Runoff.
A water cycle :]
Water leaves a watershed by natural surface run-off (streams and rivers) artificial drainage (ditches and tile drains) and by percolation into groundwater aquifers.
Runoff water
The network of rills, gullies, streams, and rivers in an area is often referred to as a watershed or river basin. This system of interconnected waterways ultimately drains into a common body of water, such as a lake or ocean.
The network of streams that drains an area of land is called a watershed or drainage basin. It encompasses all the land where precipitation collects and flows into a common outlet, such as a river, lake, or ocean. Watersheds are crucial for managing water resources, as they influence local ecosystems and water quality. Each watershed is defined by its topography and the flow of water through its streams and rivers.
Eutrophication?
It is usually the other way round. Moving in the direction of water flow, streams merge together to form larger rivers, the streams are called the river's tributaries. The point of joining is called a "confluence".
The water that flows in streams and rivers into the ocean and lakes is called runoff or freshwater.
All streams including rivers run downhill, so melting snow, rain, etc. produce water and this water drains into rivers and the rivers run downhill until they empty into a larger river or a major body of water like a gulf or ocean.
A watershed
After a flood, the water level drops as water recedes or evaporates. This process occurs as the excess water drains away through rivers and streams or soaks into the ground. The decrease in water level is a natural response to the balancing of water supply and demand.
I believe it is called Runnoff...