https://phys.org/news/2018-07-climate-river-basins.html

The difference, they found, boils down to the local availability of groundwater. In general, river basins are shaped by rainfall, which erodes the land as it drains down into a river or stream. In humid environments, a large fraction of rainfall seeps into the Earth, creating a water table, or a local reservoir of groundwater. When that groundwater seeps back out, it can also cut into a basin, further eroding and shifting its shape.

The researchers found that smaller basins that are formed in humid climates are heavily shaped by the local groundwater, which acts to carve out shorter, wider basins. For much larger basins that cover a more expansive geographic area, the availability of groundwater may be less consistent, and therefore plays less of a role in a basin's shape.