Choaspes: river in Iran, identical to the modern Karkheh, Seymareh, and Kashkan.
From a geological point of view, a tour upstream - essentially, from ancient Susa to Ecbatana - is one of the most exciting trips one can make in Iran. The river crosses through many spectacular landscapes, which are (literally) textbook examples for geologists.
In the Zagros area, there is sufficient rain to create rivers, but not enough to create forests. As a result, the rivers can cut deep into the landscape and creates impressive canyons. Of course, they are full of mud, which is eventually deposited on the plains of Khuzestan. Everywhere, you can see the layers of the earth. Some stratums are harder than other, which explains this pattern in the Karkheh river. It also explains the many bends in the river's course; a typical name is Cham Girdab ("swirl’s corner").
The beauty of this valley - the part above is known as Darreh-e Khazine ("valley of the hot bath"), and it can be seen on a satellite photo here - has always been recognized. According to the Greek researcher Herodotus of Halicarnassus (fifth century BCE), the Persians regarded the Choaspes as a sacred river and considered its water to be the most pure in the world. The Achaemenid king was supposed to drink only water from the Choaspes.note[Herodotus, Histories 1.188.]
The river was crossed by the Royal road, which connected the Persian capitals Pasargadae, Persepolis, Susa with Arbela, Nineveh, Gordium, and Sardes. This road was essentially west of the Zagros. During the Sasanian age, the route was through the mountains, and connected the capitals of that age: Istakhr, Bishapur, Ctesiphon, Arbela, Hatra, Nisibis, and Edessa.
Before the Karkheh enters Khuzestan, it changes from a swift mountain river into one of the slow and muddy streams that have created the fertile alluvial plain. It passes west of Susa; east of this city was the Eulaeus (modern Dez). Together with the Karun (ancient Pasitigris), the Karkheh gives life to Khuzestan.
Finally, a remark that I can not place somewhere else: there is no place in Iran where I have so many blue herons.