The Nile and Mississippi Rivers are recognized as examples of deltas due to their formation at the point where they meet larger bodies of water, such as seas or oceans. A delta is a landform that occurs at the mouth of a river, where sediment carried by the river is deposited as the flow velocity decreases upon entering standing water. This deposition creates various landforms, including islands and wetlands, which can incorporate diverse ecosystems.
In the context of rivers, the Nile River forms a large delta in Egypt where it meets the Mediterranean Sea, while the Mississippi River's delta, primarily located in Louisiana, is similarly characterized by the buildup of sediment and the creation of unique habitats. These features are significant because they influence regional ecology, agriculture, and human settlement.
The other options describe different geographic features. Streams are smaller bodies of flowing water that do not have the same sediment deposition characteristics as deltas. Basins are low-lying areas that can collect water but do not necessarily indicate the same sediment layering as deltas. Estuaries are partially enclosed coastal bodies of water where freshwater from rivers meets and mixes with saltwater from the ocean, distinct from the delta's sedimentary processes.