Mississippi River Delta
Silt, deposited by annual floods along the 尼罗河, created the rich, fertile soil that sustained the 古埃及ian civilization. Silt deposited by the 密西西比河 throughout the 20th century has decreased due to a system of 堤, contributing to the disappearance of protective 湿地 and in the 密西西比河三角洲 region surrounding 新奥尔良.