A shocking amount of Columbia River mercury (rapidly rising in the Columbia) is from gold mines in Idaho and Nevada that basically pulverize and then heat-vaporize tons of earth to get gold out of it.  Mercury, which tends to be plentiful where there's gold, is thus put into the river systems in two ways, in the tailings and also, more insidiously, through the air as the mercury is carried off as a vapor, later to condense on things, which is then washed into the rivers.  The Snake is becoming heavily mercury laden and dumps that load into the Columbia.

So, while Boardman and the cement plant are huge sources, these mines are a surprise to most people.

The 5% Project Let's live on the planet as if we intend to stay.