"Though it's widely assumed that Thomas Jefferson sent Lewis and Clark to explore the land of the Louisiana Purchase, Jefferson had actually harbored plans to explore the West for years. The reasons for the Lewis and Clark Expedition were more complicated, but planning for the expedition actually began before the great land purchase had even happened."

Preparations for the expedition took a year, and the actual journey westward and back took roughly two years. This timeline provides some highlights of the legendary voyage. Collected by Robert McNamara

April 1803

Meriwether Lewis traveled to Lancaster, Pennsylvania, to meet with surveyor Andrew Ellicott, who taught him to use astronomical instruments to plot positions. During the planned expedition to the West, Lewis would use the sextant and other tools to chart his position.

May 1803

Lewis stayed in Philadelphia to study with Jefferson's friend, Dr. Benjamin Rush. The physician gave Lewis some instruction in medicine, and other experts taught him what they could about zoology, botany, and the natural sciences. The purpose was to prepare Lewis to make scientific observations while crossing the continent.

July 4, 1803

Jefferson officially gave Lewis his orders on the Fourth of July.

July 1803

At Harpers Ferry, Virginia (now West Virginia), Lewis visited the US Armory and obtained muskets and other supplies to use on the journey.

August 1803

Lewis had designed a 55-foot long keelboat which was constructed in western Pennsylvania. He took possession of the boat, and began a journey down the Ohio River.

October - November 1803

Lewis met up with his former U.S. Army colleague William Clark, whom he has recruited to share command of the expedition. They also met with other men who volunteered for the expedition, and began forming what would be known as "Corps of Discovery." One man on the expedition was not a volunteer: a slave named York who belonged to William Clark.

December 1803

Lewis and Clark decided to stay in the vicinity of St. Louis through the winter. They used the time stocking up on supplies.

1804:

In 1804 the Lewis and Clark Expedition got underway, setting out from St. Louis to travel up the Missouri River. The leaders of the expedition began keeping journals recording important events, so it's possible to account for their movements.

May 14, 1804

The voyage officially began when Clark led the men, in three boats, up the Missouri River to a French village. They waited for Meriwether Lewis, who caught up to them after attending some final business in St. Louis.

July 4, 1804

The Corps of Discovery celebrated Independence Day in the vicinity of present-day Atchison, Kansas. The small cannon on the keelboat was fired to mark the occasion, and a ration of whiskey was dispensed to the men.

August 2, 1804

Lewis and Clark held a meeting with Indian chiefs in present day Nebraska. They gave the Indians "peace medals" which had been struck at the direction of President Thomas Jefferson.

August 20, 1804

A member of the expedition, Sergeant Charles Floyd, became ill, probably with appendicitis. He died and was buried on a high bluff over the river in what is now Sioux City, Iowa. Remarkably, Sergeant Floyd would be the only member of the Corps of Discovery to die during the two-year expedition

August 30, 1804

In South Dakota a council was held with the Yankton Sioux. Peace medals were distributed to the Indians, who celebrated the appearance of the expedition.

September 24, 1804

Near present-day Pierre, South Dakota, Lewis and Clark met with the Lakota Sioux. The situation became tense but a dangerous confrontation was averted.

October 26, 1804

The Corps of Discovery reached a village of the Mandan Indians. The Mandans lived in lodges made of earth, and Lewis and Clark decided to stay near the friendly Indians throughout the oncoming winter.

November 1804

Work began on the winter camp. And two vitally important people joined the expedition, a French trapper named Toussaint Charbonneau and his wife Sacagawea, an Indian of the Shoshone tribe.

December 25, 1804

In the bitter cold of a South Dakota winter, the Corps of Discovery celebrated Christmas day. Alcoholic drinks were allowed, and rations of rum were served.

1805:

January 1, 1805

The Corps of Discovery celebrated New Year's Day by firing the cannon on the keelboat.

The journal of the expedition noted that "16 men danced for the amusement of the Indians, who enjoyed the performance immensely. The Mandans gave the dancers "several buffalo robes" and "quantities of corn" to show appreciation."

February 11, 1805

Sacagawea gave birth to a son, Jean-Baptiste Charbonneau.

April 1805

Packages were prepared to send back to President Thomas Jefferson with a small return party. The packages contained such items as a Mandan robe, a live prairie dog (which survived the trip to the east coast), animal pelts, and plant samples. This was the only time the expedition could send back any communication until its eventual return.

April 7, 1805

The small return party set off back down the river toward St. Louis. The remainder resumed the journey westward.

April 29, 1805

A member of the Corps of Discovery shot and killed a grizzly bear, which had chased him. The men would develop a respect and fear for grizzlies.

May 11, 1805

Meriwether Lewis, in his journal, described another encounter with a grizzly bear. He mentioned how the formidable bears were very difficult to kill.

May 26, 1805

Lewis saw the Rocky Mountains for the first time.

June 3, 1805

The men came to a fork in the Missouri River, and it was unclear which fork should be followed. A scouting party went out and determined that the south fork was the river and not a tributary. They judged correctly; the north fork is actually the Marias River.

June 17, 1805

The Great Falls of the Missouri River were encountered. The men could no longer proceed by boat, but had to "portage," carrying a boat across land. The travel at this point was extremely difficult.

July 4, 1805

The Corps of Discovery marked Independence Day by drinking the last of their alcohol. The men had been trying to assemble a collapsible boat which they'd brought from St. Louis. But in the following days they could not make it watertight and the boat was abandoned. They planned to construct canoes to continue the journey.

August 1805

Lewis intended to find the Shoshone Indians. He believed they had horses and hoped to barter for some.

August 12, 1805

Lewis reached the Lemhi Pass, in the Rocky Mountains. From the Continental Divide Lewis could look to the West, and he was greatly disappointed to see mountains stretching as far as he can see. He had been hoping to find a descending slope, and perhaps a river, that the men could take for an easy passage westward. It became clear that reaching the Pacific Ocean would be very difficult.

August 13, 1805

Lewis encountered the Shosone people.

The Corps of Discovery was split at this point, with Clark leading a larger group. When Clark did not arrive at a rendezvous point as planned, Lewis was worried, and sent search parties out for him. Finally Clark and the other men arrived, and the Corps of Discovery was united. The Shoshone rounded up horses for the men to use on their way westward.

September 1805

The Corps of Discovery encountered very rough terrain in the Rocky Mountains, and their passage was difficult. They finally emerged from the mountains and encountered Nez Perce Indians. The Nez Perce helped them build canoes, and they began to travel again by water.

October 1805

The expedition moved fairly quickly by canoe, and the Corps of Discovery entered the Columbia River.

November 1805

In his journal, Meriwether Lewis mentioned encountering Indians wearing sailor's jackets. The clothing, obviously obtained through trade with whites, meant they were getting close to the Pacific Ocean.

November 15, 1805

The expedition reached the Pacific Ocean. On November 16, Lewis mentioned in his journal that their camp is "in full view of the ocean."

December 1805

The Corps of Discovery settled into winter quarters in a place where they can hunt elk for food. In the journals of the expedition, there was much complaining about the constant rain and poor food. On Christmas Day the men celebrated as best they could, in what must have been miserable conditions.

1806:

As spring came, the Corps of Discovery made preparations to begin traveling back toward to the East, to the young nation they had left behind nearly two years earlier.

March 23, 1806: Canoes Into the Water

In late March the Corps of Discovery put its canoes into the Columbia River and began the journey eastward.

April 1806: Moving Eastward Quickly

The men traveled along in their canoes, occasionally having to "portage," or carry the canoes overland, when they came to difficult rapids. Despite the difficulties, they tended to move quickly, encountering friendly Indians along the way.

May 9, 1806: Reunion With the Nez Perce

The Corps of Discovery met up again with the Nez Perce Indians, who had kept the expedition's horses healthy and fed throughout the winter.

May 1806: Forced to Wait

The expedition was forced to stay among the Nez Perce for a few weeks while waiting for the snow to melt in the mountains ahead of them.

June 1806: Travel Resumed

The Corps of Discovery got underway again, setting off to cross the mountains. When they encountered snow that was 10 to 15 feet deep, they turned back. At the end of June, they once again set off to travel eastward, this time taking three Nez Perce guides along to help them navigate the mountains.

July 3, 1806: Splitting the Expedition

Having successfully crossed the mountains, Lewis and Clark decided to split the Corps of Discovery so they can accomplish more scouting and perhaps find other mountain passes. Lewis would follow the Missouri River, and Clark would follow the Yellowstone until it met up with the Missouri. The two groups would then reunite.

July 1806: Finding Ruined Scientific Samples

Lewis found a cache of material he had left previous year, and discovered that some of his scientific samples had been ruined by moisture.

July 15, 1806: Fighting a Grizzly

While exploring with a small party, Lewis was attacked by a grizzly bear. In a desperate encounter, fought it off by breaking his musket over the bear's head and then climbing a tree.

July 25, 1806: A Scientific Discovery

Clark, exploring separately from Lewis's party, found a dinosaur skeleton.

July 26, 1806: Escape From the Blackfeet

Lewis and his men met up with some Blackfeet warriors, and they all camped together. The Indians attempted to steal some rifles, and, in a confrontation that turned violent, one Indian was killed and another possibly wounded. Lewis rallied the men and had them travel quickly, covering nearly 100 miles by horseback as they fear retaliation from the Blackfeet.

August 12, 1806: The Expedition Reunites

Lewis and Clark reunited along the Missouri River, in present-day North Dakota.

August 17, 1806: Farewell to Sacagawea

At a Hidatsa Indian village, the expedition paid Charbonneau, the French trapper who had accompanied them for nearly two years, his wages of $500. Lewis and Clark said their goodbyes to Charbonneau, his wife Sacagawea, and her son, who had been born on the expedition a year and a half earlier.

August 30, 1806: Confrontation With the Sioux

The Corps of Discovery was confronted by a band of nearly 100 Sioux warriors. Clark communicated with them and told them the men will kill any Sioux who approaches their camp.

September 23, 1806: Celebration in St. Louis

The expedition arrived back at St. Louis. The townspeople stood on the riverbank and cheered their return.