1. Chapter 10 Streams and Floods
GEOS 101
4/23/2017
GEOL 101
Introductory Geology
Chapter 10 Streams and Floods

2. GEOS 101
4/23/2017
Hydrologic cycle
The hydrologic cycle is a summary of the circulation of Earth’s water supply
Processes involved in the hydrologic cycle
Precipitation
Evaporation
Infiltration
Runoff
Transpiration

3. The Hydrologic Cycle

4. Channel flow and sheet flow
Longitudinal profile of a stream
Head area erosion predominant
Mouth area, deposition mainly

5. Downcutting

6. Drainage Pattern
Radial
Dendritic
Trellis
Rectangular

7. Dendritic Drainage

8. Factors affecting erosion and deposition
Velocity
Gradient – rise over run
Channel shape
Channel roughness
Discharge – amount of water flow per unit time, as discharge increases, load increases.

9. Velocity

10. Velocity

11. Channel shape and roughness
Semicircular
Flow fast
Wide channel
Flow slow
Rough bottom
Flow slow

12. Channel shape and roughness
Narrow channel
Flow fast
Wide channel
Flow slow

13. Stream Valleys Most common landform on Earth’s surface
Two general types of stream valleys
Narrow valleys
V-shaped
Downcutting toward base level
Features often include rapids and waterfalls
Wide valleys
Stream is near base level
Downward erosion is less dominant
Stream energy is directed from side to side forming a floodplain

14. Stream Valleys Features of wide valleys often include: Floodplains
Erosional: river erodes laterally
Depositional: fluctuation in conditions, base level
Meanders: sweeping bends in river channel
Cut bank: active zone of erosion
Point bar: zone of deposition
Cutoffs: shortened channel segment
Oxbow lakes: abandoned bend

15. Stream Valleys
Features of narrow valleys often include: Rapids & Waterfalls
Both occur where stream profile drops rapidly,
Rapids: resistive bed acts as temporary base level upstream, downcutting downstream
Waterfalls: stream makes vertical drop
Resistive rock underlain by erosive rock
Water plunges and erodes the underlying rock
Niagra Falls

16. Changes from Upstream to Downstream
Profile
Cross-sectional view of a stream
Viewed from the head (headwaters or source) to the mouth of a stream
Profile is a smooth curve
Gradient decreases downstream
Factors that increase downstream
Velocity
Discharge
Channel size
Factors that decrease downstream
Gradient
Channel roughness

17. Stream erosion Lifting loosely consolidated particles
Abrasion: particles scrape, rub, bump together and wear down
Dissolution: dissolve soluble rock by chemical reaction
Stronger currents lift particles more effectively

18. Active Stream Erosion

19. Base level and graded streams
Base level: lowest point to which a stream can erode
Two general types of base level
Ultimate (sea level)
Local or temporary
Changing conditions causes readjustment of stream activities
Raising base level causes deposition
Lowering base level causes erosion

20. Adjustment of Base Level to Changing Conditions

21. Local Base Level (waterfall)

22. Sediment Transport by Streams
Rolling
Suspension
dissolution

23. Sediment Deposition by Streams
Caused by a decrease in velocity
Competence is reduced
Sediment begins to drop out
Stream sediments
Generally well sorted
Stream sediments are known as alluvium
Channel deposits
Bars
Braided streams
Deltas

24. Sediment Deposition by Streams
Floodplain deposits
Natural levees: form parallel to the stream channel by successive floods over many years
Back swamps: marsh
Alluvial fans
Develop where a high-gradient stream leaves a narrow valley
Slopes outward in a broad arc
Deltas
Forms when a stream inters an ocean or lake
Consists of three types of beds (Foreset, Topset, Bottomset)

25. Deposition
Bar

26. Braided stream

27. Braided River Resurrection River, AK

28. Erosion and Deposition Along a Meandering Stream

29. Meander Loop on the Colorado River

30. Flood plains

31. Stream Valley in Arid Region Owens Valley, CA

32. Creation of oxbow lake
Creation of oxbow lake

33. Formation of a Delta

34. Delta southern, AK

35. Nile and Mississippi Deltas
Triangle shape
Bird-Foot

36. Alluvial Fan Death Valley, CA

37. Floods Floods are the most common and most destructive geologic hazard
Causes of flooding
Naturally occurring
Human-induced factors
Types of floods
Regional floods
Flash floods
Ice-jam floods
Dam failure

38. Flooding Salt River, AZ

39. Floods What is a 100-year flood?
It is not a flood that occurs every 100 years
Flood of a given size that has the probability of 1 in 100 of occurring in that year
Better term: 1-in-100 chance flood
Urban planning based on FEMA 100-yr flood maps

40. Flood Control Engineering efforts
Artificial levees
Flood-control Dams
Channelization
Nonstructural approach through sound floodplain management
Identify high risk areas
Zoning regulations for development

41. Flood-Control Dams Store water for slow release
Lowers crest of flood, spread out over time
Often have other functions
agricultural irrigation water
hydroelectric power
recreation
Reservoir covers previous land use: fertile farmland, historic sites, scenic valleys
Sediment deposition behind dam
Impediment for fish migration

42. Channelization
Altering a stream channel to speed flow of water to prevent reaching flood height
Clearing channel of debris
Dredging to widen and deepen channel
Lining channel with concrete
Artificial cutoff: straightening the channel
shorter stream increases gradient and velocity of water flow
larger discharge associated with flooding dispersed more quickly
Army Corp of Engineers

43. Artificial Levees
Earthen mounds on river banks to increase the volume capacity of river
Steeper slopes than natural levees
Trap sediment that otherwise would have been deposited in floodplain
River bed build up often requires raising the height of levee over time
Many artificial levees not built to withstand extreme flooding
Levee failure numberous on Miss. (1993)

44. Erosional Floodplain

45. Incised Meanders and Stream Terraces
Meanders in steep, narrow valleys
Caused by a drop in base level or uplift of the region
Stream Terraces
Remnants of a former floodplain
River has adjusted to a relative drop in base level by downcutting
Results in horizontal plane above current floodplain

46. Stream terraces

47. Incised Meanders Delores River, CO

48. Incised Meanders Colorado River, Canyonlands NP, UT