The 12-foot-long thresher shark was probably caught in the ocean and then dumped in the river.
"You can tell by that long tail that is has some ferocious way to thrash things around a little bit, so I wouldn't want to be hit by it or attacked by it, for sure," said Les Holcomb with state Department of Fish and Wildlife.
Dr. Dayv Lowry is the state's shark expert with the Department of Fish and Wildlife. Despite the shark's intimidating size, Lowry said thresher sharks only eat small fish and definitely don't eat people.
"People are a little big and a little outside their purview," he said.
Lowry's job is to study marine life, and he said threshers aren't uncommon off the Washington coast.
It's perfectly legal to catch them, though it's not easy to find them. Commercial boats harvest nearly 200 cubic tons of the shark each year for eating.
Fish and Wildlife officials say they'll conduct an investigation to try to determine exactly how the shark ended up at the boat launch.