Freeing the Elwha: Science Unit 1

Unit 1: Weather and River Erosional Processes

Lesson 1: Weather Patterns of the Pacific Coast

Guiding Question: How can learning about weather patterns help us to understand our water resources?

Overview: The weather in the Pacific Northwest is dependent on the Pacific Ocean. Generally speaking, weather in the region is mild, with cool wet winters, and warm dry summers with extremes in temperature and precipitation being unusual. However, despite the moderating effects of the Pacific Ocean, the mountains are very important in terms of the distribution of precipitation across the region and the development of water storage in the form of a snowpack during the summer drought. The mountain, via topographical lifting and the rainshadow effect, causes some areas to be temperate rainforests, while others become deserts.

Topics: Precipitation/Climatic Patterns in the Northwest, the Rainshadow Effect, Snowpack and Glaciers

Time: One class period


Lesson 2: What is a watershed?

Guiding question: Healthy watersheds are vital for a healthy environment and economy. How can understanding watersheds and particularly the Elwha watershed help you protect the water and other natural resources?

Overview: A watershed is an area where all precipitation either drains on the surface or underground into an outlet stream or river. Watersheds can vary by scale, with a large watershed containing many smaller ones. In addition to surface waters, much of the precipitation filters through the soil and bedrock into the aquifer. Aquifers are bounded by impermeable layers and once water reaches those, they begin to flow more horizontally. Groundwater flows along the rock layers until it reaches a surface point that is below the water table. Then, the water reemerges as springs or seep that flow over the surface.

Time: One class period


Lesson 3: River Flows and Sediment Movement

Guiding Question: Seasonal weather patterns affect the flow patterns of water into the rivers (and watersheds) of western North America. What landform features can form when erosion is caused by fast -moving water from Spring and Fall rains, and snowpack melt off?

Overview: The focus of this lesson is to learn about the seasonal flow patterns of rivers in western North America, and the erosional effects of fast-moving water. Rivers in the west tend to experience high flows during the spring rains and snowpack melt off, low flows during the long summer drought, increased flows during the return of the fall rains, and slightly lower flows in winter when much of the precipitation is trapped as snow. However, strong storms can result in sudden spikes of water flows and flash flooding.

Fast-flowing rivers can move large stones and carry a lot of sediment, so they have the ability to pluck stones from the bedrock and can carve deep canyons. Eventually fast-flowing rivers form V-shaped valleys, with terraces/benches forming along former riverbeds. Where flows reach steep gradients, rapids and waterfalls will form. The goal of this lesson is to demonstrate the features that can form in these fast-flowing streams.

Time: One class period


Lesson 4: Sediment Deposition and River Structures

Guiding Question: As rivers age and slow they deposit sediment and form sediment structures, how are sediments and sediment structures important to the river ecosystem?

Overview: The focus of this lesson is the deposition and erosional effects of slow-moving water in low gradient areas. These "mature rivers" with decreasing gradient result in the settling and deposition of sediments and the formation sediment structures. The river's fast-flowing zone, the thalweg, causes erosion of the river banks forming cliffs called cut-banks. On slower inside turns, sediment is deposited as point-bars. Where the gradient is particularly level, the river will branch into many separate channels that weave in and out, leaving gravel bar islands. Where two meanders meet, the river will straighten, leaving oxbow lakes in the former meander bends.

Time: One class period


Lesson 5: Sediment Deposition at the Sea

Guiding Question: As rivers carry sediment to the sea different sediment structures are formed along shorelines, how is sediment important to both river and the shoreline ecosystems?

Overview: The focus of this lesson is depositional and erosional effects as rivers meet the sea. As a river meets the sea, the sediment it carries is deposited in a fan-like formation called a delta. As longshore drift picks up and transports the sediment, it can be carried and deposited down current to form shoreline sediment features such as sand bars, spits, and barrier islands. These sediments can protect areas behind them from the effect of ocean waves to form estuaries, salt marshes, and lagoons. Features such as headlands and sea stacks can intercept and deflect the currents, allowing sediments to be deposited on beaches in sheltered coves.

Time: One class period


Lesson 6: Effects of Elwha Dams on Sediments

Guiding Question: The 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?

Overview: 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.

Time: One class period



This webpage was made possible in part by a grant from Washington's National Park Fund.

Last updated: April 6, 2015

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