A Growing Oxbow - Juneau Earthcache #9

This site offers a unique opportunity to witness the ongoing transformation of a meandering river bend into a beautiful oxbow lake. Let's delve into the geological processes and environmental factors that contribute to the creation of this natural wonder. This is an earthcache - there's no container to find here, rather you'll need to walk to the coordinates and make observations to answer some questions and claim your find! Please note the no trespassing signs near GZ and please make your observations from the public trail.

What is an Oxbow Lake?

An oxbow lake is a U-shaped lake or pool that forms when a wide meander of a river is cut off, creating a free-standing body of water. These lakes sometimes have regional names - in Australia you'd encounter a Billibong while in south Texas you might know these lakes as resacas. These U-shape lakes play a crucial role in supporting diverse ecosystems and conversing biodiversity. These water bodies provide habitats for a wide range of plant and animal species, including migratory birds, amphibians, and aquatic vegetation. They also serve as important breeding and feeding grounds, contributing to the overall health of the surrounding environment. 

How do Oxbow Lakes form?

There are four main steps that occur during the formation of an oxbow lake. 

1. Meandering: As the river flows, it erodes the outer bank through hydraulic action, abraision, and solution - which deepens the channel on that side of the river. For more information on meandering, please see Juneau Earthcache #9.
2. Neck Formation: Over time, the continual erosion and deposition process causes the river to gradually shift its course creating a characteristic "U" shape. This shape creates a neck of land between two bends in the river.
3. Cut Off Event: During periods of high water flow, such as intense flooding, the river may breach the narrow neck as it seeks the path of least resistance, creating a new, straighter channel. 
4. Lake Formation: Once the cut-off event occurs, the abandoned meander bend becomes an oxbow lake. Initially the lake will be connected to the main river by a narrow channel called a swamp cut or an oxbow neck. Over the years as deposition occurs this channel will become filled with sediment and eventually the lake will be completely separated from the river.

Questions

An earthcache is an opportunity to read about an earthscience topic and apply it to a real-world observations. Here at GZ you'll be able to see an oxbow lake that is in the process of forming. Make your observations at the posted coordinates and combine that with the information you learned above to answer the questions below. Please send your answers to the CO in order to successfully claim the find!

1. Observe the movement of water in the main portion of the river (to your left) and compare it with the rate of water movement in front of you. Which is moving faster?
2. Look at the surrounding area - what phase of oxbow lake formation do you think we're currently in? Has the cut-off event occurred yet? 
3. Based on your observations at GZ and by viewing a satellite image of the area, will the island in front of you ever be reachable from the mainland without getting wet? Why or why not?
4. (Optional) Take a photo at GZ and post it in your log!

References

• https://eros.usgs.gov/media-gallery/earthshot/oxbow-lakes#:~:text=Sediment%20builds%20up%20on%20one,water%2C%20called%20an%20oxbow%20lake.
• https://education.nationalgeographic.org/resource/oxbow-lake/
• https://www.geographyrealm.com/how-oxbow-lakes-form/

​This cache was placed with the permission of CBJ department of Recreation.