Sitting high above the Colorado River, the Nankoweap Granaries may be the best-known archaeological site within the Grand Canyon, stopped at by nearly every commercial river trip. But a new study by the U.S. Geological Survey (USGS) has found that hundreds of other archaeological sites up and down the Colorado River, some thousands of years old, have been degraded by nearly half a century of operation of the Glen Canyon Dam. In a paper published this spring, researchers found that 68% of archaeological sites along the river have been impacted by increased erosion as a result of dam operations. That’s up from 2000, when surveys showed only 56% of sites had such impacts. The study comes after researchers looked at 50 years of aerial photography over sites and data collected over 30 years during site visits and surveys.