Lab 7: Geomorphology Lab

Part A. Case Study: Elwha River, Washington

For millennia, the Elwha River ran wild, connecting mountains and seas in a thriving ecosystem. The river proved to be an ideal habitat for anadromous (sea-run) fish, with eleven varieties of salmon and trout spawning in its waters. These fish thrived in the cold, clear waters of the Elwha River and historically served as an important food source for the Lower Elwha Klallam Tribe living along its banks.[footnoteRef:1] [1: Text by National Park Service is in the public domain]

American expansion spurred a continual demand for lumber. The growth of the logging industry in the region brought rapid change to the Olympic Peninsula and specifically to the Elwha River with the construction of two dams. The Elwha and Glines Canyon dams were built in the early 1900s, generating hydropower to supply electricity for the emerging town of Port Angeles and fueling regional growth on the Peninsula. However, construction of the dams blocked the migration of salmon upstream, disrupted the flow of sediment downstream, and flooded the historic homelands and cultural sites of the Lower Elwha Klallam Tribe.12 Figure 15.12 provides a locator map of the Elwha River in the state of Washington circa 2010.

Figure 15.12: Elwha River Locator Map.[footnoteRef:2] [2: Figure by USGS is in the public domain]

For over a century, the web of ecological and cultural connections in the Elwha Valley were broken—then the river’s story changed course. In 1992, Congress passed the Elwha River Ecosystem and Fisheries Restoration Act, authorizing dam removal to restore the altered ecosystem and the native anadromous fisheries therein. After two decades of planning, the largest dam removal in U.S. history began on September 17, 2011. Six months later the Elwha Dam was gone, followed by the Glines Canyon Dam in 2014. Today, the Elwha River once again flows freely from its headwaters in the Olympic Mountains to the Strait of Juan de Fuca.12

It was the world’s largest dam-removal project. Over the next five years, water carrying newly freed rocks, sand, silt and old tree trunks reshaped more than 13 miles of river and built a larger delta into the Pacific Ocean. Of the 33 million tons of sediment trapped behind the dams, about 8 million tons resettled along the river or at the mouth, and another 14 million dispersed into the ocean. It would take more than 70 dump trucks running 24 hours a day for five years to move that much dirt and debris downstream. Piled up, the sediment would form a cone about one-third of a mile in diameter and taller than a 50-story building (Figure 15.13).[footnoteRef:3] [3: Text by USGS is in the public domain]

Figure 15.13: Size Comparison of Elwha River Sediment.[footnoteRef:4] [4: Figure by USGS is in the public domain]

Figure 15.14 shows the Elwha River delta after dam removal. Notice the brown sediment flowing into the darker colored water of the Strait of Juan de Fuca.

Figure 15.14: Elwha River Delta after Dam Removal.[footnoteRef:5] [5: Figure by Jeff Duda, USGS is in the public domain]

Let’s use topographic maps to explore the fluvial geomorphology of the Elwha River before and after dam removal.

If you need a quick review of topographic maps, check out the Topographic Map Symbols document from the USGS.

Step 1

Go to the file in the google drive folder to access the topographic map from 1950 (before the dams were removed).

Step 2

Use the plus (+) sign and minus (-) sign to zoom in and out of the map, as needed. When zoomed in, you can click and drag your mouse pointer to view different portions of the map.

Step 3

Locate the Elwha River, the Upper Elwha Dam, and the Olympic Power Plant, which is at the site of the Elwha Dam. Note that the Upper Elwha Dam is also known as the Glines Canyon Dam.

1. Does the Elwha River flow to the north or to the south? How can you tell?

2. Where do you find evidence that the Elwha River has changed its course over time?

3. Calculate the gradient of the Elwha River between the Upper Elwha Dam and the Olympic Power Plant.

a. Use the graphic scale at the bottom of the topographic map to find the distance between the Upper Elwha Dam and the Olympic Power Plant. What is the estimated distance in miles?

b. Find the contour line at the Upper Elwha Dam. What is the dam’s elevation?

c. Find the contour line at the Olympic Power Plant. What is the power plant’s elevation?

d. What is the difference in elevation (in feet) between these two locations? Show your work.

e. Divide the difference in elevation by the distance between the two locations. Show your work. Your answer should be in feet per mile.

Step 4

Go to the file in the google drive folder to access the topographic map from 2020 (after the dams were removed). Note: this is a very large file and may take a few moments to load.

Step 5

Use the plus (+) sign and minus (-) sign to zoom in and out of the map, as needed. When zoomed in, you can click and drag your mouse pointer to view different portions of the map.

4. Where does the Elwha River have the steepest channel? At this location, what is the difference in elevation from the riverbank to the top of the cliff at the side of the river?

Step 6

Keep both maps open in different tabs in order to compare the Elwha River between 1950 and 2020. Refer to the selected topographic map symbols shown in Appendix 15.1.

5. What are three major differences along the Elwha River that you notice between the 1950 and 2020 topographic maps?

Step 7

Search the Internet to find out about any dams, dam construction/maintenance projects, and dam removal projects in the county where you live.

6. Would you tend to support the construction of new dams in your county? Explain your response in two to three sentences.