Final answer:
The delta at the end of the Rio Grande was formed by sediment accumulation at the river's mouth where it flows into the Gulf of Mexico.
Step-by-step explanation:
The delta at the end of the Rio Grande formed through natural geological processes typical of river deltas. Over time, sediment carried by the river accumulates at the river's mouth, where the water slows down as it enters a larger body of water, in this case, the Gulf of Mexico. The sediments then deposit and gradually build up, creating a delta—an area of land formed from sediment deposits at the mouth of a river. Additionally, flow changes, cyclic flooding, tides, and waves contribute to shaping the delta over centuries, and these processes remain active today, meaning the delta is constantly changing.