Watershed and its divide

3. Watershed and its divide

5. What’s the situation at points A and B in terms of: a. gradient b. stream velocity c. erosive power? Headwaters

6. What does a stream carry and how? • Total load (visible and invisible) • Suspended load (~ 90% of total, by weight) • Bed load (~ 10% of total) • Dissolved load (chemical ions) • Discharge • Volume of water passing a point in a given amount of time; usually cubic feet (or meters) per second • Depends on cross sectional area; becomes higher when area is reduced

7. 1 3 4 2 Note the asymmetry of the channel and the stream bed. Where is the fastest velocity and why? What size particles are carried in the different locations in the water?

8. Characteristics of a meandering river

9. Anatomy of a stream • Where do velocity lines lie in the stream? • Where do erosion and deposition take place?

10. Depositional feature – alluvial fan

11. Depositional feature - delta

12. Changes in sinuosity (curvedness)

13. Entrenched stream What differences do you notice in these two rivers? Meandering stream D Best

14. Braided streams High sediment load

15. Floodplains

16. Floodplains

17. Erosion and deposition along meanders floodplain

18. Formation of a cutoff Note how increased sinuosity causes an eventual cutoff due to the stream taking the shortest course

19. Natural levees

20. Levees- natural and artificial

21. Types of drainage patterns

22. Types of floods • Flash • Localized areas, usually thunderstorms; common in deserts • Examples: • Antelope Canyon, AZ August 1997 11 died • Big Thompson Canyon, CO July 1976 145 died • Need to consider where storms are—seldom directly overhead but up drainage area • Regional, downstream • Prolonged rain which saturates ground or delayed snowmelt • Floodplain covered

23. Flash flood vs downstream flood Typically a narrow Channel; water rises Fast; usually flash type Wider area, often farmland that gets flooded; generally regional type

25. Flood frequency • Recurrence interval • Length of time in years separating floods of a similar size [measured as volume of flow of stream] • Larger floods have a longer time interval • We speak of the 100-yr flood, one that has a 1% probability of occurring once in a given year • 20-yr flood would have a 5% likelihood of occurring in a given year • Oak Creek to the south of Flagstaff experienced three 100-yr floods in a decade due to combinations of rare conditions

26. Flood activity in the U. S.

27. Large rivers in conterminous U.S.

28. Mississippi River basin facts • Mississippi River drains 42% of the U. S. • Greatest inundation floods in US occur here • 3rd largest river basin in world • 11 of 28 largest rivers in US are part of the Mississippi River system • Avg flow 645,000 cu ft/sec • 1 cu ft = ~7.5 gal, so this is 4.83 million gals/sec or 100+ swimming pools PER SEC

29. Drainage of Mississippi River

30. Mississippi River flooding 1992-93

31. Effects of flooding

32. Major flooding between 1993 and 1997

33. Missouri River in 1993

34. Our response to flooding • Dams • Only provide some flood protection—not control; if full, lakes must be drained; failure of dam is major problem • Levees • Create false sense of security; if water breeches levee, it doesn’t drain well • Sandbagging • Very short term solution • Zoning and insurance • Can’t build in 100-yr floodplain; most people don’t buy floodplain insurance knowing fed govt will bail them out • Channelization • Clear, deeper, wider, straighter paths for water – everything goes against idea of graded stream [one seeking equilibrium]