Final answer:
Mark Twain's memoir that begins with Hernando DeSoto and his first sightings of the river is 'Life on the Mississippi.'
Step-by-step explanation:
The Mark Twain memoir that begins with Hernando DeSoto and his first sightings of the river is Life on the Mississippi. This memoir is a personal account of Twain's years as a steamboat pilot on the Mississippi River. It explores the narrator's experiences and observations as he learns to pilot a steamboat and delves into the culture and history of the region