To prevent frost penetration, the till core was covered with a layer of gravel. Over the years, however, techniques for insulating till cores during winter were perfected, and artificial snow was also used to cover the till. Second, the insulation doesn't have to be removed . . . Check the middle section of the dam. That's where the La Grande Rivi re used to flow.
Today there is a valley on one side and the reservoir on the other.
The river returns to its natural bed several kilometres downstream, at the outlet of the tailrace tunnels. Sealing the dam Dikes and dams have to be watertight. They also have to be able to withstand the enormous pressure of the water in the reservoir. The goal, when building a large dam, is to get the best possible head of water with the fewest possible structures. This is how costs are kept down.