River Bank Erosion - Juneau Earthcache #10
The Mendenhall River is a 6 mile long river that winds its way from the Mendenhall Glacier towards the inside passage. While this stream meanders through the Valley it crosses beneath the bridge you find yourself standing on, which offers us a unique chance to observe bank erosion from two different perspectives! Please take a few minutes to park and to observe some of the features of the riverbank here and answer some questions. Please note that as an Earthcache this geocache does not have any container; in order to log this cache as "found" please send the CO answers to the answers at the bottom of this page. You may send these answers via the message center or via email.
It's all about Energy
As a river meadners from its origin to the sea, they serve as a transport mechanism for not only water but all the particulates and sediment deposit that are picked up in the water and eroded from the banks of the river. These bodies of water don't move in a straight
line. Meanders are produced when water in the stream channel erodes the sediments of an outer bend of a streambank and deposits this and other sediment on subsequent inner bends downstream. This process reinforces the riffle-pool structure of a stream.
Erosion in a stable stream can be minimal from year to year, but as a meander moves outward and becomes more looped, flows intensify on the outer bends. Meander patterns follow a remarkably predictable geometry described by wavelength, radius of curvature, and channel bank-full width – the height on the bank where, at higher flows, water begins to tip onto the floodplain (see figure below). No matter the size of the stream, the wavelength is approximately 11 times the channel width and, invariably, between 10 and 14 times the width. The radius of curvature of the central portion of the channel’s bend averages about 1/5 of the wavelength. Channels throughout the world follow this meander geometry so closely that the form of a large river resembles that of small streams.
Examining the Erosion
Riverbank erosion tends to fall into two primary categories: point bars and cut banks. Erosion around meanders or bends in the river occur because currents move faster along the outside of the curve than they do along the inside, which causes sedimentation to deposit on the inside banks forming large flat areas of silty deposits known as point bars and to erode the outside edge of the curve often forming cliff known as cut banks. In nature trees often serve to stabalize the erosion of the banks as their root systems tightly compact soil which slows the erosion process. In very specific situations this type of erosion can form an Oxbow Lake - for more on this type of erosion, please check out the nearby Juneau Earthcache #9.
LOGGING REQUIREMENTS:
To log this Earthcache: Read the geology lesson above. Answer all three questions posted below. Answers can be sent via e-mail or messenger contacts on my Geocaching profile within a reasonable time. Group answers are fine, just list the members of your party with your answers. Do not post the answers to the questions in your logs.
QUESTION 1. Look to the North and look at the meander ahead of you. Examine the left side of the meander. Is this an example of cut bank erosion or a point bar? Is this an outer bend or an inner bend?
QUESTION 2. Continue looking to the North and look at the meander ahead of you. Examine the right side of the bank. Is this an example of cut bank erosion or a point bar? Is this an outer bend or an inner bend?
QUESTION 3. What features can you observe that are slowing the rate of bank erosion?
Additional Logging Requirement: Post a photo that readily indicates that you (and anyone else logging the find) are at the location. This photo should be of you (face not required) or a personal item. Please ensure you do not include the observable river bank in any of your photos.
References:
- https://www.ausableriver.org/blog/why-do-streams-meander
- https://www.sciencedirect.com/topics/earth-and-planetary-sciences/bank-erosion