Hydrology and Watershed Analysis of the South Fork Nooksack River Watershed

Jonathan Picchi-Wilson

The goal of the model created was to use elevation data to model where streams should be and to delineate sub-watersheds and catchments for the South Fork Nooksack River Watershed unit and to be able to apply this model to any watershed with DEM data. The first step in the analysis was to clip a large western Washington DEM to the 10 digit HU10 hydrological unit using an NHD dataset, making sure both the DEM and the NHD data are in the same projection and then re-projecting the data into NAD 83 Zone 10N, which can be changed depending on the location of the watershed. A hillshade was created from the DEM for cartographic effect.

The fill sinks function was used to modify the elevation values of the DEM so that areas with a lower elevation than all of the surrounding cells did not prevent the flows from continuing on. The flow direction of each cell was then calculated using the Flow Direction Tool which assigns a value corresponding to the direction of the steepest decent in elevation from that cell to the surrounding cells (Figure 1).

Figure 1. The flow direction of each cell in the DEM of the South Fork Nooksack Watershed area determined by the largest difference in cell values between the given cell and surrounding cells.

Next, the flow accumulation tool computed the number of cells that drained into each cell using the output of the flow direction tool. A NHD dataset of streams for the watershed was used to determine the threshold for accumulation to determine where streams form (Figure 2).

Figure 2. Accumulation of flow for each cell determined by flow direction with areas that have an accumulation greater than 1,500 shown in blue.

The streams were assigned numbers using the Stream Link tool to give each stream segment its own unique identity for determining the stream network later in the analysis (Figure 3).

Figure 3. Numbered stream segments of the watershed assigned by the Stream Link tool.

Stream Order was used to determine the hierarchy of the stream network using the flow direction output with the Strahler method (Figure 4). Flow Length was determined from the flow direction raster as well to determine which cells are most upstream or downstream in the watershed raster (Figure 5). Sub-basins of the watershed unit were delineated using the Basin tool with the flow direction raster and vector copies of the streams and basin raster files were created.

Figure 4. The stream order from low to high, with 6 being the highest, highlighting the hierarchy of stream flow in the watershed area.

Figure 5. The flow length in the watershed area showing how far upstream or downstream each cell within the watershed is with areas in red with the longest lengths.

A dataset containing outlet points just below where high order streams intersected (a 6 meets a 5, two 5s meet, and two 4s meet) was created in editor and the Snap Pour Point tool created sub-basins where flow headed to these points (Figure 6).

Figure 6. Sub-basins of the watershed determined from pour points shown on the map where streams with a high order meet. The area for each sub-basin as well as the primary basin are also shown on the map.

Catchments were created using the Watershed tool with flow direction raster and the numbered streams raster as the pour point data (Figure 7).

Figure 7. Catchments of the watershed symbolized with different hues of blue.

The geometric network of the streams (Figure 8) was done outside of the model. A feature dataset was created to store all the associated files which included a copy of the vector stream dataset, the vector watershed boundary, and the raster catchments. A new points feature class was created to contain the outlet point where the stream network left the watershed boundary and was set as the sink of the geometric network created for the feature dataset and display arrows were added to show the direction of flow for each river after setting the AncillaryRole for the outlet point to “sink.”

Figure 8. Catchments of the watershed symbolized with different hues of blue with the stream network shown. The arrows show the direction of flow for each stream.