Oxbow Lake Reclamation EarthCache
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The coordinates for this cache take you to a bridge over what used to be part of the main channel of the Willamette River in Salem Oregon.
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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. 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. Eventually, oxbow lakes are silted up to form marshes and finally meander scars, marked by different vegetation or the absence of cultivation. The lakes commonly are filled with clay-sized sediment that is less easy to erode than surrounding material and thus have a markedly different appearance when seen from aerial views such as satellite images.
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 inundated 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.
Due to the rerouting of the channel, an oxbow lake was created and is where you are standing today.
The evolution of this oxbow lake into a meander scar is the subject of this earthcace.
A constant water level in a lake is maintained only if inflows from precipitation(rainfall), runoff and groundwater percolation balance losses from outflow, evaporation or groundwater movement.
By its' very nature, an oxbow lake will not have sufficient water inflow to balance the losses it will have. Over the time of existence of an oxbow lake, inflows carry dissolved and suspended material washed from the surrounding high ground, and biological productivity, such as trees and aquatic plants, add organic material to the accumulating sediments. These sediments gradually fill the basin as the water level lowers and allows further growth to intrude upon the edges of the lake. As these intrusions continue, the natural systems begin to form a wetland.
Wetlands generally include swamps, marshes, bogs and similar areas. We see many examples at this Earthcache sight of aquatic plants. This oxbow lake is occasionally flooded by the Willamette under extreme high water situations. As the wetland continues to infill with organic matter and begins to dryout, trees will intrude further and the cycle continues until, in this case, a meander scar is formed.
As oxbow lakes become meander scars, the soil types within the meander scar differ from the surrounding soil. Knowing where oxbow lakes and meander scars are located can help update and produce soil survey maps. Oxbow lakes and meander scars can be used to help determine past river channels. They can also be a source of fossils as organic material can become trapped in the oxbow lakes during periods of deposition.
Oxbow lakes and meander scars are also evidence that rivers and streams do migrate, or change course.
You will need to answer 3 questions in your online "Found It!" log and send us your answer to number 4 in an email. Post your answers for questions 1 -3 with your log but do not post your answer to number 4 or your log will be deleted. Please answer these questions as accurately as you can so we can get a true picture of what is taking place at this location over time. We can potentially watch the whole geologic process from river channel to oxbow lake to meander scar as there is a record of when the area was cutoff during the flood of 1861.
1) Estimate the distance below the bridge to the water level. Use of a tape measure is encouraged but not required. Say whether this was with a tape measure or an estimate.
2)Looking North, estimate the distance across from waters edge to waters edge where the natural riverbank was not modified to install the bridge.
3)On the North side of the bridge, can you tell the depth of the water? If not, describe why.
4)At the northeast and southwest corners of the bridge is a plaque. What is the number at the bottom of the plaque?
For extra Geocaching points but not required, post a picture of you and your GPS at the cache location.
Sources:
http://www.cityofsalem.net/Residents/Parks/ParkTour/Documents/History%20of%20Minto.pdf
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