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, change rock into sediment, are driven by the Sun and gravity. Internal processes, such as mountain building and volcanic activity, derive their energy from Earth’s interior

3. Mass Wasting: The Work of Gravity Mass wasting is the downslope movement of rock and soil due to gravity, no transport medium required walls of canyon extend far from the river Controls and triggers of mass wasting Water—Reduces the internal resistance of materials (lubricates) 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 water cycle 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 16.3 Figure 3.5

7. Distribution of Earth’s Water Figure 3.4

8. Running Water Streamflow The ability of a stream to erode and transport materials is determined by velocity Factors that determine velocity Gradient, or slope-drop over distance Channel characteristics including shape, size, and roughness

9. Running Water Streamflow Factors that determine velocity Discharge—The volume of water moving past a given point in a certain amount of time 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

10. Running Water Changes from upstream to downstream Profile- cross-sectional view from head to mouth Profile is a smooth curve Gradient decreases downstream Factors that increase downstream Velocity Discharge Channel size

11. Longitudinal Profile of a Stream Figure 3.8

12. 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

13. Base Level Base level and stream erosion Changing conditions causes readjustment of stream activities Raising base level causes deposition Ex. Reservoir behind dam Lowering base level causes erosion ex. Incised meanders

14. Adjustment of Base Level to Changing Conditions Figure 3.9

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

16. The Work of Streams Stream erosion Lifting loosely consolidated particles Abrasion Dissolution - is of least significance Stronger currents lift particles more effectively from bed and banks

17. The Work of Streams Transport of sediment by streams Transported material is called the stream’s load Types of load Dissolved load- contributed by groundwater Suspended load -usually silt and clay Bed load – to large to be carried in suspension Capacity— the maximum load a stream can transport, is directly related to discharge

18. 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

19. 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 Stream sediments Generally well sorted Stream sediments are known as alluvium

20. The Work of Streams Deposition of sediment by a stream Delta—Body of sediment where a stream enters a lake or the ocean Results from a sudden decrease in velocity main channel divides into smaller distributaries Mississippi delta-7 coalescing subdeltas Natural levees—Form parallel to the stream channel by successive floods over many years

21. Formation of Natural Levees Figure 3.14

22. The Work of Streams Deposition of sediment by a stream Floodplain deposits Back swamps Yazoo tributaries

23. Stream Valleys The most common landforms on Earth’s surface Two general types of stream valleys Narrow valleys V-shaped 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

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

25. Stream Valleys Features of wide valleys often include Floodplains Erosional floodplains Depositional floodplains Meanders Cut banks and point bars Cutoffs and oxbow lakes

26. Erosion and Deposition Along a Meandering Stream Figure 3.17

27. A Meander Loop on the Colorado River

28. Floods and Flood Control Floods and flood control Floods are the most common and most destructive geologic hazard Causes of flooding Result from naturally occurring and human-induced factors Causes include heavy rains, rapid snow melt, dam failure, topography, and surface conditions

29. Floods and Flood Control Floods and flood control Flash flood-occurs with little warning Flood control Engineering efforts Artificial levees-earthen mounds Flood-control dams-store water Channelization-alter stream channel Nonstructural approach through sound floodplain management

30. Drainage Basins and Patterns Drainage networks Land area that contributes water to the stream is the drainage basin Imaginary line separating one basin from another is called a divide, range in size from a ridge; which separates gullies, to continental divide which separates continents into enormous drainage basins

31. Drainage Basin of the Mississippi River Figure 3.21

32. Drainage Basins and Patterns Drainage pattern Pattern of the interconnected network of streams in an area Common drainage patterns Dendritic-”treelike” Radial-diverge from a central area Rectangular-many right angle bends Trellis-rectangular pattern in which tributary streams are nearly parallel to one another. Alternating bands of resistant and less resistant rock.

33. Drainage Patterns Figure 3.22

34. Water Beneath the Surface Largest freshwater reservoir for humans Geological roles  As an erosional agent, dissolving by groundwater produces  Sinkholes  Caverns  An equalizer of stream flow

35. 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  Zone of aeration  Unsaturated zone  Pore spaces in the material are filled mainly with air

36. 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

37. Features Associated with Subsurface Water Figure 3.25

38. Water Beneath the Surface  Movement of groundwater  Porosity  Percentage of pore spaces  Determines how much groundwater can be stored

39. Water Beneath the Surface  Movement of groundwater  Permeability  Ability to transmit water through connected pore spaces  Aquitard—An impermeable layer of material  Aquifer —A permeable layer of material

40. Water Beneath the Surface  Springs  Hot springs  Water is 6–9ºC warmer than the mean air temperature of the locality  Heated by cooling of igneous rock  Geysers  Intermittent hot springs  Water turns to steam and erupts

41. Water Beneath the Surface  Wells  Pumping can cause a drawdown (lowering) of the water table  Pumping can form a cone of depression in the water table

42. Water Beneath the Surface  Artesian Wells  Water in the well rises higher than the initial groundwater level  Artesian wells act as “natural pipelines” moving water from remote areas of recharge great distances to the points of discharge

43. Formation of a Cone of Depression Figure 3.28

44. An Artesian Well Resulting from an Inclined Aquifer Figure 3.29

45. Water Beneath the Surface  Environmental problems associated with groundwater, being exploited at 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)

46. Water Beneath the Surface  Geologic work of groundwater  Groundwater is often mildly acidic  Contains weak carbonic acid  Reacts with calcite in limestone to form calcium bicarbonate, which is soluble & is carried away in solution  Caverns  Formed by dissolving rock beneath Earth's surface  Formed in the zone of saturation

47. Water Beneath the Surface  Caverns  Features found within caverns  Form in the zone of aeration  Composed of dripstone (speleothem)  Calcite deposited as dripping water evaporates  Common features include stalactites (hanging from the ceiling) and stalagmites (growing upward from the floor )

48. Water Beneath the Surface  Karst topography  Formed by dissolving rock at, or near, Earth's surface  Common features  Sinkholes—Surface depressions  Sinkholes form by dissolving bedrock and cavern collapse  Caves and caverns  Area lacks good surface drainage (streams)  Runoff is quickly funneled belowground through sinks, flows through caverns until it reaches the water table.

49. End of Chapter 3