The river is 245 km (152 mi) in length, from its source in the high ground of the former Arrouaise Forest [fr] at Fonsomme near Saint-Quentin, to the Bay of the Somme, in the English Channel.

Private A. S. Bullock in his wartime memoir recalled his first sight of it in early April 1918: "... we reached a small place called Hengest sur Somme.

"[7] The great battles that 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.

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.

This picture [nl], 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.

Once exploited for peat cutting, the fen is now used for fishing and shooting In 2001, the Somme valley was affected by particularly high floods, which were in large part due to a rise in the water table of the surrounding land.

The St Quentin Canal, famous for the 1918 battle, links the Somme to northern France and Belgium and southward to the Oise.