Almost every day hunting parties left the camp and brought back buffaloes. The weather grew very cold in December, and several times the thermometer fell to forty degrees below zero. As spring advanced, however, the weather became very mild, and as early as April 7, 1805, they were able to leave their camp at Fort Manden and start on again. The upper Missouri they found was too shallow for the large barge they had used the previous summer, so this was now sent back down the river in charge of a party of ten men who carried letters and specimens, while the others embarked in six canoes and two large open boats that they had built during the winter. So far the country through which they had passed had been explored by a few Hudson's Bay trappers, but as they now turned westward they came into a region entirely unknown, which they soon found was almost uninhabited. The party had by this time three interpreters, one a Canadian half-breed named Drewyer, who had inherited from his mother the Indian's skill in woodcraft, and who also knew the language of the white explorers. The other two were a man named Chaboneau and his wife, a young squaw called Sacajawea, the "Bird-woman," who had originally belonged to the Snake tribe, but who had been captured in her childhood by Blackfeet Indians. This Indian girl had married Chaboneau, a French wanderer, who like many others of his kind had sunk into an almost savage state. As the squaw had not forgotten the language of her native people the two white leaders thought she would prove a valuable help to them in the wild country westward, and persuaded her and her husband to go on with them. As the weather was fine the party traveled rapidly, and by April 26th reached the mouth of the Yellowstone. They were now very far north, near the northwest corner of what is the state of North Dakota. Game was still plentiful but the banks of the river were covered with a coating of alkali salts, which made the water of the streams bitter and unpleasant for drinking. Occasionally they came upon a deserted Indian camp, but in this northern territory they found few roving tribes. When there was a favorable wind they sailed along the Missouri, but most of the time they had to use their oars. Early in May they drew up their birch canoes for the night at the mouth of a stream where they found a large number of porcupines feeding on young willow trees. Captain Lewis christened the stream Porcupine River. Here there were quantities of game, and elk and buffalo in abundance, so that it was an easy matter to provide food for all the party.