Watershed Geomorphology
Motivation
• Effect of water on landscape:
– Whereas hydrologists are mostly concerned with the movement
of water, a common task is to assess the impact of water flow
on the transport of materials – sediment and solutes
• Effect of landscape on water
– Runoff is controlled by inputs from climate, and the physical
properties of the watershed (geology, soils, land use, land
cover…)
• A long-term dynamic equilibrium exists between climate
and earth materials, resulting in watershed “shape”, that
dictates the short-term hydrologic response
Outline
• Motivation
– Form and process
– Form and hydrologic response
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Watershed Similarities
Watershed Delineation
Hillslope processes
Fluvial processes
Channel head
Landscape evolution
Expectations
• Me: Lecture, provide readings, design project
and occasional homework – Provide learning
opportunities
• You: Listen, READ and be ready to discuss, do
project and homework - learn
What is a watershed?
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A topographically delineated area drained by a stream system – that is, the total
land area above some point on a stream or river that drains past that point.
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The watershed is a hydrologic unit often used as a physical-biological unit and a
socioeconomic-political unit for the planning of and management of natural
resources.
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A watershed is essentially a network of hillslopes connected by streams.
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Watersheds operate as transfer function that convert precipitation into
streamflow.
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Under the driving force of gravity, river basins take in moisture from the
atomosphere in a distributed and intermittent manner, store it in the soil, and
release it gradually into a network o stream channels where is is concentratred
and delivered more-or-less continuously to the outlet.
What is a watershed
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The area that appears on the basis of topography to contribute all the water that passes through a
given cross section of a stream.
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The area of land that catches rain and snow that drains or seeps into a common marsh, stream,
river, or lake.
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The term watershed describes an area of land that drains downslope to the lowest point. The
water moves through a network of drainage pathways, both underground and on the surface.
Generally, these pathways converge into streams and rivers, which become progressively larger as
the water moves on downstream, eventually reaching an esturay and the ocean. Other terms used
interchangeably with watershed include drainage basin or catchment basin.
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All land is part of one watershed or another. When rain falls, much of the water runs across the
surface of the land toward a stream, river, pond, or lake as surface runoff. The land area that drains
runoff to the stream, or other surface water body, is called a watershed, or drainage basin.
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" that area of land, a bounded hydrologic system, within which all living things are inextricably
linked by their common water course and where, as humans settled, simple logic demanded that
they become part of the community."
Components of a watershed
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Ridges/divides
Hillslopes
Gullies, rills,
Ephemeral channels
Floodplains
Riparian zones
Perennial channels
Vegetation
Soils
Aquifers
Bedrock
Pattern and Process
Form and function
• Watersheds are natural partitions in the landscape that
evolve under deterministic and stochastic controls towards
self-similar critical states under the guiding principles of
energy minimization.
Water, sediment, and
solutes are byproducts of
watershed processing
Hydrologic driver
q
t
Hydrologic response
Why do watersheds appear similar Visually AND statistically?
Watershed Properties
• Watersheds are natural partitions in the
landscape that evolve under deterministic and
stochastic controls towards self-similar critical
states under the guiding principles of energy
minimization.
• What are critical states?
Horton’s Laws of Drainage Networks
𝑁𝑤
𝑅𝑏 =𝑁
𝑤+1
𝑅𝐴 =
𝐴𝑤
𝐴𝑤−1
𝐿𝑤
𝑅𝐿 =𝐿
𝑤−1
Bifurcation ratio
Law of stream areas
Law of stream lengths
Ratios tend to be similar for all rivers
Hack’s Law
L = CA0.6
Exponent tends to be near 0.6 for all rivers. What does this mean?
WITHIN a drainage basins, bankfull flow tends to scale log-linearly with area.
Hydraulic Geometry
w  aQ
b
d  cQ
m
v  kQ
f
𝑄 = 𝑤𝑑𝑣 = 𝑎 𝑥 𝑐 𝑥 𝑘 𝑄𝑏+𝑓+𝑚
𝑎 𝑥 𝑐 𝑥 𝑘) = 𝑏 + 𝑓 + 𝑚 = 1
Hydraulic Geometry
w  aQ
b
d  cQ
m
v  kQ
f
𝑄 = 𝑤𝑑𝑣 = 𝑎 𝑥 𝑐 𝑥 𝑘 𝑄𝑏+𝑓+𝑚
𝑎 𝑥 𝑐 𝑥 𝑘) = 𝑏 + 𝑓 + 𝑚 = 1
At-a-Station
b
0.1-0.2
Downstream
0.5
f
0.4
0.33-0.4
m
0.4-0.5
0.10-0.17
1
Region 2
Region 3
Region 4
Slope (m/m)
Region 1
S = 2.8691A-0.22
R2 = 0.74
SA1
SA2
SA3
0.1
10
100
1000
10000
Drainage Area (m2)
100000
1000000
Cumulative Area Distribution
1
Region Region
1b
1c
Region 1a
Region 2
Region 3
0.1
P = 200A-1.17
P(A>A*)
P = 0.718A
-0.40
0.01
0.001
CAD1 CAD2
CAD3
CAD4
0.0001
10
100
1000
10000
Drainage Area m2
100000
1000000
Exceedence Probability
1
Region 1
Region 2
Region 3
0.1
0.01
0.001
EID1
EID2
0.0001
1
10
100
Energy Index
1000
1
2
3
5
7
1
4
8
6
10
12
19
Slope (m/m)
11
15
16
20
14
18
13
17
9
Overland Flow Erosion
Groundwater Seepage Erosion
Landslide
0.1
1000
10000
Drainage Area (m2)
100000
1
2b
A2
2c
A3
2c-d
s1
Slope (m/m)
3
1a
2a
1b
2d
EID2
A1
A4
EID1
0.1
100
1000
10000
Drainage Area m2
100000
1000000
Watershed Evolution
• “Universal” properties exist arrangement of slopes and
channels in watersheds
– Statistically similar despite visual differences
• Landscape evolution governed by common underlying
principles
• Watersheds are natural partitions in the landscape that
evolve under deterministic and stochastic controls towards
self-similar critical states under the guiding principles of
energy minimization.
• There is a dynamic relationship between water flow and
landscape form
What are the implications?
Basic Principles of Geomorphology
(Ritter et al., 1995)
1.
A delicate balance or equilibrium exists between landforms and processes. The character of this
balance is revealed by considering both factors as systems or parts of systems.
2.
The perceived balance between process and form is created by the interaction of energy, force,
and resistance.
3.
Changes in driving force and/or resistance may stress the system beyond the defined limits of
stability. When these limits of equilibrium, or thresholds are exceeded, the system is temporarily
in disequilibrium and a major response may occur. The system will develop a different
equilibrium condition adjusted to the new force or resistance controls, but it may establish the
new balance in a complex manner.
4.
Various processes are linked in such a way that the effect of one process may initiate the action
of another.
5.
Geomorphic analysis can be made over a variety of time intervals. In process studies the time
framework utilized has a direct bearing on what conclusions can be make regarding the
relationship between process and form. Therefore, the time framework should be determined by
what type of geomorphic analysis is desired.
Next Steps
• Watershed delineation in DEMS
• Watershed denudation processes
– Hillslope
– Fluvial
• Return to “universal” properties