Surface Water
Fresh water on Earth may be moving, as in
streams and rivers, or still, as in pond and lakes.
All fresh water, however, comes from precipitation.
The many small streams that come together at the
source of a river are called its headwaters.
Streams and smaller rivers that feed into a main
river are called tributaries. Tributaries flow
downward toward the main river, pulled by the force
of gravity. A river and all of its tributaries
together make up a river system. The land area that supplies water to a river system is
called a watershed. Another name for a watershed is a drainage basin. One watershed
is separated from another by a ridge, or continuous line of high land, called a divide.
Streams on each side of the divide flow in different directions.
Ponds and lakes are bodies of fresh water. Unlike the moving water of streams and
rivers, ponds and lakes contain still, or standing water. Ponds and lakes form when water
collects in hollows and low-lying areas of land. People can also create a lake by building a
dam across a river. The lake may be used for supplying drinking water, for irrigating fields,
or for recreation. A lake that stores water for human use is called a reservoir.
Ponds contain many different habitats for the organisms that live there from its
muddy bottom to the tall grasses that grow along its edges. The main producer in a pond
is algae which also produces oxygen for the organisms that live in the water. Because the
water is shallow enough for sunlight to reach the bottom, plants grow throughout the pond.
Lakes, on the other hand can be so deep in the center that no light can reach there and no
plant life can grow. Lakes, like ponds, provide homes for many kinds of fish, insects, and
other wildlife.
Lakes experience changes. When the temperature drops during the change of
seasons, the more dense cold layer on the surface sinks to the bottom causing waters to
mix. This mixing, also called turnover, causes materials to rise from the lake bottom.
Turnover occurs twice each year as the seasons change. Lake turnover refreshes the
supply of nutrients throughout the lake. Nutrients are substances such as nitrogen and
phosphorus that enable plants and algae to grow. This is common in cool, northern areas
of North America.
The second type of change that may occur in a lake happens over a period of time
caused by eutrophication sometimes leading to the death of a pond or a lake.
Eutrophication is the process by which a body of water acquires a high concentration of
nutrients, especially phosphates and nitrates. This can cause an algal bloom (over growth of
algae). As the algae die and are decomposed by bacteria, dissolved oxygen in the water is
depleted, causing the death of other organisms, such as fish. Sediment from the decaying matter
builds up. As the water becomes shallower aided by evaporation, the lake becomes a soggy
marsh and eventually a filled in meadow. Eutrophication is a natural, slow-aging process for a
water body, but human activity greatly speeds up the process. Many times these excess
nutrients are a result of human activities, particularly, when fertilizers from farms or lawns
are washed into the lakes by runoff or when phosphates from laundry detergents are
allowed in wastewater.
Name_________________________________________ Date: ________
Surface Water
1. Into what two categories can fresh water be divided? ________________ and
_________________
Match the term with its definition.
_____ 2. tributary
a. A river and all its tributaries together
_____ 3. river system
b. The land area that supplies water to a river system
_____ 4. watershed
c. A smaller steam or river that feeds into a main river
_____ 5. divide
d. The ridge that separates one watershed from another
6. What is another name for a watershed? ____________________________
7. The many small streams that come together at the source of a river are ______________________.
8. Label the diagram comparing lakes and ponds.
____________________
Smaller
and
shallower
_____________________
Standing
Water
Larger
and
deeper
9. When do ponds and lakes form? _____________________________________________________
__________________________________________________________________________________
10.True or False? The main food producer in ponds is algae.
11. Explain why plants grow throughout a pond and not always throughout a lake.
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
12. A lake that stores water for human use is called a(n) ___________________________________
13. True or False? Compared with the center of a pond, the center of a lake has more organisms.
14. What occurs during lake turnover? ____________________________________________________
___________________________________________________________________________________
___________________________________________________________________________________
15. True or False? Lake turnover occurs only once a year.
16. True or False? Lake turnover happens because the density of layers of the water in it changes.
17. What is eutrophication?
________________________________________________________________________________
18. What is an overgrowth of algae called? _____________________________________________
19. Why can eutrophication lead to the death of a pond or lake?
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
20. What nutrients increase the growth of plant life in a lake that may cause eutrophication?
______________________________ and _______________________
21. What human activities contribute to eutrophication?
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________