Dennis was last seen on a canoe trip with his roommate. They started in Torrington, Wyoming, and planned to canoe down the North Platte that weekend back to Scottsbluff, Nebraska. At around 7 p.m. on May 19, 1973, their canoe overturned when it went over an irrigation diversion dam, about one mile west of Henry, Nebraska. Dennis' roommate was able to make it to shore but when he couldn't find Dennis he went to a nearby farmhouse and notified authorities. Dennis' life jacket was later discovered down the river, but Dennis' body has never been found.