columbia
The name of the fort near the mouth of the Columbia River is Fort Columbia. It is located in Washington state and was originally built to defend the entrance to the Columbia River from potential threats.
The Columbia River forms a major part of the border between the states of Oregon and Washington.
The proper name is Washougal, a US city on the Columbia River in Washington state.
The name given to soil deposited at the mouth of a river is called silt. The mouth of a river is called a delta.
The Columbia River separates Washington and Oregon. It runs from British Columbia, Canada, through Washington state, and forms part of the border between Washington and Oregon before emptying into the Pacific Ocean.
Delta
That would be the Columbia River. Back in the old days, the settlers decided on Columbia as the name of the new state that would be split off from the Oregon Territory. Someone pointed out that this would be easily confused with District of Columbia, so they named the new state, "Washington." The river was named after that first attempt at naming the state.
it is called the mouth of the river
That's the river mouth - or 'delta'.
Delta
Delta
the delta