1. Streams and Floods Physical Geology, Chapter 10

2. Chp. 10 Running Water
Running water is the most important geologic agent of eroding, transporting and depositing sediment
Nearly every landscape on Earth shows the results of stream erosion or deposition

3. Hydrologic Cycle
Hydrologic cycle - the movement and interchange of water between the sea, air, and land
Evaporation
Condensation
Precipitation
Transpiration
Runoff
Infiltration

5. Streams
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

6. Drainage Basins
Drainage basin - the total area drained by a stream and its tributaries
Tributary -a small stream flowing into a larger one
Can be outlined on a map by drawing a line around the region drained by all of the tributaries of a stream
Divide - ridge or strip of 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

8. 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
1. Dendritic - drainage pattern resembling the branches of a tree
2. Radial pattern - streams diverge outward like the spokes of a wheel
Typically form on high conical mountains
3. Rectangular pattern - tributaries have frequent 90° bends and join other streams at right angles
4. Trellis pattern - parallel streams with short tributaries meeting at right angles

9. Stream Erosion
Stream erosion and deposition are controlled mainly by a river’s velocity and discharge
Stream velocity is largely controlled by stream gradient (slope), channel shape and channel roughness
Stream reaches maximum velocity near center of channel
Floods involve an increase in velocity and total discharge (volume of water passing a particular point in a stream over time)
Higher flow velocities in a stream allow it to
transport coarser sediments

10. Stream Erosion and Deposition
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

11. 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
1.Hydraulic action - ability of flowing water to pick up and move rock and sediment
2.Solution - dissolving of rocks (e.g., limestone)
3.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

12. Sediment Transportation
Sediment load transported by a stream can be subdivided into bed load, suspended load, and dissolved load
1.Bed Load - large or heavy particles that travel on the streambed
A.Traction load - large particles that travel along the streambed by rolling, sliding or dragging
B.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

13. Sediment Transportation Continued
2.Suspended load - sediment that is small/light enough to remain above the stream bottom by turbulent flow for an indefinite period of time
3.Dissolved load - dissolved ions produced by chemical weathering of soluble minerals upstream

14. Sediment Deposition
Bars - ridges of sediment (usually sand or gravel) deposited in the middle or along the sides of a stream
Braided streams contain an overabundance of sediment deposited as numerous bars around which water flows in a system of highly interconnected rivulets
Meandering streams flow faster along the outside of bends (meanders) and more slowly along the inside, during floods.

15. 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.
The Main channel has slightly raised banks with respect to the floodplain known as natural levees

16. Sediment Deposition
Deltas - a body of sediment deposited at the mouth of a river when the flow velocity decreases
Shape of a delta will depend on whether its growth and evolution are wave-dominated,
tide-dominated, or stream-dominated

17. Sediment Deposition
Alluvial fans - large, fan- or cone-shaped piles of sediment that usually form where a stream’s velocity decreases as it emerges from a narrow mountain canyon onto a flat plain
Particularly well-developed in desert regions, such as the southwestern U.S.
Larger fans show grading from large sediments nearest the mountains and finer sediments deposited progressively farther away

18. 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 in a flood causes flood erosion
Slowing of waters as flood waters recede causes flood deposits (usually of silt or clay-sized particles) to be laid down in the floodplain

19. 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

20. 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 levels overtop artificially raised levees, leading to extremely rapid flooding and erosion than would have occurred otherwise
Wise land-use planning, including prevention of building within 100-year floodplains, is most effective

21. Stream Valley Development
Downcutting
Process of deepening a valley by erosion of the streambed
V-shaped valleys typically form from downcutting of river channel combined with mass wasting and sheet erosion of valley walls
Streams cannot erode below the elevation of the mouth of the stream (the base level)
Base level can be sea level, a lake, or the bottom of a closed basin (like Death Valley, CA)

22. Graded streams
Characteristic concave-up longitudinal profile
Rapids and waterfalls have been smoothed out by prior downcutting
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

23. Stream terraces Incised meanders Superposed streams
Step-like landforms found above a stream and its floodplain
Occurs when river cuts downward into its own floodplain deposits
Represents change from deposition to erosion
Can be caused by
rapid uplift, drops in base level, or climate changes
Incised meanders
Retain their sinuous pattern as they cut vertically below original level
May be produced by profound base level changes, as due to rapid uplift
Superposed streams
Rivers superposed onto mountain range from an eroding floodplain that used to exist above it
Several rivers cutting through the previously partly-buried Appalachians are examples (Potomac, Susquehanna)

24. Stream Valleys on Mars Evidence of different climate in past
Liquid water is 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 of larger channels is puzzling, but these do exist for channels networks in more ancient terrains on Mars
Requires warmer, wetter Mars in past
NASA missions targeting such locations