Thanks for your rapid answer and for the references. I would say, both spellings are usal. Even the eciclopèdia mentions "riu del Bolès". It seems that Bolès is more frequent in french context, for example: http://www.sandre.eaufrance.fr/geo/CoursEau_Carthage2017/Y0460600
I will add loc_name="El Bolès" and add both name="El Bulès" and loc_name="El Bolès"  to all parts of the waterway, not only just to one single part of it.   