CHAPTER 14 - STREAMS AND FLOODS
Overview
This is a long chapter that introduces many significant concepts on these important geological
processes supported by 46 figures and three boxes. It covers the mechanics of stream flow
erosional and depositional features of streams, and associated landforms and flooding.
Running water is the most important geologic agent on earth for erosion, transportation and
deposition of sediment, and landscape development. According to the hydrologic cycle, 15% to
20% of rainfall typically becomes surface runoff, usually forming streams, but also occurs as
sheetwash where conditions are favorable (i.e. deserts). All streams remove water from a
drainage basin separated from other basins by divides. Dendritic, radial, rectangular or trellis
drainage patterns develop depending on the nature of underlying bedrock and its structure.
Stream erosion and deposition are affected by stream velocity and to a lesser extent
discharge. Velocity is controlled by gradient, channel shape, and channel roughness. Discharge
increases downstream with the addition of water and causes streams to widen and deepen.
Stream erosion is produced by hydraulic action, solution and abrasion, which causes potholes.
Transportation occurs as bed load (movement by traction or saltation), but most material is
moved as suspended load, and dissolved load.
Deposition occurs when velocity drops (Figure 14.7). Bed load may be deposited on bars, with
associated placer deposits. Braided streams, with multiple channels, form when sediment load
(particularly bed load) is high. Meandering streams develop point bars inside meander loops,
and meandering may eventually form meander cutoffs and oxbow lakes. Flood plains are sites
of coarse and fine sediment deposition where flood waters are accommodated and natural
levees develop. Deltas form where streams enter standing bodies of water. Deltas may be
wave-dominated, tide-dominated, or river-dominated (birdfoot) depending on energy
conditions at the shoreline. Freshwater deltas, in particular, have a predictable pattern of
deposition of bottomset, foreset, and topset beds. Alluvial fans can be built where stream
velocity drops in continental, typically desert, settings.
Flooding is a common, periodic result of heavy rains and rapid snow melt. Recurrence intervals
describe the probability of a major flood, particularly 100-year floods, occurring in any given
year. High velocity and discharge cause erosion, high water destroys property, and flood
deposited silt and clay coat lawns, dwellings and machinery. Urbanization promotes flooding.
Flash floods are produced by large volumes of water over short intervals of time as occurred in
1976 on the Big Thompson River, and in 1997 on the Cache la Poudre River, both in northcentral Colorado. Flood-control structures (dams, artifical levees, protective walls, bypasses)
partially reduce the dangers of flooding, but the Great Flood of the Missouri and Mississippi
rivers in 1993 exceeded predicted 100-year discharges, and even the calculated 500 year
crest at Hannibal, Missouri.
Downcutting by streams forms valleys and is limited by base level (ultimate base level is sea
level). Ungraded streams concentrate downcutting to remove irregularities in their gradients.
Graded streams delicately balance transportation and sediment load with downcutting and
valley widening by lateral erosion. Valleys are lengthened by headward erosion and stream
piracy may occur. Stream terraces and incised meanders record changes from deposition to
erosion, and may reflect uplift, climatic change or drops in sea level. Uplift allows superposed
streams to erode into buried mountain ranges, such as the Appalachians.
Learning Objectives
1. Running water (aided by mass wasting) is the most important geologic
agent for erosion, transportation, and deposition of sediment and landscape
development on earth.
2. The longitudinal profile of a stream changes from steep to gentle as the
stream flows from its headwaters (where valleys are V-shaped) to its mouth
(where channels are surrounded by a flat flood plain). Stream channels usually
contain the stream, but unchanneled sheetwash can occur, commonly in
deserts.
3. Streams drain drainage basins separated from each other by divides.
Drainage patterns reflect rock type and structure. Dendritic drainage patterns
form on horizontal, unfractured bedrock. Radial patterns form on high conical
mountains. Rectangular patterns form on fractured or jointed bedrock. Trellis
patterns form in areas of tilted bedrock of varying resistance to erosion.
4. Stream erosion and deposition are controlled by velocity and discharge.
Velocity is the distance water travels per unit of time. Maximum stream
velocity is near the middle of the channel and is displaced to the outside of its
curves (Fig. 14.6). Figure 14.7 ( = Hjulstrom's Diagram but not labeled as
such) illustrates that as velocity increases (for example during a flood),
erosion and transportation of larger grain sizes is accomplished. The point is
also made that more velocity is required to erode silt and clay than sand.
Gradient (high vs low), channel shape (narrow vs wide), channel roughness
(smooth vs rough), and discharge (increased volume of water) influence
stream velocity.
5. Stream erosion involves hydraulic action (ability to pick up and move
sediments), solution, and abrasion (grinding of stream bed by coarse sediment
load, resulting in potholes).
6. Stream transportation of sand and gravel is accomplished as bed load
(movement by traction that maintains contact with the stream bed or saltation
that involves bouncing along stream bed). Silt and clay are transported in
suspension. Dissolved load comprises soluble ions. Suspension and solution
comprise the bulk of a stream's load.
7. Stream deposition is caused by a drop in discharge or velocity. Gravel bars
are formed in streams (usually braided streams) with high discharge and bed
load, and may contain placer deposits. Meandering in the lower reaches of a
stream produces point bars in the inside of meander loops and eroded banks
on the outside of meander loops. Flood plains are formed by a combination of
point-bar deposits, fine-grained flood deposits, and channel-fill deposits.
Natural levees form as water leaves the channel and spreads over the
floodplain during flooding, depositing coarse sediment next to the channel.
8. Deltas and alluvial fans reflect a drop in velocity as a stream enters a body
of water or encounters lower slopes at the base of mountains respectively.
Sediment supply, waves, and tides control the shape of a delta. Bottomset,
foreset and topset beds characterize deltas in freshwater lakes. Alluvial fans
usually exhibit grading, with coarsest material deposited closest to the
mountain front.
9. Flooding is a natural process caused by heavy rains and snow melt.
Recurrence intervals predict the average time separating flood events,
particularly 100-year floods. Erosion, high water levels and flood deposits are
the undesirable effects of flood events. Urbanization enhances flooding by
creating paved areas, storm sewers, and channel constrictions (bridges, docks,
buildings). Flash floods are short lived events often caused by thunderstorms.
Two catastrophic flash floods struck north-central Colorado in 1976 (Big
Thompson River) and 1997 (Spring Creek, Cache la Poudre River). Flooding
may be partially controlled by dams, artificial levees, protective walls, and
bypasses, but prohibiting building within 100-year flood plains should be
encouraged.
10. The Great Flood of 1993 exceeded 100-year discharges for many rivers in
the midwest and even the predicted 500-year flood level at Hannibal, Missouri.
11. Erosional downcutting forms stream valleys and is limited by base level,
either sea level or a local base level, such as a pond or lake. Glaciation may
lower base level promoting downcutting in stream valleys, or raise base level
promoting deposition.
12. Ungraded streams use downcutting to smooth their gradients. Graded
streams exhibit a balance between transporting capacity and sediment load
available. This balance is maintained by downcutting and deposition to fill
irregularities in the long profile. Graded streams may deepen their channels
and widen their valleys by downcutting and lateral erosion. Headward erosion
allows a stream to lengthen its valley, and may cause stream piracy.
13. Stream terraces are cut in either rock or sediment. They reflect a change
from deposition to erosion caused by either regional uplift (which lowers base
level and promotes downcutting), or change from dry to wet climate (which
increases the erosional capability of the stream).
14. Incised meanders have no flood plain, as typically found in meandering
streams, and reflect either lowered base level or simultaneous downcutting
and lateral erosion of graded streams.
15. Superposed streams occur when uplift allows a stream to erode through
sediment burying mountain ranges (e.g. folded Appalachians).
Boxes
14.1 ENVIRONMENTAL GEOLOGY - A CONTROLLED FLOOD IN THE GRAND CANYON: A
BOLD EXPERIMENT TO RESTORE SEDIMENT MOVEMENT IN THE COLORADO RIVER
A controlled flood (230 m3/sec to 1,270 m3/sec) of the Colorado River below
Glen Canyon Dam was conducted for six days beginning March 26, 1996. The
goals were to see whether flooding would scour the river bed and redeposit
sediment on bars and beaches along the channel, and to observe how rocks
move along the river bed during flooding. Glen Canyon dam eliminated
flooding and cut off most of the sediment supply to the Colorado River (limited
now to the Paria and Little Colorado tributaries downstream from the dam).
Scouring and deposition did occur as predicted during the first three days of
flooding, but erosion and redeposition occurred as flow was cut back. Boulder
movement was monitored by radio tags inserted into drilled boulders. Surface
velocities were established by kayaking, floating balls, and dye release. The
first crest of the flood actually arrived before the dye because it had been
pushed ahead by the flood. The experiment was a success: beaches can be
restored and boulders can be moved out of rapids. Other dammed rivers could
benefit from controlled floods.
14.2 – IN GREATER DEPTH – TOO MUCH RAIN, TOO MUCH SNOWMELT
Canada’s most devastating flooding event was caused by Hurricane Hazel in
1954 when rainfall induced flooding of creeks and rivers in the Toronto region
caused the drowning deaths of 81 people. In 1996, the Saguenay region of
eastern Quebec received more than 29 cm of rain in less than 36 hours,
causing extensive flooding of rivers and damage to roads, bridges, dams and
industrial facilities. Flooding can also be caused by rapid spring snowmelt and
the Red River in Manitoba is particularly prone to such events. Repeated
severe flooding along the Red River between 1948 and 1950 prompted the
construction of a $63 million floodway to divert the river around the city of
Winnipeg. The effectiveness of this floodway was seriously tested in the
spring of 1997 when melting of a record-breaking winter snow pack and snow
and freezing rain produced by a major storm caused the Red River to flood
once more. The flood protection measures significantly reduced the impact of
flooding in urban areas of Winnipeg.
14.3 IN GREATER DEPTH - EXPLAINING THE SIZE AND FREQUENCY OF FLOODS
Since people have encroached on flood plains, it has become important to be
able to predict the size and frequency of flood events. Data provided by
monitoring of stream water levels and discharge by Environment Canada can
be used to calculate a flood recurrence interval (return period) by dividing the
number of years of record for the river plus one by the rank of the particular
flood event. The Red River in Manitoba is used as an example. A table provides
data on peak discharges that can be used to calculate recurrence intervals.
The largest recorded discharge (4,600 m3/sec) occurred in 1997 and has a
recurrence interval of 109 years. The second largest discharge of 3,060 m3/sec
occurred in 1950 and has a recurrence interval of 54.5 years. Although flood
frequency curves are useful in predicting the discharge and frequency of most
small flood events, there is some uncertainty in predicting large floods as
these are rare events and available data are few.
Short Discussion/Essay
1. Why does it take as much velocity to erode gravel as it does to erode clay?
2. How can grain size decrease downstream, if velocity and discharge increase
downstream?
3. Which comes first in a stream: meandering or flood plain development, and
why is this the case?
4. What are the differences between wave-, tidal, and river-dominated deltas?
5. In terms of stream dynamics, what is actually happening during a flood
event?
Longer Discussion/Essay
1. Explain why dendritic drainage patterns are typical of the North American
midcontinent, while trellis, rectangular, and radial drainage patterns are
common in the mountainous portions of North America.
2. Expalin why base level is related to sea level for some streams but not for
others.
3. Why are streams and standing bodies of water muddy for many days after a
rain or flood event?
4. Colorado means "red" in Spanish, yet the river water in the Grand Canyon is
cold and clear. Why is this the case? The Colorado River and Grand Canyon are
the classic metaphor for the effects of erosion over time, but is this actually
the current situation?
5. Why do alluvial fans typically form in desert areas?
Selected Readings
Bull, W.B. 1991. Geomorphic Response to Climate Change. New York: Oxford
University Press.
Draper, D., 1998. Our Environment – A Canadian Perspective
Easterbrook, D.J. 1992. Surface Processes and Landforms. New York:
Macmillan Publishing Company.
Field, J.J. and Pearthree, P.A. 1997."Geomorphic flood-hazard assessment of
alluvial fans and piedmonts," Journal of Geoscience Education 45(1): 27-37.
Leopold, L.B. 1994. A View of the River. Cambridge, MA: Harvard University
Press.
Mairson, A. 1994."The great flood of '93," National Geographic 185(1): 44-87.
Orndroff, R.L. and Stamm, J.F. 1997."A laboratory exercise introducing the
concept of effective discharge in fluvial geomorphology," Journal of Geoscience
Education 45 (4): 326-330.