Aluminium refining requires huge amounts of electricity so plants are often sited close to things like hydroelectric dams or abundant supplies of coal or natural gas. Bauxite (aluminium ore) needs to be refined by electrolysis so it is very good way to make use of surplus off-peak high output electrical power and the costs of shipping the ore to the plant are trivial compared to the energy requirements to refine it.
Aluminium smelting is fairly unusual in that it specifically requires electricity rather than generic fuel such as coke and this is the main reason why it was so difficult and expensive to produce before the advent of high capacity electrical power generation.
This article suggests that Washington state does indeed have abundant hydro-electric power.