When I edited the river Seymareh, it was called Karkheh all the way up to the source near Kermanshah. I thought then that it was an act of vandalism. Your request prompted me to dig deeper and now I know why. There are diverging historical views as to what this long continuous river or sections thereof are called.
1. There is no Seymareh river, it’s one continuous 755 km long river called Karkheh https://www.iranicaonline.org/articles/karkeh-river
2. Ancient Greek sources mention river Choaspes originating from Media https://www.iranicaonline.org/articles/choaspes-or-coaspes-lat
3. Other sources think river Karkheh is the confluence of Seymareh and Kashkan rivers near the city of Darreh Shahr.
4. Rawlinson's map of the region in 1839 seems to agree with this view, although it remains vague about what the upstream is called. https://www.themaphouse.com/search_getamap.aspx?id=115107&ref=MEAST3780
5. Map of Persia in 1897 is more liberal in calling a larger expanse as Karkheh all the way up to Rudbar county, Ilam province. It assumes the remaining upstream is an extension of Qarasu (or Karasu) river. However, it recognizes the existence of a geographical region MASBADAN SAIMARA and Saimara ruins around the section of the river in current day Ilam province. https://www.geographicus.com/P/AntiqueMap/persia-surveyofindia-1902
6. There is no mention of Seymareh river in old Persian dictionaries as far as I'm concerned. 
7. We know that Kurds and Lurs know the river as Seymareh, at least the expanse crossing their territory. The name is recognized 
8. This article (in Persian) states that Seymareh river is the fluvial border of Ilam and Lurestan provinces and enters the Khuzestan province as Karkheh. https://khabaronline.ir/news/457112
9. So if we agree that the division is along provincial borders. The last port of call would be the confluence of Seymareh and Zal as the river enters Khuzestan plains.
