Geol 370: Sedimentology and Stratigraphy Topic 10a: Fluvial Systems - general

1. Photo by W. W. Little
Fluvial Systems
A fluvial system consists of a network of channels and associated
environments that transport sediment from a drainage basin to a
depositional basin.

2. Photo by W. W. Little
Discharge
Discharge is a measure of the amount of water carried by a stream.

3. Major Divisions of Fluvial Deposits
For purposes of large-scale architecture, fluvial systems can be divided into
two primary components, channel belt and floodplain. Splay deposits are also
significant. Each of these major sub-systems can be further divided into a
number of environments and sub-environments, such as channel floor, point
bar, abandoned channel fill, levee, overbank, swamp, etc.

4. Discharge is the volume of water that passes through a given cross-
sectional area of a stream during a specified period of time.
Measuring Discharge
Q = V/t = vA = vwd P = w + 2d

5. Photo by W. W. Little
Capacity vs. Competence
Capacity: reflects the potential volume of sediment transported by
a stream and is controlled by discharge and flow velocity.
Competence: measures the maximum size of clast that can be
transported by a stream and is related to flow velocity and
discharge.

6. In a straight channel, due to
hydraulic shear, velocity is
greatest in the center just
below the surface and least
along the bed and banks.
Flow Velocity
A stream obtains its velocity from a conversion of potential energy to
kinetic energy under the force of gravity and is, therefore, controlled
by stream gradient.
There is a limit to the maximum velocity and momentum of a given discharge
flowing down a specified slope due to friction between the water and channel
boundaries and between molecules within the flow.

7. In a curved channel, velocity is
greatest near the outside of a
bend just below the surface
and least along the bed and
banks.
Meandering
Because of turbulence, the focus of maximum velocity does not
maintain a straight course and the channel begins to bend. This
concentrates velocity toward the outside of the bend, increasing its
curvature.
The increase in velocity along the outside of a channel causes erosion. A
corresponding decrease in velocity on the inside of the channel results in
deposition.

8. Photo by W. W. Little

9. Photo by W. W. Little

10. Photo by W. W. Little

11. Photo by W. W. Little

13. Meander Evolution
Because of internal turbulence, all stream channels meander. The degree
of meandering is controlled primarily by discharge (energy) and nature
of the sediment (bank stability).

14. Photo by W. W. Little

17. Oxbow Lakes
Oxbow lakes form as reaches of a channel become abandoned through
meander-neck cut-off or chute entrenchment. These then become filled
with fine-grained sediment during flooding

18. Lateral Accretion
To maintain constant width, as a stream erodes along the cutbank, it
must deposit on the point bar. This results in lateral migration of
channel facies.

19. Photo by W. W. Little

20. Photo by W. W. Little

21. Photo by W. W. Little

23. Nature of Load
The load includes all material being carried by the discharge through
traction, suspension, or dissolution.

24. Photo by W. W. Little
Bed Load

25. Photo by W. W. Little
Suspended Load

26. Photo by W. W. Little

27. Floods
Floods occur when discharge exceeds a channel’s capacity to
contain it and stage height rises above bank elevation.
• Under natural conditions, flooding typically occurs every two to three years in response
to heavy precipitation (duration or intensity) and/or rapid snow melt.
• Extremely large floods can be caused by natural phenomena, such as glacial outbursts
and hurricanes, or human-related evens, like dam failures.

28. Flood Stage Flow Pattern
During flooding, the portion of flow above bank-full travels as a sheet
in a more or less straight path.

29. Mississippi River
Prairie du Chien, WI (1969) Photo by: Louis J. Maher

30. Photo by W. W. Little

31. Photo by W. W. Little

32. Photo by W. W. Little

33. Photo by W. W. Little
Swamps
Swamps develop in portions of the floodplain that remain below the
water table for extended periods of time. Deposits tend to be rich in
plant debris. Anaerobic conditions can lead to preservation in the
form of peat, which is converted to coal upon burial. Coal quality is
affected by the amount of silt and clay that accumulates during
flooding.

34. Fluvial Style (Channel Morphology)
Anastomosed
Meandering Braided
Straight

35. River Classification
Modern rivers are often classified according to channel morphology,
referred to as fluvial style. Major considerations include sinuosity (ratio
of channel length to a straight line), number of channels (single or
multiple), relative channel depth (depth/width ratio), nature of load
(bed/suspended load ratio), and bank cohesion.

36. Fluvial Style vs. Architecture

37. Photo by N.D. Smith (Williams River, Alaska )
Factors Controlling Fluvial Style
Meandering vs. braided:
Nature of sediment (erodibility, abundance, size)
Discharge (volume, variability)
Stream gradient
Vegetation
Meandering vs. anastomosed:
Base-level fluctuation (rage, amount) – accommodation production

38. Large-scale Architecture
(stacking patterns)
Sand:mud ratios
Sand body connectivity
Sand body geometry (lense vs. sheet)
Accommodation space and Preservation potential
Transgressive deposits
Lowstand deposit
Early (slow) rise deposits
Moderate rise
deposits
Rapid rise deposits
Highstand deposits
Coastal depositsTidally-influenced fluvial deposits
B

39. GEOL 553 - Introduction to
Facies Models - Kendall