When water levels from dams began to eat away at the shoreline, his burial site which must have been immed on the Columbia's then north bank, would have eroded out...but probably he was within a matter of hundreds of feet, or yards, from where he was buried...
One reason so few Amerindian remains from very long ago are found, may be that they did not often go to the trouble to bury their dead.
But: if there was nothing there, why did govt go to such great expense and folly to pour all this concrete? They must have feared there was much there indeed, maybe even a whole cemetery or burial ground of KM-type caucasoids.