Why did Lewis and Clark use square rigged boats?
Meriwether Lewis had boats designed for his expedition that could fundamentally be used in all sorts of weather and topographical situations that would probably be encountered.
Lewis and Clark's keelboat was built as a galley in Pittsburgh in 1803 for the Lewis and Clark Expedition, after detailed specifications by Meriwether Lewis. A keelboat, it could be propelled by oars, sails, poles and towlines. The boat was the expedition's main vessel until the spring of 1805, when it was returned to Saint Louis.
In May 1803, Lewis ordered that a riverboat, built to defined specifications, be built at Pittsburgh for the expedition. When he arrived at Pittsburgh, he found that the boat builder had just begun building the boat and that it would take six weeks to finish it. This worried Lewis, who wanted to get underway in the vessel before the summer drought lowered the water level of the Ohio River, and to reach the Mississippi River before it froze in the fall. - Lewis and Clark's keelboat
Good biographies about the Lewis and Clark Expedition demonstrate that the boats employed in the expedition did in fact use many different methods to travel the various rivers encountered. They actually employed all methods mentioned in the Wikipedia article mentioned above. The keelboat was actually propelled by oars, sails, poles or towlines, at different moments due to unforeseen conditions and situations.
The square rigged thus meant that they would be able to constantly make headway in many manners of situations that arose. Buoyancy was more an issue for the expedition than speed. At times the water levels were quite low and poles were employed at the same time the sails were up!
Lewis had designed the keelboat; he supervised its construction, and probably made changes and additions during the building period. The boat was basically a galley, a vessel not like any other found west of the Appalachians, although of a standard type used on inland waters in the east. It was 55 feet long, with an eight feet beam, and with a shallow draft. The mast was 32 feet high and could be lowered. The boat could carry a headsail and a square rigged sail. A ten feet long deck at the bow made a forecastle. A raised aftercastle of the same length contained a cabin. The hold had a capacity of 12 tons. Eleven benches for the oarsmen crossed the deck in front of the aftercastle.
The keelboat could be propelled by oars, sails, poles and towlines. When using the setting poles, the crew put the poles in the bottom of the river and pushed off while walking from the bow to the stern. Towlines were used by men, horses, or oxen pulling the boat.
This memorial nickel shows the use of setting poles on Lewis & Clark's Keelboat.