4 A river delta, also known as a deltaic plain, is a type of wetland where land is formed as a river empties into a larger body of water. River deltas are formed slowly over time as gravel, sand, mud, and silt wash downstream. As the sand, mud, and gravel, also known as sediment, collects near the mouth of the river, the water becomes more and more shallow. The sediment creates landforms, or deltas, and eventually the remaining water flowing through the land. HISTORY When dinosaurs still roamed the Earth, over 65 million years ago, sea levels were so high that the seaway sat on top of the continent. This type of seaway is called an epicontinental seaway. When the supercontinent called Pangea began breaking apart, there were major climatic shifts, causing the epicontinental seaway to shrink and move off of the continent. Even though the seaway was moving off of the continent, sea levels were still very high. The Mississippi River began flowing south to the Gulf of Mexico, depositing sediment and building land. However, because sea levels were still high, the river began building land from the southernmost tip of Illinois down to Louisiana where it currently empties into the Gulf of Mexico. About 436,000 tons of sediment currently travels down the Mississippi River every day. This sediment helps continue the land building process. Before levees were built, the river would occasionally overtop its banks and form a shorter route to the Gulf. Over time, the shorter route became the preferred channel, creating landforms called marshes between waterways. However, once humans began digging canals for oil and gas infrastructure, salt water was able to move into marshes with lower salinity. Salt water has a high salinity, which is the amount of salt measured in the water. The plants that grew in these areas were not adapted to so much salt and thus did not survive. Because the plants were not there to hold the sediment together, the process of erosion, or gradual land loss, was much quicker. EROSION There are many reasons the coast continues to erode today. Subsidence, or the sinking of land, causes erosion by exposing shores to greater wave activity. Sea level rise causes erosion by allowing water to come up higher on the coastal banks. Hurricanes, man-made river controls like levees, and invasive species all cause erosion as well. During hurricanes or strong storms, the coastal wetlands are the first line of defense. While this helps protect places like New Orleans, erosion happens much more quickly in the wetlands as they take the impact of the storm. Epicontinental Seaway (Deep Time Maps, ©2011 Colorado Plateau Geosystems Inc.) A TALE OF TWO RIVERBANKS Terraces at BS-11 (CWPPRA) Investigation #1
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