Hydrologic Analyses

Watershed and Stream Network Delineation

This exercise demonstrates the process to delineate a watershed and stream network for any point within a digital elevation model dataset. Once the stream network was identified, it was then broken into stream order.

The accompanying map depicts the watershed and stream network for Little Cottonwood Canyon in the Wasatch Mountain Range of Northern Utah. The watershed extent is calculated based off the chosen Pour Point, or point of lowest elevation. The threshold, or break, value for the minimum stream size was set to 750 based on how well the resulting network matched existing stream maps of the area. This threshold value means in order to be considered an actual stream, at least 750 cells, or an area of ~61,000 square meters, has to be draining to that point/stream. The mouth (pour point) of this watershed is draining an area of ~69.3 square kilometers.

High Resoultion Map

Map Creation Process / Key Geoprocessing Tools

1. Fill - The Fill tool should be ran on the original DEM first to fill in any holes or gaps in the DEM data that might interfere with flow modeling.

2. Flow Direction - Using the Fill dem as the input, this tool calculates which neighboring cell water would flow into.

3. Flow Accumulation - Using the Flow Direction as the input, this tool calculates how many cells are flowing into a given cell. Changing the symbology stretch type to "Histogram Equalize" gives a better visualization of where the streams are.

4. Delineating the Watershed

- Create a Pour Point. Create a new point shapefile in the catalog, yes to has M and Z. Create a point where you want the mouth of the watershed to be. This point must be within the white Flow Accumulation Stream. Use the Snap Pour Point tool to center the point in the stream.

- Run the Watershed tool to delineate the watershed. Use the Raster to Polygon tool to make a polygon watershed boundary. Use the Extract by Mask tool to clip the Flow Accumulation raster to just what is inside the watershed.

5. Estimate the Stream Network and Stream Order

- Choose the Threshold Value for minimum Flow Accumulation stream size. Change the symbology of the Flow Accumulation to two classes, then set the low values to "no color" and adjust this value range until the remaining stream network approximately matches the stream network on the USGS National Map basemap.

- Reclassify the Flow Accumulation Raster using the determined threshold value, changing the low values to NODATA.

- Use the Stream Order tool to calculate stream order. Us the Raster to Polyline to convert it to a polyline.