river, Emilia-Romagna region, northern Italy, rising in the Ligurian Apennines at an altitude of 4,613 ft (1,406 m), northeast of Genoa, and flowing 71 mi (115 km) north-northeast across the northern Apennines and the Po lowland to enter the Po River just west of Piacenza (ancient Placentia). Throughout its course, except for the first 2 mi, the river is followed by the Genoa–Piacenza Highway.

The river is known in Roman history for the victory gained on its bank in 218 BC by Hannibal whose troops defeated the combined armies of the consuls Scipio Africanus and Tiberius Sempronius Longus; some of the Roman troops, mainly cavalry, escaped to nearby Placentia, but most of them were driven into the river to their death. The plain of the Trebbia was also the scene in 1799 of a defeat of the French under Jacques-Étienne Macdonald (later duc de Tarente) by a Russo-Austrian army commanded by Gen. A.V. Suvorov.