Lucy Whitridge
Floodplains
The part of a valley that is inundated during a flood
Sediments composed of fine sands, silts and clays as well as course sand and gravel
that were deposited as point bars as the meanders shift.
Rivers that occupy valleys with broad flat floors can create natural levees. This
happens over time with several flooding events. A sheet of water covers the
floodplain, which reduces turbidity and speed of the stream; the courser materials
are deposited on the edges of channels. Less finer sediment is deposited creating a
gentle slope.
Parts of the Mississippi River levees reach 6 meters above the valley floor.
The areas behind the levees often form black swamps, which are poorly drained
areas. Water can’t flow over the levees and into the rivers. Tributaries that try to
enter the main channel sometime flow along the levees (in black swamps) for a long
distance before joining the channel. This is called a yazoo tributary.
The Yazoo River runs parallel to the Mississippi River for 300 km.
Often we see artificial levees along rivers for flood control. Their steep slopes are a
defining characteristic. During high flows sediment is not deposited on the valley
floor but in the channel, which causes the channel to be built up. The height of the
leeves are often raised, but sometimes that can not withstand the force of the
floodwater. This means that levees are often not a permanent solution.
Drainage Networks
Drainage Basin or Watershed Is an area of land that allows water to flow over or
under its surface into a particular body of water. The boundaries are determined by
the guiding contours of the land surrounding that stream, river, lake, or bay. Every
home, school, office, business and industry is part of a watershed.
http://www.oaecwater.org/watershed/crumpled-paper-watershed-exercise
Drainage Patterns change according to the rocks under which streams form as well
existing faults and folds
The dendritic pattern of tributaries is irregular and can be compared to the
branching pattern of a tree. This occurs where the underlying bedrock is somewhat
uniform such as flat sedimentary strata or massive igneous rocks. It is uniform in its
resistance to erosion and the pattern is therefore determined by the slope of the
land.
Radial patterns form when water diverges from a central area. This often happens
on isolated volcanoes or dome shaped uplifts.
When tributaries have right angle bends this is said to be a rectangular pattern.
This occurs when bedrock is crisscrossed with by a series of joints and faults. These
areas are more easily eroded than the unbroken rock.
The trellis pattern is also rectangular in form and looks like a garden trellis
because tributaries run parallel to each other. This occurs in areas with resistant
and less resistant bedrock that form bands.