Geology 102 Midterm 2 Study Guide
Geology 102 Midterm 2 Study Guide 102
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This 5 page Study Guide was uploaded by Chloe Nightingale on Thursday February 18, 2016. The Study Guide belongs to 102 at University of Oregon taught by Marli Miller in Winter 2016. Since its upload, it has received 35 views. For similar materials see in Geology at University of Oregon.
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Date Created: 02/18/16
GEOLOGY MIDTERM 2 Rivers ∙ Drainages and drainage networks: evolving from sheet wash to channels that grow by headward erosion Shape of a river profile, drainage networks, watersheds, divides, Continental divides, Great Basin §Begins with sheet wash (water flowing like a sheet) §Irregularity causes increased erosion §Small channel develops §hannel grows by headward erosion § evelopment of tributaries o Watershe: The area drained by a given trunk stream o Divid:A ridge or other topographic feature that separates two adjacent drainage basins. It is an imaginary line that separates two different directions of surface water flow. o Headward erosio: Takes place at steeper areas o Continental div:A continental d is a mountairidg on contine. On one side of the divide, all the water genersnowfa feeds into rivers that ocea,ba, or sea. The rivers on the other side of the divide feed into a diff rent ocean, bay, or sea. o Great BasinWestern US, no drainage to ocean o iver profile: o Drainage networks: ∙ Shape of drainage network: how does it relate to the bedrock below? o ∙ Concept of base level o Marks a local elevation below which the river channel above it can’t erode. ∙ Ephemeral vs. permanent streams: what are they and how do they relate to groundwater? o Ephemeral: Only flows after rainfall events o Permanent: Has a base that is below the water table and flows all year, typically found in temperate climates. ∙ River Discharge: what is it and how do we figure it out? Why is it important? o ∙ Rivers and sediment transport: dissolved load, suspended load, bed load. How does the grain size of each of these types vary? How does bed load actually move? o issolved l: Dissolved sediment o Suspended lo: Sediment is fine grained and suspended in the water o Bed loa: Dragged and pushed along the bottom of the channel, coarse grained, moved by “saltation,” or bouncing and rolling. ∙ eposition and erosion of sediment o Erosio occurs when a river load is below its capacity o Depositio occurs when it’s above its capacity ∙ How does water velocity and thus a river’s carrying capacity vary across a meander bend? o Fast water and erosion on the outside and slow water and deposition on the inside How does watertransported sediment (alluvium) change downstream from its source? o Sediment becomes finer grained, more rounded, rich in quartz (resistant to chemical weathering) What happens to a river’s gradient, discharge, and velocity downriver from its source? o Discharge increases o River gradient decreases o Velocity increases River types and environments: Alluvial fans, braided rivers, meandering rivers, floodplains, deltas, oxbows, abandoned channels, meander bends, meander cutoffs, tributaries, distributaries, lakes –know all those terms and what they are. o Alluvial fan or coneshaped deposit of sediment crossed and built up by streams. If a fan is built up by debris flows it is properly called a debris cone or colluvial fan. o raided rivers: o Meandering rivers: o Floodplains: o Deltas: o Oxbows: o Abandoned channels: o eander bends: o Meander cutoffs: o Tributaries: o istributaries: o Lakes: Erosion of Waterfalls as example of headward erosion. o Waterfalls and boulders that drop down from the top erode the cliff underneath, creating an overhang Rivers and Floo: ng When and why do rivers flood? What causes them? Besides their damage, how are they beneficial? o Caused by heavy rainstorms, prolonged heavy rains, dam breaks, and snowmelt o Benefits: Replenishes soil by depositing finegrained sediment on floodplains, flush out toxins. Seasonal vs flash floods and representation on hydrographs. o Season: rainy seasons, expected, temperate climates, wide area o Flash flo: Sudden, extreme, brief, small area, localized, arid climates How have floods changed as the landscape has become more urbanized? o Flood frequency is assessed by annual probability and recurrence interval, which are related to each other by a simple equation. What does each mean? o Annual probab means the probability of flooding in a given year §2% = 0.02% chance of flooding in the given year o Recurrence int: The average number of years it takes for a flood to occur Flood Control measures: levees, channeling, dams –each has benefits, each has drawbacks o Levee § rtificially deepens the channel §Can make things worse by moving the flood downriver §Can also promote a false sense of security o Channeling §Lining a channel with concrete or cutting off meander bends §Floodwaters can move out more quickly §A problem because water goes downriver without buffering § dverse impact on environment o ams §About 75,000 in the US §Hold floodwaters back §Support irrigation, recreation, and hydropower §Problems with short life expectancy (2050 years), increased erosion downriver because it fills with sediment §River has unfulfilled carrying capacity §Ecology effected § ams can fail ∙ City planning, using floodplain maps help us live with floods. o ∙ Flood control on the lower Mississippi River: its long term effects, especially on the delta and the relation of the Mississippi to the Atchafalaya. o Groundwater ∙ Make sure you know the meaning of the saturated zone and water table o Saturated zo: Where the pores are filled o ater tab: ∙ How does groundwater move through the saturated zone o Sinks into the zone of saturation, flows along curving lines from high to low hydraulic head, discharges to stream ∙ Hydraulic head, hydraulic gradient, porosity (primary and secondary), permeability o Hydraulic hea: The top of the water table o Hydraulic gradit: How steep the water table is o Primary porosi: Measure of the rock’s original porosity o Secondary porosi: Porosity that came after the rock was formed (caused by faulting, fracturing, etc.) o Permeabili: Ability of the rock to let water move through it. Depends on the interconnectedness of the pores and the natural conduits (fractures, etc.). The coarser the grain size, the more permeable the rock. §Order of permeabi (least to most): Shales, siltstone, sandstone, conglomerates. ∙ ontrols on porosity and permeability o ∙ Aquifers and Aquitards, confined and unconfined aquifers o Aquif: Rock or sediment with high porosity and high permeability § Confin: Doesn’t have passage to surface § nconfin: Open to land surface o Aquitar: Impedes the flow of water (low permeability) ∙ Wells and springs, meaning of artesian, locations of springs o Wel Hole that goes down to the water table o Sprin: Occurs when the water table intersects the land surface oArtesi: Well that rises above the water table o Location of sp: Discharge zones, fault zones juxtaposing an impermeable layer, water perched on an aquatard, at intersecting fractures, and at alluvial fans ∙ roundwater as a resource oWest of the middle meridia in the US, agriculture needs groundwater oRenewable, but takes time to replenish and is being used at a faster rate than it’s being recharged o High plains aq: Covers most of South Dakota, Kansas, Oklahoma, and much of North Dakota and North Texas. Varies from 2001000 feet thick and is being depleted at about 2 feet per year. ∙ Groundwater makes up ~30% of Earth’s freshwater, so heavily utilized as a resource ∙ When pumping exceeds recharge, wells develop a cone of depression –can lower water table o Cone of depres: Extraction is greater than recharge and the water table lowers ∙ ASR: Aquifer Storage and Recovery –becoming increasingly important oTakes surplus surface water and injects it into the aquifer oIn the PNW, during warm winters precipitation comes as rain, which makes for surplus water. ∙ Mesa Verde, Colorado –as an example of surface and groundwater resources o Springs and alcoves form at the top of aquitards o Scattered shale lenses intrude on sandstone Karsts ∙ Landscape dominated by dissolution of limestone or other easily soluble rock. ∙ Weakly acidic groundwater can dissolve limestone. ∙ Caves typically form below the water table, but get decorated by speleothems above the water table. How? o Water percolates through the cave rostalactites o Stalactites drip, stalagmit on the ground below them oThey can merge to form columns ∙ Besides caves and speleothems, (what’s a speleothem anyway?) what other features are typical of karst? o Speleothems are mineral deposits formed from groundwater within underground caves. Stalactites and stalagmites are types of these. oOther types: ∙ Cave shapes are typically influenced by features in the bedrock, typically those that influence permeability.
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