Streams and floods
Goal
To understand how surface-water-drainage
systems (streams) work and the patterns
and hazards of flooding.
Hydrologic cycle
Evaporation—Precipitation—Storage—Infiltration—
Runoff... also subduction and volcanism
Streams
Stream: Any surface water flow confined to a
channel—Everything from little creek in arboretum
to Mississippi River
Streams
1. Flow from ground water—generally very steady,
called base flow
2. Flow from surface runoff
Streams
Drainage 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
Streams
Drainage divide: Line
that divides one
drainage from
another—generally
topographic high
Drainage divide in Colorado
Streams
Gradient: Slope of stream—vertical drop over
horizontal distance
• Meters per kilometer or feet per mile
Streams
Stream velocity: Speed of the current
• Increases with increasing gradient
• Increases towards outsides of bends in channel and
towards channel bottom
Streams
Stream 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 surface
Streams 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 valleys
Stream 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 valley in New
Zealand
V-shaped valleys in the
Klamath Mountains
Drainage patterns
Dendritic drainages: Contain branching tributaries, like
branches of a tree
• Form over relatively uniform substrates
Drainage patterns
Trellis 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 patterns
Superposed drainage: Drainage pattern that cuts
across surrounding topography. Indicates stream
has cut down from a flat surface
Superposed drainage
in Wyoming
Stream channel patterns
Flood plain: Low flat area adjacent to main channel
subject to periodic flooding
Flood plain of the Kanawha River, West Virginia
Stream channel patterns
Braided 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 patterns
Meandering 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
Tributary of the Amazon R.
Owens R. in California
Oxbow Lakes
Pattern left by meanders along
Owens River, California
Meandering streams
Cut 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 streams
Once a meander gets too pronounced, it may be cut
off and bypassed to leave an oxbow lake
Meandering streams
Incised meanders: Meanders trapped in place when
stream cuts down rapidly after tectonic uplift
Stream channel patterns
Waterfalls 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 sediment
Sediment load: material moved by stream—we call
stream sediment alluvium
Stream sediment
Suspended 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 deposition
Deltas: Sediment piles formed when stream enters
standing body of water (like the ocean)
• Build outward from the coastline
Mississippi delta
Mississippi River Delta
New 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
Floods
Floods 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 prediction
100-year flood: according to best available data, 1-in100 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 prevent
Flood walls and levees: (man-made) keep water in
main channel. Must completely surrounded
inhabited area
• Expensive
• Must completely
encircle area
Flood control—Can’t prevent
Flood 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 prevent
Flood 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