Question 1

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

Accepted Answer

Erosion on the outside bends of meanders causes the stream to migrate with that erosion.Over hundreds of years,the stream will migrate across different parts of the full width of the valley,eroding it wider and maintaining its flat form near the elevation of the stream.

Question 2

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

Accepted Answer

Potholes are found in steep,rock-bound stream canyons where turbulent flow swirls rocks in whirlpools.Larger rocks caught in such turbulent eddies carve away at the bedrock,gradually drilling a cylindrical hole.Many people swim in potholes,but this is dangerous because of extreme turbulence and vortexes that render flotation devices ineffective.

Question 3

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

Accepted Answer

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 D

Question 4

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

Accepted Answer

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 5

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

Accepted Answer

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 6

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

Accepted Answer

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

Question 7

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

Accepted Answer

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

Question 8

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

Accepted Answer

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

Question 9

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

Accepted Answer

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 10

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

Accepted Answer

The answer of What is the main difference between a...

Question 11

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?

Accepted Answer

The answer of In the case study "Desert Debris Flows...

Question 12

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

Accepted Answer

The answer of Which way does groundwater flow in a...

Question 13

Meander wavelength is NOT correlated with which of the following?

Accepted Answer

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 14

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

Accepted Answer

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.

Question 15

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

Accepted Answer

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 16

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

Accepted Answer

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 17

What are oxbow lakes and how do they form?

Accepted Answer

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 18

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

Accepted Answer

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 19

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?

Accepted Answer

The answer of You are attempting to build a house...

Question 20

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

Accepted Answer

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.

Exam 12: Streams And Flood Processes