Lab 7: Bedrock rivers and the relief structure of mountain ranges

Transcription

1 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. Background Figure 1. Map showing major quaternary faults in the San Gabriel Mountains and vicinity 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 10 Be concentrations in stream sands to integrate catchment-averaged erosion rates over millennial timescales. In the San Gabriel Mountains, these rates range from ~ mm/yr. The following paper will be very helpful for this lab:, R.A., Whipple, K.X, Heimsath, A.M., and Ouimet, W.B., Landscape form and millennial erosion rates in the San Gabriel Mountains, CA. Earth and Planetary Science Letters 289,

2 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 Data type Grid size Description Source sgm30_dem raster 30 m Elevation (m) USGS NED sgm30_hillshade raster 30 m Hillshade of sgm30_dem sgm30_slope raster 30 m Slope map of sgm30_dem in degrees mill_creek_watershed polygon feature class N/A Outline of Mill Creek watershed chilao_watershed polygon feature class N/A Outline of Chilao watershed big_rock_creek_watershed polygon feature class N/A Outline of Big Rock Creek watershed fish_fork_watershed polygon feature class N/A Outline of Fish Fork watershed elevation_profiles line feature class N/A Stream long profiles used in 6.3 accumulation_profiles line feature class N/A Accumulation profiles used in 6.3 Figure 2. Summary of provided datasets 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. 2

3 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). 3

4 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. 4

5 As described in lecture, we will keep the concavity index (θ) fixed, and simply vary the normalized channel steepness index, k sn. 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 k sn. Once you have found the best-fit k sn 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. 5

6 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 6

7 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! 7

8 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) Table 2: Watershed parameters Watershed name Erosion rate (mm/yr) Mean hillslope angle (deg) Channel steepness index (m 0.9 ) Mill Creek 0.13 Chilao Creek 0.04 Big Rock Creek 0.43 Fish Fork

9 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. 9