What is the term for an area of highland that forms the edge of a river basin?

The term for an area of highland that forms the edge of a river basin is known as a watershed. A watershed is essentially a ridge or boundary that separates different drainage basins, directing the flow of water to different rivers or streams.

When rain falls on a watershed, the water will flow downhill towards the river and its tributaries located within that basin. This geographic feature is crucial for understanding how water is collected and funneled through various terrains, ultimately impacting ecosystems, agriculture, and water management.

The other terms do not fit this definition: the source refers to the original point where a river starts, typically in a higher elevation area; the mouth is where a river flows into a larger body of water, such as an ocean or lake; and a tributary is a smaller stream or river that feeds into a larger river. Therefore, the watershed is the correct term to describe the highland boundary that defines a river basin.