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 Flow from ground water—generally very steady, called base flow 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 To view this animation, click “View” and then “Slide Show” on the top navigation bar.

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

Yellow River in China is ~50% sediment by volume 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 To view this animation, click “View” and then “Slide Show” on the top navigation bar.

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

Combined with global 1–4mm/yr global sea level rise 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-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 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