Seeing Leon Russell’s name gracing a marquee some time ago, I facetiously commented to my younger companion, “I didn’t know Leon Russell was still alive.” “Who’s Leon Russell?” she countered. It’s an enigma how Russell, who bestrode the ’70s music scene like a colossus, faded so completely. The Tulsa piano prodigy with the endearingly slurry drawl has always been a polymath. As musical director, he launched Joe Cocker’s career with the legendary Mad Dogs and Englishmen tour, and was de facto majordomo for George Harrison’s Concert for Bangladesh band. Solo, Russell broke new ground with his swampy fusion of rock, delta soul, rustic country and R&B, in the process creating the template for modern genre-jumping pop. His Americana album from 1978 presaged an entire musical genre, and he has influenced artists as varied as The Black Keys and former protégé Elton John. The Union, John’s 2010 collaboration with Russell, launched his commercial rehabilitation and criminally belated induction into the Rock and Roll Hall of Fame. Still, titles like “legend” and “icon” don’t do the man justice. Rediscovering Russell is like finding the headwaters of the Nile.