A survey by B.C. Environment Minister Barry Penner of the damage caused by a massive landslide near Chilliwack has found that a powerful wave washed out thousands of trees on the shoreline of Chehalis Lake.

About 600 vertical metres of rock and trees slid down a mountainside, washed into Chehalis Lake and triggered a giant wave.

Penner said it looked like heavy rain and snow that fell last weekend triggered Monday's rockslide on one of the surrounding mountains.

He said it appeared 600 vertical metres of rock and trees slid down the mountainside, washed into the lake and triggered a giant wave.

"Unbelievable," Penner said after his visit to the area on Friday. "It snapped tall standing cedar trees. Full, mature, strong cedar trees just got uprooted and thrown around like matchsticks."

The wave, which officials estimated was several metres high, demolished trees several metres above the normal waterline, leaving only bare rock ringing the lake.

"Very impressive in terms of the power of nature — beautiful and terrifying at the same time," Penner said. 

A number of campsites along the lake were badly damaged by the wave, but nobody was in the area at the time.

Officials are concerned that the thousands of trees washed into the lake could dam up a connecting stream and threaten to cause flooding in a nearby First Nations reserve, Penner said.

Officials are also concerned about local fish stocks, Penner said.