1. The Fluvial Geomorphic System Fluvial (Latin fluvius: river) Streams 85% sediment transport, glaciers 7%, waves 1.5%, wind 1% Lab Hose, sand in beaker

2. Conversion Potential energy (elevation) to Kinetic Energy (motion). Most energy lost to friction, but 2-4% of total potential energy converted to work of erosion and transportation Tributaries Trunk Stream Drainage Divide Distributaries And Delta Gradient decreases downstream Factors that increase downstream: Velocity Discharge Channel size

3. Infiltration Infiltration Infiltration is controlled byInfiltration is controlled by –Intensity and duration of rainfall –Prior wetted condition of the soil –Soil texture –Slope of the land –Nature of the vegetative cover http://plantandsoil.unl.edu/croptechnology2005/soil_sci

4. Stream water feeds Stream water feeds Infiltration (rain and snowmelt soak into soil) –water moving in pore spaces Throughflow water moving through voids in soil –In cracks, burrows, root casts, etc. Runoff Sheetflow = overland flow thin slow surface layer transitions into rill erosion Sheetflow

5. Rill Erosion, Gully formation Source: Ramesh Venkatakrishnan Rill Erosion. Individual Channels do not persist Gully. Larger channels, they persist, grow headward and widen. Rill: a trickle of water, a very small stream Gully: a small watercourse with steep sides, usually cut into soil

6. Discharge Discharge The amount of water passing through a channel cross-section in unit timeThe amount of water passing through a channel cross-section in unit time Q = v * A where v is the average flowQ = v * A where v is the average flow velocity [meters / second] through a cross section A [meters 2 ] velocity [meters / second] through a cross section A [meters 2 ] Thus Q has units meters 3 /second

7. Bedrock Channels Cut down by: Solution Weathering - LimestoneSolution Weathering - Limestone Plucking (= quarrying) esp. jointed rocksPlucking (= quarrying) esp. jointed rocks –Fast moving water gets in cracks and pushes blocks free. Cycles of wetting and drying, or freezing and thawing, loosen blocks Abrasion Abrasion –the impact of rolling and bouncing detritus. Notice I skipped page 200 lower right beginning "Soil Erosion" and resumed page 203 right "Bedrock Channels" Photo by Natalie Romanoff Used with Permission

8. Waterfalls p 205 right "In stratified rocks of contrasting lithology … each resistant layer forms a waterfall in a bedrock channel, usually with a plunge pool" that scours the underlying beds.p 205 right "In stratified rocks of contrasting lithology … each resistant layer forms a waterfall in a bedrock channel, usually with a plunge pool" that scours the underlying beds. p206 left (paraphrased for clarity) As the waterfalls migrate headward, if the resistant layer dips headward, waterfalls will [become smaller]; if layers dip seaward, the waterfall may grow in height.p206 left (paraphrased for clarity) As the waterfalls migrate headward, if the resistant layer dips headward, waterfalls will [become smaller]; if layers dip seaward, the waterfall may grow in height.

9. Local change in base level affects river profiles: nickpoints Resistant layer dips headward

10. Alluvial Channels Channels usually lined with alluviumChannels usually lined with alluvium Removed and carried further downstream during floodRemoved and carried further downstream during flood Re-deposited during waneRe-deposited during wane

11. "As mean discharge of a river increases downstream, channel width, channel depth, and mean current velocity all increase." Tributaries Trunk Stream Drainage Divide Distributaries And Delta Gradient decreases downstream Factors that increase downstream: Velocity Discharge Channel size p 207 r para 1 Width, Depth, Current Velocity and Longitudinal Profile Water flows with less friction in larger channels maintains velocity with less slope p209 l

12. Ultimate Base Level Control At sea level, no further conversion of potential energy to stream work is possible.At sea level, no further conversion of potential energy to stream work is possible. Sea level lowered at least 100 m during peaks of glaciation. Headwaters were higher, stream water had more potential energy, faster streams cut deep canyonsSea level lowered at least 100 m during peaks of glaciation. Headwaters were higher, stream water had more potential energy, faster streams cut deep canyons All rivers now enter the sea via estuaries (drowned lower valleys) or deltas (sediment filling lower valleys).All rivers now enter the sea via estuaries (drowned lower valleys) or deltas (sediment filling lower valleys).

13. Overbank deposition during flooding Flood over banks into wider area, lower velocity, sand drops.(Muddy water over floodplain)

14. 6_6 Fine-grained sediment on floodplain Flood water Erosion of uppermost fine-grained sediment Older sediment 1Pre-flood Flood stage 2 Post-flood 3 Fining-upward flood deposit Bedding plane is an erosional surface Floods change the local conditions We can recognize past floods Bounders on bottom, sands and muds suspended Flood: One source of Graded Bedding skip 211r, 212, 213

15. Load, Competence Streams transport sedimentStreams transport sediment Transported material is called loadTransported material is called load Types of loadTypes of load –Dissolved load – ions in solution from weathering –Suspended load – fine particles* –Bed load Moves during high velocity events Sandy Portion – Saltation (bouncing) Sandy Portion – Saltation (bouncing) Cobbles – Traction (rolling) Cobbles – Traction (rolling) Competence – Largest particles it can moveCompetence – Largest particles it can move Proportional to velocity squared KE = 1/2 mV 2 *Our Buckingham Pi studies of grain in fluid Important definitions

16. Deposition of sediment by a streamDeposition of sediment by a stream Caused by a decrease in velocityCaused by a decrease in velocity –Competence is reduced –Sediment begins to drop out http://www.geo.cornell.edu/geology/classes/Geo101/graphics/AKbraid_stream.gif

17. Similitude For model studies that teach us about the real world, Similitude (similarity) is neededFor model studies that teach us about the real world, Similitude (similarity) is needed Dimensionless numbers such as theDimensionless numbers such as the Reynolds Number Re =  vR/  Reynolds Number Re =  vR/  must be the same in model and real world must be the same in model and real world R is the channel cross sectional area divided by the length of the wetted perimeterR is the channel cross sectional area divided by the length of the wetted perimeter skip 216, 217, 281 left Page 215

18. Dimensionless Numbers for Streams Froude Number Fr = v type of turbulence (gd) 1/2 Tranquil, or supercritical flow. Latter standing wave, resistance to flow Supercritical mostly in narrow bedrock channels in flood Reynolds Number Re =  vR/  laminar or turbulentReynolds Number Re =  vR/  laminar or turbulent Drag Coefficient C D = F grain suspensionDrag Coefficient C D = F grain suspension  V 2 D 2  V 2 D 2 These are related to competence and erosion

19. Capacity Maximum mass of sediment a stream can transport.Maximum mass of sediment a stream can transport. Measured by amount of suspended load.Measured by amount of suspended load. skip 219 220 left

20. , Habit, Channel Shape, Habit, Solid Load Q determines cross-sectionalQ determines cross-sectional channel shape and channel shape and HABIT (shape from space) measured using thalwegHABIT (shape from space) measured using thalweg Straight: Suspended load channels (<3% bedload)Straight: Suspended load channels (<3% bedload) Meandering: single sinuous channel, carries mostly suspended load, with point bars,Meandering: single sinuous channel, carries mostly suspended load, with point bars, loops loops Braided: bedload > 11% total, high channel width to depth, mid-channel barsBraided: bedload > 11% total, high channel width to depth, mid-channel bars Braided Stream Background: Figure 10-15 from text, p.221, after Schumm, 1981

21. Erosion and deposition along a meandering stream Meanders get more extreme with time. Note the THALWEG (blue arrows)

22. Floodplain Meandering Stream Oxbow http://hays.outcrop.org/gallery/rivers/arid_meander?full=1 Channel moves in direction of cutbank Pointbar advances as crossbed drapes

23. Reminder: Point Bar Sequence Erosion Gravel of bed Crossbeds of Bar Fines of Floodplain

24. Braided Streams Braided: high total load; bedload > 11% total, high channel width to depth; mid-channel barsBraided: high total load; bedload > 11% total, high channel width to depth; mid-channel bars Braided Stream

25. Graded Streams Grade: natural tendency toward self regulationGrade: natural tendency toward self regulation Changing conditions cause adjustmentChanging conditions cause adjustment –Sudden extra sediment narrows channel: so faster flow and much more erosion during flood, so faster flow and much more erosion during flood, so blockage removed and previous channel shape reestablished. so blockage removed and previous channel shape reestablished. Demo: narrow channel of lab hose http://www.tweed.nsw.gov.au/Educ ation/rm_4_bankerosion.htm