The Male Marvel character first name group and the Kings of England group need a little work, to eliminate interchangeable answers. Maybe swap James for another name which isn't also the name of a King of England, as every single name in the Kings group is also the first name of a male Marvel character, which means there are at least five valid solutions to the grid, when there should only be one.

Good. Although capitals should be enough for the descriptions ... how many capitals dont have a river?