If a 100-year flood has recently occurred along a river, it is now less likely to experience a similar event in the near term.

The statement is correct.

When a flood is giving a time-bonding term, such as the 100-year flood, in this statement, it means that it has been noticed that the floods from the particular river occur on approximately that much of a time span.

If there has been a flood recently, than it most probably means that there isn't going to be a flooding caused by that river for the foreseeable future.

Now, there's always exceptions, as the climate patters are changing constantly, and there's always surprises, but in general, in lot of regions of the world, there has been noticed a certain consistency in some natural occurrences.