Saône

The Saône (, ; Sona; ) is a river in eastern France. It is a right tributary of the Rhône, rising at Vioménil in the Vosges department and joining the Rhône in Lyon, at the southern end of the Presqu'île.

The name Saône derives from that of the Gallic river goddess Souconna, which has also been connected with a local Celtic tribe, the Sequanes. Monastic copyists progressively transformed Souconna to Saoconna, which ultimately gave rise to Saône. The other recorded ancient names for the river were Brigoulus and Arar.

Geography
The Saône rises at Vioménil at the foot of the cliff of the Faucilles in the Vosges at an elevation of 392 m; it flows into the Rhône at Lyon at an elevation of 158 m.

Its length is 473 km. Its largest tributary is the Doubs; upstream of receiving the Doubs at Verdun-sur-le-Doubs in Saône-et-Loire, the Saône is called the "Petite Saône" (lesser Saône), which reflects the large contribution of the Doubs to the Saône. In fact the Doubs's mean annual flow rate is slightly stronger than that of the Petite Saône, 175 m3/s compared to 160 m3/s; some thus assert that it is in fact the Saône that flows into the Doubs. Nonetheless, the Saône has a substantially larger watershed than the Doubs, at 11500 km2 vs. 7500 km2.

At 30000 km2 the Saône has the largest watershed of any French river that does not flow directly into the sea, covering approximately 1/18 of metropolitan France.

In pre-Roman times the river's name was "Arar", a doubling of the Indo-European root ar (water). According to Julius Caesar's Commentaries on the Gallic War this doubling reflected the idea that it was difficult to identify the direction of the river due to its slow rate of flow. The Battle of the Arar was the first major battle of the Gallic Wars. Its current name came from a sacred spring, Sauc-Onna, located at Chalon, which was used by Roman legionaries to refer to the entire river.

Departments and cities traversed by the Saône

 * Vosges: Darney, Monthureux-sur-Saône, Châtillon-sur-Saône
 * Haute-Saône: Jonvelle, Corre, Jussey, Port-sur-Saône, Scey-sur-Saône, Gray
 * Côte-d'Or: Auxonne, Saint-Jean-de-Losne, Seurre
 * Saône-et-Loire: Verdun-sur-le-Doubs, Chalon-sur-Saône, Tournus, Mâcon, Crêches-sur-Saône
 * Ain: Thoissey, Montmerle-sur-Saône, Jassans-Riottier
 * Rhône: Belleville-sur-Saône, Villefranche-sur-Saône, Anse, Neuville-sur-Saône, Fontaines-sur-Saône, Caluire-et-Cuire, Lyon

Main tributaries of the Saône
R indicates a right tributary, L indicates a left tributary.

Navigation
The Saône is navigable from its confluence with the Coney at Corre in the north of the département Haute-Saône all the way to its confluence with the Rhône (itself a navigable river) at La Mulatière, in Lyon. The navigable stretch is 367 km long, of which 206 km has been redeveloped to European high-capacity dimensions from Saint-Symphorien-sur-Saône to Lyon. It has 5 locks. The 161 km long part upstream from Saint-Symphorien-sur-Saône to Corre, also named Petite Saône, is navigable for Freycinet gauge ships and has 19 locks.

The Saône is linked with the Loire by the Canal du Centre, with the Yonne by the Canal de Bourgogne, with the Marne by the Canal entre Champagne et Bourgogne (previously the Canal de la Marne à la Saône), with the Meuse by the Canal de l'Est, whose southern branch has been renamed the Canal des Vosges, and with the Rhine by the Canal du Rhône au Rhin. All the canals are Freycinet gauge.

Also navigable are the small Canal de Pont-de-Vaux (3 km), the Seille, navigable in a 40 km stretch up to Louhans, as well as the lower part of the Doubs. None of these three connect the Saône to any other waterway.

The lesser Saône (Petite Saône)
The lesser Saône has a tendency to flood (sometimes influenced by snow), with a very strong oceanic effect. The soils are not susceptible to much infiltration, so that they saturate quickly which contributes to surface runoff. The flow rate grows very quickly, and after receiving the waters of the Lanterne, the Saône already becomes a powerful river.

The mean annual flow rate, or discharge, of the Saône has been measured over 50 years (as of 2013) at the Ray-sur-Saône hydrological station, situated about 30 km after the Lanterne confluence between Port-sur-Saône and Gray. The figure is 59.7 m3/s for a watershed area of 3740 km2 (the upper basin of the lesser Saône), and has an annual maximum of 64.5 m3/s and a minimum of 54.8 m3/s.

The river exhibits seasonal variations in flow rate, with winter floods from 84 to 108 m3/s from December to March inclusive, and summer reductions in July/August/September falling to a monthly average of 16.9 m3/s in August.

The runoff curve number in the upper basin of the lesser Saône is 505 mm annually, cf. 687 mm for the Lanterne, an elevated figure resulting from the very high rainfall in the Vosgian part of its watershed. The specific flow rate rises to 16.0 litres per second per square kilometre of watershed.

The maximum instantaneous recorded flow rate was 930 m3/s on December 19, 1982.

The greater Saône (Grande Saône)
The greater Saône is formed by the confluence of the Doubs and the lesser Saône at Verdun-sur-le-Doubs. The Doubs brings a mean annual flow rate of 175 m3/s, and the lesser Saône, 160 m3/s.

The greater Saône has only modest tributaries which have little effect on floods or other hydrological properties. It flows in a vast plain approximately 3 km wide as far as Lyon in the basin of the former Bressan lake. The slope is very gradual, and without hydraulic projects up to the north of Chalon aimed at guaranteeing a deep navigation channel, overflows would be more frequent.

At the Couzon-au-Mont-d'Or hydrological station, where the river enters the Lyon area, measurements taken between 1969 and 1986 revealed a mean annual flow rate of 473 m3/s, with a 100-year flood flow rate of 3180 m3/s The runoff curve number from the river's entire watershed is 501 mm, and the specific flow rate rises to 15.8 litres per second per square km of watershed.

Average flow rate
Overall, the average flow rate in Lyon is 475 m3/s, with a minimum of 153 m3/s, in August, and a maximum of 954 m3/s, in February.

Historic floods
When the Saône floods, the impact varies considerably over the course of the river. A large flood with a strong flow rate upstream can be largely attenuated in the Bressan plain so as to have only moderate impact at Mâcon, particularly if it carries a limited volume of water. By contrast, a medium-sized flood of the lesser Saône can turn into a significant flood downstream, if the Doubs brings in a similar contribution at about the same time.

Historic floods include:
 * The Lyon flood of 580
 * The floods of 1602 during the autumn equinox and of 1711
 * The flood of November 1840, with an estimated flow rate of almost 4000 m3/s, destroyed numerous habitations along the river valley. Many plaques marking this serious event are still visible in the villages affected. The high water measured at flood scales reached 8.05 m at Mâcon and 7.28 m at Chalon, or about 6 and 5.5 m respectively above normal levels).
 * The flood of May 1856
 * The largest floods in the last 50 years as of 2006: January 1955, March 1970, December 1981 and 1982, May 1983, March 2001 and 2006.

The reference flood in town planning is the 100-year flood. This reference was in the course of being modified as maps linked to modelling the 1840 flood in modern town planning conditions were distributed to local mayors in December 2008, and as new prevention plans were ordered for 2012.