Geology Chapter 18
A
A 100-year old flood is more catastrophic than a 50-year flood a) True b) False
potholes
A hemispherical hole in the bedrock of a streambed, formed by abrasion by small pebbles and cobbles rotating in a swirling eddy.
A
A meander that is cut off to become completely isolated from the main channel, but which retains water, is a(n) ____. a)oxbow lake b) meander neck c) cutbank d) abondoned meander
A
All else being equal, water will flow faster in a stream with ____. a) a straight channel b) well-developed meanders
C
Braided streams____. a) have flowing water either episodically or during a portion of the year b) have a channel that is highly sinuous (curvy) c) consist of a series of intertwined channels that are overloaded with sediment d) are those that divert flow from streams that they have, thoguh steam erosion, intersected
C
Divides such as high ridges are often divides outlining drainage basins; why might a sream cross a topological high such as a ridge? a) it might have established its course before tectonic forces formed the ridge b) erosion may gradually remove sediment from a region with a stream already established by flow over horizontal strata, gradually revealing an underlying resistant bad c) both answers are correct d) neither are correct
B
Ephermal streams _____. a) consist of a series of intertwined channels that are overloaded with sediment b) have flowing water either episondically or during a period of the year c) have a main channel that is highly sinious (curvy) d) are those that "steal" water from other streams which they intersect through erosion
A
Given enough time, the stream profile adjusts to the: a)elevation of the mouth b) eleveation of the source c) elevation of resistant rocks along the stream course d) common profile for all streams
they occur at many scales --> continent has several that are separated by major divides
How big are drainage basins?
thousands of Km in area
How big can a delta be?
the influences of temperature and precipitation on weathering and erosion
How does climate affect a longitudinal profile?
D
How might ocean currents or tides affect delta formation a)strong currents can transport sediment along the shore, turning the delta into a small feature like a beach b) tides may alter the delta shape, creating long bars of sand c) tides may prevent delta formation d) all of these answers are correct
C
In a delta, which of the following sediment layers forms gentle inclines? a) topset beds b) bottomset beds c) foreset beds d) none of the above
D
Moving away from its headwaters the flow of a stream becomes more: a)turbulent b) chaotic c) competent d) laminar
true
T/F: a stream can show turbulent flow over much of its width and laminar flow along its edges where the water is shallower and slower
False- happens in many types of flows
T/F: meandering only happens with streams?
True
T/F: the erosion of unconsolidated material is relatively easy to observe
B
The deltas of all major rivers consist of multiple radiating lobes of sediment in a "bird's foot" configuation, similar to the Mississippi River delta a) True b) false
B
The flat-lying area surrounding a river channel is termed the: a) base level b) floodplain c) stream gradient d) thalweg
C
The lowest elevation to which a stream can downcut is the: a) floodplain b) stream gradient c) base level d) thalweg
B
The sediment load of a stream consists of only those grains that are fine enough to stay in suspension a) true b) false
D
The toal sediment load carried by a stream is called: a) competence b) package c) saltation d) capacity
B
Ultimately the base level of a stream valley can be no lower than _____. a) the average elevation of the continent on which it is found b) sea level c) the average elevation of the ocean basins
-topography -climate -streamflow -resistance of rock in streambed to weathering+erosion -sediment load
What 5 factors control the dynamic equilibrium in a stream?
-uplift in drainage basin -crustal subsidence *why waves and tides can prevent a delta from forming as well as plate tectonic processes
What are the two preconditions for delta formation?
velocity, geometry (depth), viscosity (resistance to flow)
What determines is a stream is laminar or turbulent?
-climate of the region -width of the floodplain -size of the channel
What does the recurrence interval of floods of a certain discharge depend on?
streams have steep walls and channel occupies the whole valley; actively cutting the bedrock along with chemical and mass weathering
What happens in high mountains with a stream?
streams shapes a valley by eroding sediment particles and transporting them downstream
What happens in the lowlands with a stream
can cause damaging floods or destroy wetlands (without it there are floods but that are more frequent, but are also less damaging)
What happens when engineers artificially straighten and confine a meandering river?
steams with high sediment load and with banks that are easily eroded
What types of streams/environments do braided streams form?
where hard rocks resists erosion or faulting offsets the stream bed
Where do waterfalls develop?
at any location where rainfall or discharging groundwater contributes to the stream
Where is discharge matches with recharge
B
Which as a greater capacity? a) a small mountain stream b) the mississippi river
A
Which has greater competence a) a small mountain stream b) the mississippi river
B
Which of the following events would lower the base level of a major river? a) damming of the river b) worldwide glaciation c) local tectonic subsidence d) melting of the world's glaciers
B
Which of the following terms describes the intermittent jumping motion of sand grains along a river bottom? a) meandering b) saltation c) suspension d) bounce
they leave behind standing pools of water that evaporate and yield fine-grained flood plain deposits that are rich in mineral and organic nutrients = good for agriculture
Why are receding flood waters potentially good?
they sore floodwaters and provide habitat for many diverse species of plants and animals
Why are the wetlands found in delta planes valuable natural resources?
oxbow lake
a crescent shape, water-filled loop
meanders
a curve or bend in a stream that develops as the stream erodes the outer bank of a bend and deposits sediment against the inner bank
floodplains
a flat area about level with the top of that channel that lies on either side of the channel
terraces
a flat, step like surface in a stream valley that parallels a stream above its floodplain, often paired one on each side of the stream, marking a former floodplain that existed at a higher level before regional uplift or an increase in discharge caused by the stream to erode into the former floodplain
turbulent flow
a more complex patter on movement, in which streamlines mix, cross, and form swirls and eddys
dendric drainage
a random drainage pattern--> is typical of terrains where the bedrock of a uniform type such as horizontally bedded sedimentary rock or massive igneous or metamorphic rock
Natural leeves
a ridge of coarse material built up by successive floods that confines a stream within its banks between floods, even when water levels are high
divide
a ridge or high ground along which all rainfall runs off down one side or the other
graded stream
a stream in which the slope, velocity, and discharge combine to transport its sediment load, with neither net sedimentation nor net erosion in the stream or its floodplain
tributaries
a stream that discharges water into a larger stream
superposed stream
a stream that erodes a gorge in a resistant formation because its course was established at a higher level on uniform rock before downcutting began
antecedent stream
a stream that existed before the present topography was created and so maintained its original course despite the changes in the structure of the underlying rock and in the topography
braided streams
a stream whose channel divides into an interlacing network of channels, which then rejoin
bottomset beds
a thin, horizontal bed of mud deposited seaward of a delta and then buried by continued delta growth
downstream erosion
a type of erosion that is much less common and best expressed in rare catastrophic events, such as well an earthquake collapses a natural dam and sends scouring waters plunging downstream
suspended load
all material temporarily or permanently suspended in the flow of a current
drainage basin
an area of land, bounded by divides that funnels all its water into the network of streams draining that area
floods
an extreme case of increased discharge that results from a short-term imbalance between inflow and outflow
saltation
an intermittent jumping motion along the bed of a stream
stream
any body of water that flows over the land surface, large or small
xxxx
as a delta builds outward into the ocean the mouth of its river advances seaward, leaving new land in its wake
delta
as the floor of the lake or ocean slopes to deeper water away from the shore, the deposited materials build up a large, flat-topped deposit
x
because most streams and rivers and broad and deep and flow quickly, their flows are always turbulent
point bar
curved sandbars on the inside of banks where current is weaker
rectangular drainage
drainage developed on a strongly jointed rocky terrain, tends to follow the joint pattern
trellis drainage
drainage that develops in a valley and ridge terrain, where rocks of varying resistance to erosion are folded into anticlines and synclines
dunes
elongated ridges of sand up to many meters high that form in flows of wind or water over a sandy bed
competence
flow's ability to carry material of a given particle size
by the addition of sediment and sink as the sediment becomes compacted and Earth's crust subsides under weight of sediment load
how do deltas grow?
as a current moves sand grains by saltation they are eroded from the upstream side and deposited on downstream
how do dunes migrate?
the relationship between size and force the flow exerts on particles suspended in the bed loads *use to help design dams and bridges
how do you study how a stream carries sediments?
B
in the region surrounding an isolated volcano, a _____ drainage network is expected. a) dendritic b) radial c) rectangular d) trellis
A stream that has cut deeply into the curves and bends of its channel
incised meanders
downstream (increased width, depth, velocity)
is discharge for most areas increased upstream or downstream?
channels
long well defined troughs that allow water to flow over long distances
topset beds
materials deposited on top of the delta, typically sand
radial drainage
patterns develop on a single large peak, such as a large dormant volcano
rapids
places in a stream where the flow of velocity increases because the slope of the streambed suddenly steepens, typically at rock ledges
D
potholes in river bottom bedrock form by: a) cascading water from a waterfall that wears away the rock b) the impact of a large rock moved by a small current that makes a crater c) expansion of ice in cracks d) the grinding action of a pebble or cobble in a swirling eddy
alluvial fans
sharply defined mountain fronts, typically at steep fault scarps, streams drop large amounts of sediment in cone or fan shaped accumulations
ripples
small dunes whose long dimension is formed at right angles with the current
distributaries
smaller streams that receive water and sediments from the main channel, branch off downstream, and thus distribute the water and sediment into many channels
xxxxx
streams are dynamic geosystems that are continually changing in response to the influences of climate and plate tectonic processes
headward erosion
streams progressively cut their channel upstream into higher land, commonly accompanies widening and deepening of valleys
base level
the elevation at which a stream ends by entering a large standing body of water, such as a lake or ocean, or another stream
valley
the entire area between tops of the slops on both sides of a stream
abrasion
the erosive action that occurs when suspended sediment particles move along the bottom and sides of a stream channel
xx
the greater the velocity of a current, the larger the particle it can carry as suspended load and bed load
river
the major branches of a large stream system
bedload
the material the current carries along the bed by sliding and rolling
drainage network
the pattern of connection of all the large and small streams in a drainage basin
laminar flow
the simplest kind of fluid movement, straight or gently curved streamlines run parallel to one another without mixing or crossing between layers
longitudinal profile
the smooth, concave-upward curve that represents a cross-sectional view of a stream, from notably steep near its head to almost level near its mouth
settling velocity
the speed with which suspended particles of various weights settle to the bed (low for small grains, high for big grains)
discharge
the volume of water that passes a given point in a given time as it flows through a channel of a certain width and depth
capacity
total sediment load carried by a flow
xxx
very high velocities will erase the dines and form a flat bed below a dense cloud of rapidly saltating sand grains
velocity and volume
what affects the competence and capacity of a stream?
discharge =cross section x velocity
what is the formula for discharge
we don't know
what makes streams form incised meanders versus wider planes?
sand grains --> smaller grains
what types of grains are most likely to be moved with saltation
noncohesive particles
what types of particles are easier for a current to life (cohesive or noncohesive)
at the floodplain
where does water reach at high levels?
where the rock layer is horizontal and softer rocks make up the lowest layers
where is erosion by falls the fastest?