What evidence from the map suggests that the pediment is covered with a mantle of alluvium?

The evidence is when there is degradation. it is characteristic of channel networks in which either bedrock erosion is taking place, or in systems that are sediment - starved and are therefore entraining more material than is being deposited. When a stream degrades, it leaves behind a fluvial terrace. This can be further classified as a strath terrace - a bedrock terrace that may have a thin mantle of alluvium - if the river is incising through bedrock.