Streams and Caves Objectives (Ch. 5)

Question 1

What is meant by the drainage basin of a stream? What is a divide?

Answer 1

A drainage basin is all the land area that contributes water to a stream.
A divide separates two drainage basins.

Question 2

Name and recognize four stream drainage patterns.

Answer 2

Dendritic: controlled by slope of land; develops on relatively uniform surfaces; think tree branches
Radial: produced from domes or volcanoes; think star
Rectangular: controlled by underlying joints and faults; develops on highly jointed bedrock; think etch-a-sketch
Trellis Appalachians: occurs in areas of folded mountains; develops in areas of alternating weak and resistant bedrock; think crossed lines

Question 3

Name and describe factors that control stream velocity.

Answer 3

1. Gradient: the slope of the stream channel (vertical drop per unit of horizontal flow)
2. Channel characteristics (size, shape, and roughness): Friction slows down the water moving in a stream, so anything that increases the friction (boulders, trees, etc.) also decreases the stream velocity.
3. Discharge: the volume of water flowing past a point per second; equals width x depth x velocity

Question 4

What river has the world’s largest discharge? Where does the Mississippi River rank?

Answer 4

Amazon R. has world’s largest discharge.

Mississippi R. ranks 8th of the world.

Question 5

How do the gradient, discharge, width, depth, velocity, and sediment size of a stream change from upstream to downstream?

Answer 5

From upstream to downstream (aka from headwaters to mouth):

• gradient decreases
• discharge increases (in arid regions, may decrease)
• width increases
• depth increases
• velocity increases
• sediment size decreases

Question 6

Calculate the gradient or discharge of a stream given the appropriate information.

Answer 6

gradient=vertical drop/unit of horizontal flow (think rise/run)
discharge(ftcubed/sec)=width(ft) x depth(ft) x velocity(ft/sec)

Question 7

What is a hydrograph? How does it change when a stream’s drainage basin becomes urbanized? What does this mean in terms of the stream itself and the area it drains?

Answer 7

A hydrograph is a graph showing the rate of flow (discharge) versus time past a specific point in a river, or other channel or conduit carrying flow. The rate of flow is typically expressed in cubic meters or cubic feet per second (cms or cfs).
When a stream’s drainage basin becomes urbanized, then the lag time until the peak discharge decreases and the peak discharge itself increases for the same amount of waterfall. This means there is a greater chance of flooding. After urbanization (which is when fields and forests are replaced with impermeable surfaces such as streets, parking lots, rooftops), more water runs off and less water soaks into the ground, so the water table is lowered and the dry season discharge is decreased.

Question 8

Describe the effect of retention basins on the hydrograph of a stream and how they affect dry weather stream flow.

Answer 8

Retention basins (ponds) delay the entry of water into the stream runoff. 
-increase lag time
-decrease peak discharge
They increase the infiltration of water into the ground. 
-the water table is maintained higher than it would be otherwise and the dry season discharge is higher than it would be otherwise.

Question 9

Distinguish between ultimate and temporary/local base level.

Answer 9

The base level is the lowest level to which a stream can erode its channel.
Ultimate base level = sea level
Temporary/local base level = a lake, a larger stream, a layer of resistant rock; anything other than the ocean that restricts the downward erosion of a stream.

Question 10

What will happen in a stream if its base level is lowered? Raised?

Answer 10

In general, if base level is lowered, the stream cuts downward into its channel and erosion is accelerated. If base level is raised, the stream deposits sediment and readjusts its profile to the new base level.

Question 11

Name and discuss the three types of “work” of streams.

Answer 11

1. Erosion - most occurs during high-flow periods; can be rapid if bed and channel sides are made of poorly consolidated material; typically dominates in upstream area; if stream has bedrock channel, most of the erosion is by abrasion by its load of sediment
2. Transportation of sediment - a stream’s ability to transport sediment is determined by competence (max size particles that can be transported; depends on velocity) and capacity (max load that can be transported; depends on discharge)
3. Deposition - occurs when the stream’s velocity is reduced; heavier particles sort out first; all stream-deposited sediment is called alluvium

Question 12

Which of the “works” of a stream is more important in each of the three “ages” of a stream? Which is more important in the headwaters, middle section, and farthest downstream areas of a stream?

Answer 12

young - erosion?
mature - tranportation?
old - deposition? re-erosion
headwaters - erosion
middle section - transportation
farthest downstream - deposition

Question 13

Describe the three “loads” transported by a stream. Which accounts for most of the material moved by a stream?

Answer 13

Dissolved load - most dissolved minerals come from ground water
Suspended load - fine sand, silt, and clay particles in suspension
Bed load - particles that are too heavy to be carried in suspension and that roll or bounce along the bottom
Suspended

Question 14

What term describes all the sediment deposited by a stream?

Answer 14

alluvium

Question 15

What do the competence and the capacity of a stream each measure?

Answer 15

competence: maximum size particles that a stream can transport; depends on velocity
capacity: maximum load that can be transported; depends on discharge

Question 16

What is a braided stream? What does it indicate about the stream?

Answer 16

A braided stream is is a stream which is so choked full of sediment that it produces a bunch of little channels within the main channel. It indicates that the stream load is greater than the capacity.

Question 17

What is the difference between a delta and an alluvial fan?

Answer 17

A delta forms where a sediment-charged stream enters the relatively still waters of a lake or ocean/ is deposited at the mouth of a stream as it enters a lake or the ocean where its velocity is suddenly reduced. (On a delta the main channel splits into several smaller streams called distributaries.) forms ON WATER
An alluvial fan is a fan or cone shaped shaped accumulation of alluvium deposited where the gradient of a stream is suddenly reduced as it emerges from a mountainous, narrow valley onto a flatter terrain. forms ON LAND

Question 18

What are distributaries?

Answer 18

the smaller streams which the main channel of a delta splits into to continue carrying sediment

Question 19

Describe the formation and location of natural levees, backswamps, and yazoo tributaries.

Answer 19

Natural levees are built by successive floods over many years. When a stream overflows its banks, its velocity immediately diminishes, leaving coarse sediment deposited in strips bordering the channel. Coarse dumped first, then finer sediment spread out over the valley which is what produces the gentle slope of the levee.
Backswamps are marshes formed in the area behind the levee which is poorly drained for the obvious reason that water cannot flow up the levee and into the river.
Yazoo tributaries are tributaries that cannot enter a river because levees block the way often has to flow parallel to the river until it can breach the levee.

Question 20

List and describe the idealized stages in the development of a river and its valley and discuss several distinguishing characteristics of each stage.

Answer 20

Young Stream:
-shaped valley with steep valley walls
-occur where the stream is well above its base level and the gradient is steep
-has a narrow channel, the stream fills the valley floor, and there are no meanders
-shape indicates that down-cutting is taking place faster than valley widening
-rapids and waterfalls result where stream flows over more resistant rock layers which serve as a “temporary” base level
Mature Stream:
-downcutting has nearly stopped because the gradient is much less
-no rapids or waterfalls
-floodplain begins to develop as valley widening becomes the more dominant erosion
-stream begins to meander as it reworks the alluvium in its floodplain
Old Stream:
-has a low gradient which results in a broad, flat floodplain
- meandering becomes significant
-stream is not usually near its valley walls
-meandering streams transport most of their load in suspension
-deposition and re-erosion produce more rapid changes in the stream

Question 21

Describe how meanders, point bars, and oxbow lakes are formed.

Answer 21

Erosion is focused at the outside of a river bend where velocity and turbulence are greatest. Over time, the outside bank is undermined, especially during periods of high water.
Point bars develop as debris acquired by the stream at the cut bank moves downstream and the coarser material generally deposits on the inside banks of the meanders.
Oxbow lakes form when the river erodes through the narrow neck of land of a meander, forming a new, shorter channel segment and cutting off the meander.

Question 22

What is an entrenched (incised) meander? What causes a rejuvenated stream to form?

Answer 22

Entrenched (incised) meanders are meanders which flow in steep, narrow valleys because and old-age stream has become “rejuvenated.” “Rejuvenated” streams are caused when an old-age stream’s base level drops or if the land is uplifted which causes downcutting to resume.

Question 23

What are stream terraces and how are they formed?

Answer 23

Stream terraces are remnants of previous floodplains. They form with rejuvenated streams.

Question 24

Describe some general factors involved in the formation and development of caves.

Answer 24

Acidic groundwater follows lines of weakness in the rock, such as joints and bedding planes. As time passes, the dissolving process slowly creates cavities and gradually enlarges them into caverns.

Question 25

In what kind of rock do most caves form? What acid is most important in cave formation?

Answer 25

limestone

carbonic acid

Question 26

Where do caves form relative to the water table? Where do speleothems form relative to the water table?

Answer 26

at/below the water table
above the water table (needs air)
“Although the formation of caverns takes place in the zone of saturation, the deposition of dripstone is not possible until the caverns are above the water table in the unsaturated zone.”

Question 27

List some characteristics of a karst topography.

Answer 27

shaped by the dissolving power of groundwater; produced by the action of acidic water over a large area

• caves
• disappearing (sinking) streams
• springs
• sinkholes
• Tower karst - form in wet climates where there are thick layers of highly jointed limestone. So much rock is dissolved away that only steep, tall towers are left.
• some natural bridges

Question 28

How is a natural bridge formed in a karst area?

Answer 28

from the collapse of the roof of a cave tunnel

Question 29

Thought question: If things happen in a meandering stream to cause the meanders to become entrenched or incised, the stream is often said to be “rejuvenated.” Why do you think this term is used?

Answer 29

because it’s an “old” stream which has been made “young” again

Question 30

In what direction do the meander loops of a meandering stream move? (Find in book.)

Answer 30

laterally and down the valley