Answer:
The Columbia River Plateau extends across the US states of Washington, Oregon, and Idaho.
From the late Cenozoic to the early Pliocene, a large eruption occurred in this area. One of the largest flood basalts ever blown over the earth has covered about 160,000 km2 in the Pacific Northwest. The lava formed huge igneous rocks. Over a period of 10 to 15 million years, the lava flow covered the chilled and hardened magma, and it was repeatedly cooled and solidified. Adding that thickness, the lava became 1.8 km thick. As the magma emerged on the earth's surface, the crust gradually sank into the space lost by the springing lava. The crust subsidence created a large and slightly depressed lava plain.
The location is now known as the Columbia River Basin or the Columbia River Plateau.
The ancient Columbia River was pushed into its current course by lava rushing northwest. Part of the lava also flowed into the Columbia River basin, and the lava dammed the canyon, built a dam, and finally created a dam lake. In these ancient lake bottoms there can be found foliage impression fossils, petrified trees, insect fossils, vertebrate bones and more.
