Question

how do rivers become larger as they travel across land?
groundwater rises, adding water to rivers.
tributaries combine, adding water to rivers.
ice sheets melt slowly, adding water to rivers.
rainfall in an area increases, adding water to rivers.

Explanation:

A tributary is a stream or river that flows into a larger river. As rivers flow across land, tributaries combine with them, increasing their water volume. Groundwater rising (usually a local or seasonal effect), ice - sheet melting (a more specific glacial - related process), and increased rainfall (a more regional weather - related factor) are not the primary and consistent way rivers become larger as they travel across land compared to tributary confluence.

Answer:

Tributaries combine, adding water to rivers.