15_RiversWhite

15_RiversWhite - ESM 203: Rivers and the alluvial...

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ESM 203: Rivers and the alluvial environment 11/15/2005 1 Continental-scale Rivers Fall 2007 Continental-scale lowland rivers • Sustain largest concentrations of people on Earth • Typically 10s of millions of people live in each large river lowland • Most fertile, most trafficable parts of continents • Hazardous because – Subject to flooding – Subject to channel change – Close to sea level – Vulnerable to large-scale environmental change (climate/hydrology; sea level; crustal deformation; engineering) – Expensive to manage Continental-scale rivers • Big rivers are big because they drain large areas • Flow = (P-E)*Area • More generally, if [P-E] varies geographically within the basin • The boundaries are set by the tectonic construction of the continent http://serc.carleton.edu/images/research_education/katrina/drai nage.gif [ ]dA E P = Q A ? - Sources of Mississippi flow- reflecting climate Meade et al (1995) US Geol. Survey Circular 1133
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ESM 203: Rivers and the alluvial environment 11/15/2005 2 Sources of Mississippi River sediment load Before and after dam construction Also determined by the tectonic construction of the continent (distribution of terrains of differing erodibility) Meade et al (1995) US Geol. Survey Circular 1133 Area and morphology of large river basins depend on tectonic setting Create patterns of river characteristics and behavior Amazon Ganges, Paraguay Mekong Danube Congo; Niger Potter, (1979) J. Geology Some large rivers have been localized for a very long time by continental-scale crustal structures Potter, (1979) J. Geology
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ESM 203: Rivers and the alluvial environment 11/15/2005 3 South America Even very slow crustal movements can alter the low gradients of large rivers: Amazon basin topography Tectonics of the California transform margin Local river basins determined by tectonic grain of the region
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ESM 203: Rivers and the alluvial environment 11/15/2005 4 Santa Clara R. basin (4000 km 2 ) 4000 km 2 Upland zone: High sediment supply and low storage. Alluvial transport zone: sediment transport rate ˜ sediment supply rate. Significant transient sediment storage in valley floors and tributary fans. Multi-threaded channels in upper, steeper reaches; single-thread, meandering channels on lower gradients. ‘Free’ alluvial landforms. Alluvial accumulation zone: sediment transport capacity decreasing downstream; floodplain aggrading. Outlet: fans; deltas, estuaries.
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15_RiversWhite - ESM 203: Rivers and the alluvial...

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