Freeing the Elwha - (Sediment Effects of Elwha Dams)
OverviewThe building of the Elwha River Dams has had a huge effect on the natural sediment transport and sediment structures along the Strait of Juan de Fuca. What benefit will the removal of the dams have to natural and human communities along the Strait of Juan de Fuca as natural sediment transport resumes?
The focus of this lesson is on 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.
- Lesson 6- Effects of the Elwha River Dams on Sediments.pptx
- Dam Breach.pptx
- Stream Table
- Blocks or other structures for dam formation
- Reflection Journal pages (printable handout)
- Vocabulary notes (printable handout)
Review the essential Question. Introduce the Guiding Question.
Students should take a few minutes to respond to the reflection prompts. Discuss their answers and any questions they've generated.
Marmot dam removal
How Dams Work