Geography is all around us: 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 .