43 terms

Geology Chapter 7

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Alluvial Fan
A fan-shaped wedge of sediment that typically accumulates on land where a stream emerges from a steep canyon onto a flat area. In map view it has the shape of an open fan. Alluvial fans typically form in arid or semiarid climates. Shown in the photo is the Badwater Alluvial Fan of Death Valley.
Braided Stream
A stream consisting of multiple small, shallow channels that divide and recombine numerous times forming a pattern resembling the strands of a braid. They form where the sediment load is so heavy that some of the sediments are deposited as shifting islands or bars between the channels.
Carrying Capacity
The maximum number of organisms in this case people that a system can maintain on a continuing basis. This number depends to a large extent on the quality of life that is expected to be maintained. The higher the quality the smaller the number.
Channel
The bed where a natural body of surface water is usually present and the main current normally flows.
Cross-section
A graphic representation of the intersection of the geological bodies in the subsurface with a vertical plane of a certain orientation. It is a section of the terrain where the different types of rocks, their constitution and internal structure and the geometric relationship between them are represented.
Cut Bank
Also known as a river cliff or river-cut cliff, is the outside bank of a water channel (stream), which is continually undergoing erosion.[1] Cut banks are found in abundance along mature or meandering streams, they are located on the outside of a stream bend, known as a meander, opposite the slip-off slope on the inside of the bend.
Delta
A deposit of sediment that forms where a stream enters a standing body of water such as a lake or ocean. The name is derived from the Greek letter "delta" because these deposits typically have a triangular shape in map view. At a delta, streams often branch into "distributaries" that distribute their water and sediment load in multiple directions. Shown in the photo is the Nile Delta, where the Nile River enters Mediterranean Sea. The green vegetation of the well-watered delta contrasts sharply with the light brown sands of adjacent lands.
Deposition
The settling from suspension of transported sediments. Also, the precipitation of chemical sediments from mineral rich waters. Shown in the photo is the Badwater Alluvial Fan of Death Valley, where sediments are deposited as a stream, flowing down a steep slope encounters the flat surface of the valley, loses energy and drops its sediment load.
Discharge
The volume of water in a flowing stream that passes a given location in a unit of time. Frequently expressed in cubic feet per second or cubic meters per second. Calculated by the formula Q = A x V - where Q is the discharge, A is the cross sectional area of the channel and V is the average velocity of the stream. The hydrograph at left shows a peak discharge of a little over 200 cubic feet per second.
Drainage Basin
The geographic area that contributes runoff to a stream. It can be outlined on a topographic map by tracing the points of highest elevation (usually ridge crests) between two adjacent stream valleys. Also referred to as a "watershed".
Drainage Divide
The boundary between two adjacent drainage basins. Drainage divides are ridge crests (or less obvious locations where slope of the landscape changes direction). Runoff produced on one side of the ridge flows into stream "A" and runoff on the other side of the ridge flows into stream "B". The image shows the continental drainage divides of the United States.
Erosion
A general term applied to the wearing away and movement of earth materials by gravity, wind, water and ice.
Floodplain
Also called Alluvial Plain, flat land area adjacent to a stream, composed of unconsolidated sedimentary deposits (alluvium) and subject to periodic inundation by the stream. They are produced by lateral movement of a stream and by overbank deposition; therefore they are absent where downcutting is dominant.
Gradient
Slope of a stream bed or hillside. The vertical
distance of descent over horizontal distance of slope.
Hydrograph
A graph that shows the change of a water-related variable over time. Example: A stream discharge hydrograph shows the change in discharge of a stream over time.
Longitudinal Profile
A cross section of a stream or valley beginning at the source and continuing to the mouth. These profiles are drawn to illustrate the gradient of the stream.
Meander
A sharp bend, loop or turn in a stream's course.
When abandoned, called a meander scar or an oxbow.
Natural Levee
A mound of sediment that parallels a stream channel forming a levee-like deposit. When flood waters leave the normal stream channel and enter the flood plain there is a reduction of velocity that causes suspended sediments to fall to the bottom, producing this deposit.
Oxbow Lake
A crescent-shaped lake that forms when a meandering stream changes course. Such changes in course frequently occur during flood events when overbank waters erode a new channel.
Point Bar
Accumulations of sand and gravel deposited
in slack water on inside of a winding or meandering river.
Runoff
Liquid water moving over the land surface as a sheet or channelized flow. The portion of precipitation that moves over the ground instead of evaporating or infiltrating.
Sediment
A loose, unconsolidated deposit of weathering debris, chemical precipitates or biological debris that accumulates on Earth's surface.
Tributary
A stream feeding, joining, or flowing into a larger stream.
Watershed
The geographic area that contributes runoff to a stream. It can be outlined on a topographic map by tracing the points of highest elevation (usually ridge crests) between two adjacent stream valleys. The watershed of a large river usually contains the watersheds of many smaller streams. Also referred to as a "drainage basin".
Flash (or upland) Flood
A flood that rises and falls very rapidly.
Infiltration
The movement of surface water downwards into porous soil. The image shows the infiltration of water into the ground from the channel of a stream.
Lag time
The delay in the response of stream flow between
precipitation and flood peak.
Recurrence Interval
The average time interval between occurrences of a hydrologic event, such as a flood, of a given or greater magnitude.
Regional (or lowland) flood
normally occur on a seasonal basis when winter and spring rains combine with melting snow to overfill river basins and flood the banks. They also occur during periods of excessive rain when the rain saturates the soil and the runoff overflows streams and rivers.
Regulatory Floodway
Means the channel of a river or other watercourse and the adjacent land areas that must be reserved in order to discharge the base flood without cumulatively increasing the water surface elevation more than a designated height.
water velocity and stream discharge
What two things control the rate at which a river can erode?
features of low-gradient part of a stream
a low gradient part of a stream means that it is a lowland stream, which means it will have these features:
meander, cut bank, flood plain, oxbow lake
How does the cross-sectional profile of a river vary as you go from the upland area to the lowland area?
upland streams have: high gradients, v shaped valleys, higher velocities, carry larger particles

lowland streams: low gradient, floodplain, meander, velocity varies with channel shape
If you build a dam across a river creating a reservoir, what happens to the longitudinal profile? Eventually, what happens to the reservoir?
it would increase and eventually the reservoir would get filled then turn into a flash flood and the dam would break
How does the hydrograph for a regional or low-land flood differ from that of a flash or upland flood?
plot of discharge vs time or elevation of water vs time

lowland floods:
-Delta-plain floods
-Floodplain floods
-Spring time

upland floods:
-Intense rain events
-Dams breaking
-Any time of year
-cause most flood-related deaths in US
What should you do if you are hiking in a narrow river valley and you hear a flash flood headed down the valley?
climb to safety
How deep does water have to be in order to make a car float?
2 feet
What is a 100-year flood?
the chance of a flood happening each and every year is 1 in 100
What is the impact of urbanization on flood frequency and the sizes of floods?
increases the frequency of 100 year floods. more paving causes less infiltration and sewers causing more runoff
How does the hydrograph for a river change after urbanization? Why?
directing runoff into the stream instead of into the ground. in a forrest water can soak in to the ground but in cities water cannot soak in so it has to go into the stream. the peak is higher and sooner
structural approaches to mitigating floods.
- Levees and floodwalls
- if they fail the damage could be worse
- Dams
- could significantly change the habitat in the river corridor
List two "nonstructural approaches" to mitigating floods. List problems associated with each one.
- Relocation and voluntary buyouts
- Sustainable floodplain management
flood-proof a building.
Raising the lowest floor above the elevation that water would reach during a 100 year flood.
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