In conclusion, modern day interpretations vary too. Atlas of Iran's Roads printed in 1987 by Gitashenasi Institute marks the Beginning of Karkheh where Kashkan and Seymareh meet, while the TV documentary "Ba Seymareh," episode 22, states that Zal is the last junction before the river is called Karkheh. https://www.aparat.com/v/6FZnb?playlist=235633

I still like the latter interpretation as it reflects the indigenous naming conventions.