Running Water
Running Water
• Includes all water that falls on Earth as
rain or snow, and moves downhill
• Running water weathers the rock and
erosion then follows
• Bedrock is attacked by water in two ways:
– Physically wears down the rock
– Suspended material pounds and breaks up
the rock as it moves over it, called saltation
Running Water
• Abrasion is the grinding action by sand, pebbles
and boulders.
• Rapidly moving water removes loose fragments
(aiding in erosion)
• Chemical action occurs as water dissolves
minerals in the bedrock
Removal of Weathered Rock
Water removes weathered rock in 3 forms:
1. Dissolved minerals in solution
2. Slightly larger particles will float within
the moving water, in suspension (silt,
clay)
3. Bedload are those materials at the
bottom of the water flow, which are too
heavy to float along (sand, gravel,
pebbles, boulders)
Carrying Power and Load
• Carrying power of a stream is indicated by
the total amount of sediment and the size
of particles being moved by it.
A stream’s carrying power depends on:
• Its speed (or velocity)
• Its volume (or discharge rate)
• The time of year
The Life Cycle of a River
Ancient rivers leave evidence of their
existence by a “V” shaped valley and the
presence of rounded rock fragments
There are three basic stages of a river
1. Young (immature) Stage
2. Medium (intermediate) Stage
3. Mature Stage
Young Stage
• River erodes upper
valley walls first (they
are the first exposed
to weathering)
• Young rivers are
characterized by
steep walls (canyons)
in a “V” cross-section.
Middle Stage
• River has reached the
point where it no
longer weathers the
rock below (reaches its
base level)
• Erosion now begins to
work outward on valley
walls, and the river
begins to widen,
becoming shallower
and slower
Mature Stage (Old Age)
• The river begins to
“meander”
• The valley becomes
wide and flat
• It often re-routes itself
and cuts off a
meander to form an
isolated oxbow lake
River Stages
Flood Plains
• Mature rivers may overflow during heavy run-off
• The land flooded beside it is called the flood
plain
• If the river erodes the flood plains at different
rates, a series of steps or terraces may form.
Stream Erosion and Deposition
In a stream with a bend:
• velocity is greatest at
the outer edge of the
stream bend and
greater erosion
occurs here
• velocity is slower at
the inside of the bend,
resulting in reduced
erosion, and in fact
deposition can occur
Lengthening the River Valley
• A single heavy rain can form a miniature stream
valley (V-shaped and have tributaries running
into it)
• When the rain ends the stream may disappear,
but the small valley remains.
• This is called a gully
• Gullies grow in length and width every time it
rains. Headward erosion makes the gully
longer
• If the stream cuts below the water table, it will
become a permanent stream.
A Gully
Drainage Basins and Divides
• High land that separates one gully from
the next is called a divide
• A river and all of its tributaries is called a
river system
• The drainage basin (watershed) of a river
includes all the land that drains into the
river (directly or indirectly)
Four Types of Drainage Basins
Stream Piracy
• Occurs when a
stream erodes by
headward erosion
until it captures
another stream.
• Also called stream
capture
Water and Wind Gaps
• If water meets a
region of resistant
rock that erodes more
slowly than the rock
around it, a narrow
gap (water gap)
occurs
• Once water has dried
up, a dry narrow gap
(now called a wind
gap) remains.
Potholes and Plunge Pools
• Potholes occur when
boulders, pebbles and
sand swirl around and
grind out an oval or
circular basin
• When potholes are very
large, they are called
plunge pools
• This can cause the
recession of a waterfall
due to erosion.
Niagara
Falls
DEPOSITION OF RIVER
SEDIMENTS
Deposition occurs wherever velocity decreases
due to:
• Decreased slope
• Widening of the river
• Meeting with an obstruction (outcrop, curve etc)
• Areas of increased evaporation
• Increased seepage
• Diversion for irrigation or water supply
• Where rivers meet large bodies of water
Deltas
• A delta forms when the river flows into a
quiet body of water (lake, gulf or inland
sea), because the river slows down
• Most of the sediment drops at the river’s
mouth, with larger materials being dropped
first
• Deltas rarely form where a river meets an
ocean because currents/waves are too
strong and carry sediment away
Deltas
Alluvial Fans
• A fan shaped deposit
formed where a stream
ends, usually at the
bottom of a steep hill
where it meets flat land
• Forms on land, not in the
water
• Composed of sands and
gravels
• Surface is sloping and not
flat like a delta
Sediments on Flood Plains
• When a river overflows its banks, it slows down,
dropping suspended materials (sediments)
• More sediment is dropped at the banks, forming
thick deposits which build up, called levees
• Fertility of the flood plain (low areas behind the
levees) is increased due to nutrients and
materials deposited during flooding
Causes of River Flooding
• Flash floods: a large volume of rain over a
short period
• Abnormal period of excessive rain
• Natural dam where the river is obstructed
(ice dam, volcanic flow, landslides)
• Human effects like clear cut logging
Preventing Floods
Flooding can be prevented by:
• Planting trees to help rain absorption
• Dams to control river flow
• Artificial levees (eg. sandbags) to raise
banks
• Spillway channels to take the overflow and
re-route it