I seriously doubt that any local government will issue a building permit for new construction on land located on a flood plain. These days in some jurisdictions ownership of "improvements" built on flood plains can't even be transfered. In fact, a local bank had to repossess a very old farm house near here that happened to have part of its 10-acre lot designated as a flood plain (the farm had long ago been sold off in pieces). In order to sell the property the bank had to sub-divide the lot to get the house and small outbuildings "off" that flood plain. (Required some palm greasing due to the minimum lot size being 50 acres.)
And then there's the whole property insurance issue . . .
However many properties were "improved" before such designations, so there's that. And water boundaries do move over time. Rivers change course (an old river boat was once found about 8-miles from the river buried in a farmer's field, don't remember if it was along the Mississippi or the Missouri) and old islands wash away as new ones form.
