The river is 245 km (152 mi) long, from its source in the high ground of the former Forest of Arrouaise at Fonsommes near Saint-Quentin, to the Bay of the Somme, in the English Channel. It lies in the geological syncline which also forms the Solent. This gives it a fairly constant and gentle gradient where several fluvial terraces have been identified.
The river gave its name to the infamous Battle of the Somme during World War I, which lasted from July until November 1916 and resulted in more than a million casualties. The Somme also featured prominently in other historical campaigns. In 1066, the invasion fleet of William the Conqueror assembled in the Bay of the Somme at Saint-Valery-sur-Somme. The river featured in the 1346 withdrawal of Edward III of England's army, which forded the river at the Battle of Blanchetaque during the campaign which culminated in the Battle of Crécy. Crossing the river also featured prominently in the campaign which led to the Battle of Agincourt in 1415. The great battles which finally stopped the German advance in the Spring Offensive of 1918 were fought around the valley of the Somme in places like Villers Bretonneux, which marked the beginning of the end of the war.
The river is characterized by a very gentle gradient and a steady flow. The valley is more or less steep-sided but its bottom is flat with fens and pools. These characteristics of steady flow and flooded valley bottom arise from the river's being fed by the ground water in the chalk basin in which it lies. At earlier, colder times, from the Günz to the Würm (Beestonian or Nebraskan to Devensian or Wisconsinian) the river has cut down into the Cretaceous geology to a level below the modern water table. The valley bottom has now therefore, filled with water which, in turn, has filled with fen. This picture, of the source of the Somme in 1986, shows it when the water table had fallen below the surface of the chalk in which the aquifer lies. Here, the flow of water had been sufficient to keep fen from forming.
One of the fens, the Marais de l'Île is a nature reserve in the town of St.Quentin. The traditional market gardens of Amiens, the Hortillonages are on this sort of land but drained. Once exploited for peat cutting, the fen is now used for fishing and shooting