Several rivers flow across the Lombardy Plain to join the River Po which runs along the southern border of the province. The River Adda separates Cremona from the Province of Lodi and the River Oglio provides the border with the Province of Mantua. Other rivers in the north of the province include the River Serio and the River Tormo, and the River Mella makes up a short stretch of the border with the Province of Brescia. These rivers are linked by a network of canals which have been in place since at least the sixteenth century and are largely used for irrigation. The Muzza Canal takes its water from the River Adda and irrigates the land between that river and the River Ticino, converting the plain into a fertile area with rich meadows and productive agricultural land. The total area of the province is . Although the province is essentially flat, there are some undulations in the surface formed by the varying courses of the rivers over the millennia. For historical reasons, the province is subdivided into four rural districts, centred on Crema, Soresina, Cremona and Casalmaggiore. In the north, some watercourses emerge from the ground in the "line of springs", a phenomenon of the northern Lombardy Plain, where melt-water from the Alps flows underground through porous gravelly soils before being forced to the surface when it reaches impervious, clayey ground. The climate is largely uniform throughout the province. The annual rainfall is about with October and November being the wettest months and February and July being the driest. The average temperature is in January and  The principal economic resources of the province of Cremona are agricultural. Rice is grown with the help of water drawn from canals. Other crops include maize (corn), locally called "Melegot" and barley and to a lesser extent, soya and sugar beet. Grapes are cultivated and wine produced, and there is also a silk industry. The farms in the province are some of the most productive in the country. Other industries are quite developed, mostly in the northern zone, near Crema, where there are textiles, chemical, and mechanical factories. Beef and dairy cattle are kept in the province. The beef serves as an ingredient for local dishes, and milk from the dairy cows is used to create traditional cheeses as well as producing butter and cream. The area is famous for its food specialities, such as nougat (IItalian: "torrone") and mustard.