In the spring of 1926, it began to rain in the Deep South, and it kept on raining, for months, in a relentless torrent that must have felt biblical. For The Great Flood, his gorgeous new found-footage documentary, filmmaker Bill Morrison has plumbed America’s newsreel archives to stitch together a visual collage of a flood that ultimately submerged 27,000 miles of land, displacing 1 million people. Here are sights few alive have seen: black sharecroppers toiling in the cotton fields of the early 1920s (though in their look these scenes could well be the slave years); African-American men being forced at gunpoint to shore up the levees; tents lining the coast for miles, the biggest campout in U.S. history. And there are the floodwaters themselves, turbulent, terrifying, insanely beautiful. All of this ancient footage is silent, of course, yet The Great Flood is an aural feast. Jazz guitarist/composer Bill Frisell has created a score steeped in the blues tradition of the region, which means that its inspiration comes from the faces onscreen — mournful yes, but also, amazingly, full of humor and joy. Destined for a long life in museums and history classes alike, this is cinema as art, and a classic.