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