First off, that's opening a spillway, and that will still leave a substantial portion of land flooded. > A dam not holding any water back has far more strength

First off, that's opening a spillway, and that will still leave a substantial portion of land flooded. > A dam not holding any water back has far more strength than necessary and will probably last thousands of years. And, actually isn't true, there's a lot of complicated factors when building a dam, and having a consistent, low lying waterflow (like a river). Depending on the type of damn, soil conditions, etc, it can actually cause disproportionate erosion that will cause the damn to collapse in on itself (occasionally in a surprisingly short time frame, sometimes under a decade even - or in the case of heavy waterflow, hours or days). So, engineers will sometimes do what you suggested when it's feasible. But, it's not always.