Drainage Basin Morphometry
Rick Ford
Dept of Geosciences
Weber State University
2507 University Circle
Ogden, Utah 84408-2507
801.626.6942
rford@weber.edu
NAME:
GEOMORPHOLOGY (GEO 3150)
Weber State University
Spring Semester 2007
LABORATORY EXERCISE #9 (Drainage Basin Morphometry)
NEATNESS COUNTS IN SCIENCE!
SHOW ALL CALCULATIONS; DISPLAY ALL UNITS.
• Due in class on Wednesday, April 11
Objectives:
(1) Observe, collect, and interpret map data related to the focal point of fluvial
geomorphology -- the drainage basin (watershed, catchment).
(2) Practice the technique of stream ordering and observe the inherent organization within a
drainage network.
(3) Develop mathematical/statistical models (equations) to describe the morphometry of
a drainage basin.
Definitions:
• Drainage Basin: the topographic unit that collects precipitation and runoff in a given area and
serves as a reservoir, or storage area, for water and sediment. Large drainage basins are essentially
a collection of smaller drainage basins. Individual drainage basins are bounded by topographic
features called drainage divides, which are the crests of hills that separate the collection of
precipitation and runoff from adjacent basins.
• Drainage Network: the organization of a trunk channel and its tributaries into a transport system
for water and sediment within a drainage basin. The architecture of a drainage network can be
described in several ways, including the concept of stream order.
• Stream Order: expresses the hierarchal relationship between the individual stream segments that
make up a drainage network. In the "Strahler system", a stream segment with no tributaries is a
first-order stream. A second-order stream is formed by the joining of two first-order streams.
Where two second-order streams join, the stream is designated third-order, and so forth.
Procedures:
(1) Delineate the drainage basins of Strongs Canyon and Waterfall Canyon on the attached
topographic map (i.e. draw in the drainage divides). Let the 5000 ft contour line be the
lower end of each basin.
(2) Delineate the drainage network of Strongs Canyon by extending the mapped drainage
lines up valleys to the highest contour that has been crenulated to indicate the presence of a
channel. How would you characterize the drainage pattern within this basin (dendritic,
trellis, etc.)? ___________________________________________
Geomorphology – Lab 9 – Page 2.
(3) Using the Strahler system (see Fig. 5.17 in the text), indicate stream order on the
delineated network. This is most easily done by using different colors or tick marks.
(4) Using a spreadsheet program, set-up and complete a data table similar to the following:
Stream No. Of Total Length
Average
Order
Segments
Per Order (km)*
Average
Length (km)
Bifurcation
Slope**
Ratio
1
2
3
4
5
* To make things easier, measure valley length instead of stream length. Stream length is always
greater than valley length due to channel sinuosity. Measurements are made by laying a piece of
paper alongside the valley and marking off the distance, repeating the process for each stream
segment.
** Average slope - select a few typical streams for each order and measure their slope (drop in
elevation/valley length, expressed as a decimal fraction, e.g., 0.0042)
NOTE: Use the map scale to calculate actual ground distances from your measured map distances.
(5) In order to observe the relationships between stream order and drainage network
properties, graph the number of stream segments, average length, and average slope against
stream order (3 graphs). Use semi-log graph paper with stream order on the
x axis (arithmetic scale) and draw a best-fit line through the data points (see Fig. 5.18)
(6) Using a scientific calculator or statistics software, determine the regression equation
for each of your graphs. Label your graphs with the equation and r2 value. Note: we are
intentionally using semi-log graph paper in order to display a linear relationship between the
variables. We would get the same result by plotting the logarithm of the number of stream
segments versus the stream order on regular arithmetic graph paper.
Exponential function:
Y = abX
Taking the log of both sides: log Y = log a + (log b) X
linear form: y = mx + b;
On semi-log graph paper:
Y-intercept (“stream order 0") = a
Slope = (log y2 - log y1) / (x2 - x1) = log b;
Geomorphology – Lab 9 – Page 3.
b = 10slope
(7) Briefly discuss your findings in comparison to the discussion of these relationships,
sometimes called the “laws” of drainage basin morphometry, in your textbook (Fig. 9.2,
p. 209). What type of drainage pattern did you observe? Does the drainage network of
Strongs Canyon have an inherent organization? Please use a word processor in preparing
your response.
Turn in your completed map (3), data table (4), annotated graphs (5-6), and discussion (7).