Know before you go: This EarthCache is located on the Clive Greenbelt and is only accessed via the paved trail that goes underneath the overpass. Do NOT attempt to access this cache and/or park on NW 100th St. However, parking IS available at the provided parking waypoints.
What is a floodplain?
A floodplain is a relatively flat area of land adjacent to a river that experiences flooding during periods of high discharge. The soils usually consist of clays, silts, sands, and gravels deposited during floods. Floodplains such as the one located next to Walnut Creek are created by a process known as overbank flow.
What is overbank flow?
Overbank flow occurs when the river is flooded with more water than can be accommodated by the river channel. Flow over the banks of the river deposits a thin veneer of sediments on the floodplain that is coarsest and thickest close to the channel. This is described as vertical accretion since the deposits build the floodplain upwards. Slope ridges of sediment alongside streams, known as natural levees, form due to this process.
Evidence of a Former Flood: Mudcracks
After a flood deposits soil onto a floodplain, the sediment begins to settle. Floodplain soil begins to dry out in the sun when the high flood waters subside. Mudcracks form in environments where the soil dries and contracts. When the top soil layer shrinks while the bottom layers remain the same size, cracks become visible. When multiple cracks occur, they can form polygonal networks. Mudcracks are extremely prominent in areas with little rainfall, limited vegetation, and porous soil (soil is larger-grained such as sand).
Logging Tasks:
- Stand on the floodplain underneath the overpass and describe the land. Is it relatively flat or sloped? Does it contain a lot of vegetation?
- Observe the network of mudcracks. How close are they to the stream? What polygons do they seem to form in the ground?
- How do you think these mudcracks formed?
- As of June 2019, earthcaches 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://sites.pitt.edu/~cejones/GeoImages/5SedimentaryRocks/SedStructures/Mudcracks.html
https://web.archive.org/web/20131024182724/http://test.ourhomeground.com/entries/definition/overbank_deposit
https://serc.carleton.edu/NAGTWorkshops/sedimentary/images/mudcracks.html
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