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Runoff and Glaciers
When water returns to Earth as precipitation, there are several things that can happen to the water.
It can evaporate and go back into the atmosphere. It can sink into the ground and become groundwater. It
can be used by plants and animals. It can become frozen and stay on Earth’s surface as glaciers or
icebergs. It can become runoff.
Nearly forty percent of all precipitation becomes runoff. As the water runs off land, it carries
weathered particles with it, eroding the area. Even large, heavy particles may be dragged along. As they
move, the large pieces may break down more, smaller particles to be carried away.
The amount of runoff from any rain depends on different things. First, the amount of rainfall affects the
runoff. During heavy rains there is more runoff than during lighter rains. Areas with high annual
precipitation have more runoff than areas with little annual rain.
The shape of the land also affects the amount of runoff. Steep slopes will have greater runoff than
flat land. The type of surface is also an important factor. Very porous surfaces will have less runoff
because more of the water is absorbed into the surface. Surfaces covered in asphalt or concrete will not
absorb any of the rainfall. All of those surfaces will have one hundred percent runoff.
The amount of plant growth in the area is another factor. Plant roots are able to hold on to the soil. The
roots also absorb some of the water for the plants to use for themselves. Bare soil will erode very quickly.
Finally, temperature affects the amount of runoff. When temperatures are very warm, evaporation
takes care of most of the precipitation so there is little runoff. When temperatures are very cold,
precipitation is trapped in the form of snow or ice, so there is little runoff, at least until it thaws. When
thawing occurs slowly there is less runoff. When thawing occurs rapidly, there is too much water to be
absorbed at once, so runoff may be a big problem!
Glaciers are another means of erosion. Glaciers are large masses of moving ice and snow on land.
They are found in areas where there is a great deal of snowfall that does not melt from winter to winter. As
the glacier gets bigger and heavier, it begins to melt on the bottom because of all the pressure from
above. When it slightly melts, it begins to slide. The dirt and rocks frozen underneath the glacier act as
abrasives. It grinds and scrapes the surfaces that the glacier drags across. As glaciers travel across the
land, they can move boulders the size of houses as well as smaller rocks and sediment. Over time, the
rocks and sediment carried by moving glaciers can carve or deepen valleys.
Glaciers are also important agents of deposition. As the front end of a glacier moves into a warmer
area, the ice begins to melt. The glacier continues to move, but it melts faster than it moves. We say the
glacier is retreating. As the ice melts, it drops its load. Rocks and sediment are deposited, often great
distances from where they originally formed.
Material that is directly deposited by the glacier is called till. Till is a mixture of sizes. There may be
large boulders with small pebbles and sand deposited in one area. If the till is washed away by water and
then deposited farther away, it is called outwash. Outwash is sorted by the water into materials of different
sizes. The water will drop the largest pieces first, and the smallest pieces farther away. These will be
deposited in layers.
Glaciers are responsible for forming several different landscape features. Moraines are ridges of till
deposited by glaciers. Drumlins are small hills shaped like teaspoons. Glaciers may also form lakes. As
deposits pile up in low areas, they may block rivers and trap water. This trapped water will form a lake
covering most of the lowland area. Another type of lake is formed when blocks of ice are left behind as the
glacier moves. The ice blocks become covered with debris and they melt slowly. When melting occurs,
the debris on top of the water sinks into the hole and a kettle lake is formed. Sometimes glaciers will
scrape out enough surface materials so that the water table is exposed. A lake will form from groundwater
in the depression.
Icebergs can also be formed by glaciers. Sometimes as glaciers move, they may reach the sea.
Pieces of ice break off and float away. These are icebergs and rare most commonly found near
Greenland and Antarctica. The icebergs contain rock debris gathered by the glacier. When the iceberg
begins to melt, the debris settles to the ocean floor, many miles from where the rocks were formed.
Runoff and Glaciers Questions
1. Over one-third of all precipitation becomes:
a. glaciers
b. groundwater
c. runoff
d. water vapor in the atmosphere
2. Runoff contributes to erosion by:
a. The water carries weathered particles away.
b. The water deposits sediment in new locations.
c. The water dissolves minerals in rock.
d. The water washes away plants.
3. The amount of rainfall:
a. affects the amount of runoff
b. has no effect on the amount of runoff
c. only affects the amount of runoff if the
temperature is cold
d. only affects the amount of runoff if the
temperature is hot
4. Flat land will generally have:
a. more runoff
b. less runoff
c. the same amount of runoff
d. no affect on the amount of runoff
5. The amount of plants generally have:
a. no effect on the amount of runoff
b. little effect on the amount of runoff
c. some effect on the amount of runoff
d. no effect on the temperature
6. Why does temperature affect the amount of
runoff?
a. because it affects the rate of evaporation
b. it affects the amount of freezing and thawing
c. both a and b are correct
d. it doesn’t affect it
7. What are glaciers?
a. large floating chunks of ice in the sea
b. large masses of moving ice and snow on land
c. large chunks of ice and snow in the Arctic
d. large chunks of ice and snow in Antarctica
8. What is a glacier’s “load”?
a. ice and snow it carries
b. rocks, soil, and sediment it carries
c. the water it contains
d. the land it travels over
9. What is till?
a. material the glacier deposits directly
b. material carried away by water from the
melting glacier
c. a small, teaspoon-shaped hill
d. trees and other plant material moved by
glaciers
10. On land, glaciers can form:
a. lakes
b. moraines
c. drumlins
d. all of the above