The first Alaskan North Slope dinosaur bones were discovered in the mid-1980s. They were from the "duck-billed" Edmontosaurus, which as fully-grown adults could reach 10 feet tall, 40 feet long, and weigh three tons. These plant-eaters are thought to have lived in social groups or even herds. The question of how they survived so far north initially raised two possibilities: either they stayed in the north and perhaps lived by slowing their metabolism (maybe even hibernating), or they migrated southward to continuous food supplies and warmer climates.
Newer discoveries along the Colville River have thrown doubt on the migration theory. Several of the newer dinosaur types, including Troodon and Dromaeosaurus (both smaller flesh-eaters) as well as juvenile hadrosaurs, probably could not have physically migrated a round-trip distance estimated at more than 5,000 miles (8,000 km). This distance would have been greater than that covered by today's caribou which migrate less than half distance within a 250-mile-wide area.
Instead, the North Slope dinosaurs may have survived year-round in ancient long-gone river systems which supported lush summer vegetation. Enough seasonal plant matter may have grown during the 24-hour sunlit summers to last during the cool-to-cold dark days of winter (though not as harsh as today's North Slope winters). The plant-eating dinosaurs, in turn, would have been the over-wintering food source of the meat-eaters.