I was curious about what the backup of the Columbia River would have looked like when the Bonneville Landslide at Bridge of the Gods blocked the Columbia River. In the DEM (digital elevation model) I set the blue color at 80 meters corresponding with the approximate top of the landslide deposit. The result is a long lake that extended well beyond the Columbia River Gorge and backed water up to approximately present day Umatilla, Oregon.
This assumes that the dam held up until it was nearly over topped. Lewis and Clark recognized the landslide when they traveled down the river having observed stumps from a previously drowned forest. They suggested the slide was 20 years old. The forest stumps were a well known feature of the lower Columbia River Gorge prior to the redrowning of the stumps when Bonneville Dam was built.
Pat Pringle gives a good overview of the slide complex with a discussion on the dates of the slide (bonneville_landslide_explorations.pdf). The slide is more likely about 550 years old based on carbon dates and tree rings.
(Randall, 2012) provides a more detailed characterization of the landslide complexes in this area.