1. Our Hazardous Environment GEOG 1110 Dr. Thieme
Drainage Basins, River Channels, Streamflow

2. Drainage Basin
the fundamental landscape unit for collection and distribution of water and sediment.
separated from the adjacent basin by a divide.

4. FIGURE 6.9a

5. FIGURE 6.9b

6. Gradient - the vertical drop over a certain horizontal distance
“rise over run”

7. Drainage Pattern

8. Channel Pattern

9. Sinuosity reduces grade.
100’
The concept of sinuosity, or degree of meandering can be explained by looking at these three types of channels.
If three streams start from an elevation of 100 ft and drop to an elevation of 90 ft, which will have the steepest grade. Clearly, the straightest channel will have the steepest slope. If the stream is allowed to meander, it will reduce its grade by meandering.
If the energy entering the reach is very high, there will be bank cutting and deposition, that starts the meandering process.
Generally a stream in a steep valley floor is more prone to meander than a stream with little grade.
90’
Steep grade
Medium grade
Low grade

10. Sinuosity reduces velocity
More deposition will occur in a sinuous stream.
If the channel is blocked, it may change course or widen to a “braided” condition.
Excessive sinuosity may result in severe degradation of the stream.
Meandering will continue until the grade is reduced so far that deposition blocks the stream. At that point the channel’s course may change, increasing the grade by cutting to the next meander, or it may widen out.
Thus there is a limit to how wide a stream’s meander can go.
A wide shallow stream is highly unstable and generally has poor aquatic and riparian habitat.

12. Braided Rivers
The following conditions favor the braided channel pattern:
erodible banks
abundant coarse sediment supply
rapid fluctuations in discharge

13. Base Level is the lower limit to which to which a river
can downcut its channel.

14. A meander increases in amplitude by erosion of the cutbank
and deposition on the point bar.
A cut-off meander forms an oxbow lake.

15. Meander Scrolls indicate historical migration of meander bends.
Meanders rapidly abandoned by avulsion become oxbow lakes.

16. Features of a Meandering River Valley:
Cutbanks
Point Bars
Floodplains
Oxbow Lakes (Cut-off Meanders)
Natural Levees
Yazoo Streams
Backswamps
Terraces

17. Figure 4.7

18. Flood natural process of overbank flow
can be characterized by a discharge (cfs or m3s-1) at the point where water overflows the banks
also by a stage (feet or meters) or depth of flow

19. FIGURE 6.10

20. Discharge (Q) is the amount of water passing a given
cross-section of a river in a given unit of time.

21. Continuity Equation
Similar to Principle of Conservation of Mass and Energy in Physics
cross-section
time
Q = w * d * v
discharge width depth velocity

22. Hydraulic Geometry channel width, depth, and velocity all increase
in the downstream
direction
Depth increases at
the fastest rate
Why does velocity increase?

23. Mountain streams have turbulent flow
Water molecules and sediment particles are moving at many angles which are not parallel to streamflow
Boulders or large woody debris in a channel introduce more frictional drag

24. Turbulent and Laminar Flow
Deep, narrow channel at upstream end.
More of the water comes in contact with channel walls.
Velocity is decreased by frictional drag.
Wide, shallow channel at downstream end has laminar flow.
Most of the resistance to flow comes from shear between water planes.

25. Streams and Erosion
base level – the lowest level to which a stream can erode
local base level – controlled by local features such as a larger river or a lake or a resistant bed
ultimate (absolute) base level – sealevel

26. Graded Stream
has just the velocity required for the load supplied from the drainage basin
Stream adjustments toward equilibrium or “graded” condition:
Raising base level causes deposition
Lowering base level causes erosion
Increasing gradient causes meandering

27. River Terraces
abandoned floodplains formed when the river flowed at a higher level than at present

28. Terrace (T-2) Terrace (T-1) Floodplain (T-0)
This picture shows all the parts of a meandering stream. The high cut bank on the right shows where erosion occurs. In the very center of the picture on the left side of the channel there is a point bar.
This whole channel is entrenched in a flood plain.
In the distance you can see a terrace where a flood plain occurred in the past.
Floodplain
(T-0)

29. Valley Cross-Section Terrace Terrace Cut Floodplain bank
Cutting a slice across the channel, we can see all the parts. Normal flow is way down in the bottom of the channel. Only occasionally, on average once every two years, flow gets up to the flood plain level.
Less frequently it will get out of the channel into the flood plain.
It is important there be a flood plain to keep the water contained, reduce its energy, and protect areas out of the flood plain from damage.
The Terrace shown at the limits of the flood plain show where sediment was deposited in a flood plain long ago.
Flood flow channel
“Normal” flow Channel

30. Stream Gage
measure discharge in cubic feet per second (cfs) or cubic meters per second (cumecs)

31. FIGURE 6.C

32. Figure 5.D

33. Rating Curve - used to convert water height (stage) to discharge

34. Hydrograph a graph showing changes through time in
river discharge (cfs or m3s-1)
water depth (feet or meters)
stage (feet or meters) relative to some datum

35. FIGURE 6.E
Recurrence Interval - Average number of years within which a flood of a given magnitude occurs in the period of record

36. N + 1
R.I. = _______ M
N = total number of years of record
M = rank of magnitude of flood

38. FLOODPLAIN MAPPING
Based on contours that correspond to depths of gaged floods
Used for land use planning
Used for determining eligibility for flood insurance
Subject to uncertainties in the
statistical analysis of flood
frequency

39. Figure 4.8a
Morning Streamflow

40. Figure 4.8b
Afternoon Meltwater Flood

41. Flash Floods
typically occur in the upper part of a drainage basin.
generally produced by intense rainfall of short duration
falling over a small area.

42. Santa Elena Canyon of Big Bend National Park
March 20, 2004
over 2 m (6 ft) high wall of water
surprised group of five canoeists from the University of Wisconsin-Whitewater

43. Big Thompson Canyon
in the Colorado Front Range
July, 1976
triggered by system of thunderstorms
up to 25 cm (9.8 in) of rain fell in a few hours

44. Downstream Floods
cover a wide area
generally produced by storms of long duration that
saturate the soil and produce increased runoff.

45. Downstream Movement of a Flood Crest:
Chattooga River near Clayton, GA
Savannah River near Calhoun, S.C.
Savannah River near Clyo, Ga.