Erosion: A Constant Force of Change EarthCache
Erosion: A Constant Force of Change
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The Willamette River once meandered freely across it's floodplain and changed course many times. A river, when free to move, is a complex system of features: side channels, sloughs, oxbow lakes, and islands. Natural rivers are dynamic systems that are always changing.
The Willamette River is divided into three sections or "reaches".
The Upper Reach, at the southern end, from the headwaters to Albany, is a broad, unconstrained flood plain containing numerous side channels and islands.The Lower Reach, from Newberg to the mouth where it joins the Columbia, has remained relatively unchanged over the last 150 years. The river is confined within a basaltic trench limiting the degree of channel movement.The Middle Reach,, from Albany to Newberg, runs through a series of basaltic outcrops and mountains within the floodplain, causing the river to bounce back and forth between these resistant hills. Tributaries have delivered large amounts of sediment into a depositional basin created by the blockages of the Salem hills. Past floods have carved numerous channels through these sediment deposits.
Rivers erode the outer banks and deposit rocks and soil on the inner banks. Over time the two forces combine to create curves, or meanders. Given enough time the channel forms a "U" shape, or oxbow as the main channel carves the outer banks and deposits sediments along the inner banks. When growing meanders intersect each other, the meander loop is cut off. Without an active cutting stream, the meander is gradually filled in and an oxbow lake is formed.
During times of extremely high water, the river has the ability to "jump" it's banks and create a new channel. Even during normal flows, the river's powerful erosive force carves and shapes the land that surrounds it, slowly altering the channel location, sometimes in such a short time as to create civil engineering problems i.e. maintaing stable roads and bridges.
In 1861, the largest flood of the area in recorded history covered the whole Willamette Valley in a sheet of water, from Eugene to Portland. Peaking at 635,000 cubic feet per second, more than the flow of the Mississippi River. The flood innundated some 353,000 acres of land. Previous to this flood the Willamette flowed between two islands, so that Minto Island was on the east bank, and Brown Island was on the west bank of the river. After the flood subsided it was discovered that the river had changed its course to the present location. The 1964 flood was almost as great as the one 100 years previous, and many structures were destroyed at this time. Periodic flooding of the Minto-Brown Island area has allowed the site to survive to this day as a low density agricultural area. During the 1930's river control projects began to take shape. In the following decades more than 15 dams and a complex series of levees and dikes were built on most of the major tributaries of the Willamette.
By 2002, the river had been simplified and tamed. Most of the side channels, sloughs, oxbow lakes, and islands have been converted to land use.
For this Earthcache, you will be visiting an interpretive sign on the main channel of the Middle Reach of the Willamette River. It is short journey along a paved trail.
You will need to answer a few questions and log your answers in an email. Do not post your answers with your log or your log will be deleted.
1) Name two of the reasons why riverbank restoration is important.
2) Name two constructed stream bank features that help confine the river to one channel.
3) With the dams built upstream, do you think that this kind of river movement will happen again on this scale? Why?
4) What other manmade object do you see at this location?
For extra Geocaching points but not required, post a picture of you and your GPS at the cache location.
No pictures of the sign or man-made objects please.
Sources:
http://www.fsl.orst.edu/pnwerc/wrb/Atlas_web_compressed/3.Water_Resources/3c.historic_chl_web.pdf
http://www.cityofsalem.net/Residents/Parks/ParkTour/Documents/History%20of%20Minto.pdf
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