LYELL, CHAPTERS 10-11

Destructive Processes of Rivers

| Lyell Index | Present Inorganic Processes |

Chapter 10. Rivers: The destroying and transporting power of running water

Rivers may be considered as divided into two parts: upstream, where erosion and destruction predominates, and downstream, where deposition predominates. Lyell considers the downstream deposition of deltas in chapters 13-14, but first in chapters 10-11 he analyzes upstream erosion and transport.

Mechanisms of Erosion and Transport by running water:

• Water expands as it freezes, breaking up rocks (169).
• Water is the universal solvent. (Carries carbonic acid.)
• 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).

  Current terminology:
  The heft or specific gravity of a mineral reflects its degree of bouyancy. The specific gravity of quartz is 2.6, meaning that a chunk of quartz is 2.6 times heavier than an equal volume of water. Since a great deal of the earth's crust is quartz, its specific gravity makes a convenient reference point: minerals with a higher specific gravity are said to be heavy, minerals with a specific gravity lower than 2.6 are said to be light.

Illustrations of the excavating power of transported materials (170-171):

Widening of ravines into narrow valleys, of narrow valleys into broader valleys

 
• Background: Streams cut channels down into the rock, which steepens valley walls (downcutting). There are two kinds of valleys, narrow and broad, which result from the predominance of downcutting or lateral cutting, respectively. Lateral cutting by the river produces landslips and rockslides, widening the valley. As walls steepen, rock material falls into the stream bed by downslope movement, such as a landslide. Eventually the streams remove rock material by transport, like a conveyor belt. Typically, a valley is narrow in its youth and becomes wider with time.

   

• Salient and retiring angles
  • Focus: Narrow valleys result from the predominance of river downcutting. They are V-shaped, steep-walled, narrow-bottomed; their bottoms are about as wide as the stream.
  • Examples:
    Grand Canyon of the Colorado river (doesn't have to be "narrow"!)
    Grand Canyon of the Yellowstone river
  • In a narrow valley produced by downcutting, the angles or curves on one side of a valley correspond to the angles or curves of the other side.

   

• Meanders
  • Focus: Broad valleys result where lateral cutting prevails over downcutting, often in lowland areas. They are flat-bottomed, wider than they are deep.
  • Example: Mississippi

     

  • Meanders are the wide, sinuous curves a river makes back and forth across a flood plain, due to lateral cutting (170-171, 186-187). In antiquity the Meander River in Asia Minor gave rise to this name, since it exhibits pronounced "meandering." The velocity of the river is greatest on the outside of curves, which increases the lateral cutting. Lower velocities on the inner bank of a meander enhances deposition there. Meanders crawl along a valley, eventually migrating downstream because of the slope of the valley.

     

    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).

Transporting Power of Rivers

 
• Laminar flow
  • 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.
  • Example: Mississippi. Compare Lyell's discussion of this general law (173-174) with his description of the confluences of the Mississippi with the Missouri and the Ohio rivers (185). Is the Mississippi River widest at New Orleans? Why or why not?

   

• Illustrations of the transporting power of running water
   
  • Capacity to move boulders and large stones:
    • Scotland flood, 1829 (174-175). Name two means by which bridges are destroyed in floods.
    • If ice adheres to boulders (175), what is the effect upon a river's transporting power? See also the description of glacial moraines, derived from Saussure (176).

     

  • Capacity to excavate deep chasms through hard rock:
    • Valleys in central France (176-177). Streams have eroded through lava flows that once dammed their channels. What are the "decisive proofs" that the streams have done so gradually, contrary to those who invoke a sudden extraordinary event to account for this? (Lyell returned to consider this issue at length in volume 3.)

       

    • The Simeto river cutting through lavas of Mt. Etna in Sicily (177-179). Note Lyell's rhetorical fluorishes as he compares the Simeto channel to the "most ancient trap-rocks of Scotland." What was his point with this purple passage? What geologists likely were his target?

       

    • Niagara Falls (179-182).
      • What is it that creates waterfalls at Niagara rather than less dramatic river gradients such as cascades, rapids, or merely a steeply-inclined channel?
      • What causes the headward erosion of a river channel?
      • What is the evidence that the falls were once at Queenstown?
      • How far have they receded headward toward Lake Ontario?
      • At what time would they drain Lake Ontario, if they continue to recede at the current rate?
      • Why are extrapolative projections like this risky and prone to error?
      • Note: The channel downstream of Niagara Falls is an example of a "narrow valley" (described above).

  

  Chapter 11: Action of running water, continued.

  This chapter consists of several illustrations of the power of accelerated water. The examples explore how moving water decides its own course, regardless of what rocks, trees, or houses have to say about it.

   
  • Po
    • A major Italian river arising from a multitude of mountain tributariess. The Po has changed course often, even on a human time scale (183-185).
    • As early as the middle ages, people constructed banks to control the river and keep it from changing its course. Consider the effects of increasing the river height upon the following: water velocity, river erosion and transport capacity, rate of delta formation, and the rate of the silting up of river bed. Explain how this has led to the present situation, where in some places now the river is higher than the roofs of houses, much like old Roman aqueducts.
    • Natural levees are ridges of silt and sand naturally deposited along a river's banks during a flood. With each flood, they are raised up higher above the backswamp or lowland beyond, and sediment raises the riverbed higher as well. Eventually, the river will jump its banks and find a new way through the lowland to the sea. This results in building up the floodplain through widespread deposition; or, near the sea, the fan-shaped deposition of deltas, which Lyell considers later (and which McPhee explores in The Control of Nature, Part I).

     

  • Mississippi Basin
    • Lyell devotes a long section to this long river (cf. Mark Twain). The Mississippi's hydrographical basin "displays, on the grandest scale, the action of running water on the surface of a vast continent" (185-192).

       

    • Shape and location
      • Exemplifies the universal law that an increase in water volume does not necessarily lead to an increase in width (185; see above, p. 173).
      • Most tributary water flows into the Mississippi from the west (186).
        1. How does this fact account for the existence of limestone bluffs on the Mississippi's eastern banks?
        2. Why is it that the Mississippi river is migrating slowly eastward?

       

    • Floods
      • In the early nineteenth century the islands were surveyed and numbered (186-187). Would it have been wise to navigate by them in Lyell's day? What had happened to them?
      • What was the "raft" (187-188)? Where was it located? What was it made of? How large was it? How did it form? Was it floating or solidly anchored?
      • What are planters? "Sawyers" are snags that vibrate up and down, sometimes emerging above the surface.
      • Terrestrial vegetation becomes mixed with marine deposits because of floods (189-190). Should flooding in a river system like the Mississippi be regarded as an extraordinary event or as part of the ordinary course of nature? Why did Lyell engage in a long, rhetorical passage to make this point? In this context, what does Lyell mean by the phrase "analogy of the present course of nature"?

       

    • Lakes
      How can the gradual elevation of a river bed form lakes (190-191)? Describe the formation of many Louisiana lakes from Red River overflow in the spring.

       

    • Cooperating causes working in combination
      1. What other process is working to change river paths, to form lakes, and alter drainage patterns (191-192)?
      2. What was the center of the 1812 earthquake? Water briefly flowed backward (upstream) in the Ohio river as consequence of this earthquake!

  

  Floods due to bursting of lakes

  • The power water exerts on a valley over the long term depends more on which of the following: occasional obstructions that dam the river upstream, or ordinary river-water volume and velocity (192, 196)?
  • What is a common source of local deluges other than "ordinary" seasonal flooding (192)?

     

  • Examples:

    White Mountain landslide, New Hampshire, 1826 (p. 193-194).
    Three larger North American rockslides occurred after Lyell wrote:

    Gros Ventre rockslide, Wyoming, 1925.

    Near the Grand Tetons, occasioned by Gros Ventre river lateral cutting.

    Turtle Mountain rockslide, Alberta, 1903.

    Buried the town of Frank, triggered by an earthquake.

     

    Madison rockslide, Montana, 1959.

    Near Yellowstone, "occasioned by earthquake." 40 million cubic yards of rock buried a mile of river up to 220 feet deep. Three weeks later an upstream lake was 6 miles long and up to 190 feet deep.

    Current terminology:

    • Landslides may be either dry or wet, slow or fast, and made of larger or smaller components. (They may also occur under water.)
    •  

      Type of Flow	Wet or dry?	Fast or slow?	Particles small or large?
      Mudflow or debris flow	Wet	Fast	Smaller
      Debris slide or fall	Dry	Fast	Smaller
      Rockslide or rockfall	Dry	Fast	Larger
      Solifluction	Wet	Slow	Smaller
      Rock glacier	Wet	Slow	Larger
      Debris creep	Dry	Slow	Smaller
      Rock creep	Dry	Slow	Larger

       

    • Creep often results from frost heaving, where freezing expansion elevates soil and rock partic|es perpendicular to the slope, which upon thawing fall vertically down the slope.
    • Talus refers to piles of rock and debris from rock slides, usually half-cone shaped at the bottoms of cliffs or steep slopes.

     

    Valley of Bagnes ice-flow and dam, above Lake Geneva, 1818 (194-196).

     

    Tivoli flood over an artificial dike, 1826 (196-197).

    With the concluding paragraph about the Temple of Vesta Lyell's satirical and barbed rhetoric is displayed at its finest. How would you feel if Lyell had it in for you?