Over the ages, the Colorado river has run down the canyon carrying tons of silt. This constant sandblasting has dug deep into the rock of the Colarado plateau. The river elevation is now 1 mile lower than the South rim.
Only people in very good physical condition should try to get to river level and back on a day hike. In winter, the day simply isn't long enough for most people. And in summer, the heat can be deadly if you overexert yourself.