Erosion
► The
removal of rock material (weathered
rock, boulders, gravel, sand, silt and clay)
► Erosion suggests transport, but transport is
not necessarily erosion.
Transport
► Weathered
rock and sediments and
dissolved minerals are moved from the
source area to the depositional area.
► Transport agents
 Running Water
 Wind
 Ice
Deposition
► Deposition
occurs when the energy of the
sediment transport agent decreases.
► Deposition occurs when the sediment can
no longer be moved.
Lithification
•
•
Lithification involves compaction and
cementation that convert sediment into
sedimentary rock.
Silica and calcium carbonate are the most
common chemical cements, but iron oxide
and iron hydroxide cements are important in
some rocks.
Rivers
► Rills
to Rivers (channel flow)
► Sources:
 sheet flow from rain and snow
 groundwater
 soil moisture
► Topographic
High to Low (eventually to sea
level—base level)
► Drainage Basin (all sheet flow and channel
flow ends up in one major channel flow)
Water Cycle
Fig. 12-3, p. 274
Rivers
► Rivers
erode vertically and laterally
► Rivers transport sediments and dissolved
minerals
► Rivers deposit sediments
Hoover Dam and
Lake Mead
Erosion or
deposition?
Figure 1a, p. 273
Note v-shaped valley
shape
Note steep
gradient (much
downcutting,
cannot meander)
Erosion or
deposition?
Fig. 12-1, p. 270
What happens when
Niagara slips?
Horseshoe
Falls
http://www.niag
araparks.com/
nfgg/geology.p
hp
http://www.lockp
ortny.com/Museum
/rocks.htm
Niagara Falls!
Fig. 12-CO, p. 268
How do rivers erode?
► Cutting
power of rivers is recognized over
time and during periods of excessive flow
(e.g., flood stage)
► Steeper gradients increase downcutting
► Shallow gradients decrease downcutting but
increase lateral flow.
head
Stream Gradient
Rapid stream
velocity, bed load is
coarse, erosion is
downward.
Slow stream
velocity, bed load is
fine, erosion is
lateral, stream
meanders
mouth
Fig. 12-4, p. 274
Fig. 12-5b, p. 275
Stream Features
►
Load
 Sediment load (boulders, gravel, sand), also called bed load
 Suspended load (silt and clay) high energy events can carry sand
and gravel
 Dissolved load (dissolved minerals)
Primary and Secondary Channel
► Flood Plain
► Cut Bank vs. Point Bar
► Meanders and Ox-Bow Lakes
► Delta
► Alluvial Fan (intermittent stream)
►
Stream Load
http://www.indiana.edu/~g103/G103/week6/wk6.html
Braided
Stream, New
Zealand
Can be caused by
additional influx of
sediments or a
change in
topography
causing a lowering
of the stream
gradient
Fig. 12-7a, p. 278
Braided stream, Chester, California
Fig. 12-7a, p. 278
Primary Channel
Secondary
Channel
Primary and
Secondary
Channels of a
Stream
During a flood, the
secondary channel begins
to fill
Fig. 12-11, p. 281
Fig. 12-12a, p. 281
Fig. 12-12b, p. 281
Fig. 12-11b, p. 281
Fig. 12-11c, p. 281
Fig. 12-11d, p. 281
Fig. 12-9, p. 279
Fig. 12-9a, p. 279
Fig. 12-9b, p. 279
Cut Bank
Sediment load added by erosion at the river bank
Fig. 12-6a, p. 277
Fig. 12-9c, p. 279
Cut Bank
Point Bar
Crossbedding
Fig. 12-11a, p. 281
Meandering Stream, low
stream gradient, Laxa
River, Iceland
Note
preliminary
formation of an
ox-bow lake
Fig. 12-8, p. 278
Fig. 12-10, p. 280
Fig. 12-10a, p. 280
Fig. 12-10b, p. 280
Fig. 12-10c, p. 280
Fig. 12-10d, p. 280
Incised meanders—area must have undergone some uplift Fig. 12-23, p. 291
Delta
Velocity of the stream is reduced at the
mouth of the river.
The sediment and suspended load begins to
settle out
Sorting: coarser grains nearer the mouth,
finer grains further away
Fig. 12-13a, p. 282
Topset Beds
Foreset Beds
Bottomset Beds
Delta Deposition Pattern
Fig. 12-13b, p. 282
Mississippi
River
Delta—
birds foot
delta
Fig. 12-13c, p. 282
Alluvial Fans (much coarsergrained than deltas)
Form when heavy rainstorms
occur in sparsely-vegetated
semi-arid regions.
Fig. 12-14, p. 283
Fig. 12-14a, p. 283
Fig. 12-14b, p. 283
Idealized Stages in the
Development of a
Stream.
Fig. 12-21, p. 290
Younger, steeper gradient
Downcutting erosion
Fig. 12-21a, p. 290
Intermediate gradient
Fig. 12-21b, p. 290
Low gradient,
mature stream
approaching base
level
Fig. 12-21c, p. 290
Eventually, downcutting will lead to
the formation of terraces (which are
erosional remnants of floodplains.
Fig. 12-22, p. 291
Fig. 12-22a, p. 291
Fig. 12-22b, p. 291
Fig. 12-22c, p. 291
Fig. 12-22d, p. 291
Drainage Basin
Fig. 12-15, p. 285
Divide
Small scale drainage basins follow topography
Fig. 12-15a, p. 285
Wabash River
Drainage Basin
Fig. 12-15b, p. 285
Stream Drainage Patterns
Fig. 12-16, p. 286
Dendritic
Fig. 12-16a, p. 286
Rectangular—
controlled by
solution joints
Fig. 12-16b, p. 286
Trellis
Fig. 12-16c, p. 286
Radial
Fig. 12-16d, p. 286
Deranged—swampy
regions
Fig. 12-16e, p. 286
Fig. 12-17, p. 288
Fig. 12-17a, p. 288
Fig. 12-17b, p. 288
Fig. 12-17c, p. 288
Fig. 12-18, p. 289
Fig. 12-18a, p. 289
Fig. 12-18b, p. 289
Fig. 12-19, p. 289
Fig. 12-19a, p. 289
Fig. 12-19b, p. 289
Headward erosion in a valley and stream piracy.
Arrgh!
Fig. 12-20, p. 289
Fig. 12-20a, p. 289
Fig. 12-20b, p. 289
CHAPTER OUTLINE
• Introduction
• The Hydrologic Cycle
• Running Water
• GEO-FOCUS 12.1: Dams, Reservoirs, and Hydroelectric Power
• How Running Water Erodes and Transports Sediment
• Deposition by Running Water
• Drainage Basins and Drainage Patterns
• CULTURAL CONNECTIONS: Flood Stories from Around the World
• Base Level
• Graded Streams
• Valley Evolution
• Geo-Recap
CHAPTER OBJECTIVES
1 Running water, one part of the hydrologic cycle, does considerable geologic
work. Water is continuously cycled from the oceans to land and back to the
oceans.
2 Gradient measures how steep a stream is. Discharge measures the volume of
water that passes a given point per unit of time. Discharge, along with velocity,
usually increases downstream.
3 Running water transports large quantities of sediment and deposits sediment
in or adjacent to braided and meandering rivers.
4 Flooding is a natural part of stream activity that takes place when a channel
receives more water than it can handle.
5 Alluvial fans (on land) and deltas (in a standing body of water) are deposited
when a stream’s capacity to transport sediment decreases.
6 Rivers and streams continuously adjust to changes. Base level is the elevation
below which a stream cannot erode. Waterfalls and lakes are temporary base
levels, and the sea is ultimate base level.
CHAPTER OBJECTIVES
7 The concept of a graded stream is an ideal, although many rivers and streams
approach the graded condition.
8 Most valleys form and change in response to erosion by running water
coupled with other geologic processes such as mass wasting.
Available Fresh Water
Fig. 12-2, p. 270
Fig. 12-5, p. 275
Figure 1b, p. 273
Fig. 12-5a, p. 275
CHAPTER SUMMARY
• Water continuously evaporates from the oceans, rises as water vapor,
condenses, and falls as precipitation. About 20% of all precipitation falls on land
and eventually returns to the oceans, mostly by surface runoff.
• A channel’s gradient varies from steep in its upper reaches to gentle in its lower
reaches.
• Flow velocity and discharge are related, so a change in one results in a change
in the other. Velocity and discharge increase downstream in most rivers and
streams.
• Running water erodes by hydraulic action, abrasion, and dissolution of soluble
rocks.
• The larger particles transported by running water move as bed load, whereas
the smallest particles move as suspended load. Rivers and streams also
transport a dissolved load of materials in solution.
• Braided streams have complex, multiple, intertwining channels. Their deposits
consist mostly of sand and gravel.
CHAPTER SUMMARY
• Meandering streams have a single, sinuous channel in which point bars of
sand or gravel are deposited. Cutoff meanders known as oxbow lakes
eventually fill with fine-grained sediments and organic matter.
• Floodplain deposits might consist of a succession of point bars deposited by
a migrating channel, or mud deposited by water carried into the floodplain
during floods.
• Deltas form where a river or stream enters a standing body of water and
deposits its sediment load. Small deltas in lakes commonly have a three-part
division of bottomset, foreset, and topset beds, but marine deltas are larger,
more complex, and more important economically.
• In arid and semiarid regions where a river or stream flows from a mountain
canyon onto adjacent lowlands, a deposit known as an alluvial fan
accumulates. Alluvial fans consist of stream-deposited sand and gravel
and/or mudflow deposits.
• Rivers and streams along with their tributaries carry runoff from areas known
as drainage basins, which are separated by divides.
CHAPTER SUMMARY
• Sea level is ultimate base level, the lowest level to which rivers and streams
can erode. Local or temporary base levels are lakes, other rivers or streams,
or particularly resistant rocks.
• Rivers and streams tend to eliminate irregularities in their channels so that
they develop a smooth, concave profile of equilibrium. These so-called
graded streams approach this ideal condition only temporarily.
• Valleys develop and evolve by several processes, including downcutting,
lateral erosion, headward erosion, stream piracy, and mass wasting.
• The formation of a floodplain, followed by renewed downcutting by a stream,
leaves remnants of the older floodplain at higher levels known as stream
terraces.
Mass wasting
Sediment added to stream by mechanical weathering and
erosion
Fig. 12-6b, p. 277
Pothole formed by cutting power of falling
water and abrasion from rock inside the
pothole.
Fig. 12-6c, p. 277
Note roundness of the boulders inside the
pothole.
Fig. 12-6d, p. 277
Stream Deposits
► Levee
► Flood
plain deposits
► Braided streams (sediment load is greater than
velocity can move)
► Deltas (velocity decreases as stream enters larger
body of water-not channel flow, allowing for
deposition)
► Alluvial fans (“semiarid delta”)
Stream Gradient
► Slope
of the channel as it flows downhill.
► Steeper gradient in upper reaches, near
headwaters,
► Stream gradient gets progressively smaller
towards base level.
► Steep gradient streams are generally
straight and cut downward.
► Shallow gradient streams meander and
generally cut sideways (laterally).