Person, Place or Thing?

I’m a celebrated tourist attraction Geologist Major John Wesley Powell explored me in 1869 I’ve got a river running through me I’m in America I’m popular with families I’m one of Earth’s great wonders I’m called “The Spectacular Valley” I became a national park in 1919 You can ride a mule to get to the bottom of me You’ll find me in Arizona

Can you guess what I am?