A river delta is a landform created by deposition of sediment that is carried by a river as the flow leaves its mouth and enters slower-moving or stagnant water. This occurs where a river enters an ocean, sea, estuary, lake, reservoir, or (more rarely) another river that cannot carry away the supplied sediment.
Btw, here's also an easy answer to your question.
Deltas are wetlands that form as rivers empty their water and sediment into another body of water, such as an ocean, lake, or another river.
