Formation of Flood Plain
Flood plains are made by a meander eroding sideways as they travel downstream. When a river breaks its banks and floods, it leaves behind layers of alluvium (silt). These gradually build up to create the floor of the flood plain. Floodplains generally contain unconsolidated sediments, often extending below the bed of the stream. These are accumulations of sand, gravel, loam, silt, and/or clay, and are often important aquifers, the water drawn from them being pre-filtered compared to the water in the river.
The floodplain during its formation is marked by meandering or anastomotic streams, oxbow lakes and bayous, marshes or stagnant pools, and is occasionally completely covered with water. When the drainage system has ceased to act or is entirely diverted for any reason, the floodplain may become a level area of great fertility, similar in appearance to the floor of an old lake. The floodplain differs, however, because it is not altogether flat. It has a gentle slope downstream, and often, for a distance, from the side towards the center.
The floodplain is the natural place for a river to dissipate its energy. Meanders form over the floodplain to slow down the flow of water and when the channel is at capacity the water spills over the floodplain where it is temporarily stored. In terms of flood management the upper part of the floodplain (piedmont zone) is crucial as this is where the flood water control starts. Artificial canalisation of the river here will have a major impact on wider flooding. This is the basis of sustainable flood management.
Floods on the Flood Plain
Floods are usually seasonal and can be predicted months ahead of time. The ability to develop agriculture, the transportation allowed by rivers, and the normally stable flood season make flood plains ideal locations to develop urban areas. The flood plains between the Tigris and Euphrates rivers are home to some of the world's earliest civilizations and first cities, including Ur.
Logging Requirements:
Send an email or geocaching.com message with with the name of this earth cache in the first line of the message and the answer to the following questions (every person logging a cache must send the answers individually):
1. Based on the drawings above, what part of the flood plain are you standing on at the posted coordinates?
2. How wide is the flood plain from this location to the other side?
3. When this flood plain gets flooded, what is the source of the water?
4. Tell whether the flood plain has water or not. If it is flooded, estimate the depth of the water.
Optional: Post a picture of yourself or your group with the flood plain in the background.
Sources:
National Geographic (http://education.nationalgeographic.org/encyclopedia/flood-plain/)
The Info List (http://www.theinfolist.com/php/SummaryGet.php?FindGo=flood_plain)
New World Encyclopedia (http://www.newworldencyclopedia.org/entry/Floodplain)
Wikipedia (https://en.wikipedia.org/wiki/Floodplain)
Ace Geography (http://www.acegeography.com/landforms-of-deposition---lower-course.html)
Congratulations to HerrKraft on the FTF!