Post on 21-Dec-2015
Streams and floods
GoalTo understand how surface-water-drainage
systems (streams) work and the patterns and hazards of flooding.
Hydrologic cycleEvaporation—Precipitation—Storage—Infiltration—
Runoff... also subduction and volcanism
StreamsStream: Any surface water flow confined to a channel
—Everything from little creek in arboretum to Mississippi River
Streams1.Flow from ground water—generally very steady,
called base flow
2.Flow from surface runoff
StreamsDrainage basin (watershed): Total area from which
overland flow reaches a stream
Tributary: Smaller stream that feeds a larger stream
Messalonskee Stream is a tributary of Kennebec River: Its drainage basin is part of Kennebec River drainage basin
Amazon Basin
StreamsDrainage divide: Line
that divides one drainage from another—generally topographic high
Drainage divide in Colorado
StreamsGradient: Slope of stream—vertical drop over
horizontal distance• Meters per kilometer or feet per mile
StreamsStream velocity: Speed of the current• Increases with increasing gradient• Increases towards outsides of bends in channel and
towards channel bottom
StreamsStream discharge: Total volume of water that passes
through a stream channel per unit time• Cross-sectional area (ft2 or m2) x average velocity
(ft/s of m/s) = discharge (ft3/s or m3/s)• Little creek in arboretum discharge = 10’s ft3/s;
Mississippi River discharge = 600,000 ft3/s
Streams shaping Earth’s surfaceStreams are very efficient agents of erosion and
sediment transport• Will rapidly cut down to level at which they can no
longer erode their channels, or their base levels
Stream trying to reach its base level
Stream valleysStream erosion typically creates v-shaped valleys• Stream cuts downwards and sides of valley slide or
wash into main channel until slopes are stable
V-shaped valleys in the Klamath Mountains
V-shaped valley in New Zealand
Drainage patternsDendritic drainages: Contain branching tributaries, like
branches of a tree• Form over relatively uniform substrates
Drainage patternsTrellis drainages: Form in areas where rocks of very
different hardness have been folded or faulted• Streams follow less resistant rock in valleys
Trellis drainages in Virginia
Drainage patternsSuperposed drainage: Drainage pattern that cuts
across surrounding topography. Indicates stream has cut down from a flat surface
Superposed drainage in Wyoming
Stream channel patternsFlood plain: Low flat area adjacent to main channel
subject to periodic flooding
Flood plain of the Kanawha River, West Virginia
Stream channel patternsBraided streams: Channels form anastomosing
(converging and diverging) strands separated by mid-channel bars
• Develop where sediment supply exceeds normal ability for stream to transport it
Stream channel patternsMeandering streams: Stream that winds and loops its
way through the flood plain in a random pattern• Develop in response to current-speed differences
around stream bends
Owens R. in California
Tributary of the Amazon R.
Oxbow Lakes
Pattern left by meanders along Owens River, California
Meandering streamsCut banks: form on the outside of meanders where
fast current is actively eroding channel bank
Point bars: form on the inside of meanders where sediment drops out of slow current
Meandering streamsOnce a meander gets too pronounced, it may be cut
off and bypassed to leave an oxbow lake
Meandering streamsIncised meanders: Meanders trapped in place when
stream cuts down rapidly after tectonic uplift
Stream channel patternsWaterfalls and rapids: Form where stream bed locally
more resistant to erosion or where faulting, landslides, or debris from a tributary have interrupted stream gradient
Stream sedimentSediment load: material moved by stream—we call
stream sediment alluvium
Stream sedimentSuspended load: sediment carried along in water
column by turbulence of current
Bed load: Larger particles that bounce and/or roll along the bottom
Yellow River in China is ~50% sediment by volume
How Streams Move Sediment
Stream sediment deposition• Mid-channel bars and point bars• Levees: ridges of relatively coarse sediment
deposited alongside main stream channel
Stream sediment deposition• Alluvial fans: From when stream leaves a narrow
canyon and enters large, flat valley—Gradient decreases, so current slows and drops most of the sediment load
Stream sediment depositionDeltas: Sediment piles formed when stream enters
standing body of water (like the ocean)• Build outward from the coastline
Mississippi delta
Mississippi River DeltaNew Orleans sits on
Mississippi River delta that formed less than 1000 years ago
City is subsiding ~5mm/yr because sediment supply cut off by man-made levees (we’re keeping the floods out)
• Combined with global 1–4mm/yr global sea level rise
Red = below sea level
FloodsFloods occur when local precipitation runoff exceeds
normal capacity of the stream channel
Factors that influence runoff• Topography• Soil and bedrock• Land use
Flood prediction• Based on past record of yearly peak discharges• Extrapolations made from incomplete data sets
generally underestimate flood size and frequency
Peak discharge is plotted against recurrence interval
Flood prediction100-year flood: according to best available data, 1-in-
100 chance it could happen any given year
Like shuffling a deck of cards and trying to draw the ace of spades 1-in-52 chance every time
Flood control—Can’t preventFlood walls and levees: (man-made) keep water in
main channel. Must completely surrounded inhabited area
• Expensive• Must completely encircle area
Flood control—Can’t preventFlood control dams: store water in reservoirs and
release it gradually• Can impound main stream or lots of small tributaries
• Expensive• Flood large areas• Block fish migrations
Flood control—Can’t preventFlood zoning: Most municipalities don’t allow new
construction inside of areas that will be inundated by discharge predicted for a 100-year flood
• Homeowners insurance doesn’t cover flooding• Must purchase separate flood insurance