Some data I got from the St. Paul district's site:
When you open the chute and drop the pool 13 feet (exactly what I'd do if I had access to the big valves that let you "play river") the riverbed is revealed in places and the river itself moevs in ways not seen with normal pool levels. Check out the guy on the usually-submerged sandbar below.
Below - more riverbed revealed and obvious waterlines along the lock walls.
Yea, it was well below zero when these pictures were shot. Here's some local media with video.
The group followed (National Park Service Ranger Dave) Wiggins across the bridge, turning to look back at the dark-russet granite feet of the Stone Arch Bridge, which normally are nearly covered by river water. And they turned again to see the roots of Spirit Island, once an 18-foot tree-covered stone column and Dakota sacred site jutting out of the river, now little more than a gravel spit just downriver of the bridge. Farther down, past an old piling from the dismantled 10th Avenue Bridge, a faint V-shaped shore-to-shore dip marks the location of a now-abandoned 1897 dam.