Chapter 17- Streams and floods textbook
Terms in this set (32)
What role do streams have during the hydrologic cycle?
Streams collect water from runoff and discharged groundwater, and they conduct it from elevated regions of the continent down to the sea.
A drainage network that look similar to a branching tree and occur when streams cut through uniform material with a relatively constant slope.
Dendritic drainage network
A drainage network that resembles the spokes of a wheel and occurs when streams drain a central, conical mountain.
Radical drainage network
A drainage network that consists of streams that make right-angle turns, with multiple streams commonly traveling in parallel; they occur in areas where perpendicular sets of joints present paths of least resistance to flow.
Rectangular drainage network
A drainage network that has an appearance similar to vines creeping up a trellis and occurs in valley and ridge topography, with major tributaries inhabiting the valleys and traveling parallel to the ridges, and minor tributaries draining the hillsides transversely before their flows join the valley streams.
Trellis drainage network
Streams that constantly have a flow of water
Streams that are at least seasonally dry, typically flowing only after snowmelt or rainfall.
How does discharge vary according to the stream's length, and how does weather affect discharge?
In a moist climate, discharge increases steadily due to the addition of water from tributaries and rainfall on the journey to the sea. In a dry climate, discharge may decrease downstream as the stream is subjected to intense evaporation and relatively little rainfall.
Why is average downstream velocity always less than maximum downstream velocity?
At the contact with sediment, water encounters friction and moves more slowly than at the surface near the middle of the stream.
Describe how streams and running water erode the Earth's surface.
Running water can break and lift particles off the stream bottom and also use these particles as abrasive tools to scratch away additional sediment. Water can dissolve some minerals within the streambed.
What are three components of sediment load in a stream?
Dissolved load (ions and other material in solution), suspended load (small, floating grains), and bed load (larger grains moving along the stream bottom).
Distinguish between a stream's competence and its capacity.
Competence is a measure of stream energy and refers to the largest particle size that a stream can carry; capacity is the amount of sediment that a stream can carry.
Describe how the character of a drainage network changes, along its length, from headwaters to mouth.
Drainage networks near their source consist of many small tributary channels. With increasing distance from the source, these channels merge to form a single main channel. Near the delta of a river, the main channel becomes choked with sediment and is broken into many distributary channels that conduct water and sediment into the ocean.
the ultimate base level; streams do not generally erode below sea level.
Occurs where the downcutting ability of a stream is limited to a surface that is above sea level.
A local base level
All produce local base levels for streams
Natural lakes and artificial reservoirs, waterfalls atop resistant rock layers, and larger parent streams
Streams that are choked with relatively coarse sediment; they readily spill through their banks to form numerous, subparallel braided (intertwined) channel strands.
A stream that consists of an isolated, highly sinuous channel.
A meandering stream
The broad, flat region surrounding a river that is not actively downcutting; it is carved by lateral erosion of meander cutbanks and built up by deposition of sediments during floods.
Describe how meanders form, develop, are cut off, and then are abandoned.
All streams will waver from a straight line at some point. When they do, the outer bend of a turn has relatively fast flow (becoming a site of erosion) as compared to the inner bend (which becomes a site of point bar deposition). Subsequent erosion and deposition push the stream channel farther and farther off a straight course. However, surrounding cut banks can encroach each other over time, and streams can change course due to earthquakes or floods. These changes yield a shorter, straighter segment that draws flow away from the meander and isolates it, forming an oxbow lake. Over time, oxbow lakes will either dry up (arid climate) or infill with flood sediment (wet climate) to form abandoned meanders.
What is stream piracy?
Stream piracy occurs when two channels come to intersect, with the stream designated as the pirate stealing flow from the captured stream.
What causes drainage reversal?
Drainage reversal is caused by the uplift of new mountain ranges that disallow streams to continue in their previous course.
How does a stream-eroded landscape evolve over time?
Early after an episode of regional uplift, downcutting stream erosion combined with mass wasting produces steep V-shaped valleys among a rugged, mountainous landscape. With time, streams approach base level and cease downcutting, producing side-to-side erosion and flattening the elevated landscape to form a broad, low-lying floodplain. Eventually, the floodplain will widen and form a very broad plain just above base level, called a peneplain.
Describe how deltas grow and develop. How do they differ from alluvial fans?
Rivers carrying sediment meet an ocean or large lake in which the water is stagnant. Flow velocity is slowed dramatically at the interface, and the stream loses its competence, its sediment load falling out to form a wedge. Alluvial fans form at the foot of a mountain range, often from the deposits of ephemeral streams. The sediments of an alluvial fan are generally coarser than those of a delta, but each commonly displays a fanlike shape.
How are superposed and antecedent drainages similar?
Superposed and antecedent drainage are similar in that they cut across an existing mountain range.
What activities tend to increase flood risk and damage?
Building in floodplains and replacing surface sediments with asphalt and concrete increase the prevalence of floods and the damage they cause. Localized flood control measures (such as levees and channelization) can worsen flooding in downstream environments.
What is the recurrence interval of a flood, and how is it related to the annual probability?
The recurrence interval (R) for a flood is the average waiting time between successive floods that are equally great (or greater). The annual probability for such a flood is 1/R (often expressed as a percentage).
How have humans abused and overused the resource of running water?
Humans have polluted waterways, overdrawn streams for irrigation, altered sediment supply and ecosystems due to dam construction, and urbanized and agriculturalized floodplains.
the flow of a fluid when each particle of the fluid follows a smooth path, paths which never interfere with one another.
What do lakes, rapids, waterfalls, and terraces indicate about the stream gradient and base level?
Lakes/reservoirs can act as a local base level along a stream. When a natural or artificial dam impedes stream flow, the stream adjusts to the new base level. Erosion takes place downstream from the dam (especially if it is a natural dam and water can flow over the top). Just upstream from the dam the velocity of the stream is lowered so that deposition of sediment occurs causing the gradient to become lower. The dam essentially become the new base level for the part of the stream upstream from the dam.
Irregular flow that is characterized by tiny whirlpool regions. The velocity of this fluid is definitely not constant at every point.
Why do canyons form in some places and valleys often?
Because geologic processes stack strong and weak rocks, such stratigraphic variation often yields a stair step profile of the canyon walls, as seen in the Grand Canyon. Strong rocks yield vertical cliffs, whereas weak rocks produce more gently sloped canyon walls.
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