Deck 12: Streams And Flood Processes

Question 1

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

Answer 1

D

Question 2

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 valley.
B) The slope of the riverbed builds up until it is steep enough to transport the size of gravel supplied.
C) The pebbles work themselves down through sediments in the riverbed until they reach bedrock.
D) The pebbles dam the river channel.
E) The water slows down because the larger sediment gets in the way.

Answer 2

B

Question 3

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

A) must have rained 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

Answer 3

C

Question 4

Meander wavelength is NOT correlated with which of the following?

A) channel width
B) channel depth
C) radius of curvature
D) length of the meander arc between meanders
E) width of the meander belt

Question 5

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) modifying the stream by channelization.
E) deforestation within the watershed.

Question 6

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 last longer.
C) The hydrograph would be lower but last 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 7

Which of the following areas 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

Question 8

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 9

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 parts of the stream channel left after an avulsion when a stream abandons its channel.
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

Question 10

Which of the following 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 11

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 12

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

A) coarser material from a tributary channel
B) bedrock in the stream channel
C) a landslide involving the stream bank
D) higher than normal water in the stream
E) the headwaters area of many mountain streams

Question 13

With urbanization of the upstream drainage area,what changes can be expected in the height of the 100-year flood?

A) a lower height, since the water runs through faster
B) a greater height
C) the time at highest level will be longer
D) the downstream extent of the highest level will be less
E) no change; the flood level only depends on the total rainfall upstream

Question 14

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 to force 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) none of these

Question 15

Why would living on a floodplain behind a levee be safe from flooding?

A) Levees are rarely overtopped.
B) Water cannot seep under levees to flood the floodplain.
C) Levee banks are rarely eroded enough to fail.
D) Once a levee is built high enough to contain a 100-year flood, that volume of water will never flood.
E) This statement is not true because 100-year flood levels often continue to get higher with time.

Question 16

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 17

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 doesn't-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 18

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

A) next year
B) 10 years
C) 100 years
D) 200 years
E) anytime

Question 19

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 20

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 21

What are the differences in appearance between a debris flow and a mudflow deposit?

Question 22

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

Question 23

Why does stream level rise within a few hours of a heavy rain,even though you see no water running off the slopes?

Question 24

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

Question 25

Why are river floodplains often broad and flat-bottomed,many times wider than the width of the meander belt of the stream?

Question 26

What is the main difference between a water flood and a hyperconcentrated flood?

Question 27

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 28

What are natural levees? Where do they form? Why do they form there?

Question 29

A floodplain with a record since 1930 had its largest historic flood in 1999,and there is a slightly larger flood 12 years later in 2011.What,if any,change is there in the recurrence interval for the 1999 flood? Be numerically specific.

Question 30

Which way does groundwater flow in a dry climate with respect to a stream?

Question 31

How would you calculate the discharge (or flow rate)of a stream?

Question 32

What is the best long-term solution to avoiding a debris flow?

Question 33

Why are alluvial fans so broad,even though they form at the lower end of a narrow mountain canyon?

Question 34

What changes upstream from a site would likely lead to higher-level floods with the same rainfall pattern? What changes would occur for this stream's hydrograph?

Question 35

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