The development of graceful river meanders and oxbow lakes is explained in this short Two Minute Geology video. Entrenched Meanders are also explained.
Entrenched Meanders form when tectonic uplift begins underneath an meandering river system - causing the river to carve a meandering river canyon. In addition to the Yakima River Canyon in Washington, the San Juan River also features excellent entrenched meanders at Gooseneck State Park in Utah.
This episode begins with Nick standing next to a sweeping curve of the Yakima River downstream of Ellensburg, Washington. The concept of meanders being old age features is established. When rivers are youthful, they are typically linear, but as the river ages, its subtle curves become more exaggerated meanders as time goes by. The meanders are constantly shifting their locations due to continued erosion on the outside of meander curves - and continued deposition of sediment on the inside of the curves. The final stage of meander development is a cut off of the meander that almost loops back completely on itself, and the river abandons the meander - cuts a new straight channel - and an ox-bow lake is formed at the abandoned meander. Since the Yakima River has beautiful, well-formed meanders here, it is clear that central Washington was flat long ago - just like the Mississippi River system today.
The episode then switches to Nick at an overlook vista on the western rim of the Yakima River Canyon. Since the meandering river is now at the bottom of a canyon, the concept of plate tectonic uplift is introduced. The canyon cutting here is a younger event than the river meander development. Regional uplift in central Washington is due to the development of the Yakima Fold and Thrust Belt - and area of densely packed folds and faults that show that this area has been under crustal compression during the last 10 million years. The compression is the force that drives the tectonic uplift that has caused the river to become energized. The river has held its position against an uplifting section of Columbia River Basalt layers now on display in the walls of the beautiful Yakima River Canyon. The Yakima River has been here longer than the Yakima River Canyon!
Filmed in November, 2012.
Episode written by Nick Zentner and Tom Foster.
Video, Sound, & Editing: Tom Foster
28 май 2013
