Besides the frigid weather, they have to use bush planes that are capable of landing on gravel bars and inflatable boats to access the sites, and often have to repel down the side of a cliff to do the digging. (sciencedaily.com)
Gigantic gravel bars deposited within dry valleys implied deep, fast - flowing water. (discovermagazine.com)
As the fish move upstream, they can mistakenly choose the wrong channel or get pushed by their brethren into shallow water near the river bank or above a submerged gravel bar. (nps.gov)