Surface Processes and Landform Evolution (12.163/12.463) Fall, 04 -- K. WhippleLab 5: DEM Analysis: San Gabriel Mountains, CA In this lab you will work with digital topographic and geologic maps with a set of DEM analysis tools (ArcGIS and Matlab scripts) to study the topography of the San Gabriel Range in southern California. The challenge is to attempt to deconvolve the many possible controls on landscape morphology (lithology, tectonics, climate, etc). You will also have a paper on this topic by Blythe et al (2000) – this work can help guide your analysis, but you should aim for an evaluation/critique of the paper. Read it first, ask questions in class if you have any about it. They looked at topographic data, but not at stream profiles which ought to contain the most tectonic information in mountainous landscapes. Do your analyses validate their interpretation? Are there some problems? Can you upgrade/refine their interpretation? You are working on an unresolved research problem: no one knows fully the answers to the questions posed below for the San Gabriel mountains. Moreover, keep in mind that it is unlikely that analyses of topography alone can resolve all of the issues – your assignment is to explore both what you can and cannot decipher from the topography. This type of analysis can provide a useful guide to subsequent field work. Where the topography allows multiple interpretations, strive to (a) recognize this fact; (b) identify alternate hypotheses; and (c) suggest what additional data would be required to resolve the question under consideration. Motivating Questions (to be addressed in your analysis and report) • Is the San Gabriel Range subject to spatially variable uplift? If so, can you define the pattern? • If spatial patterns in channel steepness (ks) are apparent, do they correlate with mapped structures? Do they correlate with long-term exhumation rates determined from low-temperature thermochronology? (i.e., Are the “blocks” defined by Blythe et al (2000) supported? Would you draw different boundaries?) • Is the landscape in steady-state? Can you tell? • Is there evidence for a temporal change in rock uplift rate? Can you distinguish a temporal change in uplift (migrating knickpoints) from a spatial pattern (fixed knickpoints)? How? • Do lithologic variations importantly influence stream profiles and hillslope gradients in the San Gabriel Mountains? • Can topographic analyses alone discriminate the relative importance of tectonics, lithology, and climate factors? If not, what additional data is needed and how could it be obtained? For Graduate Students: • Are there spatial patterns in fluvial drainage density (that is, differences in xc as measured by the break in slope-area scaling that might indicate a transition from debris-flow to fluvial erosion processes) and do they correlate with channel steepness patterns? (i.e., are debris-flows more important in areas with steeper slopes?). • Are the channels of the upper San Gabriel River (north of the San Gabriel fault) detachment-limited? Can you tell, and if so how? • Is there evidence that a downstream transition from detachment-limited to transport-limited conditions occurs? 1 3/8/2005Surface Processes and Landform Evolution (12.163/12.463) Fall, 04 -- K. WhippleData Available • Paper by Blythe et al (2000) with discussion of interpretation of low-temperature thermochronologic data (generally young ages = faster long-term erosion). • 1:24,000 USGS Topographic Contour Maps (paper copies in map library; scanned versions in Langtang/public – ask TA if you want to see these) • Digital Geologic map, Faults map, and Thermochronologic data (ArcGIS project) • Digital Elevation map (10m), Shaded Relief map, Slope map, 50m Contours (ArcGIS project) • Stream_profiler analysis codes (ArcGIS dll and Matlab scripts) (ArcGIS project) • Matlab code for bedrock channel evolution (from bedrock channel lab) Your Assignment Deliverables: 6 page (1.5 line spacing, 12 pt font), thoughtful analysis and discussion of the San Gabriel Mountains in terms of the motivating questions above. Reports should be supported by whatever figures and tables are necessary to illustrate what you have learned and what key problems remain unconstrained. Digital figure submissions are fine (pdf or ArcGIS project) if you’d rather not bother with printing color figures (please provide black and white print). You may find that some simulations with your bedrock channel code help with your analysis/interpretation. Focus on the south-draining drainage basins within the DEM, aim for an analysis of about 20 profiles (start with larger tributaries that cover the regional pattern – you might want to look at channels representative of each of the blocks defined by Blythe). As always you are encouraged to work together and discuss your approach with your fellow students, TA, and instructor. The main point is to learn both the strengths and limitations of DEM analysis. However, I expect each of you to do your own stream profile analysis and to prepare your final reports independently. Setting up Digital Data for your Analysis Each of the lab computers has all the required data in a directory called user_data\labuser\sg_class. You will want to pick one lab computer and do all your work there (you need Matlab and ArcGIS 8.x or 9.0 (sp1) on any alternate machine you’d like to work on in your home or office – talk to your TA about what is required to set this up). Within sg_class you will find directories data, profile_codes, and my_project. Do not edit, move or rename the data or profile_codes directories – these will be used by all. You will start by copying and renaming the directory my_project as name_project or similar. Do not alter the source my_project directory in any way. Inside your new name_project directory you will find a pre-loaded ArcGIS project (san_gabes1.mxd) and two directories arcgis and matlab. The arcgis directory will store all files generated by ArcGIS stream profiler (including temporary files). The matlab directory contains the dem and drainage area arrays and will be used to store all matlab stream profiler output, including postscript files of your plots for printing. Save the san_gabes1.mxd ArcGIS project with a personalized name. Now you should be ready to go. 2 3/8/2005Surface Processes and Landform Evolution (12.163/12.463) Fall, 04