The violently turbulent motion at the base of the falls converts the river's directed energy into heat. When the river reaches the sea, its fresh water, being lighter than the sea water, floats on top of the sea, sometimes many miles beyond the river's mouth before it is diluted with sea water. Most of the fresh water on the continents is out of sight, below ground. It exists in the pores between mineral deposits and is fed by precipitation that percolates through the ground under the influence of gravity. The fluid velocity in the underground aquifers is much lower than it is in rivers, the water being impeded by the frictional force of the porous medium through which it flows.
Underground water is often the source of potable water.
Locating underground water and pumping it from the ground for human use is limited by the characteristics of the underground aquifer. Preserving the purity of this water from contamination by toxic fluids buried or dumped on the surface and subsequently traveling down into the aquifer is a major problem worldwide. At the edge of the ocean we see the ocean waves crashing on the shore. The waves carry to the shore energy generated by the wind blowing over the ocean surface. Ocean waves are called gravity waves because the pull of gravity at the air-sea interface is responsible for the propagation of these waves, which do not penetrate far below the ocean surface.