*PLEASE RECALL THAT TO LOG AN EC AS FOUND THE LOGGING REQUIREMENTS MUST FIRST BE MET, AND ANSWERS AND PHOTOS MUST BE SENT.*
To complete this Earthcache (EC), you need to be positioned/standing on the publically accessible High Rollaway Observation Deck (see WP2).
Due to nature of this EC, it must be completed during day light hours.
This is a seasonal cache and will be disabled in the winter months.
Please note: we believe that it's possible for wheelchair users to access this cache, however, the path leading to the observation deck is not paved (it's a hard-packed trail). If I am wrong, please let me know.
Please be sure to read the lesson, questions and send the answers using an in-game feature. I will read and respond to all submissions; posted finds without the accompanying answers may be removed.
Enjoy this beautiful location!
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The Lesson - What's an Oxbow?
River landforms can be divided into upper, middle and lower course features. As the river moves from the upper course to the lower course, the features in the river will change.
Standing on the Observation Deck, take a look below you at the Manistee River. What you are looking at it an Oxbow in the making. Although this is not a full Oxbow formation there are several very prominent features that show the formation.
The upper course is the section at the beginning of the river, nearest the source. Rivers usually begin in an upland area, like the top of a mountain. Snow melts or rain falls on high ground and begins to flow downhill. The upper course of a river is usually steep and narrow. In the middle course the river has more energy and a high volume of water. The gradient here is gentle and lateral (sideways) erosion has widened the river channel. The river channel has also deepened. A larger river channel means there is less friction, so the water flows faster. The lower course is the end of a river. It flows into another body of water. This could be the sea, an ocean, a lake or another river. In the lower course, the river channel is deep and wide and full of lots of water.
A Oxbow or meander in general is a bend in a sinuous watercourse. A meander is formed when the moving water in a stream erodes the outer banks and widens its valley. A stream of any volume may assume a meandering course, alternatively eroding sediments from the outside of a bend and depositing them on the inside. The result is a snaking pattern as the stream meanders back and forth across its down-valley axis. When a meander gets cut off from the main stream, an oxbow lake is formed. Over time meanders migrate downstream, sometimes in such a short time as to create civil engineering problems for local municipalities attempting to maintain stable roads and bridges.
Meander formation is a result of natural factors and processes. The waveform configuration of a stream is constantly changing. Once a channel begins to follow a sinusoidal path the amplitude and concavity of the loops increase dramatically due to the effect of helical flow sweeping dense eroded material towards the inside of the bend, and leaving the outside of the bend unprotected and therefore vulnerable to accelerated erosion, forming a positive feedback loop.
Flow of a fluid around a bend is vortex flow in order to conserve angular momentum. The speed of flow on the outside of the bend is fastest, and on the inside of the bend is slowest. The water surface is also super-elevated towards the outside of the bend, so on the floor of the channel the water pressure is greater on the outside of the bend than on the inside of the bend. This pressure gradient drives a cross-current towards the inside of the bend.
The cross-current along the floor of the channel is part of the secondary flow and sweeps dense eroded material towards the inside of the bend. The cross-current then rises to the surface near the inside of the bend and, moving near the surface, flows towards the outside of the bend, forming a helical flow.
The greater the curvature of the bend, and the faster the flow, the stronger is the cross-current and the stronger the sweeping of dense eroded material along the floor of the channel towards the inside bank.
Image: Stages of Oxbow Formation
Erosion
There are a number of natural forces that lead to erosion including wind erosion, frost action, soil creep and slumping, but the most common and powerful source is water. As a river's velocity increases, so too does the amount of erosion taking place. There are four ways that a river erodes; hydraulic action, corrosion, corrosion and attrition.
Hydraulic action – the force of the water wearing away the bed and bank of the river
Corrosion – the chemical reaction between the water and the bed and bank of the river, wearing it away.
Corrasion/abrasion – where bedload in the river wears away its bed and bank.
Attrition – where rocks in the water become smaller and rounder by hitting each other.
River bank erosion occurs when water wears away at the banks of a river or stream. While river erosion is a naturally occurring process, human impact can increase its rate. Common contributing factors to river and stream bank erosion include:
- Clearing vegetation away from the river bank.
- Flooding.
- Intense rainfall.
- River bank saturation from nonriver water.
- Stream and land use management.
- River straightening.
- River redirection around infrastructure or debris in the channel.
- Characteristics of the river bank soil.
Other forms of erosion include:
Soil Creep: Soil creep defines the slow mass wasting process of soil on a slope, under the influence of gravity.
Slumping: A slump is a form of mass wasting that occurs when a coherent mass of loosely consolidated materials or a rock layer moves a short distance down a slope. Movement is characterized by sliding along a concave-upward or planar surface.
Ice: Cycles of freezing and melting push ice onto the shoreline. Ice can cause significant shoreline damage.
Storm water: Storms remove or displace loose layers of soil causing sheet erosion.
Sheet erosion: Sheet erosion occurs as a shallow 'sheet' of water flowing over the ground surface, resulting in the removal of a uniform layer of soil from the soil surface. Sheet erosion occurs when rainfall intensity is greater than infiltration (sometimes due to crusting). Rarely seen but accounts for large volumes of soil loss.
Erosion Prevention Methods
- Revetments: an engineering technique used on a sloping surface or creating a sloping surface using stone, concrete or other material that creates a slope to absorb the energy of incoming water (click here);
- Rock armour/riprap: large boulders placed in front of a cliff or slope to deflect (wave) energy away and protect the shoreline from erosion (click here);
- Gabions: a cage-like structure filled with rocks or concrete (click here).
- Tree revetment is a river bank erosion control system that uses small fallen trees anchored horizontally in place along the river bank to prevent erosion. The trees slow the flow of water, which cuts back on the rate of erosion. They also catch sediment in the tree branches and prevent it from flowing down the river.
- Coir logs are another river bank erosion repair method made using coconut fiber. Coir logs are large in diameter, which makes them ideal for supporting river banks or being used for erosion control on hills, shorelines, and other areas prone to erosion. Even though they’re big, coir logs are easy to place. Once they’re in position, they can help establish vegetation growth. They’ve been effectively used in construction sites, restoration projects, and stabilization areas. Most coir logs will last two to five years before biodegrading. (click here)
- Natural vegetation has a major impact on bank erosion. For the most part, river banks that have vegetation erode slower than those without. This is because the roots of the vegetation generally increase the soil’s strength around the river bank, which makes the bank less prone to mass failure. Additionally, plants can act as shock absorbers during heavy rainfall, which also slows the rate of erosion.
- Soil erosion mat: While these mats are sometimes made with wood fiber or straw, one of the top materials used is coir coconut fiber. This is because coir is strong yet natural. Plus, unlike straw or wood, coir can last in the water anywhere from two to five years. (click here for an image)
Sources/Additional Information:
BBC
Wikipedia - Meander
Wikipedia - Oxbow Lake
Soil Erosion
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Questions - if you are delayed in sending your responses, please post a note. If sending responses for more than one cacher, clearly indicate the cachers' names in your submission; regardless, EACH cacher must post their own unique photo at GZ.
1. Required Photo for all cachers - to demonstrate that you were on site, post a photo of you/your caching name & date/your GPS/another caching object (etc.) standing on the Observation Deck with your log. No spoilers of the river/area, please.
2. Based on the lesson and your observations, is this a lower, middle or upper river course?
3. Using what you have learned in the lesson, as well as the diagram provided, what stage would you say this Oxbow is in? Early stages? Midway? Full Oxbow?
4. Based on your observations at GZ, provide a reason for your answer in Q3.
5. Now look below you at the river banks and hill.
a) Estimate the depth of the level of erosion from the observation deck to the river bank. Provide a measurement in metres/feet.
b) Provide 1 example of erosion prevention that you can observe at GZ.
Thanks for visiting and have fun!