After almost 3000 miles of meandering travel the expedition reached Beaverhead Rock in August of 1805. Sacagawea recognized this rock and knew that her people regularly frequented this valley and that their home was on the other side of the mountains in the background.
Lewis crossed the Divide at today's Lemhi Pass which is just west of Grant, Montana and made contact with the Shoshone. A party of Indians traveled back with him to Camp Fortunate where they agreed to trade the men horses.
The location of Camp Fortunate is under the water of a reservoir today. The reason they know where it was is that the island in the photo is actually a small mountain which Lewis described in great detail and at the foot of which they camped.
It certainly makes for a pretty picture if nothing else.