The heightmap is slightly better now, but don't just blur the whole image. Look, the details of one of estuaries were almost completely lost.
Also remember, that riverbed shoud be placed lower, than rest of the land, so pixels defining rivers must be always darker than areas directly surrounding them. Now estuaries are in fact small mountain chains

. Rivers should be darker, this will probably require to make those bright areas darker also (to eliminate "canyon effect"). Then Arangol should take care of final touches, checking the result in Blender.