Precipitation that falls on the land either evaporates, flows across the surface,
or soaks into the ground.Water moving across the surface is called
runoff. Water that moves across the land as a small body of running water
is called a stream. A stream drains an area of land known as the stream
drainage basin or watershed. The watershed of one stream is separated
from the watershed of another by a line called a divide. Water that collects
as a small body of standing water is called a pond, and a larger body is
called a lake. A reservoir is a natural pond, a natural lake, or a lake or pond
created by building a dam for water management or control. The water
of streams, ponds, lakes, and reservoirs is collectively called surface water.
Precipitation that soaks into the ground percolates downward until
it reaches a zone of saturation. Water from the saturated zone is called
groundwater. The amount of water that a material will hold depends on
its porosity, and how well the water can move through the material depends
on its permeability. The surface of the zone of saturation is called
the water table.
The ocean is the single, continuous body of salt water on the surface
of Earth. A sea is a smaller part of the ocean with different characteristics.
The dissolved materials in seawater are mostly the ions of six substances,
but sodium ions and chlorine ions are the most abundant. Salinity is a
measure of the mass of salts dissolved in 1,000 g of seawater.
An ocean wave is a moving disturbance that travels across the surface
of the ocean. In its simplest form, a wave has a ridge called a crest and
a depression called a trough. Waves have a characteristic wave height,
wavelength, and wave period. The characteristics of waves made by the wind depend on the wind speed, the time the wind blows, and the fetch.
Regular groups of low-profile, long-wavelength waves are called swell.
When swell approaches a shore, the wave slows and increases in wave
height. This slowing refracts, or bends, the waves so they approach the
shore head-on. When the wave height becomes too steep, the top part
breaks forward, forming breakers in the surf zone. Water accumulates at
the shore from the breakers and returns to the sea as undertow, as longshore
currents, or in rip currents.
Ocean currents are streams of water that move through other seawater
over large distances. Some ocean currents are density currents,
which are caused by differences in water temperature, salinity, or suspended
sediments. Each ocean has a great system of moving water called
a gyre that is centered in mid-latitudes. Different parts of a gyre are given
different names such as the Gulf Stream or the California Current.
The ocean floor is made up of the continental shelf, the continental
slope, and the ocean basin. The ocean basin has two main parts, the abyssal plain and mountain chains called ridges.
