1Surface WaterHydrologic (water) CycleSediment transport in rivers and streamsWhy do river channels bend and change location over time? Longitudinal profile – downstream change in gradient (slope)Terraces1TerracesDrainage basins and dischargeFloodingHuman interventions to prevent floodingHydrologic cycleSurface water, ground water, and water elsewhere on earth is connected together as part of a large cycle2Fig. 12.2 Understanding EarthSediment Transport by Streams• Dissolved material - from chemical weathering• Dissolved load• Solid material - clastic material from physical weathering• Suspended load - smaller particles3• Bed load - larger particles• Capacity: how much sediment a stream can transport (related to how much water or discharge)• Competence: size of the largest particles which a stream can move (related to water velocity)Streams may carry clastic sediment either as particles suspended in the water or moving along the bottom. Particle size and water velocity determine which particles will be suspended and which4suspended and which will be transported along the bed. Streams also carry dissolved material.Fig 13.2 Understanding Earth1. Steep, narrow river valley in the mountainsChanges in a river system from the mountains to the sea3 A delta and nearly2. Broader, shallower valley further downstream5from Chapter 13 Understanding EarthNile River delta, Egypt - NASA3. A delta and nearly flat land where the river flows into a lake or the seaNext: Stream Dynamics.movFormation of meandersFaster-flowing water at the outside of a river bend erodes the bank forming a cut bank. With continued erosion, the river channel moves 6toward the side where erosion occurs. Deposition at the point bar on the inside of the bend fills in behind the shifting channel.Fig 13.10 Understanding Earth27Formation of an oxbow lakeWhen a river has well-developed meanders, the river may cut a new, shorter channel. Often this occurs when the river floods. The abandoned part of the original channel becomes an oxbow lake.Fig 13.10 Understanding EarthHow river channels change position (meander) over time.Fig 13.10 Understanding Earth8Outside of bend:Cut bankInside of bend: Point barMeandering river near Anchorage, Alaska. Note the accumulation of sand (white areas) at point bars.Fig 13.11 Understanding Earth9Ridges (meander scrolls) on an old point barThese ridges record the change in river position over time.Oxbow lakes and other remnants of old river channels11Next: rivers animation12Drainage basins The entire area that contributes water to a river is called its drainage basin or watershed. Basins are separated by areas of higher elevation called drainage divides. On one side of the divide, surface water flows into one stream. On the other side of the divide, surface water flows into another stream. Fig 13.21 Understanding Earth3Drainage basin of the Colorado River13Fig 13.22 Understanding EarthFloodingFlooding is a natural river process. Almost all floods result from heavy rainfall, rapid snow melt, or a combination of these two factors. During a flood the amount of water (di h ) i th ill fit i th i h l14(discharge) is more than will fit in the river channel.Flooding becomes a problem when people get in the way.1996 flood in Liuzhou, China Flooding is a normal river and stream process. This city is built on a river floodplain as are many other cities.Chapter 13 Understanding Earth15Estimated Flood Losses in the U.S.16http://www.weather.gov/hic/flood_stats/flood_trends.JPGA river floodplain: A floodplain is the relatively flat area at the bottom of a river valley. This area may be covered in water during a flood. In larger floods, more of the floodplain is covered in water. During floods, fine sediment is deposited on the floodplain. 17Fig 13.8 Understanding EarthRiver terraces A river terrace is an old area of floodplain that was left behind when the river eroded downward and formed a new floodplain at a lower level. Fig 13.20 Understanding Earth4Discharge, the amount of water flowing in a river channel, is calculated by multiplying the velocity of water flow by the width and depth of the river.Fig 13.14 Understanding Earth19Formation of natural levees by flooding. The thickest and coarsest deposits of sediment accumulate near the edges of the river channel.Fig 13.13 Understanding Earth20Natural levees during a flood21Flood frequency curveThe recurrence interval for the largest flood often is estimated based on data for smaller floods. Forecasting based on past flood history only works if enough data has been 22gcollected and the river system hasn’t been changed significantly. Many river systems have been changed by human activity, and this can have a big effect on the accuracy of flood probabilities.Fig. 14-29 Geology 2ndedition -ChernicoffFlood-frequency curve for the Skykomish River at Gold Bar, WAA different river would have a different curve23Relationship between flood size (discharge) and frequency or probability of occurrence. A “ten-year” flood has a 10% chance of happening in any particular year.Fig 13.15 Understanding EarthVariation in discharge pattern with land use type24Variation in discharge pattern with land use typeFig. 14-28 Geology 2ndedition - Chernicoff5Making better maps of areas subject to flooding25areas subject to floodingUSGS Open File Report 98-200http://wa.water.usgs.gov/pubs/ofr/floodgis/Older, 1982 map compared to a newer, 1998 map.The newer map uses both more accurate land elevation data anda more accurate (larger) estimate of flood discharge. USGS Open File Report 98-2002627Map of flood depth using the improved data.USGS Open File Report 98-20028Mapping areas subject to floodingAerial photograph of a large flood in the pilot study area, almost as big as the projected 100-year-flood (Feb. 9, 1996)USGS Open File Report 98-200What can people do about flooding?• Research studies to better understand the problems• Make maps showing what areas are most hazardous• Educate people• Land use planning and zoning regulations for future development (e.g. avoid building in the most hazardous areas)• Disaster preparation29pp• Supplies• Emergency plans & practice• Control flood waters• Fix and improve levees•Dams• Build or improve flood channels and bypasses• Raising existing housesSome human modifications to river systemsArtificial leveesChannelization - straightening & deepeningDams and reser oirserosion deposition