A distributary.
Distributary
A backwater is a part of a river in which there is little or no current. It refers either to a branch of a main river which lies alongside it and then rejoins it or to a body of water in a main river which is backed up by an obstruction such as the tide or a dam.Source: Wikipedia
Distributary
the river that separates demerara from essequibo, is the boerasirie river (in guyana)
The river that separates Gaul and Italy is the Rubicon.
The Hudson River separates Manhattan from New Jersey.
There is no river - the Bering Strait separates Alaska and Russia. It simply separates North Amercia and Asia.
That would be the Columbia River. This river separates approximately two thirds of southern Washington from northern Oregon. At a place on the 46th parallel north this river total enters the state of Washington, and this parallel becomes the border line that separates the two states.
The Colorado River separates it.
The Hudson River.
The Red River!
The Mississippi river