The USGS Water Science School
Rainfall hits the land and starts to runoff downhill, usually into rivers (top left of photo). Rivers flow on until they empty into the oceans, often carrying lots of dirt with them, as this satellite picture shows.
Here, the Rio de la Plata estuary is saturated with sediment at the confluence of the Parana River, Argentina, which enters the river basin from the left, and the Uruguay River, seen coming straight down from top-center. Such large amounts of sediment can often be seen in the river basin, and provide nutrients to the phytoplankton blooms that are regularly observed off the coast of Argentina.
Credit: NASA Visible Earth.