The Volume and Extent of the Lake Created by the Bridge of the Gods, Colombia River

Transcription

1 Rachel Markoff 12/1/2011 GEO 327G/386G Term Project The Volume and Extent of the Lake Created by the Bridge of the Gods, Colombia River 1. Introduction The Bonneville Landslide occurred on the Colombia River before European settlement. The date is disputed and it may have occurred between 1100 and Debris from Table Mountain and Greenleaf Peak created a dam across the river, known in native legends as the Bridge of the Gods. This created a lake on the river. Eventually the earthen dam was breached and swept away, leaving the Cascade Rapids. It has previously been estimated that the lake may have extended upstream to modern-day Arlington, about 100 miles upstream. The Colombia River today is heavily altered by dams. Bonneville Dam is just downstream of the area of the Bonneville Landslide. This creates a lake that extends upstream to the Dallas Dam, which in turn creates a lake, Lake Celilo, which extends to John Day Dam. John Day Dam creates Lake Umatilla, which extends upstream beyond the estimated range of the dam. The Bridge of the Gods formed a lake that may have extended across this area. The objective of this project was to use GIS to estimate the area and volume of the prehistoric lake. 2. Data The only data necessary for this project were 1-Arc-second Digital Elevation Model data from the USGS National Map Seamless Server at 3. Outline Locate area of landslide using Google Earth Outline polygon of area of interest in Google Earth and convert to layer file to add to map Download National Seamless Data Server DEM data of the area Unzip the DEM data and add to map Convert the DEM raster to a UTM projection Create a lake outline polygon: create a contour of the lake level, edit, and convert to polygon Create mask of lake polygon Find volume between DEM and lake height, also find area of polygon 1

2 4. Procedure The first step of the project was to define the area of interest. This was done in Google Earth. I knew that the location of the landslide was at Table Mountain and Greenleaf Peak near Cascade Locks on the Colombia River. The location of the landslide was identified in Google Earth by identifying Table Mountain and Greenleaf Peak. Landslide deposits lie in front of them and have now formed an area of small lakes. The modern Bonneville dam is to the left (downstream). Figure 1: Identifying the location of the landslide using Google Earth. The area of interest was determined in Google Earth. The extent of the lake created by the landslide was 35 miles upriver based on drowned trees, but it has been estimated to have extended as far as the area of Arlington, Oregon, about 100 miles upriver. A rectangle was outlined of this part of the river using the polygon tool in Google Earth. The rectangle was saved as a kml file which was converted to a layer file in a blank ArcGIS map document. This was done using the tool ArcToolbox>Conversion Tools>From KML>KML To Layer. 2

3 Figure 2: Importing the KML file into ArcGIS. Elevation data for the area were downloaded from the National Seamless Data Server. I initially tried to download 1/3 Arc second data from the National Elevation Dataset, but I ended up using 1 Arc Second Data. I also downloaded county and state boundary line data. The metadata format was changed to XML in the Modify Data Request section before they were downloaded (Fig 3). Since the file size of the 1/3 Arc Second Data was large, I decided to make the area of interest smaller. I did not extend it as far upriver and I reduced the north-south extent greatly since it originally had been larger than necessary. 3

4 Figure 3: Outlining the area of interest on the National Seamless Data Server and selecting files to download, changing to XML. Figure 4: Downloading files. 4

5 I unzipped the data files and added the layers to the map. They were in the NAD 1983 datum and the data frame was in WGS 1984, but these datums are similar, so the difference is not a problem. I added the state and county line layers without trouble. Unfortunately, an error message showed up when I tried to unzip the DEM files, and I was unable to unzip them and add them to the map. Thus, I had to go back and download coarser, 1-Arc-Second data. I was able to unzip this data and add it without trouble, and it is simpler to work with because it is a continuous layer, whereas the finer data was separated into four layers due to file size. Figure 6: The DEM. Contour line is added later. Now I had the DEM for my area, but it was not in the right units. Horizontal units were in decimal degrees and vertical units were in meters. To perform volume calculations, I had to convert it so the horizontal units would also be in meters. I used ArcToolbox >> Data Management Tools >> Projections and Transformations >> Raster >> Project Raster to convert the raster to a raster in UTM coordinates, so all units would be in meters. This new raster was named DEM_UTM. 5

6 Figure 7: Converting the DEM raster into a raster in UTM. The next step was to create a polygon that would show the extent of the prehistoric lake. This was created from a contour line of the DEM. Judging from the elevation of the landslide deposits in Google Earth, an elevation of 70 meters above sea level was estimated for the lake height (when the lake filled to capacity). The 3D Analyst and Spatial Analyst extensions were added. To create contours, I used ArcToolbox >> Spatial Analyst Tools >> Surface >> Contour. Figure 8: Creating contours from the raster data. 6

7 I selected the 70-meter contour from the Counter_70 shapefile. This was done using the tool Select By Attributes with the code CONTOUR = 70. The selected contour was exported using the Data >> Export Data tool for the layer. The new shapefile was named BaseContour. I removed the old contour layer from the map. Figure 9: Selecting the 70-meter contour. Figure 10: The contour. The arrow indicates the landslide area, where the dam was. Next, I had to edit the contour so it would accurately represent the lake outline. I went into editing mode and selected the lake outline shapefile (named BaseContour ) to work with. First, I had to cut the lake off at the landslide deposit. I used the split tool on the editing toolbar to split the lake outline at the deposits and deleted everything downstream. Then I joined the lines at the dam by editing the vertices. 7

8 Figure 10: Result after I split the lines where the dam would have been. Figure 11: The dam. The upstream extent of the polygon is the John Day Dam. The water behind this modern-day dam is at a higher elevation than the water in the prehistoric lake would have been. Thus, the project is limited because it is impossible to accurately predict the full upstream extent of the prehistoric lake based on this data. However, I could find the extent of the lake downstream of the John Day Dam. 8

9 I continued editing the 70-meter contour. I deleted areas outlined by the contour that fell outside of the lake, because these are irrelevant to the project. Areas outlined by the contour that fall inside of the lake must be islands. After I finished editing the contour, I selected stop editing and saved the edits. Figure 12: The edited lake outline. I used the tool ArcToolbox >> Data Management Tools >> Features >>Feature To Polygon to convert the lines to polygon files. 9

10 Figure 13: Converting to polygons. I used the tool ArcToolbox >> Spatial Statistics Tools >> Utilities >> Calculate Areas to find the areas of the polygons. I now had a layer of the polygons with the area as a column in the attribute table. I selected the main polygon, which was the lake area excluding islands. The area was found to be m^2. This can be converted to mi². Figure 14: Calculating polygon areas. 10

11 Figure 15: The attribute table with areas of the polygons. The next step was to find the volume of the lake. First, I had to make an Analysis Mask to define the region where the analysis was performed. I used the tool Spatial Analyst Tools >> Extraction >> Extract By Mask. Since I only wanted to analyze the water volume, I created a new polygon that excluded the islands by selecting the water polygon and exporting the data to a new shapefile, Water_Area. Figure 16: Extracting by mask. I used the tool 3D Analyst Tools >> Functional Surface >> Surface Volume. The result was to create a table that included lake area and volume. Interestingly, the volume differed slightly from the volume I found using the Calculate Areas tool. The resulting area was m^2. The volume was m^3. 11

12 Figure 17: Finding the lake volume. Figure 18: The data table resulting from the Surface Volume tool. 5. Conclusion This table summarizes the results: Area calculate areas Area Surface Volume Volume meters 201,589, m² 195,019,768 m² 6,218,635,663 m³ miles mi² mi² 1.49 mi³ Acre-feet I don t know if the area results differ because of differences in the tools or if one analysis somehow accidently included an island. All results are incorrect for the volume and area of the prehistoric lake because the DEM included the modern-day dams and lakes. The lake area would really have extended further upstream. The estimated volume is also affected because it excludes the volumes now occupied by the modern lakes. To perform the analysis correctly, I would perform the above procedure, but use bathymetric data of the river bottom. However, I was unable to find any such data. 12