::For the collation of Golden Age as well as Las Vegas and Los Angeles (both being US cities with "English" names), I agree with you.
Indeed, a general category for riverine islands would be much better than Category:Islands by river. Sorting the article in "Islands by river" under "G" is logical, but has the disadvantage of using an element that isn't displayed in the category listing. 
As La Cartuja is also in other categories, i.e. Category:Isthmuses, we should still find a default sortkey that is consistent. Similar to "The Hague" (under "H") or "La Chaux-de-Fonds" (under "C", as on ,  and anywhere else I could find it), I'd go for "C".  User:Docu