Student Activities

Sediment Effects of Elwha Dams

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Grade Level:
Middle School: Sixth Grade through Eighth Grade


Through this activity, students will learn about the effects that building the Elwha River dams had on the natural sediment transport and deposition mechanisms in the Strait of Juan de Fuca. The formation of Lake Mills caused most of the sediment to deposit in a delta at the head of the reservoir, rather than at the mouth of the river. In addition to a large delta, a bed of fine silt covers the lake floor. The loss of this sediment has resulted in severe consequences downstream. Sandy beaches at the mouth of the river, which used to contain rich shellfish beds, have washed away. Salmon spawning beds in the lower five miles of the river have eroded away and sediment transport to Ediz Hook by longshore drift has stopped as well.

  • River-dominated delta: Rivers carrying and depositing great quantities of sediment in the delta will grow outward into the sea as deposition is greater than wave erosion (example Mississippi Delta).
  • Wave-dominated delta: Rivers carrying and depositing less sediment than what is eroded away by wave action will form rounded fans (such as the Nile delta) or flattened deltas (such as the Klamath River Delta).
  • Long-shore drift: The movement of sediment down shore in a common direction caused by the combination of the ocean currents, wind direction, tidal movements, and oblique wave action on the shore.
  • Dam: A barrier constructed across a waterway to control the flow or raise the level of water.
  • Reservoir: A man-made water containment system often the result of lake filling behind a dam, but also can be water stored in large tanks or underground storage.

Additional Resources

Dam removal

Marmot dam removal

How Dams Work



Download Activity Materials

Download Sediment Effects of Elwha Dams powerpoint

Download Dam Breach powerpoint

Last updated: October 2, 2015