A few things worth mentioning (sorry for the long post):
Life Jackets -
A life jacket is a bad idea. For some of these higher jumps, around 70ft, the impact velocity is around 45mph. If you've ever been dumped behind a boat going that speed, which is a good bit faster than typical skiing/wakeboarding/tubing speed, and hit the water feet first, you'll know that the life vest can get pushed up on your torso and make it very hard to breathe or even swim with any real progress.
Currents -
If you jump from the center of a span and not near a support piling/pier, the currents are usually not a problem. If the water is moving fast and the flow is consistent, then it's still easy to swim straight up and then laterally to shore. I've done the Cass Street Bridge over the Mississippi in Lacrosse WI, which was about 75ft in autumn when the water was low. It's narrow and deep (25+ft) there and the water moves quickly, but even at night the current wasn't a problem. Granted, this was a couple of college swimmers, but it was truly a walk in the park to swim back.
I've also done some ocean jumps, including the ~45ft cliff in one of the last episodes of Lost (at the mouth of a cave near Honolulu) and a ~35ft pier jump in southern California. There were legitimate currents due to the ocean swells, but it was large volumes of water moving, no little eddies, so it wasn't a problem to anyone in the large groups there.
Local River Depth -
I've taken a boat with a depth sounder along the Mississippi from Inver Grove Heights (well south of St Paul) all the way to Boom Island Park near downtown Minneapolis a couple times and looked at depths under the big bridges. Unlike Lacrosse, the depths around here were more around 10ft; it was two or three years ago so I don't remember exactly, but I do remember thinking that it was too shallow to be safe. This was mainly based on competitive platform diving experience. In rivers, where it's nearly impossible to swim down and check out the bottom visually, I like a decent buffer since things get stock on the bottom, things that might not show up with a depth sounder.
Lastly, with rivers, and especially in the spring after a heavy rain season, watch for trees and logs floating in the water.
If anyone wants to go jumping in the next few days, let me know. I'm up for it.