The victim's name has not been released, but police say he is a 48-year-old man who was on a sport fishing trip in Bute Inlet, a remote area approximately 100 km northeast of Campbell River.
Officials with the Transportation Safety Board say the crash likely happened because the helicopter was flying low.
"It was operating at low level. These operations are just moving fisherman from one pool on the water to another," said TSB spokesman Bill Yearwood.
The pilot and the other survivors are being interviewed by TSB officials today, Yearwood said.
Meanwhile, RCMP dive teams are attempting to recover the victim's body from the submerged wreckage.
The helicopter appeared to go down and overturn in the Homathco River around 4:30 p.m. PT on Saturday at the north end of Bute Inlet.
Five of the six on board, including the pilot — a 68-year-old man from Gold River, B.C. — escaped with non-life-threatening injuries.
Also on board were three other men aged 37, 45 and 71, all U.S. citizens.
The fifth passenger was a 44-year-old man from Gold River, a small community on the west coast of Vancouver Island.
E and B Helicopters, the company that operated the chopper, was able to rescue the five survivors, said Capt. Stu Robertson of the Joint Rescue Coordination Centre in Victoria.
"The call came from the company. When they came on scene, they found five people on the side of the river [but] they were missing one," Robertson said.
"We sent a Cormorant helicopter from 442 Squadron up. They were going to look for the sixth person, but quickly found the person deceased in the fuselage."
All were initially transported to Campbell River hospital for examination. Four have since been released, while the fifth person, a passenger, remains in a Victoria area hospital undergoing further treatment.
The B.C. Coroners Service and the Transportation Safety Board are investigating.