Earth Science
Chapter 8
Section 2
A. Runoff and Erosion:
Moving water is the major agent of the erosion that has shaped
Earth’s land surface.
Runoff is all the remaining water that moves over the Earth’s
surface.
Formations caused by runoff:
1. Rills and Gullies:
Rills – As runoff travels, it forms tiny grooves grow
larger, forming gullies.
Gullies – A large groove, or channel, in the soil that
carries runoff after a rainstorm.
As water flows through gullies, it moves soil and rock
with it, thus enlarging the gullies through erosion. Gullies
flow only after it rains.
2. Streams and Rivers:
Gullies join together to form a larger channel called a
stream.
Streams – A channel along which water is continually
flowing down slope. Unlike gullies, streams rarely dry up.
Small streams are also called creeks or brooks.
Rivers – As streams flow together, they form larger
bodies of flowing water called rivers.
3. Amount of runoff:
The amount of runoff in an area is dependent on five main
factors:
1. The amount of rain an area receives.
2. The amount of vegetation. Grass, shrubs and trees
reduce runoff by absorbing water and holding soil in
place.
3. Soil type. Some types of soil absorb more water than
others.
4. The shape of the land. Steeply sloped land has more
runoff than gently sloped land.
5. How people use the land. Pavement absorbs no
water, creating more runoff, removing vegetation
also creates more runoff and erosion.
B. River Systems:
A stream grows into a larger stream or river by receiving water
from tributaries.
A tributary is a stream that flows into a larger stream.
Example: The Missouri River is a Tributary of the Mississippi
River.
All of the tributaries to the Ohio River form a system that drains
a large part of eastern North America.
A land area from which a river and its tributaries collect their
water is known as a Drainage Basin.
A divide is the high ground between two drainage basins. The
most famous divide is the Continental Divide. The Continental
Divide separates the streams that flow into the Gulf of Mexico
from streams that flow into the Great Basin or the Pacific
Ocean.
C. Erosion by Rivers:
Scientists classify rivers by identifying certain features that
form as a result of erosion.
Through erosion, a river creates valleys, waterfalls, flood
plains, meanders, and oxbow lakes.
1. Valleys- Rivers form on steep mountain slopes. A river is
often fast flowing and follows a straight, narrow path. The
steep slopes along the river erode rapidly. The result is a
deep V-shaped valley.
2. Waterfalls – Waterfalls may occur where a river meets an
area of rock that is very hard and erodes slowly. The
softer rock wears away faster than the harder rock. A
waterfall develops where the softer rock was removed.
3. Flood Plain – The flat, wide area along a river. The river
often covers its flood plain when it overflows.
4. Meander – A loop-like bend in the course of a river. As a
river widens from side to side, it tends to erode the outer
bank and deposit sediment on the inner bank of a bend.
Over time the bend, or meander, becomes more and more
curved.
5. Oxbow lake – A meander that has been cut off from the
river.
D. Deposits by Rivers:
As water moves, it carries sediments with it.
Any time moving water slows down, it drop or deposits some
of the sediments.
Deposition creates landforms such as alluvial fans, and
deltas. It can also add soil to a river’s flood plain.
Alluvial fan – A wide, sloping deposit of sediment formed
where a stream leaves a mountain range. As its name
suggests it is shaped like a fan.
Deltas - Sediment deposited where a river flows into an
ocean or lake builds up a landform called a delta. There are
different shapes of deltas. Arc-shaped, triangle-shaped and
others known as “bird’s foot” deltas, such as the Mississippi
River Delta.
Soil on Flood Plains - Deposition of sediments on flood
plains from retreating waters makes river valleys fertile for
dense forests to grow or for growing crops.
E. Groundwater Erosion and Deposition:
Groundwater- A term used by geologists to identify water that
soaks into the soil and fills cracks in rock.
Groundwater can cause erosion through a process of chemical
weathering.
When groundwater mixes with carbon dioxide it forms a weak
acid called Carbonic Acid.
This acid dissolves the minerals in the rock gradually hollowing
out pockets in the rock.
Over time these pockets develop into caves or caverns.
Deposits from this type of chemical weathering inside the caves
occur when water evaporates and leaves a deposit of calcite.
A deposit that hangs from the roof of a cave is called a
stalactite.
A deposit that builds up from the floor of a cave is called a
stalagmite.
In rainy areas where limestone is near the surface, deep valleys
and caverns are common.
If the roof of a cavern collapses a depression called a sinkhole is
created. This type of landscape is called karst Topography.
Areas of karst topography in the U.S. are Florida, Kentucky and
Indiana.