Final answer:
Water flowing down a hill into a river during a thunderstorm is known as runoff, which is surface water moving over the land.
Step-by-step explanation:
During a thunderstorm, water that flows down a hill into a river is best described as runoff. Runoff is water from precipitation that flows over the surface of the ground and does not soak into the ground to become groundwater.
It often collects in streams, rivers, and eventually makes its way into larger bodies of water, such as lakes or oceans. This process of runoff also includes water from melted snow. Surface runoff can affect river discharge, which describes the volume of water moving through a river channel over time.