First off, why it's hard to do before the water levels match: when you were a kid, did you ever try to dam a small stream? It's pretty much impossible to do it by dumping dirt onto the edges of the dam -- you have to put stuff in that's heavy enough to withstand the rushing water. But they can't get big rocks there now, so it's pretty much impossible to fix.
Once the water levels have equalized, then it's possible to start transporting and dumping stuff into the breach. Once a stable wall has been rebuilt, they can start pumping water back over the wall to drain the city.
Please take technical repair talk to another thread. How is that helping anyone find lost relatives or friends?