Final answer:
A steep slope separating a floodplain from terraces above it is called a bluff.
Step-by-step explanation:
The steep slope separating a floodplain from terraces above it is called a bluff. A bluff is a high, steep bank or cliff, typically composed of rock or sediment, that is created by erosion or deposition by a river or other body of water. This feature can be found in river systems where the floodplain is at a lower elevation than the terraces above it.