The red flag or Bravo flag indicates that the vessel is carrying hazardous liquids or loading / discharging hazardous liquids. It is a REQUIRED day signal with a red all around light at night when loading or discharging at dock or anchor. Note that the flag also signals that the vessel is "carrying" that hazardous liquid and is flown at all times to signify that condition. It is not really "flown" in that most are welded on and painted with the proper signage.
Steve: correct on the red flags locking by themselves. You can go on line at the COE lock and dam page and find out exactly how many boats are waiting, locking time, difference in water levels, wait times, etc... updated fairly often usually real time input.
dan