At around 11 o’clock that morning Lewis departed from Pittsburgh, Pennsylvania with a a party of eleven men. They arrived shortly after at Bruno’s Island which was a mere 3 miles from Pittsburgh. Once ashore Lewis demonstrated his airgun to the men, while citizens watched. However, one citizen accidentally caused the gun to discharge, sending the bullet through a woman’s hat where it brimmed across her temple. The woman was assumed dead immediately, only to be revived moments later. Afterwards they proceeded to McKees Rock, where the water had fallen so low that they were forced to raise the boat for 30 yards. —Lewis & Clark Expedition Left Pittsburgh 31 Aug 2003 (LewisAndClark1803.com)
Since Lewis didn’t meet up with Clark until later (in their famous but unsuccessful search for a water passage through what is now the northwestern United States), most historians don’t start their account of the expedition with Lewis’s departure from Pittsburgh.
Link goes to a summary of the diary kept during the expedition. The photo of the model is from a different site, lewis-clark.org.