Freeing the Elwha (Sediment Deposition and River Structures)
- Grade Level:
- Sixth Grade-Eighth Grade
- Earth Science, Geology, Hydrology, Physical Science
- One class period
- Group Size:
- Up to 36
- National/State Standards:
- Science EALR4 6-8, ES3A, EALR 4 6-8, ES2G
Reading EALR 1, Component 1.2
Social Studies EALR 5, Component 5.2
Writing EALR 2, Component 2.1
OverviewAs rivers age and slow they deposit sediment and form sediment structures, how are sediments and sediment structures important to the river ecosystem?
BackgroundThe 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.
- Meander: When the floodplain is becomes sufficiently level rivers meander. As the river strikes an area of slightly more resistant material, it is deflected. This causes the thalweg of the river to strike the banks at angles and at higher velocity, cutting into the outside turn, while depositing material on the inside turn.
- Thalweg: Line of fastest water velocity in a river. Thalwegs strike the cut-bank in a river meander
- Cut-bank: The outside curve of a river meander, where erosion is greatest due to the higher stream velocities, causing cutting on the bank and sometimes forming a small cliff
- Point-bar: The inside-curve of a river meander, where stream velocity is slow and deposition of sediment is greater.
- Braided Stream: When a fast flowing river from high gradient encounters a broad, flat valley that is overloaded in erodible sediment, it will begin to spread out into multiple channels that are interconnected. These channels constantly migrate across the flood of the valley, as they meander through the sediment.
- Oxbow Lake:When two meander curves meet, they circumvent the meander, and straighten the flow of the river. The abandoned channel can become disconnected from the river by sediment to form a lake. These lakes eventually fill in with sediment.
- Lesson 4- Sediment Deposition and River Structures.pptx
- Mass Wasting and Flash Floods.pptx
- Stream Table
- Reflection Journal Pages (printable handout)
- Vocabulary Notes (printable handout)
Review Essential Question and introduce Guiding Question.
Hand out first Reflection Journal page and have students take a minute to consider and respond to the questions then discuss responses and questions generated.
Handout and go over the Vocabulary Notes. Students will define the vocabulary words as they watch the PowerPoint Lesson.
Present PowerPoint Lesson.
Run Demonstration on Stream Table of a low-gradient mature river.
Run Demonstration of Mass Wasting and Flash Floods.
Show features such as the thalweg, meanders, cut-banks, point-bars, and braiding.