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  • 1. Fluvial Processes and Flooding

2. Summary of Important Concepts

  • Stream erosion is a very important force shaping the earths surface. A streams ability to erode is related to its velocity and its discharge.
  • Streams transport sediment particles. Streams that end at standing bodies of water (oceans or lakes) deposit this sediment to form deltas. Intermittent streams in arid mountain areas deposit coarse sediment to form alluvial fans.

3. Summary of Important Concepts, continued

  • Flooding is the main hazard associated with streams. Flooding is measured using hydrographs-- graphs which plot water level (or discharge) over a period of time associated with the flood.
    • Upland floods (upstream)occur in the higher areas of a drainage basin. These are sudden floods (flash floods) that move quickly through narrow valleys.
    • Lowland floodsoccur in the lower areas of a drainage basin. These are slower but longer-lasting floods that spread out over broad areas of the streams floodplain.
  • Urban development increases flooding because buildings, roads and parking lots decrease the infiltration of rainwater into the ground, and increase the speed with which rainwater runs off into channels.

4. Water on Earth is Recycled:The Hydrologic Cycle

  • >97% of earths water is contained in the oceans and roughly 3% is fresh.
  • Of that three %, MOST of earths fresh water is tied up in ice (~2%), then groundwater (next chapter), lakes, ice and streams.
  • Weather patterns determine the amount and location of precipitation, and the amount and time over which precipitation occurs is not constant.
  • Evaluation of Precipitation
    • Area over which the rain falls
    • Duration of the rain
    • Intensity

5. The Hydrologic CycleThe Fate of Precipitation

    • Most water evaporates off the ocean (large volume) and enters the atmosphere.Through precipitation of snow, rain and ice, water falls either directly back into the ocean or on the land.
    • Water that falls onto land enters streams by infiltration through the ground or by runoff.
    • Infiltration
      • The movement of water into rocks or soil through cracks and pore spaces.
    • Runoff
      • Water that flows over the land.

6. The Hydrologic Cycle 7. How Water Enters a Stream

  • A cup of land bounded by areas of high relief is called the DRAINAGE BASIN.Any precipitation that falls within the cup of land flows to the stream as either runoff or by infiltration.Infiltration capacity of the soil is controlled by:
    • Slope of the land (steeper slopes cause water to flow as runoff rather than infiltrate).
    • Soil texture (we will see with groundwater, properties of soils may dictate how easily water will infiltratethink about sand versus clay).
    • Nature of the vegetative cover (plants can remove water with roots and transpire the water back into the atmosphere).

8. The United States is divided, so to speak by the Appalachian Mountains on the East and the Rockies in the Midwest.All water falling in between those two mountain ranges is in the Mississippi River Drainage basin. 9. Streams A body of running water that is confined in a channel and flows under the influence of gravity. Channel width may vary from a few cms to several kms.The term stream in geology refers toany water flowing in a channel , and so includes rivers, streams, creeks, brooks, washes, arroyos and other related features. Throughout this lesson we will use the termstreamto includeallthese types of features. 10. In general, streams begin at theheadwatersat higher elevations, and discharge into other streams and lakes (relative base level) that will eventually reach the ultimate base level (sea level). 11. Streams

  • Longitudinal Profile - Elevation changes of a stream from source to mouth.
  • Gradient - the vertical drop of a channel over a horizontal distance.

12. Streams

  • Theheadwatersare the upper part of the stream near its source in the mountains.Upstreamregions.
    • Steep Gradient.
  • Lower reaches of a stream are referred to asdownstreamregions.
    • Shallow Gradient.

13. Streams

  • The mouth is the place where a stream channel terminates and enters the sea, a lake, etc.
  • Base level is the theoretical limit to which the stream can erode.It is, in effect, the elevation of the streams mouth.

14. The ultimate goal of a stream is to erode the land down to sea level!

  • Streams BEGIN at headwater regions and either DISCHARGE (flow into) into other streams, into lakes or into the ocean.
  • The limit to which a stream can downcut the bottom of its channel (erode) is its BASE LEVEL.The ultimate base level for fluvial processes is the ocean (weather all land to sea level).Many streams have a localized base level.

15. Stream Gradation

  • Headwater streams are located at higher elevations with steep slopes or gradients (change in elevation/distance).These streams usually have more work (breaking down the mountains) to do in terms of weathering away land and carrying sediments downstream.These streams are termed youthful because they are just beginning to carve the lands surface ( DEGRADING STREAMS ).It is essentially a collecting system for water and sediment.
  • As we move further downstream, the land begins to level, slope gradient begins to become more gentle and the water velocity slows down ( GRADED STREAMS ).It is essentially a transporting system for water and sediment.
  • Ultimately a stream will discharge into a large lake or ocean.As the stream approaches its base level, the land is relatively flat, water is flowing extremely slow and sediments are deposited into many landforms ( AGGRADING STREAMS ).It is essentially a dispersing system for water and sediment.

16. Stream Gradation

  • In order to examine features of these three streams, we need to look at the relationship between stream velocity and how much stuff a stream can carry (i.e. sediment).
  • The VOLUME of sediment a stream can carry is calledCAPACITY .Think of the capacity of sediment a stream is capable of carrying.Larger streams (having a larger volume of water has a higher capacity to transport stuff).
  • A streamsCOMPETENCEis the MAXIMUM grain size a stream can transport.This is directly related to the velocity of the stream.Faster moving water has a higher competence because it can move larger sized materials.

17.

  • Erosionby streams has shaped the land surface worldwide over geologic time. This spectacular gorge in Colorado is entirely a product of stream erosion acting over several millions of years.
  • The ability of a stream to erode (competence and capacity) relates to two things:
  • Velocity-- thespeedof the water, generally measured infeet per second .
  • 2. Discharge-- thetotal amount(volume) of water carried by the stream. Discharge is generally measured incubic feet per second , orcfs .

18. Stream Velocity Profile: Friction along the bottom and sides of a stream (wetted perimeter) decreases the velocity there.The fastest moving water is therefore, in the center of the stream at the surface. 19. Stream Discharge

  • Stream discharge is the VOLUME of water that passes a given point along a stream in a given amount of time.
  • Discharge (Q) depends on two thingshow fast is the water moving (velocity, v) and cross sectional area (A) of the stream (to determine volume of water).
  • Q = A*vwhere velocity is measured in feet/sec and area is ft 2 .ft/sec*ft 2 = ft 3 /sec or cubic feet per second or CFS.
  • For example, if we have a lot of precipitation, stage level in the stream rises, increasing the cross sectional area of the stream (i.e. more water).Increase A and multiply it by v, will increase the discharge Q.

20.

  • Discharge
      • The volume of water passing a given point in a stream per unit time
      • Q = cross sectional area of the stream channel (depth x width) x velocity
      • measured in cubic meters or cubic feet per second

Upstream - Narrow V-shaped channels Downstream - wide, deep U-shaped channel 21. This graph shows the connection between discharge and the amount of sediment a stream can carry (sediment derived from erosion of the streams banks and channels).The higher the velocity, the faster the water moves, the more sediments can be transported, and the higher erosive capabilities of the stream. A 10x increase in discharge corresponds to a nearly 100x increase in erosion and sediment load carried. 22. Stream Gradation

  • Okay, considering what we have learned, we can examine the features of the different types of streams mentioned earlier.These three types