In 1804, the youngest member of the Lewis and Clark expedition got lost in the Great Plains for 16 days. Today, poet Campbell McGrath gives voice to the young man and his imagined adventures in a new epic, “Shannon: A Poem of the Lewis and Clark Expedition.” Host Steve Curwood talks with McGrath about his hero’s triumphs and losses as he wanders the prairie alone, among buffalo herds and under the big Western sky.