What is a Confluence?
In geography, a confluence occurs where two or more flowing bodies of water join to form a single channel. Upstream from the GZ of this Earthcache, you will notice that the Des Moines River and the Raccoon River form a confluence. Although this geocache will mention the phenomenon of confluences, we invite you to learn more about the definition of confluences and rivers (in more detail) from Azrag’s earthcache here: https://www.geocaching.com/geocache/GC1557G. Our earthcache will highlight the erosional and depositional aspects that occur downstream from a major confluence.
Confluence-caused Erosion → Bed Erosion:
Bed erosion, degradation, or lowering is a process by which the river's bed is eroded to a new lower level quickly. Bed erosion leads to the undermining of riverbanks, which causes the enlargement of the channel. Bed erosion typically leads to bank erosion because of the decreased water level. The height of the riverbanks relative to the bed increase, leaving them more susceptible to collapse. When another river joins a flowing body of water at a confluence, this can increase water discharge and velocity. These can promote the process of erosion downstream.
Confluence-caused Deposition → Sand Bar Build-up:
A sandbar is a deposit of sand along the shore of a river. Rivers and streams carry a steady quantity of sand and sediment, some settling along the bottom. During increased discharge events, such as large storms, sand gets stirred up on the bottom and spreads along the edges. The presence of a confluence also increases water flow, so some rivers downstream will experience a similar process of sand bar build-up.
**All logging tasks can be completed standing on the sidewalk**
Logging Tasks:
- Observe any evidence of bed erosion. Is there any significant difference in water level compared to the height of the riverbank? If so, estimate the height of the riverbank compared to the water.
- Notice any sediment deposition due to the confluence. Is there a build-up of sediment present? If so, what type of sediment does it consist of? Estimate the length and width of the sediment bar.
- Notice the turbidity of the water. Rivers with high turbidity carry more suspended sediment. Does the water appear clear, cloudy, or completely opaque? How does the turbidity of this portion of the Des Moines River affect depositional processes?
- As of June 2019, earthcaches now contain required photo logging tasks. Please provide a photo of yourself, your GPSr, or a personal item that proves that you have visited this site. Please post this in your log.
Sources:
https://www.qld.gov.au/__data/assets/pdf_file/0033/67677/what-causes-stream-bed-erosion.pdf
https://en.wikipedia.org/wiki/Confluence
https://jan.ucc.nau.edu/gaud/bio300/sandbar.htm#:~:text=Sandbars&text=A%20sandbar%20is%20a%20deposit,the%20river's%20edges%2C%20creating%20sandbars
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