The Kızılırmak flows for a total of 1,355 kilometres (842 mi),[2] rising in Eastern Anatolia around 39°48′N 38°18′E / 39.8°N 38.3°E / 39.8; 38.3 (Kızılırmak source), flowing first to the west and southwest until 38°42′N 34°48′E / 38.7°N 34.8°E / 38.7; 34.8, then forming a wide arch, the "Halys bend", flowing first to the west, then to the northwest, passing to the northeast of Lake Tuz (Tuz Gölü in Turkish), then to the north and northeast, where it is joined by its major tributary, the Delice River (once known in Greek as the Cappadox river) at 40°28′N 34°08′E / 40.47°N 34.14°E / 40.47; 34.14.

After zigzagging to the northwest to the confluence with the Devrez River at 41°06′N 34°25′E / 41.10°N 34.42°E / 41.10; 34.42, and back to the northeast, it joins the Gökırmak (Sky River in Turkish) before finally flowing via a wide delta into the Black Sea northwest of Samsun at 41°43′N 35°57′E / 41.72°N 35.95°E / 41.72; 35.95 (Kızılırmak mouth).

As the site of the Battle of Halys, or the Battle of the Eclipse, on May 28, 585 BC,[3] the river formed the border between Lydia to the west and Media to the east until Croesus of Lydia crossed it to attack Cyrus the Great in 547 BC.

In the 130s a governor of Cappadocia wrote: "long ago the Halys River was the boundary between the kingdom of Croesus and the Persian Empire; now it flows under Roman dominion.

Dams have reduced the flow of sediment to the delta, allowing coastal erosion.