Name: ___________________________________ Date: __________ Period #: ______
The Earth’s Water
The Water Cycle: Water moves constantly between the atmosphere and the Earth’s
surface in the water cycle. The water cycle is a group of processes that move the Earth’s
water between different phases (solid, liquid, and gas) and between reservoirs. A
reservoir is, in this case, anything that can hold water. Lakes, oceans, and clouds are all
reservoirs of water.
Processes of the Water Cycle:
 Evaporation is the process in which liquid water changes into a gas. Energy from
the sun causes water on the Earth’s surface to evaporate.
 Transpiration is the process in which water moves from a plant’s body to the
atmosphere. The sun’s energy causes water to evaporate of plant leaves.
 Condensation is the process in which water vapor (or gaseous water) changes into
a liquid. Clouds form when water vapor in the atmosphere condenses into water
droplets.
 Precipitation is the release of water to the Earth’s surface in the form of rain,
freezing rain, sleet, snow, or hail.
 Runoff is water that moves from land into rivers and streams.
 Infiltration and percolation occur as water sinks into, and moves downward
through permeable soil and collects in rocks. This water is referred to as groundwater.
Watershed Systems
A watershed is an area of land that drains rainfall and snowmelt into a particular body
of water. The terrain of the land around the body of water helps define the watershed. A
watershed can include many creeks, streams, and rivers. In Pennsylvania, many creeks
form as runoff moves down the Appalachian Mountains. Creeks and streams can
combine to form rivers. Some rivers eventually flow into lakes. Most rivers eventually
flow into bays or oceans. When this happens, the freshwater from the rivers mixes with
the salt water of the bay or ocean. A large part of Pennsylvania lies within the
Chesapeake Bay watershed. The Chesapeake Bay watershed covers about 64,000 square
miles of land.
Pollution anywhere in a watershed can lead to pollution in a larger body of water.
When runoff (from precipitation) moves over the land, it washes soil, fertilizers, litter,
toxic chemicals, and other pollutants into creeks and streams.
Agricultural runoff from farms in Pennsylvania can be carried by a system of creeks
and rivers into the Chesapeake Bay. These nutrients can cause an overgrowth of algae in
larger bodies of water. An overgrowth of algae in a body of water is called an algal
bloom.
Freshwater Bodies
Most bodies of water in a watershed are freshwater bodies. Freshwater bodies include
creeks, streams, rivers, and lakes containing little or no salt. Lakes are examples of lentic
systems, because the water remains almost still. Creeks, streams, and rivers are
examples of lotic systems because the water moves over the land from areas of higher
elevation to areas of lower elevation.
Stream ecosystems can have varying levels of biodiversity depending on the
conditions in a particular stream. Biodiversity refers to the variety of organisms that live
in an area. Changes in the flow rate, temperature, and water quality (pollutants) of a
stream can diminish the biodiversity. Pollutants can choke or poison organisms.
Many wetlands are freshwater bodies. Wetlands are areas of land that are under
water for all or part of the year. Wetlands include marshes, swamps, and bogs.
Wetlands filter pollutants, excess nutrients, and sediments out of the water that flows
through them, so wetlands improve water quality. Wetlands also soak up excess water,
so they can help prevent flooding.
Freshwater bodies are important natural resources. People need water for drinking,
cooking, bathing, and irrigating crops. Rivers are also used to transport products,
produce electricity, and for recreational purposes.
Saltwater Bodies
Most of the water on the Earth is found in oceans and contains dissolved salts such
as sodium chloride. Saltwater can be found in wetlands that form along ocean coasts.
Salt marshes are areas of land that are flooded during high tide. Bays and estuaries are
bodies of water that connect to oceans. Salt water and fresh water have different
properties. The salinity of salt water is higher than freshwater. Salinity is the dissolved
salt content in a body of water, usually measured in parts per million (ppm).
PROPERTY DIFFERENCES BETWEEN FRESH AND SALT WATER
DENSITY
FREEZING POINT
CONDUCTIVITY
NATURAL RESOURCES
IMPORTANCE TO
WATER CYCLE
USED FOR DRINKING
OR WATERING CROPS
Salt water is denser than fresh water because of all of the dissolved
salts it contains. Because salt water is denser than fresh water,
objects float more easily in salt water than fresh water.
The dissolved salts in salt water cause the freezing point of salt
water to be lower than the freezing point of fresh water.
When sodium chloride dissolves in water, it forms charged particles
called ions. The ions in salt water make it a good conductor of
electricity.
People get seafood from the oceans and use oceans for transportation
and recreation.
When seawater evaporates, it leaves behind salts and other particles
dissolved in it. Through evaporation, fresh water returns to the
atmosphere where it can form clouds and fall back to the Earth as
precipitation.
Unlike fresh water, salt water can not be used to drink or to water
crops. Because most of the water on Earth is not drinkable, people
must keep fresh water clean and use it wisely.