As the Corps of Discovery entered the uncharted, they had to map the geographical features of the area they explored. While they traveled, they needed to name as many rivers, streams and mountains as possible in order to form a point of reference for the maps. At first it was likely easy, if not fun. Every member of the expedition—including Seaman, Lewis's dog—had at least one geographical feature named after him.

There’s the Shields River, located near Livingston, which is named after Private John Shields, the Corps’ blacksmith and gunsmith. Little Pompey, Sacagawea’s infant son, has a looming tower near present-day Billings named after him: Pompeys Pillar.

Pompeys Pillar (Click for a larger image)

Sticking with people they knew, Lewis and Clark decided to honor America’s governmental leaders when they named the three rivers that form to create the Missouri. They named these rivers the Jefferson, Gallatin and Madison. The Judith River was named by Clark after his sweetheart back home, whom he married when the expedition was over. Lewis named a river after Clark and Clark named a river after Lewis, but neither Lewis's River nor Clark’s River retained the name the explorers gave them. They are now the Salmon River and the Bitterroot River, respectively.

Lewis and Clark often often relied on geographical descriptions for names. Montana’s Milk River was named by Lewis, who observed that it looked like “tea that has milk in it.” Another creek that was just a trickle of water was named Teapot Creek, because it only contained enough water to fill a teapot. One of Montana’s most prominent geographic features is Square Butte (near Great Falls); Lewis and Clark referred to it as "Fort Mountain" on their maps. Clark dreamed he named a creek Roloje, so he did.

And of course, Great Falls is another of Lewis's names that stuck for obvious reasons. But the equally informal names he gave each of the lesser falls at the Great Falls were scrapped by later cartographers. What Lewis called the “Beautiful Cascade” was later named Rainbow Falls, and Lewis's “Cascade of about 14 feet” was officially named Colter Falls (likely after John Colter, a member of the party). The “Upper Pitch” is now called Black Eagle Falls.

Lewis named the Gates of the Rocky Mountains when they were paddling up the Missouri near present-day Helena. On both sides of the river are steep cliffs, and to the explorers it must have felt as though they had found a passage through the Rocky Mountains— especially since the river bends there and creates an optical illusion, looking as if the river dead-ends into a cliff.

They also named geographical features after their experiences. In the Bitterroot Mountains in the fall of 1805, the Corps was forced to kill and eat three colts to survive. The creek where they were camped became “Colt-killed Creek,” which has since been renamed Whitesand Creek. Later, when food was even more scarce, they named another creek Hungry Creek. The name of that creek has remained unchanged.

But the naming didn’t end with the Corps of Discovery returning to St. Louis in 1806. Lewis and Clark County in Montana is named after the famous explorers, and Lake Sacagawea in North Dakota is named after the Shoshone woman who accompanied the expedition.