Deck 13: Streams and Flood Processes

Question 1

Lahars carry volcanic material, making them different from mudflows of clay.

Answer 1

True

Question 2

Which area is LEAST likely to have a flash flood?

A)western Washington state
B)southern Arizona
C)eastern Nevada
D)southern Texas
E)central South Dakota

Answer 2

A

Question 3

Active alluvial fans are always marked by meandering streams.

Answer 3

False

Question 4

A gaining stream is more likely to be found in a wet climate.

Question 5

How do debris flows tend to move?

A)at constant speed
B)fast initially, then slower before stopping
C)slow initially, then faster before stopping
D)in surges or waves, leading with fine sediment particles
E)in surges or waves, leading with coarse boulders

Question 6

A glacial outburst flood typically originates from a lake.

Question 7

If a side stream feeds coarse gravel into a river with a sandy bottom, what does the river do about it?

A)The river flushes it down into the valley.
B)The slope of the riverbed builds up until it is steep enough to transport the size of gravel supplied, or until a flood carries it downstream.
C)The gravel dams the river channel.
D)The gravel works down through sediments in the riverbed until it reaches bedrock.
E)The water slows down because the larger sediment gets in the way.

Question 8

An ice dam is more likely to occur in fall, when temperatures begin to drop.

Question 9

Why are flash floods more common in dry climates than wet climates?

A)The rainfall intensity is greater.
B)Individual storms last longer.
C)The soil is less permeable.
D)The river channels are not as deep.
E)The channel gradients are steeper, so the water flows faster.

Question 10

Why does a stream bottom erode more deeply when its water level rises in a flood?

A)Since its water velocity remains the same, the increased volume of water can carry more sediment.
B)Its water level rises, so water velocity increases and it can carry more sediment.
C)It does not-its stream bottom builds up to match the rise in water level.
D)Its gradient becomes steeper, so it can erode deeper.
E)To accommodate the additional water, its velocity slows, and it digs in deeper.

Question 11

High-gradient bedrock channels generally have shallow, wide cross-sections.

Question 12

Debris flows are most common in mountainous areas such as the U.S. southwest.

Question 13

A hyperconcentrated flow has between 20 and 47 percent sediment by volume. ​

Question 14

How would a hydrograph for a drainage basin change if major urban growth were to occur upstream?

A)The hydrograph would be higher and narrower.
B)The hydrograph would be both higher and longer.
C)The hydrograph would be lower but longer.
D)The hydrograph changes only with wildfire, logging, or overgrazing, not urban growth.
E)The hydrograph becomes skewed, rising more slowly but dropping very quickly.

Question 15

A river slope or gradient adjusts to what three factors that it has no control over?

A)stream width, depth, and amount of water
B)water turbulence, channel roughness, and grain size of sediment
C)water velocity, water temperature, and stream depth
D)water velocity, grain size of sediment, and amount of sediment
E)channel shape, channel cross section, and channel roughness

Question 16

A debris flow can contain less than 40 percent sediment.

Question 17

What does NOT cause an area of rapids on a stream?

A)steepening of the channel slope
B)bedrock in the stream channel
C)the presence of a thalweg
D)higher than normal water in the stream bed
E)greater channel depth and water velocity

Question 18

How does urbanization of the upstream drainage area for a floodplain affect the 100-year flood level?

A)It lowers flood height, since the water runs through faster.
B)It raises flood height.
C)It decreases duration of the peak flood level.
D)The downstream extent of the highest flood level will be less.
E)There is no change; the flood level only depends on the total rainfall upstream.

Question 19

Why are the deposits of streams graded with the largest boulders or pebbles at the bottom?

A)The largest boulders are always at the bottom; they never move.
B)The largest boulders eroded from the stream bank immediately roll to the bottom and stay there.
C)Jostling of boulders, pebbles, and sand in turbulent water moves the boulders to the bottom.
D)As the water slows after a flood, the largest boulders drop out first.
E)Tributaries add additional sediment to the stream.

Question 20

Grain sizes increase downstream as slope decreases in a stream channel.

Question 21

The technique of analyzing physical evidence of past floods (preserved in the geologic record) to reconstruct the approximate magnitude and frequency of major floods is called _______________.

Question 22

The _______________ of a stream, also known as its channel slope, is its steepness of descent from its highest elevation to its lowest.

Question 23

Instruments called _______________ more accurately approximate stream flow by measuring water velocity at hundreds of locations based on the shift in sound frequencies due to moving particles.

Question 24

What are oxbow lakes and how do they form?

A)Oxbow lakes are a stagnant part of a stream that is shaped like an ox's yoke.
B)Oxbow lakes are formed when a stream channel erodes deeply, forming a local lake.
C)Oxbow lakes are cut-off meanders, left behind as a flooding stream takes a shorter path downstream.
D)Oxbow lakes are the name given to plunge pools below wide waterfalls.
E)Oxbow lakes are watering holes for oxen and cattle near rivers, excavated by humans.

Question 25

After a major rainstorm in a humid climate and with no overland flow to a stream, why does the stream level rise almost immediately?

A)Groundwater seeps down to raise the water table and forces older groundwater out into the stream. ​
B)The groundwater moves very fast, through porous ground, to the stream.
C)The stream rise comes from farther up the drainage basin.
D)Actually it doesn't rise rapidly; it merely seems like it does.
E)The groundwater displaces older groundwater in the stream, over the water table.

Question 26

Why do braided streams form in a dry climate instead of meandering streams?

A)The stream flows on a broad, flat surface, so it spreads out and braids.
B)The stream has too little water to carry the total sediment supplied.
C)The stream channel is so shallow that the stream flows easily over its banks.
D)Water soaks down to the water table, so it doesn't ever flow downslope.
E)There are so many channels that they can't help but cross one another to form a braided pattern.

Question 27

If a 100-year floodplain was flooded in 2003, when should we expect the next 100-year flood?

A)2013
B)2053
C)2103
D)2104
E)at any time

Question 28

A(n) _______________ is similar to an alluvial fan, but its sediments are deposited underwater.

Question 29

All of the following diminish the reliability of recurrence intervals EXCEPT ____.

A)using data from a short time interval
B)separating different meteorological conditions
C)population growth and urbanization
D)cyclical or long-term variations in climate
E)deforestation within the watershed

Question 30

Which factor CANNOT be used to determine the depth of a debris flow in a canyon it moved through?

A)bark battered off the upstream sides of trees up to that height
B)sand embedded in bark up to that height
C)rocks lodged in tree branches up to that height
D)the maximum height of sand eroded from the valley sides
E)the highest rocks on natural levees deposited by the flow

Question 31

A(n) _______________ is able to maintain a dynamic equilibrium between inflow and outflow of sediment.

Question 32

A stream that is overloaded with sediment and forms broad, multichannel paths is called a(n) _______________ stream.

Question 33

Why are buried oil and gas pipelines vulnerable to damage during a flood?

A)The pipes rise with the rising water level, causing the pipes to break.
B)The large amounts of sediment flowing during a flood puts additional pressure on the pipes.
C)Channel scour can erode down to the pipeline level, and tumbling rocks can sever the pipe.
D)When the levee is overtopped, the dropping water level can expose the pipe, leading to damage.
E)If the oil and gas are shut off, the pipe becomes lighter and can float to the bankfull level.

Question 34

What keeps a stream flowing year-round, even though it may not rain for many months?

A)the rain far upstream
B)large springs near the stream channel upstream
C)groundwater flow from the adjacent ground into the stream
D)very heavy dew on cold mornings
E)melting snowpack

Question 35

What changes upstream from a site would NOT likely lead to a higher and shorter hydrograph?

A)increased urbanization ​
B)wildfire
C)clear-cut logging
D)overgrazing
E)reforestation for a Christmas tree farm

Question 36

Stream discharge is calculated by multiplying the water's average velocity by the _______________ of the stream.

Question 37

The volume of sediment a stream can carry is known as its _______________.

Question 38

A stream that sweeps from side to side in wide turns is called a(n) _______________ stream.

Question 39

What caused the catastrophic 1997 flood along the Red River in North Dakota?

A)thawing to the south of the frozen north-flowing river that backed up water behind the ice to the north
B)thawing of a heavy snowpack on the nearly horizontal surface of ice-age Lake Agassiz
C)heavy rains on a deep snowpack on nearly flat ground
D)a stalled warm front that was unusually far north for that time of year
E)a cold front from the north that intersected a warm front in the northern Mississippi valley

Question 40

The depth of sediment eroded during floods is called _______________.

Question 41

Where do we find potholes, and how do they form? Why do they pose a hazard to humans?

Question 42

In the case study "Desert Debris Flows and Housing on Alluvial Fans-Tucson, Arizona, 2006," why did the hydrograph for Sabino Creek show higher and higher discharge rates between July 27 and July 31?

Question 43

The probability that a certain discharge or flow will be exceeded in any single year is equal to the inverse of the _______________.

Question 44

You are attempting to build a house in a valley bottom, but before you break ground on your new project, you want to make sure that the area is not in danger of experiencing a debris flow. What sort of evidence would you look for to ensure that you would be safe building in that location?

Question 45

A glacial outburst flood is also called a(n) _______________.

Question 46

Which is likely to flow farther down to the flatter, wider part of a valley: a debris flow or a mudflow? Why?

Question 47

Sections of bedrock streams tend to steepen abruptly at locations called _______________.

Question 48

Engineers commonly added rock rubble, or _______________, along the banks of a straightened portion of a stream.

Question 49

Explain the relationship between stream velocity and erosion.