Imposing our will upon our natural surroundings is not the way to go. I don't think a dam is as good of a long-term solution as increasing wetlands and decreasing the amount of pavment / non-pourous surfaces.
We need to rethink development with nature in mind: green roofs which retain and store moisture (and decrease heating /cooling costs), pourous pavement which allows water to seap-in (like it used to) and enter the groundwater supply, and, for the love of God, smaller parking lots. Problem is, we're set in our ways and there's too many comfortable people and those who don't know there is a better way.
Dams fail too and when they do it is 50 times worse then stuff like this. Delhi, IA dam anyone? Imagine the benefits a widened buffer zone would have for wildlife!