i.e., becoming anaerobic), and higher levels of chlorinity and organic-induced acidity. In many lakes, especially in the zones of continental climate, summer heating and winter cooling create strong vertical temperature gradients which oppose water intermixing, resulting in the periods of summer and winter thermal lake stratification. They are intervened by bottom water overturning, which happens in autumn (autumn overturn) and in spring (spring overturn) due to equalizing of temperature gradients and the resulting easier intermixing by wind and other sources of turbulence.