What is a flat, triangular-shaped land formed when a river carries and deposits sediment at its mouth.

The answer is "Delta"

Delta is a flat triangular shaped land formed when a river carries and deposits

sediment at its mouth.

This usually happens where a river is entering an ocean, sea,

lake, reservoir,

and another river that cannot carry away the sediment supplied by the river.

The last one happens very rarely (Entering a river).