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 sea III 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 restrictions 4 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 downstream and side to side in valley bottom 5 can cut off meander loops leading to oxbow lakes b braided channels where there is oversupply of sediment 1 occasional periods of great capacity and competence 2 low discharge results in divided interwoven channel pattern VI Base level and erosion A Lower limit of erosion base level 1 Ultimate base level ocean 2 Local base level a Lake level b Resistant rock layers c Discharge into another stream 3 Affects deposition and erosion of
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