An incised valley is a river valley that has been deeply cut into the landscape, often due to a drop in sea level or tectonic uplift. These valleys are important in studying the response of river systems to environmental changes, the development of sedimentary basins, and the preservation of ancient river systems in the geological record.
Reference: Dalrymple, R. W., Boyd, R., & Zaitlin, B. A. (1994). Incised Valley Systems: Origin and Sedimentary Sequences. SEPM Special Publication No. 51.