The Corps members were almost in constant sight of cottonwoods and/or willows all along the Missouri River to Fort Mandan. The excerpts below describe the cottonwood's growth characteristics and how it was used in construction of boats and their winter quarters at Fort Mandan.
Capt. Lewis, May 25, 1804 - N° 4. Was taken at a small Village North side of the Missouri called Sharetton (ed. La Charrette) on the 25th of May 1804. this is the last settlement on the Missouri; and consists of ten or twelve families mostly hunters. this specimine is the seed of the Cottonwood which is so abundant in this country, it has now arrived at maturity and the wind when blowing strong drives it through the air to a great distance being supported by a parrishoot of this cottonlike substance which gives the name to the tree in some seasons it is so abundant as to be troublesome to the traveler. this tree arrives at great sise, grows extreemly quick the wood is of a white colour, soft spungey and light, perogues are most usually made of these trees, the wood is not durable nor do I know any other valuable purpose which it can answer except that just mentioned. this tree forms a great majority of the timber bordering the rivers Missouri and Mississippi; it extends itself throughout the extensive bottom lands of these streams and seases to appear when the land rises into hills when these rivers form new lands on their borders or Islands in their st[r]eams, which they are per[pe]tually doing, the sweet willow is the first tree or shrub which usually makes it's appearance, this continues one two or three years and is then supplanted by the Cottonwood which invariably succeedes it. this tree resembles much in it's air and appearance that beatifull and celibrated tree the Lombardy poplar; and more particularly so when in its young state, the young plants grow very close untill they have attained the age of four or five years, a proportion of them then begin to dye and the forrest opens and gives place to sundry other shrubs and plants which will be noticed in their proper places.
Sgt. Patrick Gass, November 28, 1804 - About the 16th, the weather became very cold, ... On the 19th, the hunters came up with the periogue, loaded with the meat of about thirty deer, eleven elk, and some buffaloe. In the cold weather we moved into the huts, though not finished. From the 20th to the 27th we had fine pleasant weather, and on the evening of the latter, finished the roofs of our huts. These were made of puncheons split out of cotton-wood and then hewed. The cotton-wood resembles the Lombardy poplar, and is a light soft wood. The largest trees are in thickness about eighteen inches in diameter. On the night of the 27th the snow fell seven inches deep, and the 28th was stormy.