GEOSC 340 Spring 2015
DiBiase
Lab 7: Bedrock rivers and the relief structure of mountain ranges
Objectives
In this lab, you will analyze the relief structure of the San Gabriel Mountains in southern California and
how it relates to bedrock river incision and the form of channel long-profiles.
Figure 1. Map showing major quaternary faults in the San Gabriel Mountains and vicinity
Background
The San Gabriel Mountains lie just north of Los Angeles, CA, bounded by the right-lateral San Andreas
Fault to the north, and a series of north dipping thrust faults to the south. A gradient in uplift rate,
increasing from west to east, produces a strong gradient in topographic relief and erosion rate. In contrast,
climate and lithology, which are also expected to influence erosion rate, do not vary strongly across the
range. This sets up an ideal location to study the topographic controls on erosion rate. Because bedrock
rivers define the relief structure of unglaciated landscapes, we can use the channel steepness index
(channel slope normalized to its expected dependence on upstream drainage area) to quantify relief in the
San Gabriel Mountains. To quantify erosion rates, we measured cosmogenic 10Be concentrations in
stream sands to integrate catchment-averaged erosion rates over millennial timescales. In the San Gabriel
Mountains, these rates range from ~0.1 – 1.0 mm/yr.
The following paper will be very helpful for this lab:
DiBiase, R.A., Whipple, K.X, Heimsath, A.M., and Ouimet, W.B., 2010. Landscape form and millennial
erosion rates in the San Gabriel Mountains, CA. Earth and Planetary Science Letters 289, 134-144
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6.0 Summary of provided datasets
You have been provided with a file geodatabase “lab_7_data.gdb”, which includes a 30 m digital
elevation model of the San Gabriel Mountains, as well as some topographic derivatives (slope
and hillshade). I also included 4 watershed outlines and two feature classes containing the long
profiles of a sampling of bedrock streams across the range.
Name
sgm30_dem
sgm30_hillshade
sgm30_slope
mill_creek_watershed
chilao_watershed
big_rock_creek_watershed
fish_fork_watershed
elevation_profiles
accumulation_profiles
Data type
Grid size
Description
raster
30 m
Elevation (m)
raster
30 m
Hillshade of sgm30_dem
raster
30 m
Slope map of sgm30_dem in degrees
polygon feature class
N/A
Outline of Mill Creek watershed
polygon feature class
N/A
Outline of Chilao watershed
polygon feature class
N/A
Outline of Big Rock Creek watershed
polygon feature class
N/A
Outline of Fish Fork watershed
line feature class
N/A
Stream long profiles used in 6.3
line feature class
N/A
Accumulation profiles used in 6.3
Figure 2. Summary of provided datasets
Source
USGS NED
6.1 Measuring topographic relief at different scales
Navigate to and open the tool \\Spatial Analyst Tools\Neighborhood\Focal Statistics. We used
this tool in Lab 3 to smooth the elevation data by taking the mean elevation of a moving window.
For this lab, we will use this tool to calculate the local relief at each cell on the digital elevation
model by determining the elevation range (maximum – minimum elevation) for a circular
moving window of various sizes (Fig. 3). Start with a circular window with radius 100 meters,
and go up in 3-4 intermediate steps to a radius of 5 km. Be sure you use map units rather than cell
units, and name your files appropriately!
Figure 3. Using Focal Statistics to calculate local relief. In this case over a circular 500 m-radius window.
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For each of your calculations, record the mean and maximum relief by navigating to the
Classify… window in the symbology of the layer (Fig. 4). Add these values to the table in the
included spreadsheet “lab07.xlsx”.
Figure 4. Classify window within symbology. Here the mean value of relief is 177 m, and the max is 861 m.
6.2 Extracting topographic data from analysis catchments
Now, let’s use the watersheds provided to clip out individual DEMs. First, go to the tool \\Spatial
Analyst Tools\Extraction\Extract by Mask:
Figure 5. Spatial Analyst Extract by Mask dialog box
For each of the three analysis catchments, extract a slope map using the watershed boundary as
the mask (Fig. 5). Then, open up the symbology of each, go to classify and record the mean value
for slope in Table 2 of the included Excel spreadsheet “lab07.xlsx” (Fig. 6).
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GEOSC 340 Spring 2015
DiBiase
Figure 6. Mean slope/relief estimate. The mean slope for Mill Creek is 23.2 degrees (keep aware of units!!)
6.3 Plotting long profile data and comparing to model predictions.
I have included an Excel spreadsheet containing a series of editable plots for you to determine the
channel steepness index of 4 streams that cover a wide range in tectonic forcing across the San
Gabriel Mountains.
Each spreadsheet is named according to its watershed, and you should be able to directly connect
your observations of the long profile with your map.
Figure 7. Example of long profile with BAD model fit. Adjust the ksn value until the curves roughly match.
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GEOSC 340 Spring 2015
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As described in lecture, we will keep the concavity index (θ) fixed, and simply vary the
normalized channel steepness index, ksn. Tune this value until you get a good fit by eye for the
whole stream profile, starting at the mouth (Fig 7). When you have finished, record this number
in the appropriate table located at the end of the spreadsheet.
Figure 7. Good fits for long profile and slope-area data.
For comparison, I have also included a plot of log Slope vs. log Area to help visualize the powerlaw fit between slope and drainage area that we are fitting to the data. You should be able to
visualize the variation in the fitted curve as you adjust ksn.
Once you have found the best-fit ksn value for all four streams, place the four charts on the same
page (copy/paste is easiest within Excel), and scale them such that the horizontal and vertical
scales are similar for all four. Be sure to indicate the amount of vertical exaggeration.
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GEOSC 340 Spring 2015
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6.6 3D perspective view
For two of your catchments, follow the steps outlined in Lab 3, section 3.6 (copied here) to
generate a 3D perspective view using ArcScene (Fig. 8). I suggest comparing Chilao Creek or
Mill Creek with either Big Rock Creek or Fish Fork (i.e., one low uplift rate, one high uplift rate).
Feel free to use whatever raster you wish for visualization – elevation, slope, relief… this is an
open ended exercise for you to be creative! NOTE: replace all the tennessee valley data with this
labs data, in case it isn’t obvious…
3.6 3D Visualization of data in ArcScene
Often times, it is helpful to view a 3D rendering of the landscape, as when you use Google Earth.
The program ArcScene enables you to directly load custom image and elevation datasets that you
have generated in ArcMap in a much more flexible manner than Google Earth. This part of the
lab is entirely optional, but I encourage you to experiment a bit with it.
First, open up the program ArcScene using the shortcut on the 3D Analyst toolbar: . The layout
looks quite similar to ArcMap, and you have access to the same tools and catalog. Open the
catalog window, navigate to the raster dataset “tenn2_dem”, and load it into your map window.
Right now it is a flat plane that you can rotate and move around using various mouse buttons.
To make this layer 3D, we need to assign base heights. Double click on the layer and go to the
Base Heights tab (Fig. 11).
Figure 11. Assigning base heights in ArcScene.
We want to assign the elevation values from “tenn2_dem”, so select Floating on a custom
surface and then navigate to the raster “tenn2_dem”. We can also control the resolution of the
base height assignment. Since our data set has a Cell size of 2 m, set the values to 2 in order to
show the highest level of detail. If your elevation dataset is too large, you can increase this
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number to enable faster 3D rendering. You can also adjust vertical exaggeration and offset in this
window. Try setting the conversion factor to 2.0 and see what happens.
After you adjust the base heights, reset the scene extent by clicking the large globe button on the
right hand side of the navigation toolbar (Fig. 12)
Figure 12. Navigation toolbar in ArcScene.
We can add illumination to scene by going to the Rendering tab in the Layer Properties dialog
and selecting Shade areal features relative to the scene’s light position (Fig. 13). While you
are here, crank up the slider bar on Quality enhancement for raster images. Similar to choosing
the base height resolution, this adjusts the image resolution of your raster datasets. If your
computer is getting bogged down, you can slide this back to the left.
Figure 13. Rendering properties for layer in ArcScene.
Often times, it is more useful to visualize a separate dataset, such as curvature or satellite imagery
in today’s lab. Navigate to and load up your curvature raster for Tennessee Valley, and adjust
the symbology by loading it from the layer file “curvature_symbology.lyr” (see Fig. 4). Repeat
the steps shown above to assign base heights using the DEM, shade the features using the scene’s
light position, and crank up the quality enhancement for the raster image (Fig. 14). Now you
should have a nicely-rendered curvature map to explore!
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Figure 14. Perspective view of curvature map for Tennessee Valley.
NOTE: these tables are also in the Excel spreadsheet “lab07.xlsx”
Table 1: Relief measurements
Window size (radius, m)
Mean relief (m)
Maximum relief (m)
Erosion rate (mm/yr)
0.13
0.04
0.43
1.12
Mean hillslope angle (deg)
Table 2: Watershed parameters
Watershed name
Mill Creek
Chilao Creek
Big Rock Creek
Fish Fork
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Channel steepness index (m0.9)
GEOSC 340 Spring 2015
DiBiase
Lab 7 deliverables, due Wednesday April 8 before lecture (40 pts total)
(drop-box on angel, single pdf)
(5 pts) An overview map showing topographic relief and the three analyzed watersheds.
• Use at least a 2 km-radius moving window to calculate relief.
• Make relief transparent over the hillshade, and keep watersheds as outlines.
• Be sure to label the watersheds with their appropriate name (e.g., Mill, Chilao, Big Rock, Fish
Fork).
(5 pts) A 3D perspective of two of your analyzed watersheds made in ArcScene
• You can choose which layer to display (e.g., elevation, hillshade, slope, relief…)
• This is one instance where I will accept a map without a scale bar/north arrow.
(5 pts) Four long profiles of the provided stream reaches, with steady-state stream power (constant
channel steepness) profiles overlain.
• Make sure to put all four plots on the same page, and at the same vertical and horizontal scale.
• Don’t forget to indicate the vertical exaggeration!
(5 pts) A completed data table, as shown in your excel file and copied above for convenience
(20 pts) A written report 2-3 pages long (12 pt font, 1.5 line spacing, 1” margins), which should
include the following
• A brief introduction and methods (1-2 paragraphs)
• A description of the results – how does relief vary across the San Gabriel Mountains? How does
hillslope angle vary? Channel steepness? (~2 paragraphs focused on synthesizing observations)
• A discussion focused on addressing the following questions:
1) How does the size of your moving window influence the values of relief you
measure? Is there a maximum value of relief for this landscape at very large scales?
2) Discuss, in terms of as many surface processes that you can, what happens when you
move from a low-uplift zone to a high-uplift zone. Be specific, and include all you
have learned in this class about hillslopes, alluvial rivers, and bedrock rivers. It’s
okay to guess!
3) Describe how a positive relationship between bedrock river incision and relief (or
channel steepness) results in a negative feedback that limits the height of mountain
ranges.
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