Dams trap a higher proportion of runoff in drier regionsU.S. Dam construction leveled of in 1980s at almost a billion acre-feetTiming of Dam construction varied regionally, but was fastest between 1940 and 1980.Extent of global forestsArmy Corps of Engineers aggressively “de-snagged” American RiversHow big does a log have to be in order to influence a river? The “key member” logs that anchor log jams tend to have a diameter ≥half the channel depth and a length ≥ half the channel widPosition in Channel NetworkEffects of Wood in RiversChannelmorphologyBankmaterialFlowobstructionsValleyslopeConfinementSediment Water WoodRiparianvegetationHuman Impacts to RiversHuman Impacts on Rivers• dams• channelization• loss of woody debris/riparian forestsMore than 80,000 dams affect > 90% of the nation's 5.8 million km of rivers.QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.Dams trap a higher proportion of runoff in drier regionsU.S. Dam construction leveled of in 1980s at almost a billion acre-feetTiming of Dam construction varied regionally, but was fastest between 1940 and 1980.QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.Hoover DamQuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.Timber crib dam in MichiganQuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.Impact of Humans on the Flux of Terrestrial Sediment to the Global Coastal Ocean, James P. M. Syvitski, Charles J. Vörösmarty, Albert J. Kettner, Pamela Green, Science, v. 308, p. 376-380.Reservoirs often trap 40% to >80% of the sediment carried by large rivers, reducing the sediment delivered to coastal environments despite increased soil erosion in upland environments.QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.Human’s increased annual sediment delivery to rivers by 2.3 billion tons, from about 6.5 billion tons to almost 9 billion tonsQuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.Human’s decreased annual sediment delivery to oceans by 1.4 billion tons, from about 6.5 billion tons to about 5 billion tonsQuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.Less sediment reaches the coast in Asia and parts of the Americas due to dam constructionHumans have simultaneously increased the sediment transport by global rivers through soil erosion (by 2.3 ± 0.6 billion metric tons per year), yet reduced the flux of sediment reaching the world's coasts (by 1.4 ± 0.3 billion metric tons per year) because of retention within reservoirs.Splash damsHuman Impacts on Rivers• dams• channelization• loss of woody debris/riparian forestsQuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.Los Angeles River at VernonQuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.Los Angeles River at Canoga Ave.QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.ConnecticutQuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.MissouriQuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.IllinoisQuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.CaliforniaPort of Seattle circa 1870Lower Duwamish River and estuaryLower Duwamish River todayHuman Impacts on Rivers• dams• sediment input• channelization• loss of woody debris/riparian forestsExtent of global forestsForests have covered about one-third of the Earth’s land surface during the Holocene.But the extent of forest cover has changed substantially ... Oregon AmazonCameroonFew of the worlds forests retain “frontier” conditionsLog jams were significant obstacles to navigation and land development in the western U.S.02000400060008000100001880 1900 1920 1940 1960Snags/YearRivers & HarborsSkagit RiverAll RiversArmy Corps of Engineers aggressively “de-snagged” American RiversThousands of snags were removed from Puget Sound rivers between 1880 and the mid-20th CenturyLWD can control the formation of pools and bars, and thereby channel reach morphologyGreater wood loading leads to more poolsFor channels we’ve surveyed in Alaska and Washington, a plane-bed morphology occurs only at low LWD loading1.51.00.50.005101520pool spacing (channel widths/pool)SE AlaskaNW Washingtonplane-bed pool-riffleLWD frequency (pieces/m)Log jams trap copious amounts of sediment and aggrade entire reaches of channel.A100 mHow big does a log have to be in order to influence a river?The “key member” logs that anchor log jams tend to have a diameter ≥half the channel depth and a length ≥ half the channel width.NISQSNOHSTILLPieces/KilometerJamsOtherPosition in Channel NetworkQueets River, WashingtonLog stepsBankfull benchMeander jamValley jamEffects of Wood in RiversSpatial scale (meters)11010010,00011001000In-ChannelPoolsCoverBankComplexityReachChannelSwitchingIslandsSloughsValley BottomWater,Sediment & Wood RoutingYearsEnvironmental History of Puget Sound RiversTwo dominant types of river valleys: Pleistocene (glacial) and Holocene (fluvial)Pleistocene (glacial) valleys were incised by meltwater beneath the Puget Lobe glacierHolocene (fluvial) valleys were incised by rivers into the Puget Lowland after deglaciationPleistocene (glacial ) valley: Snoqualmie River Lidar DEM~1870 Landscape68101214160 1,000 2,000 3,000Horizontal distance (m)*26060018101214160 400 800 1,200 1,6002606021Horizontal distance (m)*Channel pattern: MeanderingHabitats: Oxbows and large depressional wetlandsCross valley profile: Convex***= Snoqualmie river channelHolocene (fluvial) valley: Nisqually River1999 aerial & 2000 field810121416180 400 8008Horizontal distance (m)*1315171921230 400 800Horizontal distance (m)*11Channel pattern: AnastomosingHabitats: Multiple channels & floodplain sloughsCross valley profile: “Corrugated” from channels & islands***= main channelExamples of cross-valley topographyPleistocene valleys Holocene valleysGreen River in Tukwila246810120 1,000 2,000 3,000xs 89Horizontal Distance (m)**= regraded areas (e.g. Southcenter Mall)2628303234360500Horizontal distance (m)*27Cedar River near Elliott*River at low point of floodplainRiver perched above floodplainComparative river dynamics• Avulsion type: Meander cutoff Channel switching• Floodplain occupation rate: Slow Rapid• Migration &
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