Final answer:
The flat area where a river ends and deposits sediments, forming a delta or an estuary, is known as a river delta. Deltas are triangular-shaped and deposit sand and silt to form fertile lands, while estuaries are coastal areas where fresh and saltwater mix.
Step-by-step explanation:
The flat area on to which a river eventually empties out is often referred to as a delta or an estuary. A delta is typically a triangular-shaped section of land found at the mouth of a river. It forms as the river's waters slow down and spread into many channels, depositing sand and silt that the river has transported downstream. When a river flows into an ocean, sea, or lake, and the waters significantly slow, the sediments carried by the river accumulate, forming this feature.
Estuaries, on the other hand, are coastal areas where the fresh water from rivers mixes with saltwater from the sea. The mix of freshwater and saltwater, along with sediment deposits, create this distinct environment. These two flat areas are crucial environments that support diverse ecosystems and facilitate sediment transportation and deposition.
In some cases, rivers can create large deltas that become fertile farmlands, as seen historically in Egypt's Nile Delta, which the ancient Egyptians referred to as Kemet due to the deposition of rich black silt. Similarly, rivers may also create estuarine areas even without high silt content, especially in regions where ocean currents or wave action is significant.