Chapter 3 Landscapes Fashioned by Water

1. Chapter 3 Landscapes Fashioned by Water

2. Earth’s External Processes • Weathering, mass wasting, and erosion are all called external processes because they occur at or near Earth’s surface • Internal processes, such as mountain building and volcanic activity, derive their energy from Earth’s interior

3. Mass Wasting: The Work of Gravity • Mass wastingis the downslope movement of rock and soil due to gravity • Controls and triggers of mass wasting • Water—Reduces the internal resistance of materials and adds weight to a slope • Oversteepening of slopes

4. Mass Wasting: The Work of Gravity • Controls and triggers of mass wasting • Removal of vegetation • Root systems bind soil and regolith together • Earthquakes • Earthquakes and aftershocks can dislodge large volumes of rock and unconsolidated material

5. Water Cycle • The watercycle is a summary of the circulation of Earth’s water supply • Processes in the water cycle • Precipitation • Evaporation • Infiltration • Runoff • Transpiration

6. The Water Cycle Figure 3.5

7. Distribution of Earth’s Water Figure 3.4

8. Running Water • Drainage Basins • Land area that contributes water to the stream is the drainage basin, every stream has one regardless of size • Imaginary line separating one basin from another is called adivide, range is size from a ridge; which separates gullies, to continental divide which separates continents into enormous drainage basins

9. Drainage Basin of the Mississippi River Figure 3.6

10. Running Water • River Systems • a zone of erosion – dominant process in upstream area • a zone of sediment transport – trunk stream • a zone of deposition – where river enters a large body of water, forms delta

11. Running Water • Streamflow – (laminar & turbulent flow) • The ability of a stream to erode and transport materials is determined by velocity • Factors that determine velocity (which is distance traveled per unit of time) is measured at gaging stations • Gradient, or slope (drop over distance) • Channel characteristics including shape, size, and roughness

12. Running Water • Streamflow • Factors that determine velocity-Discharge—The volume of water moving past a given point in a certain amount of time ( intermittent & ephemeral streams) • Changes along a stream • Cross-sectional view of a stream is called the profile • Viewed from the head(headwaters or source) to the mouth of a stream

13. Running Water • Changes from upstream to downstream • Profile (from headwaters to mouth) • Profile is a smooth curve • Gradient decreases downstream • Factors that increase downstream • Velocity • Discharge (Colorado River is an exception) • Channel size (wider, deeper, smoother bottom) • Factors that decrease downstream • gradient • channel roughness

14. Longitudinal Profile of a Stream Figure 3.9

15. The Work of Streams(erosion, transport, deposition) • Stream erosion-Earth’s most important erosional force, consists of: • Lifting loosely consolidated particles by • Abrasion • Dissolution • Stronger currents lift particles more effectively from bed and banks, than weaker currents

16. The Work of Streams • Transport of sediment by streams • Transported material is called the stream’s load • Types of load • Dissolved load-most of the load is from groundwater • Suspended load-consists mostly of silt and clay, usually the largest part of the load • Bed load-particles too large to be carried in suspension • Capacity—the maximum load a stream can transport, is directly related to discharge

17. The Work of Streams • Competence • Indicates the maximum particle size a stream can transport • Determined by the stream’s velocity • If the velocity doubles, the competence increases four times. Greatest erosion and transportation of sediment occurs during floods (inc. in discharge results in a greater capacity, and an inc. in velocity results in greater competence)

18. The Work of Streams • Deposition of sediment by a stream • Caused by a decrease in velocity • Competence is reduced • Sediment begins to drop out, each particle size has a critical settling velocity, resulting in sorting of particles • Stream sediments • Generally well sorted • Stream sediments are known as alluvium, which is well sorted material

19. Stream Channels • Bedrock Channels – stream actively cuts into solid rock • steep, winding nature- (white water rafting) • Alluvial Channels – channel composed of loosely consolidated sediment (alluvium), can undergo major changes in shape due to erosion, transport, and deposition • two common types – meandering & braided channels

20. Stream Channels • Meandering Channels – these streams transport mainly mud (silt and clay) as suspended load in a deep, smooth channel • Meanders-migrate laterally by eroding the outside of the bend and depositing on the inside • Cut banks and point bars • Cutoffs and oxbow lakes

21. Erosion and Deposition along a Meandering Stream Figure 3.11

22. Stream Channels • Braided Channels –complex network of converging and diverging channels, have and interwoven appearance, the channel is wide and shallow • large portion of the stream’s load consists of coarse material (sand & gravel), transported as bedload • one circumstance in which braided streams form is at the end of a glacier where there is a large seasonal variation in discharge

23. Base Level • Base level and stream erosion • Base level is the lowest point to which a stream can erode • Two general types of base level • Ultimate (sea level) • Local or temporary includes lakes, resistant rock layers, and main streams that act as base level for their tributaries

24. A Waterfall Is an Example of a Local Base Level

25. Base Level • Base level and stream erosion • Changing conditions causes readjustment of stream activities • Raising base level causes deposition ex.- Reservoir behind a dam • Lowering base level causes erosion ex.- Incised meander

26. Adjustment of Base Level to Changing Conditions Figure 3.16

27. Stream Valleys(consists of the channel and surrounding terrain that contributes water to the stream) • The most common landforms on Earth’s surface • Two general types of stream valleys • V-shaped (narrow valleys) • valley deepening • Downcutting toward base level • Features often include rapids and waterfalls, due to variations in the erodibility of the bedrock into which a stream is cutting, causing the stream profile to drop rapidly • waterfalls are where the stream profile makes an abrupt vertical drop

28. Stream Valleys • Two general types of stream valleys • Wide valleys • valley widening • Stream is near base level • Downward erosion is less dominant • Stream energy is directed from side to side forming a floodplain

29. Stream Valleys • Changing Base Level and Incised Meanders • meanders originally developed on floodplainof a stream that was relatively near base level. Then a change in base level caused the stream to begin downcutting. Either base level dropped (ex. During the Ice Age) or the land upon which the river flowed was uplifted (ex. Uplift of Colorado Plateau).

30. A Meander Loop on the Colorado River

31. Depositional Landforms • Deposition of sediment by a stream • Bars – deposits on sand and gravel (temporary features) • Delta—Body of sediment where a stream enters a lake or the ocean • Results from a sudden decrease in velocity, (river seeks a shorter, higher gradient route to base level) • main channel divides into smaller distributaries • Mississippi delta consists of 7 coalescing subdeltas • Natural levees—Form parallel to the stream channel by successive floods and deposition over many years

32. The Work of Streams • Deposition of sediment by a stream • Floodplain deposits • Back swamps (marshes) • Yazoo tributaries

33. Formation of Natural Levees Figure 3.22

34. Drainage Patterns • Drainage pattern • Pattern of the interconnected network of streams in an area • Common drainage patterns • Dendritic – (treelike) most common • Radial – diverge from a central area • Rectangular –many right angle bends • Trellis – rectangular pattern in which tributary streams are nearly parallel to one another, due to alternating bands of resistant and less resistant rock.

35. Drainage Patterns Figure 3.23

36. Floods and Flood Control • Floods and flood control • Floods are the most common and most destructive geologic hazard, the discharge exceeds the capacity of the channel • Causes of flooding • Result from naturally occurring and human-induced factors (failure of dams and levees) (urban areas) (mountainous areas) • Causes include heavy rains, rapid snow melt, dam failure, topography, and surface conditions

37. Floods and Flood Control • Floods and flood control • Flash flood -occurs with little warning, unlike regional flooding • Flood control • Engineering efforts • Artificial levees-earthen mounds • Flood-control dams (store water) • Channelization – alter stream channel • Nonstructural approach through sound floodplain management • identifying high risk areas • minimize development • promote more appropriate land use

38. Groundwater: Water Beneath the Surface • Largest freshwater reservoir for humans • One of our most important and widely used resources • Geological roles • As an erosional agent, dissolving by groundwater produces • Sinkholes • Caverns • An equalizer of stream flow for streams when rain does not fall

39. Water Beneath the Surface • Distribution and movement of groundwater • Distribution of groundwater • Belt of soil moisture – surface film on soil particles, used by plants in life functions some evaporates back into the atmosphere • Zone of aeration • Unsaturated zone – above the water table • Pore spaces in the material are filled mainly with air

40. Water Beneath the Surface • Distribution and movement of groundwater • Distribution of groundwater • Zone of saturation • All pore spaces in the material are filled with water • Water within the pores is groundwater • Water table—The upper limit of the zone of saturation

41. Features Associated with Subsurface Water Figure 3.28

42. Distribution and movement of groundwater • Factors influencing the movement and storage of groundwater • Porosity • Percentage of pore spaces • Determines how muchgroundwater can be stored • Permeability • Ability to transmit water through connected pore spaces, the smaller the pore space the slower the groundwater moves

43. Water Beneath the Surface • Movement and storage of groundwater • Aquitard—An impermeable layer of material, such as clay • Aquifer—A permeable layer of material,the layer that well drillers seek • Groundwater movement – is mostly very slow, from pore to pore. This is in response to gravity. • water percolates into a stream from all directions, some paths are clearly upward. This is due to water pressure in the zone of saturation.

44. Springs-(where the watertable intersects the Earth’s surface) • Springs • Hot springs – over 1000 in the U.S. • Water is 6–9ºC (10-15̊ F) warmer than the mean air temperature of the locality • heated due to inc. depth (1̊̊ F / 100 ft.) • Heated by cooling of igneous rock • over 95% of hot springs (&geysers) are heated by igneous cooling & occur in the west • Geysers • Intermittent hot springs • occurs when underground chambers exist within hot igneous rock • Water turns to steam and erupts

45. Wells • Wells-serve as small reservoirs into which groundwater migrates and from which it can be pumped • Pumping can cause a drawdown (lowering) of the water table • Pumping can form a cone of depression in the water table

46. Formation of a Cone of Depression Figure 3.32

47. Water Beneath the Surface • Artesian Wells • Artesian wells act as “natural pipelines” moving water from remote areas of recharge great distances to the points of discharge • Water in the well rises higher than the initial groundwater level • requires a confined, inclined aquifer so that one end is exposed at the surface • requires aquitards above & below aquifer

48. An Artesian Well Resulting from an Inclined Aquifer Figure 3.33

49. Water Beneath the Surface • Environmental problems associated with groundwater • isbeing exploited at an increasing rate • overuse threatens groundwater supply • Land subsidence caused by its withdrawal (San Joaquin Valley) • Contamination (septic tanks, farm wastes, sewer systems) • In some areas, is treated as a nonrenewable resource, ex. High Plains (extends from So. Dakota to western Texas) recharge is less than what is withdrawn

50. Water Beneath the Surface • Land subsidence caused by groundwater withdrawal • As water is withdrawn, water pressure drops, transferring weight of overburden to the loose sediments. • results in compaction & subsidence • Ex. San Joaquin Valley, New Orleans, Baton Rouge & the Houston-Galveston area of Texas • Outside of U.S.- Mexico City, which is built on a former lake bed.