Thank you for correcting my mistake

I tend to agree with @Manohar.
The pertinent area should probably be tagged with "landuse=reservoir", "intermittent=yes".
Dam only refers to the actual dam and not to the reservoir area.

In "Map Features" it clearly states that the riverbank feature should usually be reserved only for wider rivers. In this case the river width is about 5 meters (I only checked one stretch of the river). "Map Features" does not define what is a wide river (e.g. >10 meters) so @airv interpretation could well be valid.