An alluvial fan.
a stream that empties into a larger river is a Tributary
tributary
It is called a River Delta.
mouth
A river that empties into a lake is most often known as the headwaters-if it is the primary water source for body of water= or = tributary
The Nile River delta.
One river is the Ganges River. Technically it is a distributary of the Ganges River, the Padma that empties into the Bay. Also there are several other rivers that flow through the Ganges Plain that enter the Bay of Bengal such as the Meghna and the Jamuna.
A floodplain.
The flat, wide area along a river is called its flood plain. This is sometimes part of a larger area called the alluvial plain, which formed as the river shifted over time and deposited soil when it flooded.
An estuary is an inlet of water where a river empties into an ocean or sea. It is formed from the freshwaters of rivers and stream, protected from storms, ocean waves and winds.
A river is a large stream of water that flows across the land and empties into a larger body of water. A river usually empties into a sea or ocean.
That triangular area is called a delta. It's named after a triangular shaped letter of the Greek alphabet.Note: A river that empties into a lake is most often known as the headwaters if it is the primary water source for body of water= or = tributary.