In summer, the Yukon River teems with life. Ducks and geese raise their young in quiet sloughs. Moose graze amid shoreline willows. Beavers splash along the muddy banks. Concealed by waters milky with glacial silt, hundreds of thousands of salmon surge upstream from the Bering Sea toward the river’s origins in northwestern Canada, bound for the streams where they were born, will lay their own eggs and will die.