I used to do this trip regularly. I left Chicago in 1992 but for about 10 years I had a boat at the old Crowley's yacht yard location. They have since moved to the Calumet river.
The bridge tenders announce bridge openings and delays. There are often delays due to electrical
problems. The bridge tenders are not allowed to reset circuit breakers - they must call an electrician. Probably union rules. As said before, they start at one branch and bring them to where the branches join, then they bring up the boats from the other branch. So many boats are left to tie up as best they can or just to mill around 'til the other boats join them. The radio announcements help the first group of boaters know how soon they are getting under way.
In the old days, before Crowley's relocation far more boats came up from the south branch and they always started the south branch flotilla first. Also, the south branch has a lot more bridges to get through that the north branch so it takes longer.
Those bridges are very old and there are often delays due to problems.
A bit of trivia: There used to be a cement ship named the Medusa Challenger that made regular deliveries to Chicago. Very often the bridges got stuck in the open position during it's transits messing up the heavy Chicago automobile traffic. This situation was called the curse of the Medusa.