Transcript of How a Delta is Formed

[music] since the beginning of human civilization people have settled along rivers fertile deltas created by them. The sediment carried and deposited by mighty rushing waters creates land rich in nutrients and ideal for crops and livestock. Where there are uninhibited rivers there's new rich land and where such resources abound there are people. The problem with people settling along rivers and deltas is that rivers inevitably flood. This is good for the land because sediment from the water is deposited on the previously dry earth and replenishes the soil. However, this flooding can destroy the human homes established there. Thus we build levees around these river titans in an attempt to keep the lumbering giants at bay. Louisiana has two of these breathing rivers, the Mississippi, and its distributary, the Atchafalaya. The Mississippi has made Louisiana an international port of trade and commerce, and both rivers have contributed to the lush habitat Louisiana's people and animals. Many smaller rivers feed into the Mississippi creating a vast watershed that drains nearly 40 percent of the continental United States, and all combine to bring the Mississippi and its delta to life. The long-term consequence of levees, however, is prevention of land formation. Rivers deposit sediment in one area over hundreds or thousands of years, until the accumulation is so great that the new topography of the land causes the river to change course and begin to deposit sediment somewhere new. When a river is prevented by levees from flooding or changing course, the sediment it carries cannot build up the land, and over time erosion begins to wear away the already-existing earth. in the case of the Mississippi River, these levees also mean that all of the sediment carried by the river is dumped uselessly into the Gulf of Mexico and over the continental shelf, illustrated here. As a result, the land of Louisiana's coast is sinking and eroding at an alarming rate, and people's homes and livelihoods are again in jeopardy. Here is a simplistic depiction of how a delta unrestricted by levees is formed. This model represents a "bird's foot" delta similar to the Mississippi. Here we see the sediment being churned up and carried by the rushing waters. As more and more sediment rides with the water, the sand, silt, and clay begin to build up along the banks and in certain parts of the river mouth until channels are formed. Finally, the sediment has piled up so much that the water has had to divert itself around the newly created islands, and we now have an emerging bird-foot delta. Now, vegetation begins to form on the newly-created and nutrient-enriched land around the river and on top of the islands. Before and after. Natural land production is a slow process. This delta would have taken approximately 1,000 years to form.