Question

structure built to prevent a stream from flowing onto a floodplain
artificial levee
base level
delta
gate belt

Explanation:

An artificial levee is a man - made embankment. It is constructed alongside a stream. Its main purpose is to contain the water within the stream channel. By doing so, it prevents the stream from overflowing onto the adjacent floodplain. A base level is the lowest level to which a stream can erode. A delta is a landform formed at the mouth of a river where sediment is deposited. A gate belt is not a relevant geological or hydrological structure in this context.

Answer:

artificial levee