Sometime during the past two millennia, the expression "cum grano salis" has come to mean that one accepts an observation or a statement, if not with dread of a sudden accident, then at least with a mite of reservation or skepticism. You may, of course, choose to take that explanation with . . . a grain of salt.
Although "true believers" may take umbrage at the suggestion, there are in fact a number of "facts" in the journals of the Lewis and Clark expedition that may justifiably be taken with a grain of salt. For example, Clark calculated (August 3, 1806) that the distance from today's Livingston, Montana, to the mouth of the Yellowstone River was 837 miles. Today, measured by the Talweg, or deepest part of its channel, it measures only 438 miles between those two points, and it is the only one of the rivers the expedition measured that seems to have changed its course or length little or none in the past 200 years.