Which river valley feature is characterized by steep sides and a narrow bottom?

The feature characterized by steep sides and a narrow bottom is an incised meander. This geological formation occurs when a river cuts deeply into the landscape, often due to a drop in base level or increased erosion from the river's flow. As the river incises into the valley, it leaves behind steep, almost cliff-like sides, creating a very pronounced and narrow valley floor that differs significantly from surrounding features.

Incised meanders are formed over time through the processes of erosion and sediment transport, often resulting in a dynamic landscape that shapes the flow and behavior of water within the valley. This is particularly distinct from other options like a broad floodplain, which features wide, gentle slopes and extensive flat areas that provide a contrasting environment to the steep, narrow incised meander. Valley widening refers to a process where the sides of a valley are eroded away to create a broader valley floor, and a delicate ridge is not a recognized geological feature associated with river valleys, making incised meander the most fitting choice.