Ch. 14 Review Questions

How are superposed and antecedent drainages similar? How are they different?

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How do people try to prevent flood damage? Are he methods successful? Why?

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What is the recurrence interval of a flood? Why can't someone say that "the hundred year flood happened last year so I'm safe for another hundred years"?

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Describe how deltas grow and develop. How do they differ from alluvial fans?

Deltas develop where the running water of a stream enters standing water, the current slows, the stream loses competence, and sediment settles out. Geologists refer to any wedge of sediment formed at a river mouth as a delta, even though relatively few have the triangular shape of the Nile Delta. Where a fast-moving stream abruptly emerges from a mountain canyon into an open plain at the range front, the water that was once confined to a narrow channel can spread out over a broad surface. As a consequence, the water slows and abruptly drops its sedimentary load, and a gentle sloping apron of sediment (sand, gravel, and cobbles) called an alluvial fan accumulates.

What is the difference between a seasonal flood and a flash flood?

A FLASH FLOOD is an event that occurs WITHIN 6 hours following the END of the causative event (such as rains, ice jams, or dam breaks) which result in fatalities, injuries, and/or significant damage to property. Examples of Flash Floods include damage to buildings, roads, gravel shoulders, bridges, railways or other landscape features including soil erosion. Generally, flash flooding events develop rapidly and can occur anywhere water collects, especially areas of steep terrain, and water runoffs. Flash Floods rarely last more than 12 hours. A FLOOD is an event that occurs AFTER 6 hours following the END of the causative event (rains, ice jam, dam breaks) which result in fatalities, injuries, and/or significant damage to property. Examples of Floods include damage to buildings, roads, gravel shoulders, bridges, railways or other landscape features including soil erosion. Generally, flooding events usually take longer to develop and they usually occur along or near larger rivers. The duration of flooding events may extend longer than 24 hours, perhaps several days.

How does a braided stream differ from a meandering stream?

A braided stream have numerous, subparallel braided channel strands. A meandering stream consists of a single highly sinuous channel. Thus, during normal flow, the sediment settles out and the channel becomes choked with sediment. As a consequence, the stream divides into numerous strands weaving back and forth between elongated mounds or bars of gravel and sand. The result is a braided stream. Meandering streams wind back and forth like a snake.

Distinguish between a stream's competence and its capacity.

A stream's competence is how big the particles are in a stream and capacity is how much sediment is transported.

How does a stream-eroded landscape evolve as time passes?

At first the streams in the mountain range have steep gradients, drop over numerous rapids and waterfalls, and flow in deep valleys. As time passes, however, slumps and landslides move debris down mountainsides and into stream channels, and floods carry the debris away. All the while, downcutting continues to lower the channel floor. Overall, erosion and transportation transforms the rugged mountains into low, rounded hills. As this happens, once-narrow valleys broaden into wide floodplains with gentle gradients. As more time passes, even the hills erode away, and the landscape evolves almost into a new plain at an elevation close to the streams' base level.

Why do canyons form in some places and valleys in others?

Canyons form where the rate of stream downcutting is faster than the mass wasting of the rock lining the walls of either side of the channel. Valleys form where less resistant rock yields to mass wasting at rates faster than stream downcutting.

Describe how meanders form and are cut off and the landforms result from the process.

Meandering streams form where running waters travels over a broad floodplain underlain with soft substrate, in a region where the river has a very gentle gradient. If the stream starts out straight, natural variations in the water's depth causes the fastest moving current to swing back and forth. The water begins to cut away at the outer arc of where it is moving the fastest. Each curve begins to move sideways and to become more pronounced over time.The stream continues to cut away at the outside of the bank, forming a cut bank. On the inside, sediment settles and a point bar is eventually formed. Eventually, the meanders get so big that they erode into eachother and cut off from the rest of the stream, forming an oxbow lake, or an abandoned meander (if it dries out).

What are the components of sediment load in a stream?

The sediment load consists of three components, dissolved load, suspended load, and bed load.

What factors determine the position of the base level? What is the difference between a local base level and the ultimate base level?

Upstream of the drainage mouth determines the position of the base level. Sea level determines the ultimate base level.