Answer:

Tower Bridge, whose colorful supports rise steeply above a winding river that is always churning, is a rewarding place.

Commentary:

The relative clause "whose colorful supports rise steeply above a winding river that is always churning" refers back to Tower Bridge, a well-specified noun (there is only one Tower Bridge—in Sacramento at least). Therefore, this non-restrictive clause should be set off with commas.

The relative clause "that is always churning" refers back to river, but "that is churning" specifies which river is being talked about. Therefore, this relative clause does not need to be set off with commas.