When the sluices are closed, the reservoir fills apparently overnight. It would take many months to fill the reservoir behind a dam that large.
Whenever the dam builders are looking at Campbell's Kingdom, they look distinctly upwards from the top of the dam. You can't use a dam to flood land which lies higher than the top of the dam.
In an early scene, the truck driver sits on the right side of the vehicle as in England, not the left as in Canada. The movie was filmed in Italy, not in Canada.
When Morgan reaches the landslide and discovers it was caused by dynamite he and his lt clamber over it to a jeep brought down by his men and head up the mountain but the road was too narrow for them to have turned it round as they did.