Sockdolager means a heavy, decisive blow, one that can knock someone down. The word isn’t used today but was common in reference to boxing matches. For example, take this article from a boxing match from 1869: Allen fought cautiously, evidently husbanding his strength. McCoole got in heavily on Allen’s ribs and latter went down to avoid another sockdolager. Source.
On Powell’s second expedition, the crew coined this rapid Sockdolager.
How to run Sockdolager
Sockdolager has big holes and waves throughout the rapid. The general line is to start center and hit everything straight. There are large holes and waves that flank the sides of the rapid, both on river right and left.