The United States government has a unique legal relationship with Indian tribes, defined by treaties, statutes, executive orders, court decisions and the U.S. Constitution itself. The U.S. Army Corps of Engineers interacts with tribes on a government-to-government level within this framework.
Indian tribes are considered independent nations according to U.S. law and court decisions and must be dealt with by the U.S. government, including the Corps of Engineers, in the same way we would deal with any other nation.
The U.S. entered into treaties with the original citizens of the Columbia River Basin in 1854 and 1855. During treaty negotiations, the tribes reserved certain rights for themselves, particularly regarding fishing. The Corps of Engineers has a responsibility to uphold those rights and treaty provisions.