Either A) Blow a hole in a levee on the other side of the river, giving the water somewhere else to go, preferably where it will do less damage, or
B) FIgure out where the water is going and blow part of a levee to let the water *out*.
Both are viable techniques used on Old Man River over the years.
Not a good idea if the flooded river is higher than the lake. The flooding in the city shouldn't rise any more than the level of whatever is behind the breach.