LOIRE, the longest river of France, rising at 4,500 ft. in the recent volcanic peak of Gerbier de Jonc on the central plateau, and flowing north and west to the Atlantic. After a course of 18 m. it follows a picturesque channel along the foot of the basaltic rocks of Le Puy district, through narrow gorges and small plains. At Vorey, it is joined by the Arzon (left) and be comes navigable for rafts. The north-westerly direction of the Loire and its affluent the Allier is due to a tilting of the central plateau of France in Tertiary times so that the river valleys are filled with Tertiary deposits, on to which the Loire flows after passing the gorges of St. Victor. It again penetrates the hills of Carboniferous rocks before reaching the plain of Roanne. In this course it is joined by a large number of streams, the most im portant being the Coise (right), the Lignon du Nord and the Aix (left). Below Roanne the Loire is accompanied by a canal to Digoin (35 m.), thence by the so-called "lateral canal of the Loire" to Briare (122 m.).
At Digoin the Loire receives the Arroux, and gives off the canal du Centre (which utilizes the valley of the Bourbince) to Chalon-sur-Saône. Before reaching Nevers the Loire passes off the central plateau and on to the Jurassic rocks which form the rim of the Paris basin, over which rocks it flows in a northerly direction. Just beyond Nevers it is joined by the Allier (left) ; this river rises 3o m. S.W. of the Loire and follows an almost parallel course. Above Nevers the Loire is joined by the Aron (right), along which a canal proceeds northward, and below the confluence of the Allier gives off the canal du Berry to Bourges. Near Sancerre the river leaves the Jurassic rocks and has worn a valley through the Cretaceous uplands. At Briare it gives off a canal northward to the Seine. Between Gien and Blois the river flows in a wide arc across the Tertiary rocks of the Paris basin, passing Orleans, whence the canal d'Orleans, following the river Cens, communicates with the Briare canal. Passing Blois the Loire receives (right) the Cisse, and, after passing Tours, the three important left-hand tributaries of the Cher, Indre and the Vienne. Below Saumur it is frequently divided by long sandy islands fringed with osiers and willows ; while upon arriving at Les Ponts-de-Ce (south of Angers) it is split into several distinct branches and studded with islands. At Angers it passes off the
Cretaceous and on to the Palaeozoic and Archaean rocks of the Armorican massif.
The principal tributaries are : left, the Thouet at Saumur, the Layon and the Eyre; right : the Authion, and, most important tributary of all, the Maine, formed by the junction of the rivers Mayenne, Sarthe and Loir. It receives the Erdre (right) at Nantes and the Sevre-Nantaise (left), and farther on the canalized Achenau (left) and the navigable Etier de Mean (right) near St. Nazaire. Below Nantes, between which point and La Martiniere (below Pellerin) the channel is embanked, the river is known as the Loire Maritime and widens out between marshy shores, passing Paimboeuf on the left and finally St. Nazaire, where it is 12 m. broad. The length of the channel of the Loire is about 625 m. A lateral canal, known as the Maritime canal of the Loire, between Le Carnet and La Martiniere enables large ships to ascend to Nantes. It is 93- m. long, and 194 (capable of being increased to 24) ft. deep. At each end is a lock 405 ft. long by 59 ft. wide. The canal de Nantes a Brest connects this city with Brest.
The Loire is navigable only in a very limited sense. During the drought of summer thin and feeble streams thread their way between the sandbanks of the channel; while at other times a stupendous flood submerges wide reaches of land. When the flood waters of two or more tributaries arrive serious inundations result. Attempts to control the river began at a very early date, and in the middle ages the bed between Orleans and Angers was enclosed by dykes io to 13 ft. high. In 1783 a double line of dykes 23 ft. high was completed from Bec &Allier downwards. In modern times embankments, aided by dredging operations ex tending over a large number of years, have ensured a depth of 18 ft. in the channel between La Martiniere and Nantes. Several towns have constructed special works to defeni themselves against the floods; Tours, the most exposed of all, being surrounded by a circular dyke ; and in the upper Loire reservoirs have been con structed to store flood waters.