Captain Meriwether Lewis was a worried man as the terrible winter of 1804-05 approached, but not because of the weather. Even though the Corps of Discovery had made good progress in reaching North Dakota, he was annoyed by a problem that manifested itself every time the group met a tribe of Native Americans there. Sign language and George Drouillard's limited ability to speak Indian tongues enabled Corps members to communicate with indigenous people such as the Teton and Yankton Sioux, Missouris, Omahas, Otoes and Poncas downstream. Farther upriver, however, the explorers came to the land of the Amahamis, Mandans, Arikara (or Ree) and Hidatsa (also known as Minitari or Gros Ventres, which is French for "Big Bellies"). The language of these tribes was more difficult for the company to understand. Conversely, the explorers had difficulty making themselves understood.