The principal rivers are the Sangarius (mod. Sakaria), which traverses the province from south to north; the Rhyndacus, which separated it from Mysia; and the Billaeus (Filiyas), which rises in the Ala-Dagh, about 50 m. from the sea, and after flowing by Boli (anc. Claudiopolis) falls into the Euxine, close to the ruins of the ancient Tium, about 40 m. north-east of Heraclea, having a course of more than 100 m. The Parthenius (mod. Bartan), the boundary of the province towards the east, is a much less considerable stream.
The natural resources of Bithynia are still imperfectly developed. Its vast forests would furnish an almost inexhaustible supply of timber, if rendered accessible by roads. Coal also is known to exist near Eregli (Heraclea). The valleys towards the Black Sea abound in fruit trees of all kinds, while the valley of the Sangarius and the plains near Brusa and Isnik (Nicaea) are fertile and well cultivated. Extensive plantations of mulberry trees supply the silk for which Brusa has long been celebrated, and which is manufactured there on a large scale.
According to ancient authors (Herodotus, Xenophon, Strabo, &c.), the Bithynians were an immigrant Thracian tribe. The existence of a tribe called Thyni in Thrace is well attested, and the two cognate tribes of the Thyni and Bithyni appear to have settled simultaneously in the adjoining parts of Asia, where they expelled or subdued the Mysians, Caucones, and other petty tribes, the Mariandyni alone maintaining themselves in the north-east. Herodotus mentions the Thyni and Bithyni as existing side by side; but ultimately the latter must have become the more important, as they gave their name to the country. They were incorporated by Croesus with the Lydian monarchy, with which they fell under the dominion of Persia (546 B.C.), and were included in the satrapy of Phrygia, which comprised all the countries up to the Hellespont and Bosporus. But even before the conquest by Alexander the Bithynians appear to have asserted their independence, and successfully maintained it under two native princes, Bas and Zipoetes, the last of whom transmitted his power to his son Nicomedes I., the first to assume the title of king. This monarch founded Nicomedia, which soon rose to great prosperity, and during his long reign (278-250 B.C.), as well as those of his successors, Prusias I., Prusias II. and Nicomedes II. (149-91 B.C.), the kingdom of Bithynia held a considerable place among the minor monarchies of Asia. But the last king, Nicomedes III., was unable to maintain himself against Mithradates of Pontus, and, after being restored to his throne by the Roman senate, he bequeathed his kingdom by will to the Romans (74 B.C.). Bithynia now became a Roman province. Its limits were frequently varied, and it was commonly united for administrative purposes with the province of Pontus. This was the state of things in the time of Trajan, when the younger Pliny was appointed governor of the combined provinces (103-105 A.D.), a circumstance to which we are indebted for valuable information concerning the Roman provincial administration. Under the Byzantine empire Bithynia was again divided into two provinces, separated by the Sangarias, to the west of which the name of Bithynia was restricted.