Rivers 3

A delta is a landform that is created at the mouth of a river where that river flows into an ocean, sea, estuary, lake, reservoir, flat arid area, or another river. Deltas are formed from the deposition of the sediment carried by the river as the flow leaves the mouth of the river. Over long periods of time, this deposition builds the characteristic geographic pattern of a river delta.


River deltas form when a river carrying sediment reaches a body of standing water, such as a lake, ocean, or reservoir. When the flow enters the standing water, it is no longer confined to its channel and expands in width. This flow expansion results in a decrease in the flow velocity, which diminishes the ability of the flow to transport sediment. As a result, sediment drops out of the flow and deposits. Over time, this single channel will build a deltaic lobe (such as the bird's-foot of the Mississippi or Ural River deltas), pushing its mouth further into the standing water.

Different types of deltas:


1. Arcuate delta


2.Bird Foot delta

3.Estuarine delta

An arcuate delta forms when a river meets the sea in a place where the waves, currents, and tides are strong. It is often bow shaped and has a number of distributaries flowing across it. An example is the Nile delta of Egypt.




















Bird's Foot Delta
A bird's foot delta forms where sediment is deposited in relatively calm offshore waters. An example of a bird's foot delta is the Mississippi river delta.
























Estaurine Delta
When the mouth of a river enters the sea and is inundated by the sea in a mix with freshwater and very little delta, it is called an estuary. An example of a estuarine delta is the River Shannon































Levees are earthen embankments whose primary purpose is to furnish flood protection from seasonal high water for a few days or weeks a year. Levees are broadly classified as either urban or agricultural because of the different requirements for each. Urban levees provide protection from flooding in communities; including their industrial, commercial, and residential facilities. Agricultural levees provide protection from flooding in lands used for agricultural purposes.



There are five main types of levees:
1. Mainline and Tributary levees: generally parallel the main channel and/or its tributaries.



2. Ring levees: completely encircle or "ring" an area from all directions.


3. Setback levees : generally built as a backup to an existing levee that has become endangered due to such actions as river migration.


4. Sublevees: constructed for the purpose of underseepage control. Sublevees encircle areas landward of the main levee that are flooded, generally by capturing seepage water, during high-water stages thus counterbalancing the hydrostatic pressures beneath the top stratum.


5. Spur levees: project from the main levee and provide protection to the main levee by directing erosive river currents riverward .



Click on the link below to some common lower course features:

http://images.google.ie/imgres?imgurl=http://www.sln.org.uk/geography/schools/blythebridge/images/21_levees.jpg&imgrefurl=http://www.sln.org.uk/geography/schools/blythebridge/GCSERiversRevisionLC.htm&usg=__8S5YTzWIxAd02VcKZ9GLdA1fdZo=&h=283&w=281&sz=11&hl=en&start=2&sig2=vjufL2J42Ql5hufiEWbDpw&um=1&tbnid=fDwCH-fYLN60JM:&tbnh=114&tbnw=113&prev=/images%3Fq%3Dlevees%26hl%3Den%26sa%3DN%26um%3D1&ei=1c_DSentOdeD_gaJ2pCUCw

Rivers 2

Meanders and Ox Bow Lakes

A meander is formed when the moving water in a river erodes the outer banks and widens its valley creating a meander. A stream of any volume may assume a meandering course, alternatively eroding sediments from the outside of a bend and depositing them on the inside. The result is a snaking pattern as the stream meanders back and forth across its down-valley axis. When a meander gets cut off from the main stream, an ox bow lakeis formed. Over time meanders migrate downstream, sometimes in such a short time as to create civil engineering problems for local municipalities attempting to maintain stable roads and bridges.

Formation:

Meander formation is a somewhat equivocal term referring to the natural factors and processes that result in meanders. The waveform configuration of a stream is constantly changing. Once a sinusoidal channel exists it undergoes a process during which the amplitude and concavity of the loops increase dramatically due to the effect of helicoidal flow in increasing the amount of erosion occurring on the outside of a bend. In the words of Elizabeth A. Wood
... this process of making meanders seems to be a self-intensifying process ... in which greater curvature results in more erosion of the bank, which results in greater curvature ...
The helical flow is explained as a transfer of momentum from the inside of the bend to the outside. As soon as the flow enters the bend some of its momentum becomes angular, the conservation of which would require an increase of velocity on the inside and a decrease on the outside, exactly the opposite of what happens. Instead centrifugal force superelevates the surface on the outside, moving surface water transversely into it. This water moves down to replace the subsurface water pushed back at the end of the bend. The result is the scouring helical flow, and the greater the curvature, the greater the angular momentum and the stronger the cross-current.
The question of formation is why streams of any size become sinuous in the first place. There are a number theories, not necessarily mutually exclusive.

Oxbow lakes
Oxbow lakes are created when growing meanders intersect each other and cut off a meander loop, leaving it without an active cutting stream. Over time, these oxbow lakes tend to dry out or fill in with sediment.