An elevated, relatively flat surface flanking a river valley, representing a former floodplain or channel level that was abandoned as the river incised to a lower elevation. Terraces record the geomorphic history of a valley.
A fluvial terrace is a step-like landform composed of a flat or gently sloping surface (tread) and a steep slope or scarp (riser) that represents a former active floodplain of a river now elevated above the current channel due to stream incision. Terraces form when changes in climate, tectonics, base level, or sediment supply cause a river to erode downward into its previously deposited sediments, abandoning the former floodplain surface. Paired terraces (matching surfaces on both sides of the valley at the same elevation) indicate uniform incision, while unpaired terraces result from lateral migration during progressive downcutting. Fill terraces are composed entirely of alluvial sediments, while strath terraces are cut into bedrock with a thin veneer of alluvial cover. Multiple terrace levels in a valley record successive episodes of aggradation and incision, providing a chronological record of river behavior that can be dated using radiocarbon, optically stimulated luminescence (OSL), or cosmogenic nuclide methods. Terrace deposits are important aquifer systems in many mountain valleys and are often targeted for groundwater development. Understanding terrace stratigraphy is valuable for archaeological investigations, as ancient human settlements frequently occupied elevated terrace surfaces near water sources.