Human impacts on rivers Water Sediment Wood riparian veg flow obstructions rate of change hydrology sediment wood inputs channel morphology geology gradient fast catchment vegetation valley form slow climate Channelization direct human intervention riparian veg flow obstructions rate of change hydrology sediment wood inputs channel morphology geology gradient fast catchment vegetation valley form slow climate Channelization Los Angeles River Channelization Illabot Creek Skagit River Watershed Human impacts on rivers Water Sediment Wood Human impacts on rivers Globally we have increased upland erosion by about ten times while simultaneously cutting sediment delivery to oceans by half Hydraulic Mining Sierra Nevada Hydraulic Mining Sierra Nevada Yuba County 1870 Sacramento river bed elevations 18501910 Rising river bed exacerbates flooding on Sacramento River beginning in 1860s K Street looking east 1862 Sacramento river bed elevations 18501910 Farmers get mad and outlaw hydraulic mining Hydraulic mining Half century lag time to move sediment through system Sacramento and Yuba river bed elevations 1850 1950 Sedimentation in San Francisco Bay delta estuary 40 of SF Bay filled in with mud 1 5 billion yards of sediment 8 times the amount excavated to build the Panama Canal Hydraulic Mining and Dam Construction San Francisco Bay watershed Declining sedimentation in San Francisco Bay delta estuary due to passing of sediment wave and dam construction San Pablo Bay San Francisco Bay Post mining levees and subsidence in Sacramento River delta 20 ft Flooding on Sacramento River 2004 1997 Human impacts on rivers Water Sediment Wood Green River Washington Water Dam built for downstream flood control Extensive floodplain development from 19612009 Howard Hanson Dam Green River flood protection Flood control plans on the Green River depended upon maintenance of the dam Dam operations limited the flow in the river to 12 000 cfs for most years equal to pre dam 2 yr recurrence interval flow With dam in place the 2 yr 100 yr and 500 yr floods were all determined to be 12 000 cfs Green River flood protection Control of overbank flows led to the promise of no more valley bottom flooding and so the floodplain land values skyrocketed and commercial and residential development replaced farms on most of the valley bottom But Howard Hanson dam started leaking almost as soon as it was finished Two depressions formed in right abutment Anatomy of Howard Hanson Dam Green River Landslide Debris Small Depression Location t ro u Drainage Tunnel Spillway t en m nk a b Em gG n i t in is Ex Curta ht A Rig Big Depression Location Engineered Filter Blanket Flood Control Tower Fish Passage Facility Excavation ent m t bu Eagle Gorge Reservoir Inside the dam s drainage tunnel Normally dye placed in the reservoir seeps through in 12 hours it s now happening in 2 hours Potential Internal Erosion How to fix the dam Grout curtain and additional wells November 2009 Cutoff wall Future Green River flood protection To fix the dam will require time as the reservoir is drawn down and can store less water With dam unable to store the equivalent of a several year RI flood let alone a 500 yr flood what is the prognosis for the levees being able to contain floodwaters Green River Valley Flooding Extensive annual flooding along valley bottom Pre development flood extent in 1965 after the dam was in place Natural levees along the river built up to contain the bankfull flow 12 000 cfs 1965 Flooding Along East Valley Highway 1965 Flooding Along East Valley Highway 13 900 cfs 17 600 cfs 25 000 cfs Supersacks as flood protection Supersacks add a bit of height to the levees Green River Valley Flooding Other styles of flood control include getting infrastructure out of harms way levee setbacks and designing floodways along which flood flows would be routed For this year there are not many options other than to warn the public set up a flood warning system enhance the levees and pray it doesn t rain a lot Green River Valley Flooding FEMA estimates that flooding from a levee breach at 12 000 cfs could displace 15 000 to 20 000 people and cause 1 3 to 3 7 billion in damage How does flood control affect flood risks Interactions between land use and increased flood damage in the Russian River watershed
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