The Biga Çayı begins its life on the slopes of Mount Ida — the mountain called Kaz Dağı in Turkish, sacred in antiquity as the throne of Zeus, the place from which the gods were said to watch the Trojan War unfold on the plains below. From those heights the river descends through the Troad, the region of northwestern Turkey that takes its name from ancient Troy. It collects tributaries as it moves northeast, passing through Çan, then bending toward Biga, before reaching the coast at Karabiga — a small port on the Sea of Marmara whose name preserves the old identity of the river that flows through it. The river is also known locally as the Çan Çayı and the Kocabaş Çayı, alternative names that reflect the towns along its course rather than its ancient identity. Along the way it passes through terrain that has been farmed for thousands of years: wheat fields, olive groves, villages of stone and tile whose inhabitants live in proximity to one of the most historically layered landscapes on earth without necessarily thinking about it every day.

In May 334 BC, Alexander the Great crossed this river and defeated the Persian satraps waiting on the far bank. It was his first major victory against the Achaemenid Empire, the opening move in a campaign that would eventually carry him from Macedonia to the borders of India. The battle was called the Battle of the Granicus. Ancient writers described the crossing as difficult — steep banks, a strong current, javelins raining down from the defenders on the eastern shore — though modern visitors have noted that the river at its likely crossing point is not especially formidable. The discrepancy may be the result of seasonal flooding, changes in the riverbed over two millennia, or the tendency of ancient historians to amplify the obstacles their heroes overcame. The strategic consequences of the crossing were not exaggerated. Within days of the battle, Sardis, the former Lydian capital and a key Persian administrative center, had surrendered to Alexander without a fight.

Scholars have argued for decades about exactly where on the Granicus the battle took place. The river has shifted in its valley over the centuries; the ancient course may have run somewhat further east than the modern channel. The classicist N. G. L. Hammond proposed that the ford was located between the modern villages of Gümüşçay and Çeşmealtı, near a long ridge that matches the ancient historians' description of where the Persian infantry was stationed. That theory has found wide support. Visitors to the area today cross the Biga Çayı on bridges and travel roads that follow the same general corridors that ancient armies used, through a landscape whose agricultural character has not changed fundamentally since the fourth century BC, even if the specific fields and the specific villages are entirely different.

The Granicus lent its name to more than a battle. On Mars, a long sinuous valley system carries the name Granicus Valles — a channel carved by ancient water or lava, running for 750 kilometers at 29.72° North, 131.0° East. Planetary scientists who named it reached back to the classical world, as planetary nomenclature often does, connecting this small Turkish river to the surface of another planet. The naming says something about the Granicus's place in cultural memory: it is famous enough, in the long tradition of classical education, to serve as a landmark on Mars. The river itself flows on in Çanakkale Province, indifferent to its celestial namesake, carrying snowmelt from Mount Ida toward the blue expanse of the Sea of Marmara just as it has for millennia.