Streams and Rivers

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Assignments

Chernicoff, Chapter 13, "Streams and Floods."

Mark Twain, excerpt from Life on the Mississippi.

Reminders

Mineral Collections due Monday after spring break. Copies of the mineral template are available from a paper tray at the entrance to my office in Rm 202.

Reading Focus Questions

Read the entire chapter. Study the illustrations. Focus your attention on the following questions. Be prepared to answer any of these questions should they occur on a quiz or on the final exam. For multiple choice questions select the single best answer. (Parenthetical notes are my remarks of related facts not found in the textbook.)

True or False? The total amount of water on the Earth has remained fairly constant throughout recent geological time.
Study Figure 13-2, the water cycle.
True or False? The total area from which precipitation reaches a stream is known as a drainage divide.
True or False? The boundary between adjacent drainage basins is a watershed.
The first, smallest channels of a stream are known as:
1. distributaries
2. headwaters
3. tributaries
4. rills
True or False? The longitudinal profile of a stream from its headwaters to its mouth is characteristically convex.
A stream's vertical drop in elevation over a given horizontal distance is known as its:
1. discharge
2. turbulence
3. velocity
4. gradient
True or False? When considering stream velocity, we are interested in the time required for water to flow a certain distance and are not concerned with the volume of water the stream contains.
All of the following statements about stream velocity are correct EXCEPT:
1. Velocity of the water within a stream is uniform.
2. Due to friction, water velocity is greatest in the center just below the surface.
3. When a stream curves, water velocity is greatest on the outside of the curve.
4. Velocity increases downstream, after leaving steeper mountain gradients behind.

Supplemental Notes
(To be discussed in class. Quotes from Charles Lyell's Principles of Geology, 1830)
- Laminar flow
  - There is a "universal law" that "the velocity at the bottom of the stream is everywhere less than in any part above it, and is greatest at the surface" (172).
  - Therefore a question "naturally arises," as to "how the more tranquil rivers... flowing on comparatively level ground," can transport to the sea the load brought down into them by the mountain tributaries (173). How is effective transport on relatively level ground possible?
- River volume and surface width
  - A "general law" of rivers is that two equal streams, when combined, "do not occupy a bed of double surface" (173). Rather, "the space which it occupies (surface width) decreases relatively to the volume of water; and hence there is a smaller proportion of the whole retarded by friction against the bottom and sides of the channel. The portion thus unimpeded moves with great velocity, so that the main current is often accelerated in the lower country, notwithstanding that the slope of the channel is lessened" (173).
  - Example: McPhee, Control of Nature, p. , on the significance of water velocity for triggering debris flows from the San Gabriel mountains.
  - Is the Mississippi River widest at New Orleans? Why or why not?

True or False? The volume of water passing a given point on a stream bank per unit of time is known as the stream's discharge.
True or False? If the sediment load entering a graded stream increases or decreases, the stream's gradient will be altered to maintain sediment transport and flow velocity.
The collapse of a bank in a graded stream...
1. immediately decreases the sediment load
2. immediately reduces the channel gradient
3. reduces stream velocity
4. results in increased erosion and transport until equilibrium is restored
Construction of a dam in a graded stream causes an increase in...
1. sediment load
2. downstream erosion
3. stream gradient
True or False? A beheaded stream has lost its mouth.
A beheaded stream has lost its headwaters as a result of:
1. aggradation
2. trellis drainage
3. stream piracy
4. meander scars
The term meander refers to a stream that:
1. winds across a flat landscape
2. carries a large sediment load
3. carries a small load
4. has many sand and gravel bars
(Meanders do not remain in the same place, but migrate, continually reworking the surface of the floodplain. Study Figure 13-10 and reflect on this question: In which direction would meanders tend to migrate, upstream or downstream?)
An oxbow lake is formed by:
1. stream piracy
2. formation of a water gap
3. deposition in a meander scar
4. cut-off of a meander loop
Read the excerpt from Mark Twain, Life on the Mississippi. Explain how Twain's humor reflects a basic knowledge of meanders in the Mississippi.
- Meander terminology:
  
  Isthmus (on the Mississippi, called a "cut off").
  
  A narrow bridge of land between two adjacent meander loops. If the downstream meander has encountered more resistant rock, the upstream meander will overtake it and cut through the isthmus, forming an oxbow lake.
  
  Island.
  
  Arises if a river cuts through an isthmus while also continuing to follow the meander.
  
  Oxbow lake.
  
  Forms from an abandoned meander whose ends silt up.
  
  Meander scar.
  
  A dried up oxbow lake, perhaps filled with vegetation.
  
  Braided stream.
  
  Some streams are become braided rather meandering if the sediment load is great.
  
  Incised (or entrenched) meanders
  
  Incised meanders form when there is downcutting with little lateral cutting.
  Example: Moselle River meanders 600-800 feet deep in rock.
  G. Poulette Scroupe, "Excavation of Valleys": a great flood or a single strong torrent would produce straight river channels; incised meanders are evidence of gradual, progressive formation (171-172).
How might you calculate the future date when Niagara Falls will drain Lake Erie?
- What uncertainties attend such a prediction?
True or False? Most of the sediment in a stream is carried as the dissolved load.
Fine, solid particles are transported by a stream as a _____________ load.
1. suspended
2. bed
3. dissolved
4. saltatory
Sand grains in a stream's bed load move with a bouncing motion known as:
1. hydraulic lifting
2. dissolution
3. saltation
4. traction
Hydraulic lifting is caused by a stream's:
1. dissolved minerals
2. sediment load
3. water pressure
4. drainage pattern
- Note: Suspended materials weigh less in water than they would in air. An added "mechanical power is obtained by the attrition of sand and pebbles, borne along with violence by a stream" (170). "We are often surprised at the facility wherewith streams of a small size, and which descend a slight declivity, bear along coarse sand and gravel; for we usually estimate the weight of rocks in air, and do not reflect sufficiently on their comparative bouyancy when submerged in a denser fluid. The specific gravity of many rocks is not more than twice that of water, and very rarely more than thrice, so that almost all the fragments propelled by a stream have lost a third, and many of them half of what we usually term their weight" (172). Quotes from Charles Lyell, Principles of Geology (1830).
Deposited stream sediments are known collectively as:
1. alluvium
2. bedrock
3. bed load
4. levees
Note: The concluding section on Mars will help you to answer some of the focus questions for The Mars Show. The Mars Show presents photographs and animations of some of the features described here. The surface of Mars:
1. receives a very small amount of rainfall
2. has many river-related landforms
3. has floodwaters during its frequent ice melts
4. has permanent, water-filled braided streams with teardrop-shaped islands

Class Agenda

Set time for special daytime presentation of The Mars Show.
Review focus questions (and supplemental notes).
View slides as time permits.