Imagine this basin full of water (those railroad tracks were not there). This was Lake Conemaugh. At the time of the flood, it held approximately 20 million TONS of water. The lake was surrounded by a large clubhouse and lake houses owned by members of the South Fork Fishing and Hunting Club, a summertime haven for wealthy Pennsylvania executives and entrepreneurs, including Andrew Carnegie, Henry Clay Frick, and Andrew W. Mellon.
For a detailed account of the flood and its aftermath, I highly recommend reading David McCullough's book, The Johnstown Flood. I'll leave you with one more neat image I found on Wikimedia Commons. It is an 1889 stereoscope of the dam site after it collapsed (looking upstream).