When did Lewis and Clark arrive back at St Louis?

In spring 1805, they continued to the headwaters of the Missouri River, struggled across the Continental Divide at Lemhi Pass, and headed west along the Salmon, Clearwater, Snake, and Columbia Rivers to the Pacific. They landed at the mouth of the Columbia River, Astoria, Oregon on December 3rd 1805.

The explorers began their journey home on March 23, 1806. On July 3, after crossing the Continental Divide, the Corps split into two teams so Lewis could explore the Marias River. Lewis and Clark stayed separated until they reached the confluence of the Yellowstone and Missouri Rivers on August 11. Once reunited, the Corps was able to return home quickly via the Missouri River. They reached St. Louis on September 23, 1806.