Abbeville, Saint-Valery-sur-Somme, Le Crotoy Tributaries The tributaries listed comprise: Hydrology 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. This satellite photograph shows the fenny valley crossing the chalk to the sea on the left. The sinuous length at the centre of the picture lies downstream from Péronne. 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 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. Flow-rate data (external links) Monthly flow rates Catchment area . at Abbeville. Flow rates at Hangest-sur-Somme Daily flow rates compared with mean rates for the time of year at Hangest-sur-Somme (m³/s). Catchment area . for the year - 1993.1994. 1995. 1996. 1997. 1998. 1999. 2000.2001.2002.2003.2004.2005. Flow rates at Péronne Mean flow rates monthly and daily at Péronne (m³/s). Catchment area . for the year - 1986.1987.1988.1989.1990.1991.1992.1993.1994.1995.1996.1997.1998.1999.2000.2001.2002.2003.2004.2005. Navigation and canals The construction of the Canal de la Somme began in 1770 and reached completion in 1843. It is long, beginning at St.Simon and opening into the Bay of the Somme. From St.Simon to Froissy (near Bray sur Somme, south of Albert), the canal is alongside the river. Thence to the sea, the river is partly river and partly navigation. From Abbeville, it is diverted through the silted, former estuary,