Even though its a pretty major closure it is still what they consider routine.
They don't seem to be in the habit of hiding closures to make sure people come anyways, on the contrary they are usually pretty good about putting the information out so people can plan around that sort of stuff.
I don't think they would want to deal with people that spend a lot of money to come expecting stuff to be open and then find out when they get here its closed.
Dates change on a fairly frequent basis, but they are seem to be pretty good at getting that out once it is definitive (stuff even opens back up ahead of the scheduled time on occasion)
As to why such a long time frame for when the might refill it... I would assume with maintenance on a scale as huge as this that they don't have a definitive deadline on when it will be finished until the river is drained and they can actualy see the extent of the work that needs to be done and/or get a feel for how long it will take once they actually get started.
If its like refilling the lake at Paradise Pier they had to coordinate with the city and county Water depts so they might not be able to set a date until they get closer and get it coordinated with everyone else involved...
Of course a lot of that is my opinion... Just keep checking the schedule online, that seems to be the most accurate place to get info!