what documentary evidence would make it.

"To complete his survey across the continent, on the line of travel
between the State of Missouri and the tide-water region of the
Columbia, was Fremont's object in this expedition; and it was all that
he had obtained orders for doing; but only a small part, and to his
mind an insignificant part, of what he proposed doing. People had been
to the mouth of the Columbia before, and his ambition was not limited
to making tracks where others had made them before him. There was a
vast region beyond the Rocky Mountains--the whole western slope of our
continent--of which but little was known; and of that little, nothing
with the accuracy of science. All that vast region, more than seven
hundred miles square--equal to a great kingdom in Europe--was an
unknown land--a sealed book, which he longed to open, and to read.
Leaving the frontier of Missouri in May, 1843, and often diverging
from his route for the sake of expanding his field of observation,
he had arrived in the tide-water region of Columbia in the month of
November; and had then completed the whole service which his orders
embraced. He might then have returned upon his tracks, or been brought
home by sea, or hunted the most pleasant path for getting back; and if
he had been a routine officer, satisfied with fulfilling an order, he
would have done so. Not so the young explorer, who held his diploma
from nature, and not from the United States Military Academy. He was
at Fort Vancouver, guest of the hospitable Dr. McLaughlin, Governor of
the British Hudson Bay Fur Company; and obtained from him all possible
information upon his intended line of return--faithfully given, but
which proved to be disastrously erroneous in its leading and
governing feature. A southeast route to cross the great unknown region
diagonally through its heart (making a line from the Lower Columbia to
the Upper Colorado of the Gulf of California), was his line of return;