of it goes directly back into the air to form water vapor and clouds again. However, most
of this solid water sits atop mountains and slowly melts over time to provide a steady flow
of freshwater to streams, rivers, and lakes below. Some of that water enters the Earth’s
groundwater, seeping below the surface through pores in the ground. This water can
formaquifersthat store freshwater for centuries. Alternatively, it may come to the surface
through springs or find its way back to the oceans.
When water falls from the sky is rain it form streams and rivers that flow downward to
oceans and lakes. People use these natural resources as their source of water. They also
create canals, aqueducts, dams, and wells to direct water to living areas to meet their needs
(Figure13.7). Sometimes, our manipulation or pollution of water greatly affects other
species. Many scientists are seeking better ways of using Earth’s water in a sustainable and
efficient way.
Obviously, people are not the only creatures that rely on water. Plants and animals also
depend on this vital resource. Plants play an important role in the water cycle because
they release large amounts of water vapor into the air from their leaves. This process of
transpirationmoves liquid water from plants into the air. You can see transpiration in
action if you cover a few leaves on a plant with a plastic bag. Within a few hours, water
vapor released from the leaves will have condensed onto the surface of the bag.
Figure 13.7: Hoover Dam on the Colorado River. ( 24 )Lesson Summary
- Earth’s surface is mostly water covered. Most of that water is in our oceans, leaving
only 3% freshwater.