September 1, 1803

Morning Fog

The Pilott informed me that we were not far from a ripple which was much worse than any we had yet passed, and as there was so thick a fogg on the face of the water that no object was visible 40 paces he advised remaining untill the sun should acquire a greater altitude when the fogg would asscend and disappear; I conscented; we remained untill eight Oclock this morning when we again set out—
—Meriwether Lewis

Reflections on Fog

the Fog appears to owe it’s orrigin to the difference of temperature between the air and water the latter at this seson being much warmer than the former; the water being heated by the summer’s sun dose not undergo so rapid a change from the absence of the sun as the air dose consiquently when the air becomes most cool which is about sunrise the fogg is thickest and appears to rise from the face of the water like the steem from boiling water—
—Meriwether Lewis

Lewis would later confirm his hypothesis by comparing the water temperature with the air temperature.

Horsetail Ripple and Woollery’s Trap

we passed the little horsetale ripple or riffle with much deficulty, all hands laboured in the water about two hours before we effected a passage; the next obstruction we met was the big-horse tale riffle, here we wer obliged to unload all our goods and lift the emty Boat over, about 5 OCock we reach the riffle called Woollery’s trap, here after unloading again and exerting all our force we found it impracticable to get over, I therefore employed a man with a team of oxen with the assistance of which we at length got off we put in and remained all night having made only ten miles this day.—
—Meriwether Lewis