Stream isn't a Water obstacle, unless flooded, in which case it is treated as a pond.
B21.1 ... Any non-stream hex whose center dot is emplaced in a blue background is a Water Obstacle hex.
B20.44 Flooded: If a stream is flooded, it is one level higher than it would normally be (i.e., usually level 0), and is treated as a flooded pond. All mapboard-depicted ponds in a scenario that has a flooded stream are also one level higher than normal, as per 21.21. See 16.6 for a flooded stream's effect on marsh.