ES142: Exam 2 Study Guide.docx

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Boston University
Earth Sciences
CAS ES 142
Duncan Fitz Gerald

Professor FitzGerald Spring 2014 Introduction to Beaches and Shorelines ES 142 SUBJECT: RIVERS AND DELTAS 1. What factors affect the volume of sediment delivered by rivers to to the ocean? a. Drainage Area b. Climate (precipitation, evaporation) c. Vegetation d. Relief (Topography, glaciation) 2. What two factors affect the morphology (shape) of a delta? a. Discharge Effectiveness i. Sediment discharge ii. Wave power 3. How did “Delta” get its name? a. Shaped like the greek letter delta, herodotus 4. Be able to define Suspended load and Bedload. What factors controls whether sediment will be transported by suspended versus bedload? a. suspended load – sediment moving downstream. Suspended in water colum (weak flow conditions) b. bedload – sediment moving close to bed (channel bottom – higher flow conditions) 4. Define Capacity and Competency. a. Capacity – volume of sediment a river can carry (increases as discharge and velocity increase) b. Competency – largest sixed sediment a flow can carry (increases with flow velocity) 5. Give examples of Birdfoot (also called digitate), Cuspate, and Deflected deltas. a. Birdfoot/Digitate – Mississippi River b. Cuspate – Niger and Nile River c. Deflected – Senegal River 6. What are the drainage divides of the Mississippi River. a. East: Appalachian mountains b. West: Rocky Mountains c. North: End Moraine; what separates great lakes from Mississippi 8. The Mississippi River has changed the location where it discharges sediment to the Gulf of Mexico many times during the past 4,000 years. Why? a. The long course of the missiissippi is elongating the pathway to which the sediment deposits b. Because of this long course there is a short cut that was found due to water finding the most direct route to the ocean 9. The Mississippi Delta is eroding and land is being loss at a rate of football field every 25 minutes. What processes and conditions are responsible for this land loss? a. decrease in sediment load b. subsidence – lowering of land surface c. hurricanes – strip away vegetation d. oil and gas industry barges cut through land e. rise in sea level SUBJECT: EFFECTS OF GLACIATION (Refer to Chapter 17) 1. What are drowned river valleys and how do they form? a. Drowned river valley – valley enlarged when sea level was lower and rivers extended pathways across continental shelf i. Expansion of ice sheets lowers sea level ii. When water returns when ice sheets melts, the water drowns the river valley b. Sea level lowering due to growth of continental glaciers; when ice melted and and returned to ocean basins the rising sea level flooded river valley 2. What are fjords and how do they form? a. Fjord – flooded coastal mountain valley b. Formed when alpine glaciers excavate bedrock valleys below sea level (flooding glacial troughs) c. Deepened valleys are flooded when glaciers retreat flooded valley 3. What are end moraines and outwash plains and how do they relate to Cape Cod and the Islands? a. ice as a conveyor belt bringing sediment to front b. when ice stagnates, sediment accumulates producing moraine consisting of till c. water flowing from a glacier front produces braided streams d. braided streams deposit sandy sediment forming an outwash plain e. moraines create the backbone of cape cod 4. How do the erosional effects of continental glaciers differ from valleys glaciers? a. Continental Glaciers (Ice sheets) – strip away sediment cover to bedrock - create rocky coast -bedrock islands and few beaches b. Valley Glaciers (alpine glaciers) – concentrated erosion in mountain valleys -excavation of bedrock leads to deepening of valleys 5. Why is the coast of Maine rocky with numerous islands but few beaches? a. Laurentide Ice sheet from the Hudson Bay stripped away sediment cover, creates rocky and irregular coast with many bedrock islands 6. Where do barriers occur along the sediment-starved coast of Maine? a. Hoppum Beach (where his cottage is) -Pathway for sediment to find its way to coast -River brings sand to coast after glaciation 7. What are drumlins and what influence do they have in the Boston area? a. Drumlins – oval shaped hills of till formed beneath ice sheets -Formed from ice molding the underwater till b. They form the topography in Boston and the Islands in Boston Harbor 8. What does till consist of? a. till – gravel in a matrix of sand, silt, and clay 9. Why are the compositions of till and outwash sediment so different? (Course reading material) a. Outwash sediment comes from the reworking of till with additional sediment from glacial tunnels b. Outwashed are formed from out water streams, there is a lot of melt water that comes from underneath the glaciers when they retreat. -Competently carry silk sand and maybe fine gravel -Deposited on the way and become out washed grains -Finest material is carried out all the way 10. What is a kettle and how do they form? a. Kettle – pond or wetland/bog in outwash plain, formed with ice blocks melt within outwash plain leaving a depression 11. What are the Milankovitch Cycles (perturbations of the Earth’s orbit about the Sun; there are three: Eccentricity, Obliquity, and Precession) and how do these cycles relate to glaciation? a. Eccentricity – how circular or elliptical the earth’s orbit is a. How orbit around sun is elliptical vs circular (only 6%) b. Obliquity – amount of tilt the Earth’s axis of rotation from vertical (greater the tilt, greater severity of winter and summer) a. Tells earths tilt on axis b. Greatest tilt, northern
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