Human Impacts to Rivers - University of...
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Channelmorphology
Bankmaterial
Flowobstructions
Valleyslope
Confinement
Sediment Water Wood
Riparianvegetation
Human Impacts to Rivers
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Human Impacts on Rivers
• dams
• channelization
• loss of woody debris/riparian forests
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More than 80,000 dams affect > 90% of the nation's 5.8 million km of rivers.
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Dams trap a higher proportion of runoff in drier regions
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U.S. Dam construction leveled of in 1980s at almost a billion acre-feet
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Timing of Dam construction
varied regionally, but was fastest
between 1940 and 1980.
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Hoover Dam
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Timber crib dam in Michigan
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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.
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Human’s increased annual sediment delivery to rivers by 2.3 billion tons, from about 6.5 billion tons to almost 9 billion tons
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Human’s decreased annual sediment delivery to oceans by 1.4 billion tons, from about 6.5 billion tons to about 5 billion tons
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Less sediment reaches the coast in Asia and parts of the Americas due to dam construction
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Humans 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.
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Splash dams
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Human Impacts on Rivers
• dams
• channelization
• loss of woody debris/riparian forests
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Los Angeles River at Vernon
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Los Angeles River at Canoga Ave.
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Connecticut
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Missouri
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Illinois
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California
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Port of Seattle circa 1870
Lower Duwamish River and estuary
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Lower Duwamish River today
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Human Impacts on Rivers
• dams
• sediment input
• channelization
• loss of woody debris/riparian forests
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Extent of global forests
Forests have covered about one-third of the Earth’s land surface during the Holocene.But the extent of forest cover has changed substantially ...
Oregon Amazon Cameroon
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Few of the worlds forests retain “frontier” conditions
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Log jams were significant obstacles to navigation and land development in the western U.S.
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0
2000
4000
6000
8000
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1880 1900 1920 1940 1960
Snag
s/Ye
ar
Rivers & HarborsSkagit RiverAll Rivers
Army Corps of Engineers aggressively “de-snagged” American Rivers
Thousands of snags were removed from Puget Sound rivers between 1880 and the mid-20th Century
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LWD can control the formation of pools and bars, and thereby channel reach morphology
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Greater wood loading leads to more pools
For channels we’ve surveyed in Alaska and Washington, a plane-bed morphology occurs only at low LWD loading 1.51.00.50.0
0
5
10
15
20
pool
spa
cing
(ch
anne
l wid
ths/
pool
)
SE AlaskaNW Washington
plane-bed pool-riffle
LWD frequency (pieces/m)
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Log jams trap copious amounts of sediment and aggrade entire reaches of channel.
A
100 m
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How big does a log have to be in order to influence a river?
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The “key member” logs that anchor log jams tend to have a diameter ≥half the channel depth and a length ≥ half the channel width.
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NISQ
SNOH
STILL
Pieces/Kilometer
JamsOther
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Position in Channel Network
Queets River, Washington
Log steps
Bankfull bench
Meander jam
Valley jam
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Effects of Wood in Rivers
Spatial scale (meters)
1
10
100 10,000
1
100
1000
In-ChannelPoolsCoverBank
Complexity
ReachChannel
SwitchingIslandsSloughs
Valley BottomWater,
Sediment & Wood RoutingYears
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Environmental History of Puget Sound Rivers
Two dominant types of river valleys: Pleistocene (glacial) and Holocene (fluvial)
Pleistocene (glacial) valleys were incised by meltwater beneath the Puget Lobe glacier
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Holocene (fluvial) valleys were incised by rivers into the Puget Lowland after deglaciation
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Pleistocene (glacial ) valley: Snoqualmie River
Lidar DEM ~1870 Landscape
6
8
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12
14
16
0 1,000 2,000 3,000
Horizontal distance (m)
*
2606001
8
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12
14
16
0 400 800 1,200 1,600
2606021
Horizontal distance (m)
*
Channel pattern: MeanderingHabitats: Oxbows and large depressional wetlandsCross valley profile: Convex
*
** = Snoqualmie river channel
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Holocene (fluvial) valley: Nisqually River
1999 aerial & 2000 field
8
10
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14
16
18
0 400 800
8
Horizontal distance (m)
*
13
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0 400 800
Horizontal distance (m)
*
11
Channel pattern: AnastomosingHabitats: Multiple channels & floodplain sloughsCross valley profile: “Corrugated” from channels & islands
**
* = main channel
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Examples of cross-valley topography
Pleistocene valleys Holocene valleys
Green River in Tukwila
2
4
6
8
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12
0 1,000 2,000 3,000
xs 89
Horizontal Distance (m)
*
*
= regraded areas (e.g. Southcenter Mall)
26
28
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32
34
36
0 500
Horizontal distance (m)
*
27
Cedar River near Elliott
*
River at low point of floodplainRiver perched above floodplain
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Comparative river dynamics
• Avulsion type: Meander cutoff Channel switching• Floodplain occupation rate: Slow Rapid• Migration & avulsion zone: Narrow Wide
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Metric Prediction
Pleistocene HoloceneMORPHOLOGY
1. Channel gradient Low High
2. Stable forested islands Few Many(surrogate for branching pattern)
3. Sinuosity Meandering Strait
4. Cross-valley topography Convex “Corrugated”HABITATS
5. Wetland area High Low
6. Oxbow ponds Many Few
7. Total length offloodplain sloughs Variable High
DYNAMICS
8. Dominant avulsion type Meander cutoff Channel switching
9. Channel migration zone Narrow WideSlow rate Rapid rate
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Nooksack River
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Nooksack River
Glacial valley
Fluvialvalley
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mid 1800s mid 1900s
0
10,000
20,000
NKS SKG STL SNH
P
RT
ESS
EEM
NKS SKG STL SNH
Wet
land
are
a (h
ecta
res)
EEM=estuarine emergent; ESS=estuarine scrub-shrub; RT=riverine-tidal; P=palustrine
Change to wetland area in four North Sound estuaries/deltas
1870s 2000
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SkykomishSkykomish River, 1917River, 1917White (now Green) River near Kent, 1906
Hazard prediction and zoning
Holocene valleysRivers avulse & migrate with high frequency within ~60-70% of the valley
a “Holocene valley” river channelwill knock on your door before long
Pleistocene valleysRivers migrate slowly within ~10% of the valley,but floodwaters fill the valley with regularity
in a “Pleistocene valley” the river channel won’t visit but the waterwill--frequently
In other words:
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Restoring rivers, habitats, and species
Without a planning framework