Chapter 16: Streams and Flooding

What happens to the erosive power of a river as velocity increases?

Erosion increases.

In which location would a geologist look for a river confined to a single channel?

Gentle plains

True or false: The position of the Mississippi River delta has been remarkably stationary for the past 7,000 years.

False

What is a floodplain?

A broad strip of land built up by sedimentation on either side of a stream channel and covered with water during a flood

What is a point bar?

A sandbar deposited along the inside of a stream meander due to lower velocity

What is a stream terrace?

A stream terrace is an old floodplain perched outside and above the current floodplain.

These are formed when a steep drainage enters a broad valley and the sediments and debris are deposited.

Alluvial fans

What is a natural bridge?

An arch-shaped features that span a (former) stream

Where do deltas form?

At the mouth of a river that empties into an ocean or lake

Which curve (A or B) on the hydrograph represents a steeply sloped basin with respect to runoff of water after a storm?

B

The image shows a stream hydrograph of a flood where discharge is on the vertical axis, time is on the horizontal axis, and the red line is the flood stage. Which labeled point shows where the flood begins?

B, where the stream rises to the flood stage

On a field trip, you measure the following: river depth = 7 feet; river width = 12 feet; and river velocity = 3 feet/sec. What is the discharge? ft3/sec

Blank 1: 252

The gradient of this stream is m/km.

Blank 1: 7 or seven

Lake Mead and Lake Powell were created by damming the River.

Blank 1: Colorado

When a river enters an ocean and begins depositing sediments to form deltas, the first particles to settle out are large particles and sand followed by and , which settle farther offshore.

Blank 1: clay Blank 2: silt

Human-constructed barriers such as levees are beneficial because they can help prevent ; however, it is difficult to construct an affordable levee that will not fail during the worst situations.

Blank 1: flooding or floods

The type of graph used to display the change in a river's discharge over time is a(n) .

Blank 1: hydrograph

When a river floods and deposits sediment parallel to and along the river channel, a(n) is formed.

Blank 1: levee

Flooding can be prevented by building parallel to river channels. However, these structures do fail, and construction to prevent the worst possible flooding events is not affordable.

Blank 1: levees

Meanders, floodplains, and river terraces are commonly found along -gradient rivers.

Blank 1: low

The production of ice sheets during global cooling results in sea levels. These ice sheets can depress the , causing river drainages to flow toward the ice sheets.

Blank 1: lower, decreased, reduced, or lowered Blank 2: crust or land

The base level of a river is the elevation to which the river can erode.

Blank 1: lowest or minimum

A river is formed as a river incises into the surrounding rocks and sediments, abandoning the floodplain and leaving a record of the river's historical size and location.

Blank 1: terrace

Older floodplains that are perched above current floodplains are called stream .

Blank 1: terraces

The flat benches that are perched above a river or stream and that stair-step upward and outward from the active channel are .

Blank 1: terraces

The property of a fluid that is defined as the resistance to flow is .

Blank 1: viscosity

An important and widespread geologic agent in eroding, transporting, and depositing sediment is flowing .

Blank 1: water or H2O

A nickpoint is a location where a stream has an abrupt change in gradient and can form a(n) .

Blank 1: waterfall

______ are formed as flowing water collects in natural cracks or low spots. Once formed, these capture additional runoff within the drainage basin.

Channels

Select the answer that is not a factor in the deposition of deltas.

Chemical makeup of sediment

Which of the following is not a characteristic of a braided river or stream?

Constant flow

Select the human-caused flooding events from the list of flood causes below.

Dam failure Urbanization

______ are formed as rivers slow down when they reach their mouth and empty into an ocean.

Deltas

What problems are associated with straightened or channelized stream channels?

Increased erosion downstream Reduced sediment deposition downstream Downstream flooding

Which of the following describe the largest known flood along the Colorado River?

It occurred before humans were in the area. Its discharge is estimated using geomorphology. It had a discharge of approximately 8,500 m3/sec.

Select from the list below the two large reservoirs created by damming the Colorado River.

Lake Powell Lake Mead

Select all the natural causes for flooding from the list below.

Local heavy precipitation Eruption of snow-covered volcano Snowmelt Regional precipitation

What types of streams typically deposit clay- to sand-sized sediments?

Low-relief streams

Which of the following physical parameters show a decrease from the headwaters to the mouth of a river?

Maximum grain size

______ are formed beside low-gradient rivers when floods occur and sediments build up along the channels. These features become barriers to water returning to the river from the floodplain.

Natural levees

From the list below, select the factors that control the deposition of a delta.

River and ocean ice Wave action Discharge Vegetation Sediment load

Select common features located along low-gradient rivers.

River terraces Single channels Floodplains Meanders

Which of the following physical parameters show an increase from the headwaters to the mouth of a river?

Sediment load Discharge Channel size

What three factors of a drainage basin influence the flow response to rainfall?

Shape Size Slope

Select consequences to a river when a dam is constructed.

The dam causes the river to deposit its sediment load upstream of the dam. The dam creates a temporary base level for the river. Water released from the dam is clear (no sediment) and has a new capacity to erode and transport sediments downstream.

Select the two conditions that resulted in the 1993 upper Mississippi River flood.

The ground was already saturated from early spring rains. Severe precipitation was caused by warm moist air from the Gulf of Mexico and cold Arctic air merging.

What is a drainage basin?

The naturally defined area that a stream drains

When are stream eddies formed?

When turbulence increases

Do changes in tectonics, human development, and geology affect river systems?

Yes

A decrease in the size of sediments carried by a river usually reflects ______ in the gradient of the river.

a decrease

Geologists use large water tanks in laboratories to study braided river dynamics. They control the slope, the ______, and the consistency of water flow to study how the variables affect the stream system.

amount and type of sediment

The erosional base level of a river can be ______.

an ocean a lake a closed land basin

Streams that are associated with and predate the formation of a geologic structure are ______, whereas a stream that is established in association with but after the formation of a geologic structure is ______.

antecedent; superposed

The headwaters of the Colorado River ______, and the mouth is at the Gulf of California.

are in the Rocky Mountains

The process of meander formation begins when a difference in roughness on the ______ causes water to move faster on one side of the channel than on the other.

bottom

Flat-bottomed valleys between mountains would be good places to look for ______ rivers or streams.

braided

A stream ______ erode below its base level.

cannot

Rivers on gentle plains usually occupy a single ______ rather than being braided.

channel

As water flows, it accumulates in natural cracks, which eventually form ______ rather than spreading across the land.

channels

The Upper Mississippi River ______.

is young was created by glacial meltwaters

A flash flood ______.

often results from intense rainfall lasts a short time

Slope failures from the surrounding mountain slopes and tributaries are the two sources of ______ in mountain streams.

sediments

We disrupt the balance of a river system when we discourage the process of migration and meandering by ______ streams.

straightening

The Salton Sea in California was formed in 1905 when ______.

the Colorado River flooded

The term "100-year flood" signifies ______.

the size of a flood that is predicted to have a 1 in 100 probability of occurring in a given year

When the gradient of a stream steepens or the channel is constricted by bedrock or obstacles, the flow of water becomes ______ forming ______.

turbulent; rapids

What is an incised stream?

One that has eroded into the underlying rock

The particle size transported by water that is in between clay and sand is .

Blank 1: silt

A river cannot erode below ______.

sea level

Use the following information to calculate the gradient of River X. If the vertical change is 20 meters in 1 kilometer, then the gradient expressed in m/km is ______.

20

What is a drainage divide?

A ridge or strip of high ground separating one drainage basin from another

What is a braided stream?

A stream that flows in a network of many interconnected rivulets around numerous bars

The formation of a natural arch requires which of the following?

A strong type of rock The formation of fins Erosion by water and/or wind

Over the last 7,000 years, the mouth of the Mississippi River has moved. What evidence supports the different end points of the river?

As least six huge abandoned sediment deposits mark where the river created a delta.

Which of the following is the correct relationship between river velocity and sediment load?

As velocity increases, so does the river's capacity to carry a larger load.

Where is the peak discharge on this hydrograph?

B

The area that a stream naturally drains is called a drainage .

Blank 1: basin

Viscosity is the resistance to exhibited by fluids.

Blank 1: flow, movement, or motion

Which drainage basin covers the largest area in the United States?

Gulf of Mexico

Which two drainage divides are separated by the Continental Divide?

Gulf of Mexico Pacific Ocean

Which of the following are considered to be causes for severe flooding events other than precipitation alone?

Prolonged drought followed by intense precipitation Failure of a dam Rainfall coinciding with snowmelt and ice dams in a river Stalled hurricane

What are distributaries?

Small, branching channels that carry water away from the main channel

Select two ways that mountain streams obtain their sediments.

Soil and/or sediment slides down into the drainage area. Tributaries bring sediments into the drainage area.

From the list below, select the characteristics of a braided river or stream.

Steep gradient Variable flow Abundant sediments

What is stream abrasion?

The process of grains chipping, scraping, and sandblasting the streambed as they move downstream

Which two of these do you multiply to determine the discharge of a river?

Velocity of the river Area of the river

Select the three measurements needed to calculate discharge of a river at a certain point.

Velocity of the water River width River depth

What is a flood?

Water overflows a stream's channel.

The lowest level to which a river can erode is its ______.

base level

Smaller subsidiary channels that feed the main channels of rivers are ______.

tributaries

If you examined a hydrograph and saw peak discharge in a small amount of time, then a quick decline in discharge, what might have been a possible explanation for the peak?

A heavy rainstorm event upstream

What is a bedrock stream?

A mountainous stream flowing through a carved bedrock channel

Besides sea level and rock type, the other two primary factors that influence a stream or river's profile are and .

Blank 1: climate or weather Blank 2: tectonics

In a meandering river, the deeper outside bend is eroded into a steep riverbank referred to as a(n) , and on the shallower opposite side of this is a(n) bar.

Blank 1: cutbank Blank 2: point

As the gradient decreases in a river, the size of the sediment that can be transported .

Blank 1: decreases or declines

The feature that is formed where a river meets the sea, slows down, and deposits its sediments is a(n) .

Blank 1: delta

When the amount of sediment exceeds the ability of the current to carry it, the stream the sediment.

Blank 1: deposits

What types of sediments or rocks are deposited by low-relief streams?

Clay Sand Silt

Deposition in bedrock channels occurs along riverbanks and behind obstacles as ______.

water velocity decreases

A stream deposits the sediment it is carrying ______.

when it no longer has the capacity to do so

The Fall Line is located between the ______ Mountains and the coast, and is marked by ______.

Appalachian; waterfalls

Steep slopes in a(n) narrow canyon plus an unusually large thunderstorm resulted in the 1976 Big Thompson River flood near Estes Park, .

Blank 1: Colorado

Which large river runs from the high Rocky Mountains to western Mexico passing lakes, dams, reservoirs, geologic structures, incised meanders, and finally a delta on its journey? The River.

Blank 1: Colorado

What is the name for small, branching channels that carry water away from the main river channel and distribute it over the surface of the delta?

Blank 1: Distributaries

The red line marks the Line, the boundary between hard bedrock in the mountains and soft sediments in the coastal plains. Stream gradient changes here, and it is marked by a series of .

Blank 1: Fall Blank 2: waterfalls

What is the word used to describe the change in elevation for a given stream or river over a horizontal distance?

Blank 1: Gradient

The part of the Mississippi River that carved itself a river valley over Cenozoic sedimentary rocks when sea level was low, and then filled the valley with deposits when sea level rose, is the Mississippi.

Blank 1: Lower

The Colorado River starts in the Rocky Mountains in the United States and has its mouth in . (Use a single word for your answer.)

Blank 1: Mexico

During the 1993 flood on the upper River, floodwaters flowed onto the floodplain because levees did not hold the water back. It took months for the floodplains to dry out because the trapped the water, preventing it from flowing back into the river channel.

Blank 1: Mississippi Blank 2: levees

What is the name of the subsidiary channels that join up with the main river channel?

Blank 1: Tributaries

When steep, narrow drainages enter broader, more gentle valleys and streams deposit the large sediments they are carrying and form .

Blank 1: alluvial Blank 2: fans

Multiplying the width of a channel by its average depth will give you the of the channel. (Use a single word for your answer.)

Blank 1: area

Oceans, lakes, or the bottoms of closed basins are all examples of levels of rivers.

Blank 1: base

A stream in a mountainous area that is carved from bedrock is called a(n) stream.

Blank 1: bedrock

A stream that flows in a network of many interconnected rivulets around numerous bars is a(n) stream.

Blank 1: braided

The type of rivers that are characterized by a network of interweaving sinuous channels, with the overall channel being fairly straight, are rivers.

Blank 1: braided

This is a natural .

Blank 1: bridge or arch

One stream may divert the flow of another stream into a different basin through the process of stream . This occurs during erosion.

Blank 1: capture Blank 2: headward

The drainage pattern associated with areas where the rocks have the same resistance to erosion is a pattern, whereas drainage patterns are found on symmetrical mountains.

Blank 1: dendritic Blank 2: radial

Potholes or bowl-shaped pits are formed when flowing water and sediments swirl in small in the rock.

Blank 1: depressions, holes, pits, or cavities

The volume of water flowing through a stretch of a river is the and is measured by multiplying the river depth by river width and river velocity.

Blank 1: discharge

A ridge or strip of high ground that separates one drainage basin from another is referred to as a drainage .

Blank 1: divide

As the velocity of water increases so does the turbulence. This results in the formation of swirls in the current called .

Blank 1: eddies

The gradient of a stream or river is defined as the change in for a given horizontal distance.

Blank 1: elevation or height

As meanders develop on a floodplain and either the base level drops or a tectonic event causes uplift, the meanders deepen and become .

Blank 1: entrenched or incised

Acting as major sculptors of Earth's surface, rivers and streams the landscape, carry the materials, and later deposit the sediments.

Blank 1: erode

A flood that is characterized by high discharge over a short time frame is called a(n) flood.

Blank 1: flash

An event in which the amount of water flowing through a river channel overflows its banks is a(n) .

Blank 1: flood

A broad strip of land on either side of a stream channel that is covered by water during a flood and replenished by sedimentation is a(n) .

Blank 1: floodplain

When a river is in a steady state in which the balance between the supply of sediment and the amount of sediment that the river can carry is in a near equilibrium, the river is referred to as a(n) river.

Blank 1: graded

Rivers are dynamic systems driven by precipitation and . (Hint: the answer is just one word.)

Blank 1: gravity

The force behind a river's flow from high to low elevations is .

Blank 1: gravity

The origin of a river is called its , and the end of a river is called its .

Blank 1: headwaters or head Blank 2: mouth

Many historic floods are associated with the large precipitation that accompanies a(n) ; a good example is the storm named Mitch in 1998 that brought 78 inches of rain in some places in a few days.

Blank 1: hurricanes or typhoons

The uplift of high mountain ranges increases slope and precipitation causing a subsequent in the amount of sediment available for a stream to transport.

Blank 1: increase

Braided rivers carry more volume and larger size ranges of sediment types than rivers.

Blank 1: meandering

Because of continued erosion and deposition that leave behind scars and point bars, river migrate back and forth across a flat valley floor.

Blank 1: meanders

The discharge of a river is measured in cubic per second.

Blank 1: meters or feet

A cutoff meander may develop into a(n) lake.

Blank 1: oxbow

A stream that flows year-round is a(n) stream, whereas a(n) stream does not flow the entire year.

Blank 1: perennial Blank 2: ephemeral or intermittent

The bowl-shaped pits that are created when flowing water and sediments swirl in small depressions are .

Blank 1: potholes

The gradient, or of a stream is steeper near the headwaters than at the mouth.

Blank 1: profile or slope

When water flows chaotically through narrow channels or over large rocks and other debris that partially block the channel, are formed.

Blank 1: rapids

Changes in water or sediment supply, change in slope, or changes to base level elevation can begin or end the life of a(n) .

Blank 1: river, stream, creek, or waterway

The principal processes that sculpt Earth's landscape are erosion and deposition by .

Blank 1: rivers or streams

The movement of sand grains by a series of short leaps or bounces along a streambed is .

Blank 1: saltation

Meandering rivers leave behind exposed curved ridges called meander marked by lines of vegetation or as curved dry depressions. When the depressions are filled with water, they are lakes.

Blank 1: scars Blank 2: oxbow

A tributary that itself has a tributary within a drainage system is a(n) -order stream.

Blank 1: second

The higher the velocity of a flowing stream, the larger its capacity to carry .

Blank 1: sediment

A delta is formed where a river meets an ocean because is deposited as the river loses speed.

Blank 1: sediment, sand, or load

Construction of dams causes an interruption of river flow. River are deposited upstream of the dam, shortening the life of the reservoir.

Blank 1: sediments

The amount that a river or stream channel curves in a given length is called its .

Blank 1: sinuosity

Dissolution is the process by which materials are removed and transported by flowing water.

Blank 1: soluble, dissolved, or water-soluble

With respect to the seasons, the highest discharge of a river in the Upper Midwest is usually in the and the lowest discharge is during the .

Blank 1: spring Blank 2: winter

Rivers and streams are similar bodies of flowing water. A(n) is a body of water with a current that flows downstream within a channel. A(n) is a large body of water with considerable volume and permanent or seasonal flow.

Blank 1: stream Blank 2: river

The 1993 flood of the Mississippi River at St. Louis began June 26 when the river reached the flood stage, and peaked August 1st. The flood peak was about feet above flood stage.

Blank 1: twenty or 20

When comparing their ages, the Upper Mississippi is than the Lower Mississippi.

Blank 1: younger

Which of the following locations would be the two most common areas in which to find a braided stream or river?

Broad, sloping plains that flank mountain ranges Flat-bottomed valleys between mountains

In mountain streams that are steep with high stream velocities, which pattern of erosion is the most common?

Channel bed is eroded faster than the channel sides.

Why do the meanders on a stream migrate?

Continuing erosion and deposition

What is a very common and important geologic agent in eroding, transporting, and depositing large amounts of sediment daily?

Flowing water

What is happening to coarse-grained sand in a stream velocity of 70 cm/s?

It is being transported as bedload.

What is happening to silt and clay in a stream velocity of 5 cm/s?

It is being transported as suspended load.

How can the melting of an ice sheet or glacier affect a river's drainage?

Melting of the ice releases tremendous amounts of water and sediment that lead to new or larger channels. Uplift of the land after the glacier melts can change drainage patterns and direction.

On the diagram shown, look at the channel profile at the place labeled "cutbank." Where is the deepest water? The shallowest water?

On the outside of the bend; on the inside of the bend

How long was the Mississippi River above flood stage in St. Louis during the summer of 1993?

Over 3 months

What occurred to decrease the gradient of the Lower Mississippi River, causing it to deposit sediment within the valley it has previously incised?

Sea level rose.

Select the conditions that caused the 1976 Big Thompson River flood near Estes Park, Colorado.

Steep slopes of the river canyon Narrow canyon Unusually severe thunderstorm

Which of these would indicate a flood on a stream hydrograph?

Stream discharge that is greater than the flood stage for the channel

Which of these has a concave-up shape?

Stream elevation profile

Which of the following are primary factors that influence a stream's profile?

Tectonics Climate Sea level Rock type

Select conditions that have an effect on the flow of river systems over time.

Tectonism Geology Climate Dams Runoff

What process generally defines a nickpoint and forms waterfalls?

The channel bed below the nickpoint has a faster erosional pattern than above the nickpoint.

A community experiences two floods with stream discharge equal to the 100-year recurrence interval. The floods occur in 1980 and 1990. When will or did the next flood occur?

The next 100-year flood could occur in any year.

Which of the following are ways in which tectonism, such as an uplift of a mountain, can affect drainage?

The uplift can increase the slope of the drainage. The uplift can increase precipitation into the drainage. The uplift can increase the supply of coarse sediment. The uplift can cause a rain shadow on the other side, causing a decrease in the runoff into the drainage.

What characteristics do rivers and streams share?

They flow downhill due to gravity. They flow within a channel. They can be braided or meandering.

True or false: Increasing the rate of runoff increases the likelihood of flooding in local streams.

True

When conditions such as climate remain stable, a river may approach a dynamic equilibrium state. The river would then be considered ______.

a graded stream

Most streams have ______ profiles.

concave-up

Features that occur along the Colorado River include ______.

dams natural lakes geologic structures lava flows

The feature formed when a river deposits its sediment near its mouth is a(n) ______.

delta

When rivers enters a lake or an ocean, they slow down and lose their capacity to carry sediments, forming ______.

deltas

Bedrock channels occur where erosion by a stream has cut down into the local bedrock. Although erosion is the dominant process, ______ tends to occur where and when flow velocity decreases. Lower velocities can often be found behind objects, such as boulders, in the stream.

deposition

The amount of water flowing through a channel over a given amount of time is called its ______.

discharge

Soluble materials in flowing water are removed and transported by a process called ______.

dissolution

On the outside of a meander, the water velocity is faster and causes ______, and on the inside of a meander, the water velocity is slower, which causes ______.

erosion; deposition

Meanders cause differences in water velocity in the river channel. On the outside of a meander, water velocity is ______ and causes erosion, while on the inside of a meander, water velocity is ______ and deposition of sediments occurs.

faster; slower

A stream with no tributaries is a ______-order stream.

first

Large water tanks in laboratories are used by geologists to study water ______ in river systems because the tanks can be used to control the slope of the river and the type of sediments.

flow

The discharge represented by B on this hydrograph is for a basin with ______ slopes.

gentle

The driving force behind a river's flow is ______.

gravity

In flowing water, the upstream side of an obstruction is subjected to the ______ of abrasion.

greatest amount

Streams can expand the area of the basin by ______ which may lead to ______.

headward erosion; stream capture

The profile of a river is steeper at its ______.

headwaters

The area of the origin of a river is its ______, and the ______ the end of the river.

headwaters; mouth is

Channel beds erode faster than the channel sides in ______ streams and rivers. These are generally steep mountain streams.

high-gradient high-velocity highly turbulent

The more years of river flow data that are collected, the ______ the probability of predicting a major flooding event.

higher

The ______ the velocity of a river, the ______ its capacity to carry sediment.

higher; larger

The probability that a discharge of a certain size will occur during any given year is calculated using ______.

historic raw data

Streams that have cut down into bedrock and flow through bedrock channels are called ______ streams.

incised

As water velocity in a stream increases, turbulence increases, resulting in a(n) ______ in the water's ability to erode and transport sediment within a channel.

increase

The estimates for a 100-year flood on the Colorado River near Lee's Ferry are based on ______.

inferred discharge of historic floods

The size of sediments in braided rivers typically is ______ than that carried by meandering rivers.

more varied

The cross-sectional area of the channel of a stream or river is calculated by ______.

multiplying the channel's width by its average depth

The volume of water flowing through any part of a river per unit of time (discharge) is calculated by ______.

multiplying the velocity by the cross-sectional area of the river

A large opening that goes all the way through a narrow panel of rock is called a ______. A smaller, but similar opening is called a ______. These features may form in areas where a narrow panel of rocks, called a ______, has formed due to weathering controlled by joints on both sides.

natural arch; window; fin

When meander scars are filled with water, they are ______.

oxbow lakes

A stream or river that flows all year is ______.

perennial

A depositional feature that has been built on the inside of a stream channel curve because of lower velocity is a ______.

point bar

Streams are systems driven by ______.

precipitation and gravity

The ability to determine the ______ of a flooding event is based on river flow data. The more data collected and the greater the time span for data collection, the better understanding we have of the chance of future flooding events.

probability

The type of sediment transport that involves a series of leaps or bounces off the bottom of a streambed is ___________.

saltation

The ______ load is the total amount of sediment carried by a river.

sediment

The size of the drainage basin and the ______ the drainage basin influence the flow response to rainfall.

shape of

Changes in the ______ can begin or end the life of a river.

slope the river flows across elevation at the base level water or sediment supply at the source

River ______ are relatively flat benches that are perched above a river or stream and stair-step upward and outward from the active channel.

terraces

The formation of the Salton Sea occurred in 1905 during a flood of the Colorado River. It occurred in an area ______.

that historically had a lake

Sinuosity refers to ______.

the amount that a river or stream channel curves in a given length

A hydrograph shows ______.

the change in a river's discharge over time

Waterfalls, lakes, and large boulders are landforms that characterize ______.

the headwaters of mountain streams

The graphs geologists use to estimate river flow probability are ______.

unique to each stream

The turbulence of water increases as the water's ______ increases, which can increase the water's ability to erode and transport materials within the channel.

velocity