1. Running Water
Running water is the most important geologic agent in eroding, transporting and depositing sediment
Nearly every landscape on Earth shows the results of stream erosion or deposition

2. Hydrologic Cycle
Hydrologic cycle - the movement and interchange of water between the sea, air, and land
Evaporation
Solar radiation provides energy
Precipitation
Rain or snow
Transpiration
Evaporation from plants
Runoff
Water flowing over land surface
Infiltration
Water soaking into the ground

4. Running Water
Stream - a body of running water, confined to a channel, that runs downhill under the influence of gravity
Headwaters - upper part of stream near its source in the mountains
Mouth - place where a stream enters sea, lake or larger stream
Channel - a long, narrow depression eroded by a stream into rock or sediment
Stream banks - sides of channel
Streambed - bottom of the channel
Floodplain - flat valley floor composed of sediment deposited by the stream
Insert revised Fig. 10.2

5. Drainage Basins
Drainage basin - the total area drained by a stream and its tributaries
Tributary - a small stream flowing into
a larger one
Divide - ridge or high ground that divides one drainage basin from another
Continental Divide separates the streams that flow into the Pacific from those that flow into the Atlantic and Gulf of Mexico

6. Mississippi River Drainage Basin

7. Missouri River Drainage Basin

8. Yellowstone River Drainage Basin

9. Powder River Drainage Basin

10. Drainage Patterns
Drainage pattern - the arrangement, in map view, of a stream and its tributaries
Most tributaries join the main stream at an acute angle, forming a V or Y pointing downstream
Dendritic - drainage pattern resembling the branches of a tree
Radial pattern - streams diverge outward like the spokes of a wheel
Typically form on conical mountains (volcanoes)
Rectangular pattern - tributaries have frequent 90° bends and join other streams at right angles
Trellis pattern - parallel streams with short tributaries meeting at right angles

11. The Parana River, Brazil: rectangular drainage?

12. The Big Sandy: a meandering stream

13. Sediment Deposition
Meandering streams flow faster along the outside of bends and more slowly along the inside, depositing point bars on the insides of the meanders
Meander cutoffs may form when a new, shorter channel is cut through the narrow neck of a meander (as during a flood)
Insert Fig

14. Deposition of sand bars, formation of cross bedding and Ox-bows (not related)

15. Stream Erosion
Stream erosion (and deposition) controlled by flow velocity and discharge
Stream velocity controlled by stream gradient (slope), channel shape and channel roughness
Maximum velocity near center of channel
Floods involve increased velocity and discharge (volume of water passing a particular point in a stream over time)
Higher stream velocities promote erosion and transport of coarser sediments
Erosion of very small particles difficult due to molecular binding forces

16. Stream Erosion Stream gradient is the downhill slope of the streambed
Typically measured in feet per mile in the U.S., and in meters per kilometer elsewhere
Usually decreases downstream
Channel shape and roughness
Both effect stream velocity due to drag
Narrower, deeper channels allow faster flow
Smoother channels allow faster flow
Wider, shallower channels decrease flow speed
Rougher channels decrease flow speed
Stream discharge is the volume of water flowing past a given point in a unit of time

17. Stream Erosion
Streams cut their own valleys, deepening and widening them over time and carrying away the sediment
Stream erosion occurs by three mechanisms: hydraulic action, solution, and abrasion
Hydraulic action - ability of flowing water to pick up and move rock and sediment
Solution - dissolving of rocks (e.g., limestone)
Abrasion - grinding away of stream channel by the friction and impact of the sediment load
Potholes are eroded into streambed by the abrasive action of the sediment load in the stream

18. Sediment Transportation
Sediment load transported by a stream can be subdivided into bed load, suspended load, and dissolved load
Bed load - large or heavy particles that travel on the streambed
Traction load - large particles that travel along the streambed by rolling, sliding or dragging
Saltation load - medium particles (typically sand-sized) that travel downstream by bouncing along - sometimes in contact with the streambed and sometimes suspended in the flowing water
Suspended load - sediment that is small/light enough to remain above the stream bottom by turbulent flow for an indefinite period of time
Dissolved load - dissolved ions produced by chemical weathering of soluble minerals upstream

19. Sediment Deposition
Sediments are temporarily deposited along stream course as bars and floodplain deposits, and at/near its end as deltas or alluvial fans
Bars - ridges of sediment (usually sand or gravel) deposited in the middle or along the sides of a stream
Braided streams contain sediment deposited as numerous bars around which water flows in highly interconnected rivulets

20. A Braided River: typical of rivers just disgorging from mountainous areas. A rapid drop in current velocity and too much stuff to carry

21. The North Platte River: A Braided Stream

22. Sediment Deposition
Floodplains are broad strips of land built up by sedimentation on either side of a stream channel
Floodplain sediments are left behind as flood waters slow and recede at the end of flood events
Main channel has slightly raised banks with respect to the floodplain known as natural levees

23. Sediment Deposition
Delta - body of sediment deposited at the mouth of a river when flow velocity decreases
Surface marked by shifting distributary channels
Shape of a delta depends on whether its wave-dominated, tide-dominated, or stream-dominated

24. The Ganges River Delta

25. Sediment Deposition
Alluvial fan - large, fan- or cone-shaped pile of sediment that forms where stream velocity decreases as it emerges from a narrow mountain canyon onto a flat plain
Well-developed in desert regions, such as the southwestern U.S.
Larger fans show grading from large sediments nearest the mountains to finer sediments farther away

26. Flooding
When water levels rise and overtop the banks of a river, flooding occurs
Natural process on all rivers
Described by recurrence intervals
A 100-year flood is, on average, the size of the largest flood within a 100-year period of time
Can cause great damage in heavily populated areas
High velocity and large volume of water causes flood erosion
Slowing of waters as flood ends causes flood deposits (usually of silt or clay-sized particles) to be deposited in the floodplain

27. Flooding Urban flooding Flash floods
Paved areas and storm sewers increase runoff by inhibiting infiltration
Rapid delivery of water to streams increases peak discharge and hastens occurrence of flood
Flash floods
Local, sudden floods of large volume and short duration
Typically triggered by heavy thunderstorms

28. Flooding Flood control
Dams designed to trap flood waters in reservoirs upstream and release it gradually over time
Artificial levees designed to increase capacity of river channel
Works well until stream overtops artificially raised levees, leading to extremely rapid flooding and erosion
Wise land-use planning, including prevention of building within 100-year floodplains, is most effective

29. Stream Valley Development
Downcutting
Process of deepening a valley by erosion of the streambed
V-shaped valleys typically form from downcutting combined with mass wasting and sheet erosion
Streams cannot erode below their base level
Basel level can be sea level, a lake, or the bottom of a closed basin (e.g., Death Valley, CA)
Downcutting rate can be rapid if a stream is well above base level (e.g., Grand Canyon, AZ)

30. Stream Valley Development
Graded streams
Characteristic concave-up longitudinal profile
Rapids and waterfalls have been smoothed out by extensive erosion over a long period of time
Delicate balance between available sediment load and transport capacity
Lateral erosion widens stream valleys by undercutting of stream banks and valley walls as stream swings from side to side across the valley floor
Headward erosion is the slow uphill growth of a valley above its original source by gullying, mass wasting, and sheet erosion

31. Stream Valley Development
Stream terraces
Step-like landforms found above a stream and its floodplain
Occurs when river rapidly cuts downward into its own floodplain
Represents relatively sudden change in rate of erosion
Can be caused by rapid uplift, drops in base level, changes in underlying lithology or climate changes

33. The Colorado: a meandering stream

34. The Colorado: downcutting due to tectonic uplift

35. Stream Valley Development
Incised meanders
Retain sinuous pattern as they cut vertically downward
May be produced by profound base level changes, as when rapid tectonic uplift occurs

36. Stream Valleys on Mars Evidence of different climate in past
Liquid water not stable on surface of Mars under present conditions
Too cold
Atmospheric pressure too low
Stream channels and terraces suggest long-term erosion by flowing water
Lack of smaller tributaries is puzzling, but these do exist for channels networks in more ancient terrains on Mars
Requires warmer, wetter Mars
NASA missions targeting such locations

37. Lecture Outlines Physical Geology, 11/e
Plummer, McGeary & Carlson

38. Mass Wasting
Mass wasting is downhill movement of masses of bedrock, rock debris or soil, driven by the pull of gravity
Landslides have been far more costly in the U.S., in terms of both lives and dollars, than all other geologic and weather hazards combined
Mass wasting is, with proper planning, perhaps the most easily avoidable of all major geologic hazards

39. Classification of Mass Wasting
Types of mass wasting are classified based on:
Rate of movement
Wide range from < 1cm/year to >100 km/hour
Type of material
Did moving mass start out as solid bedrock or as debris (unconsolidated material at Earth’s surface)
Type of movement
Flow, slide, or fall

40. Classification of Mass Wasting
Types of movement
Flow
Descending mass moves downhill as a viscous fluid
Slide
Descending mass remains relatively intact, and descends along well-defined surfaces
Translational slide - movement along plane parallel to motion
Rotational slide (slump) - movement along a curved surface
Fall
Material free-falls or bounces down a cliff

41. Factors Controlling Mass Wasting
Factors making mass wasting likely:
Steep slopes
Shear forces maximized by gravity
Large relief
(large elevation change from top of
mountains/hills to valley floor)
Thick layer(s) of loose rock,
debris, soil
Presence of water
Lubricates moving rocks/debris/soil
Lack of vegetation
No roots to hold rock/soil in place
Seismic (earthquake) activity

42. Factors Controlling Mass Wasting

43. Common Types of Mass Wasting
Creep (or soil creep)
Very slow downslope movement of soil
Major contributing factors include water in soil and daily freeze-thaw cycles
Can be costly to maintain homes, etc., on creeping ground as foundations, walls, pipes and driveways crack and shift downslope over time

44. Common Types of Mass Wasting
Debris flow - mass wasting in which motion takes place throughout the moving mass (flow)
Earthflow - debris moves downslope, slowly or rapidly, as a viscous fluid
Commonly occurs on steep hills, with thick debris cover, after heavy rains
Solifluction is an example
Mudflow - flowing mixture of debris and water, usually down a channel
Most likely to occur on steep unvegetated slopes with thick debris cover
Heavy rains on the slopes of stratocone volcanoes with fresh ash layers often triggers
Debris avalanches are very rapid and turbulent
Can reach speeds of several hundred km/hr

45. Types of Mass Wasting
Rockfall - when a block of bedrock breaks free and falls or bounces down a cliff
Commonly an apron of fallen rock fragments (talus) accumulates at cliff base
Rockslide - the rapid sliding of a mass of bedrock along an inclined surface of weakness
Rock avalanche - a very rapidly moving, turbulent mass of broken-up bedrock
Debris slide - a coherent mass of debris moving along a well-defined surface
Debris fall - a free-falling mass of debris

46. Preventing Landslides
Preventing mass wasting of debris
Construct retaining wall with drains
Don’t oversteepen slopes during construction
Preventing rockfalls and rockslides on highways
Remove all rock that is prone to sliding
“Stitch” together outcrop
Important to know the susceptibility of land to mass wasting before building any road or structure!