Another unexpected fork in the course of the Missouri posed a problem for the Corps of Discovery. On June 2, 1805, Lewis and Clark had their first encounter with the Marias River near present-day Loma. The instructions they had gotten from the Mandan tribe in North Dakota made no mention of the river. The waterway was especially confusing because of its size. Its wide, powerful nature in high water made it seem as likely a main river as the Missouri arm.
The expedition camped near present day Fort Benton on June 4, 1805, and spent nine anxious days trying to decide which fork was the Missouri. The wrong route could have cost them weeks. Lewis climbed to what is now called Decision Point Overlook for a better view of the surrounding land, in hopes of finding the right route. Lewis finally reasoned that the thick, dark waters of the Marias traveled too much to the north to be the Missouri. He felt that the clearer waters of the westward river must move through the high mountains. In the end, it was exploration that decided the matter, and Lewis was proved right. Finally, Lewis took the correct southern fork to the Great Falls of the Missouri, while the rest explored the northern (the Marias).
From their encounter with the Marias, near present-day Loma, to the exploration that sent them up the correct southern fork, the expedition spent the nine days only 25 miles from the first of the five "Great Falls." Expedition journals identified this area as "a judicious position for the purpose of trade."
Lewis cached supplies at the mouth of the Marias for later use. His 1806 return trip took him on an extended exploration of the Marias to the northernmost point reached by the expedition. Jefferson had requested that the expedition look for a natural boundary for a land treaty with the British. Lewis hoped the Marias would end near the 49th Parallel, but his journey ended at Camp Disappointment. Lewis named it Maria's River for his cousin, Maria Wood, but it changed over time to Marias.