A series of small explosions will mark the beginning of a very big demolition project.

Today's blasts will be across the top level of Marmot Dam, on the Sandy River. In quick succession, 68 blasts will fracture the dam's concrete. It won't destroy the dam completely, though. The plan is to crack and crumble the top part of the dam so that workers can take it apart, piece by piece.

After all that rubble is cleared away, workers will plant more, and deeper, charges to take another layer out.

All this activity is expected to send more than a million tons of sediment downstream. All that sediment is expected to smother salmon, steelhead and trout eggs, wreak havoc on fish habitat, and cause other problems. It will also allow hatchery-bred fish to swim upstream and, for the first time, to inter-breed with wild fish.

However, in time, the theory is that wild salmon and steelhead, which have been blocked by the Marmot and by the nearby Little Sandy Dam - which will also be removed - will have access to miles of upriver spawning and living areas.

Some fishermen, boaters and other lake users are upset about losing Roslyn Lake, a popular recreation facility. Some had proposed building fish ladders and taking other measures to keep the dams in place, while still helping the anadromous fish, which often live most of their lives in the ocean, but which return to the rivers of their birth to spawn. But Portland General Electric, which operates the dams, said that would be too expensive, so they'd have to go. The Marmot Dam demolition will be completed in September. It will be the biggest dam removal project ever undertaken in Oregon.

Environmentalists say the benefits will far outweigh the losses - especially since PGE will donate 1,500 acres of land in the Sandy River Basin to the Western Rivers Conservancy. It will eventually become part of a planned 9,000-acre natural resource and recreation area, to be operated by the U.S. Bureau of Land Management. The company will also give its water rights to the public.

At one time, Marmot Dam provided electricity to 12,000 Oregon homes and businesses.