Earth Science, 13e

1. Earth Science, 13e Tarbuck & Lutgens

2. Running Water and GroundwaterEarth Science, 13eChapter 5 Stanley C. Hatfield Southwestern Illinois College

3. Li River, China’s Guilin DistrictNote karst topography, in which groundwater has dissolved large volumes of limestone (more later).

4. Earth as a system: the hydrologic cycle • Illustrates the circulation of Earth’s water supply • Processes involved in the cycle • Precipitation • Evaporation • Infiltration • Runoff • Transpiration

5. The hydrologic cycle

6. Running water • Drainage basin • Land area that contributes water to a river system • A divide separates drainage basins

7. Drainage basins and divides

8. Drainage basins and divides exist for all streams, regardless of size.

9. River systems are divided into 3 zones.

10. Running water • Streamflow • Factors that determine velocity • Gradient, or slope • Channel characteristics • Shape • Size • Roughness • Discharge – volume of water flowing in the stream (generally expresses as cubic feet per second)

11. Gradient (slope) • Parts of lower Mississippi: 10 cm/km • Mountain streams: 40 m/km • Steeper gradient has more energy,  more velocity

12. Discharge • Measured in m3 or ft3 per second • Changes over time due to amount of precipitation in drainage basin

13. Measuring stream velocity 1 kph – 30 kph Straight – highest at center Curved– highest at outer bank

14. Running water • Upstream-downstream changes • Profile • Cross-sectional view of a stream • From head (source) to mouth • Profile is a smooth curve • Gradient decreases from the head to the mouth • Factors that increase downstream • Velocity • Discharge

15. Running water • Upstream-downstream changes • Profile • Factors that increase downstream • Channel size • Factors that decrease downstream • Gradient, or slope • Channel roughness

16. Longitudinal profile of a stream

17. Stream Properties – Headwaters to Mouth

18. Running water • The work of streams • Earth’s most important erosional agent • Downcut, widen streams • Transport sediment which can erode banks, channel, bedrock • MG: Sediment transport by streams

19. Running water • Transportation – transported material is called the stream’s load • Dissolved load • From groundwater, dispersed through flow • Expressed in ppm • Velocity of streamflow has no effect on stream’s ability to carry dissolved load • Precipitation only if water chemistry changes

20. Running water • Transportation – transported material is called the stream’s load • Suspended load • Biggest portion of river’s load • Usually fine particles s/a silt, clay but could be sand or gravel, especially during flood (which can also increase quantity) • Controlled by flow velocity and settling velocity (speed @ which particle falls through still fluid) • Slow settling + high flow = longer suspension • Bed load

21. Running water • Suspended load, Colorado River

22. Running water • Transportation – transported material is called the stream’s load • Bed load – solids are to large to be carried in suspension, settle along stream bed • Erosional action – move by rolling, sliding, saltation (jumping or skipping) • < 10% of total load

23. Running water • The work of streams • Transportation • Load is related to a stream’s • Competence • maximum particle size • increases proportionately to square of velocity (swift streams have greater competence • Capacity • maximum load • related to discharge

24. Running water • The work of streams • Transportation • Deposition • Caused by a decrease in velocity • Competence is reduced • Sediment begins to drop out • Stream sediments • Known as alluvium • Well-sorted deposits

25. Running Water • Bedrock channels vs. alluvial channels • Bedrock – headwater, steep • May contain rapids/waterfalls • Rapid • section of river where river bed has a relatively steep gradient  • increase in water velocity and turbulence • river becomes shallower and has some rocks exposed above the flow surface

26. Running Water

27. Running Water • Bedrock channels vs. alluvial channels • Bedrock – headwater, steep • May contain rapids/waterfalls • Waterfall – place where water flows over a vertical drop in the course of a stream or river

28. Running Water • Bedrock channels vs. alluvial channels • Alluvial – loosely consolidated sediment (alluvium) • Meandering • mostly suspended load • evolve over time as bends migrate floodplain • most erosion @ outside of bend

29. Running Water Cut bank – zone of active erosion Point bar – coarser material deposited

30. Running Water

31. Running Water • Braided Streams • Complex network of diverging channels • Coarse grains are transported as bed load

32. South-looking photograph showing diamond-shaped bars and meandering braided stream channels, East Fork Toklat River, Alaska Range, Denali National Park, Alaska. South-looking photograph showing diamond-shaped bars and meandering braided stream channels, East Fork Toklat River, Alaska Range, Denali National Park, Alaska. Rakaia River, South Island New Zealand

33. Running water • Base level • Lowest point a stream can erode to • Two general types • Ultimate – sea level • Temporary, or local • Changing causes readjustment of the stream – deposition or erosion

34. Adjustment of base level to changing conditions

35. Running water • Stream valleys • Valley sides are shaped by • Weathering • Overland flow • Mass wasting • Characteristics of narrow valleys • V-shaped • Downcutting toward base level

36. Running water • Stream valleys • Characteristics of narrow valleys • Features often include • Rapids • Waterfalls • Characteristics of wide valleys • Stream is near base level • Downward erosion is less dominant • Stream energy is directed from side to side

37. Running water • Stream valleys • Characteristics of narrow valleys • Features often include • Rapids • Waterfalls • Characteristics of wide valleys • Stream is near base level • Downward erosion is less dominant • Stream energy is directed from side to side

38. V-shaped valley of the Yellowstone River

39. Continued erosion and deposition widens the valley – see next slide

41. Land is uplifted – meandering river downcuts

42. Running water • Features produced by deposition • Deltas • exist in ocean or lakes • formed from the deposition of the sediment carried by the river as the flow leaves the mouth of the river • Human activities s/a diversion of water, damscan radically alter delta ecosystems • dams block sedimentation which can cause the delta to erode away • use of water upstream can greatly increase salinity levels as less fresh water flows to meet the salty ocean water • Nile Delta and Colorado River Delta are some of the most extreme examples of the ecological devastation caused to deltas by damming and diversion of water.

43. Nile River delta

44. Running water • Features produced by deposition • Natural levees • form parallel to the stream channel • commonly form around lowland rivers and creeks without human intervention • Area behind levee is characteristically poorly drained (water can not flow up the levee and into the river) • Marshes called backswamps result. • Yazoo tributaries • Since tributary stream can not enter river, it has to flow parallel to the river until it can breach the levee • Name comes from the Yazoo River, which runs parallel to the Mississippi River for 280 km (170 mi) before converging

45. Formation of natural levees by repeated flooding

46. Running water • Features produced by deposition • Alluvial Fan - fan-shaped deposit formed where a fast flowing stream flattens, slows, and spreads, typically at the exit of a canyon onto a flatter plain • As stream's gradient decreases, it drops coarse-grained material • Reduces capacity of channel • Forces it to change direction and gradually build up a slightly mounded or shallow conical fan shape. 

47. Alluvial Fan – Lake Louise, Alberta

48. Running water • Floods and flood control • Floods are the most common geologic hazard • Causes of floods • Floods are caused by many factors and can be exacerbated by increased amounts of impervious surface or by other natural hazards such as wildfires, which reduce the supply of vegetation that can absorb rainfall.

49. Causes of floods • Heavy rainfall • Highly accelerated snowmelt • Severe winds over water • Unusual high tides • Tsunamis • Failure of dams, levees, retention ponds, or other structures that retain water

50. Running water • Floods and flood control • Engineering efforts • Artificial levees • Steeper slope than natural levee • Sometimes made of concrete • Flood-control dams • Store water, then let it out slowly • Destroy farmland, etc. • Trap sediment leading to erosion downstream