Where the river meets the sea.
Here we can see the river water from Copper River in Alaska meet the Ocean. The river water is rich in sediment and has a lighter blue to it than the dark blue of the sea. The water does eventually mix. Sometimes this phenomenon can be seen from space. This photo is sometimes inaccurately labelled as where two oceans meet.
Source: Alaska Dispatch News
These river images make it clear and easy to understand what the different features are and they offer a powerful illustration of the way that rivers shape our land.