ES 105 Streams and Floods I. Hydrologic cycle A. Distribution 1. +97% in oceans 2. >3% surface water a. +99% surface water in glaciers b. >1/3% liquid, fresh water in streams and lakes~1/10,000 of water B. Cycle 1. evaporates a. 84% from sea surface b. Transpiration is 1) plants releasing moisture to atmosphere 2) “Evapotranspiration” is combined effect c. Becomes atmospheric moisture moved by winds 2. condensation into clouds allows precipitation a. 75% over sea, 25% over land b. Concentrated in tropical and midlatitudes c. Much falls as snowfall, 1) ‘storage’ of solid water on land surfaces 2) Glaciers hold over 2% of Earth’s water a) Most of fresh water on land b) If it melted, sea level would rise 75 meters+/- 3. runoff, infiltration a. back to the sea—about 1/3 of land precipitation runs off b. (most of the other 2/3 of land precipitation is returned to atmosphere by evapotranspiration) c. Groundwater: storage of water from cycle for long times II. Running water A. Runoff is only 0.00005% of total water on Earth, but vital to civilization B. Understanding source of runoff to be rainfall realized in 1500s C. Drainage basin 1. land that contributes water to stream 2. basins separated by divides D. River systems 1. erode channels in which they flow a. most erosion is in headwater area of stream b. variety of erosional landforms including V-shaped valleys 2. transport sediment delivered to them by mass wasting a. solid particles transported in suspension, and as bedload b. dissolved material, released by weathering, important component of stream transportation 3. deposit material in temporary sites on the way to the seaIII. Streamflow A. Types of streamflow 1. Laminar flow in smooth, straight-line paths at consistent velocity 2. Turbulent flow is erratic a. differing directions lead to localized areas of greater velocity b. lifts material from streambed—enhances erosion B. ability to erode and transport controlled by velocity of flow 1. Channel characteristics a. gradient b. channel roughness, shape and size 2. amount of water in channel (discharge) C. channel characteristics 1. gradient—drop in feet, meters or centimeters divided by distance of stream channel in miles or kilometers a. lower Mississippi River < 10 cm/km b. Columbia River 1) elevation 10 ft @ Portland 2) 100 miles to the sea 3) 0.1 ft.mile = 1.9 cm/km 2. shape, roughness contribute to frictional drag of channel on water a. large channels 1) have less surface area per volume of water 2) more efficient because there is less drag b. channel roughness 1) smooth channels have less obstacles promotes smooth laminar flow 2) rough channels prone to turbulent flow a) slowed b) more erosive 3. discharge—the amount of water flowing in the stream a. cross sectional area x velocity of water flowing b. cubic meters per second c. Willamette discharge— 1) About 1700 m3/s Feb 2, 2008 2) About 510 m3/s Mar 1, 2008 3) Website--http://waterdata.usgs.gov/or/nwis/uv? format=gif&period=31&site_no=14191000 d. Most rivers have seasonal fluctuations of discharge 1) High flow during snowmelt or rainy season 2) Some are ‘intermittent’ or ‘ephemeral’e. Floods 1) Discharge is greater than bank-full level a) Measured in feet above flood stage b) Reported as cubic feet per second or cubic meters per second c) Recurrence interval i. X year flood a. 10 year flood—10% chance of occurring in any given year b. 25 year flood—4% chance c. 100 year flood—1% chance d. 500 year flood—0.02% chance ii. Not an absolute event. a. Not ‘will occur only every 500 years’ b. Probability based on sedimentary records 2) Types of floods a) Riverine i. Slow due to protracted rainfall ii. Flash due to sudden rainfall b) Coastal i. Storm surge ii. High tide iii. Cyclonic storm rainfall c) Catastrophic i. Landslide or lava flow damming river ii. Washout of dam—natural or manmade 3) Flood Effects a) Infrastructure damage i. buildings ii. Utilities iii. Transportation systems b) Disease and pollution c) Crop and food supply d) Natural vegetation e) Renewal of nutrients in farmland 4) Flood control a) Containment levees and reservoirs b) Water management in reservoirs and sacrificial areas c) Flood-plain development restrictions4. longitudinal profile of stream a. changes from headwaters to the mouth b. constantly decreasing gradient 1) smooth, concave upward curve over length of stream 2) some local irregularities present are usually temporary c. increases in discharge, width, depth, velocity also downstream IV. Work of Running Water A. Most important erosion agent—even in deserts!! 1. Downslope sheetflow of precipitation coalesces into rills and gullies 2. Becomes stream that continues to gain water from tributaries 3. velocity of water can erode banks and channel a. hydraulic force of water can cut into bedrock b. particles carried by stream enhance its erosive ability B. transportation of eroded material 1. loads of stream a. dissolved load—in solution b. suspended load—carried as ‘mud’ 1) fine particles in normal flow 2) sand and pebbles in flood stage c. bedload—bounces and rolls along bottom of channel 2. carrying ability—competence vs. capacity a. competence— 1) maximum size of particle that can be moved 2) determined by velocity of flow b. capacity— 1) amount of material that can be moved 2) determined by discharge 3. greatest transportation occurs at floodstage a. greater discharge b. greater velocity 4. transportation will cease—‘deposition’—when velocity slows a. largest particles deposited first—creates sorting of material b. occurs within channels, adjacent to channels, at mouth, etc.V. Stream Channels A. Bedrock channels 1. where gradient is steep—velocity carries all loose particles away 2. undulating gradient in headwaters allows local accumulations B. alluvial channels 1. in deposited material—‘alluvium’: the loose material deposited by streams 2. streamflow reflected in ability to transport and erode this material 3. results in numerous characters of channel patterns a. meandering channels in fine sediments 1) transport much material as suspended load 2) wide sweeping bends eroded on outside curve—cutbank 3) slower velocity on inside curve—deposition of pointbar 4) results in migration of the meander loops
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