Pheneus or Pheneos was a town in the northeast of ancient Arcadia. Its territory, called Pheniatis, was bounded on the north by that of the Achaean towns of Aegeira and Pellene, east by the Stymphalia, west by the Cleitoria, and south by the Caphyatis and Orchomenia. This territory is shut in on every side by lofty mountains, offshoots of Mount Cyllene and the Aroanian chain; and it is about 7 miles (12 km) in length and the same in breadth. Two streams descend from the northern mountains, and unite their waters about the middle of the valley; the united river bore in ancient times the name of Olbius and Aroanius. There is no opening through the mountains on the south; but the waters of the united river are carried off by subterranean channels (katavóthra) in the limestone rocks, and, after flowing underground, reappear as the sources of the river Ladon. In order to convey the waters of this river in a single channel to the katavóthra, the inhabitants at an early period constructed a canal, 50 stadia in length, and 30 feet in breadth. This great work, which was attributed to Heracles, had become useless in the time of Pausanias, and the river had resumed its ancient and irregular course; but traces of the canal of Heracles are still visible, and one bank of it was a conspicuous object in the valley when it was visited by William Martin Leake in 1806. The canal of Heracles, however, could not protect the valley from the danger to which it was exposed, in consequence of the katavóthra becoming obstructed, and the river finding no outlet for its waters. The Pheneatae related that their city was once destroyed by such an inundation, and in proof of it they pointed out upon the mountains the marks of the height to which the water was said to have ascended. Pausanias evidently refers to the yellow border which is still visible upon the mountains and around the plain: but in consequence of the great height of this line upon the rocks, it is difficult to believe it to be the mark of the ancient depth of water in the plain, and it is more probably caused by evaporation; the lower parts of the rock being constantly moistened, while the upper are in a state of comparative dryness, thus producing a difference of colour in process of time. It is, however, certain that the Pheneatic plain has been exposed more than once to such inundations. Pliny the Elder says that the calamity had occurred five times; and Eratosthenes related a memorable instance of such an inundation through the obstruction of the katavóthra, when, after they were again opened, the water rushing into the Ladon and the Alpheius overflowed the banks of those rivers at Olympia. The account of Eratosthenes has been confirmed by a similar occurrence in modern times. In 1821 the katavóthra became obstructed, and the water continued to rise in the plain till it had destroyed 7 to 8 square miles of cultivated country. Such was its condition till 1832, when the subterraneous channels again opened, the Ladon and Alpheius overflowed, and the plain of Olympia was inundated. Other ancient writers allude to the katavóthra and subterraneous course of the river of Pheneus.

Orchomenus or Orchomenos was an ancient city of Arcadia, Greece, called by Thucydides the Arcadian Orchomenus, to distinguish it from the Boeotian town.