In rec.boats.paddle Steve wrote:

I'm looking for the narrowest, deepest river canyons in Northern California.

What would be ideal is a 200 foot deep canyon that is only 200 feet
wide at the top. So this may exist only in the true slot canyons made
of sandstone in the desert. But I'm in NorCal so I'm looking for as
close to that as I can get around here.

Mill Creek would probably qualify as the canyon with narrowest top.
In some places you can jump across. But it's not very deep.

Balloon Dome Gorge on the Middle Fork San Joachin river might be
the deepest and steepest combo. I haven't been there, but the
Holbek/Stanley guidebook says it's like Yosemite with the walls
a river's width apart.

Hells Canyon (between Oregon and Idaho) is supposedly the deepest
in the 48 states, but it doesn't look like much from river level,
which shows how specious these comparisons can be.