1. Streams and floods

2. Goal
To understand how surface-water-drainage systems (streams) work and the patterns and hazards of flooding.

3. Hydrologic cycle
Evaporation—Precipitation—Storage—Infiltration—Runoff... also subduction and volcanism

4. Streams
Stream: Any surface water flow confined to a channel—Everything from little creek in arboretum to Mississippi River

5. Streams Flow from ground water—generally very steady, called base flow
Flow from surface runoff

6. 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

7. Streams
Drainage divide: Line that divides one drainage from another—generally topographic high
Drainage divide in Colorado

8. Streams
Gradient: Slope of stream—vertical drop over horizontal distance
Meters per kilometer or feet per mile

9. Streams Stream velocity: Speed of the current
Increases with increasing gradient
Increases towards outsides of bends in channel and towards channel bottom

10. 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

11. 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

12. 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

13. Drainage patterns
Dendritic drainages: Contain branching tributaries, like branches of a tree
Form over relatively uniform substrates

14. 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

15. Drainage patterns
Superposed drainage: Drainage pattern that cuts across surrounding topography. Indicates stream has cut down from a flat surface
Superposed drainage in Wyoming

16. Stream channel patterns
Flood plain: Low flat area adjacent to main channel subject to periodic flooding
Flood plain of the Kanawha River, West Virginia

17. 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

18. 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

19. Oxbow Lakes
To view this animation, click “View” and then “Slide Show” on the top navigation bar.

20. Pattern left by meanders along Owens River, California

21. 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

22. Meandering streams
Once a meander gets too pronounced, it may be cut off and bypassed to leave an oxbow lake

23. Meandering streams
Incised meanders: Meanders trapped in place when stream cuts down rapidly after tectonic uplift

24. 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

25. Stream sediment
Sediment load: material moved by stream—we call stream sediment alluvium

26. 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

27. How Streams Move Sediment
To view this animation, click “View” and then “Slide Show” on the top navigation bar.

28. Stream sediment deposition
Mid-channel bars and point bars
Levees: ridges of relatively coarse sediment deposited alongside main stream channel

29. 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

30. Stream sediment deposition
Deltas: Sediment piles formed when stream enters standing body of water (like the ocean)
Build outward from the coastline
Mississippi delta

31. Mississippi River Delta
New Orleans sits on Mississippi River delta that formed less than 1000 years ago

32. 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

33. Floods
Floods occur when local precipitation runoff exceeds normal capacity of the stream channel
Factors that influence runoff
Topography
Soil and bedrock
Land use

34. 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

35. 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

36. 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

37. 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

38. 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