In both snapshots 14w17a and 14w18a, the courses of rivers change compared to the snapshot before. In the 14w17a Snapshot, the customized world type was added, but no changes to the standard world generation were intended (as far as the Wiki tells). The "fix" of this bug (https://bugs.mojang.com/browse/MC-53616) in 14w18a didn't return the river generation to the pre-14w17a state, so this bug isn't completely done, I think.