You are not allowed to enter by car the Zion Canyon Scenic Drive. A bus can get you up to this point to start your walks of the Riverside and the Narrows.
At this point, at the temple of Sinawava, the river valley narrows abruptly. More north, the river didn't cut yet through a hard layer to reach the soft one, so it could undercut the cliffs and widen the valley. South of the temple of Sinawava, the river did reach the softer Kayenta formation, and could undercut the harder Navajo sandstone. After undercutting, slabs of hard sandstone fall down and the widening of the canyon is a fact. You must have noticed the wider valley floor from Springdale to this point.