Chapter 16: The Fluvial Processes

1. Chapter 16: The Fluvial Processes

2. The Fluvial Processes The Impact of Fluvial Processes on the Landscape Streams and Stream Systems Stream Channels Structural Relationships The Shaping and Reshaping of Valleys Floodplains Theories of Landform Development 2 � 2011 Pearson Education, Inc.

3. The Impact of Fluvial Processes on the Landscape Moving water is widespread Effective as an agent of erosion and deposition Influence on landscape dominant Smoothes irregularities 3 � 2011 Pearson Education, Inc.

4. Streams and Stream Systems Any channeled flow of water is a stream Fluvial processes are those which involve running water Unchanneled downslope movement of water� overland flow Channeled movement of water�streamflow 4 � 2011 Pearson Education, Inc.

5. Streams and Stream Systems Valleys and interfluves Valley�portion of terrain in which a drainage system is established Interfluve�higher land above valley walls that separates adjacent valleys Drainage basins Watersheds Areas that drain to the valley Drainage divide 5 � 2011 Pearson Education, Inc.

6. Streams and Stream Systems Stream orders Small streams join larger ones Relationship called stream orders First order versus second and third order Smaller numbers in stream order are more frequent Larger numbers are longer in length and watershed order 6 � 2011 Pearson Education, Inc.

7. Streams and Stream Systems Fluvial erosion and deposition Splash erosion Sheet erosion Rill erosion Gully erosion Erosion by streamflow Once channeled, erosion is greatly enhanced Erosion increased by debris the streamflow picks up Chemical weathering�corrosion 7 � 2011 Pearson Education, Inc.

8. Streams and Stream Systems Perennial and intermittent streams Perennial streams�permanent, in humid regions Intermittent streams�seasonal Ephemeral streams The role of floods Amount of water in a stream, discharge Most excavating of valleys accomplished during floods Stream gages used to measure flow characteristics Flood recurrence intervals (i.e., the �100-year flood�) 8 � 2011 Pearson Education, Inc.

9. Stream Channels Streamflow normally confined to channels, giving it a three-dimensional complexity Friction effect of flow along bottom and sides of channel Highly turbulent flow irregularities produce local currents which modify streamflow 9 � 2011 Pearson Education, Inc.

10. Stream Channels Stream channel patterns, four categories Straight channels: short and indicative of strong geologic structure control Sinuous channels: irregular and gentle curvature that is sinuous in appearance Meandering channels: highly variable curvature Braided streams: multiple interwoven channels separated by loose bars or islands 10 � 2011 Pearson Education, Inc.

11. Structural Relationships Stream development affected by many factors Most important factor is geologic-topographic structure Consequent versus subsequent streams Antecedent streams�those which existed before new uplift occurs Superimposed streams 11 � 2011 Pearson Education, Inc.

12. Structural Relationships Stream drainage patterns Dendritic pattern: treelike pattern with random merging of streams Trellis pattern: forms in areas of hard and soft bedrock in close contact, shape modified by structure of bedrock 12 � 2011 Pearson Education, Inc.

13. Structural Relationships Stream drainage patterns (cont.) Radial pattern: streams descend a concentric uplift Centripetal pattern: streams converge into a uniform basin Annular pattern: forms in areas of hard and soft domes or basins, flow follows soft bedrock and is confined by hard bedrock 13 � 2011 Pearson Education, Inc.

14. The Shaping and Reshaping of Valleys Valley deepening Downcutting�hydraulic power of the moving water to lower the streambed Base level�lower limit to the amount of downcutting that can occur�ultimate base level is sea level Knickpoints�channel irregularities such as rapids and waterfalls Knickpoint migration 14 � 2011 Pearson Education, Inc.

15. The Shaping and Reshaping of Valleys Valley widening Widening slows when above the base level As gradient decreases, stream flow meanders and lateral erosion occurs Water moves fastest on outside of curves (cut bank) Slowest water on inside of curves accumulates alluvium (point bar) 15 � 2011 Pearson Education, Inc.

16. The Shaping and Reshaping of Valleys Valley lengthening Headward erosion: fast moving water from slope break between interfluve and valley wall causes material collapse Results in decrease of interfluve area and increase in valley area Causes a headward extension of the valley into the interfluve area 16 � 2011 Pearson Education, Inc.

17. The Shaping and Reshaping of Valleys Stream capture Headward erosion causes one stream to be diverted to the basin of another stream naturally Captor versus captured stream Beheaded stream Elbow of capture 17 � 2011 Pearson Education, Inc.

18. The Shaping and Reshaping of Valleys Delta formation Slowed flow when it reaches an ocean or lake results in deposited sediment Debris builds up and forms a delta Distributaries Deposition in valleys Aggradation 18 � 2011 Pearson Education, Inc.

19. Floodplains Low-lying near flat alluvial valley floor that is periodically inundated with floodwaters Floodplain landforms Bluffs Cutoff meander Oxbow lake Meander scars Natural levees Backswamps Yazoo streams 19 � 2011 Pearson Education, Inc.

20. Floodplains Modifying rivers to control flooding Humans live on floodplains due to flat land, abundant water, and productive soils Manmade levees, dams, and overflow floodways created to avert disaster Example of the Mississippi River Human-induced changes to the deltas and floodplains 20 � 2011 Pearson Education, Inc.

21. Stream Rejuvenation Lowering of sea level during ice ages or increase of elevation from tectonic uplift increases gradients Gradient increases restart or rejuvenate vertical incision by streams Stream terraces Entrenched meanders 21 � 2011 Pearson Education, Inc.

22. Theories of Landform Development Davis�s geomorphic cycle Continuous sequence of uplift, fluvial erosion, and denudation Youth stage, streams become established and drainage pattern develops Mature stage, streams approach equilibrium Old age stage, erosion reduces landscape to near base level Rejuvenation, new uplift restarts the cycle 22 � 2011 Pearson Education, Inc.

23. Theories of Landform Development Penck�s Theory of Crustal Change and Slope Development Criticisms of Davis� geomorphic cycle Stated the land slope has a parallel retreat, same slope angle over time Many ideas have been substantiated by subsequent works 23 � 2011 Pearson Education, Inc.

24. Theories of Landform Development Equilibrium theory Variations in influence of crustal movement and resistance of underlying rock Slope forms adjust to geomorphic processes to achieve energy balance Shortcomings in areas that are tectonically stable Prime theory used today 24 � 2011 Pearson Education, Inc.

25. Summary Fluvial processes are those which involve moving water Fluvial processes impact the landscape through erosion and deposition Valleys, interfluves, and drainage basins describe the impacts of fluvial processes on landscape Stream orders help describe the structure of stream systems Streams affect the landscape through numerous types of erosion and deposition Friction along stream channels and turbulence within the channels affects their patterns 25 � 2011 Pearson Education, Inc.

26. Summary There are four primary types of stream channels Consequent and subsequent streams develop along areas of new land formation Antecedent and superimposed streams result from slow uplift of land affecting the pattern There are five patterns of stream drainage Streams affect valleys by both widening them and deepening them through downcutting The base level is the lowest level at which a stream will downcut 26 � 2011 Pearson Education, Inc.

27. Summary Knickpoints are irregularities in the channel flow, such as waterfalls or rapids Valleys can be lengthened by headward erosion, stream capture, and delta formation Floodplains are flat land areas that have fertile soil but are prone to occasional flooding from nearby streams Numerous landforms related to floodplains exist Stream rejuvenation occurs when uplift increases the slope gradient of a stream There are numerous theories of landform development and their relationship to fluvial processes 27 � 2011 Pearson Education, Inc.