Salinity is defined as the mass of salts dissolved in 1.0 kg, or 1,000 g of seawater. This is the same concentration as a 3.5 percent salt solution. An aquifer is a layer of sand, gravel, sandstone, or other highly permeable material beneath the surface that is capable of producing water. In some places an aquifer carries water from a higher elevation, resulting in a pressure on water trapped by impermeable layers at lower elevations. Groundwater that is under such a confining pressure is in an artesian aquifer.
Some artesian wells are under sufficient pressure to produce a fountainlike flow or spring.
The Atlantic Ocean and Pacific Ocean watersheds. Two adjacent watersheds are separated by a line called a divide. Rain that falls on one side of a divide flows into one watershed and rain that falls on the other side flows into the other watershed. A continental divide separates river systems that drain into opposite sides of a continent. The North American continental divide trends northwestward through the Rocky Mountains.