of Silesia is relatively flat, although its southern border is generally mountainous. It is primarily located in a swath running along both banks of the upper and middle Oder (Odra) River, but it extends eastwards to the upper Vistula River. The region also includes many tributaries of the Oder, including the Bóbr (and its tributary the Kwisa), the Barycz and the Nysa Kłodzka. The Sudeten Mountains run along most of the southern edge of the region, though at its south-eastern extreme it reaches the Silesian Beskids and Moravian-Silesian Beskids, which belong to the Carpathian Mountains range. Historically, Silesia was bounded to the west by the Kwisa and Bóbr Rivers, while the territory west of the Kwisa was in Upper Lusatia (earlier Milsko). However, because part of Upper Lusatia was included in the Province of Silesia in 1815, in Germany Görlitz, Niederschlesischer Oberlausitzkreis and neighbouring areas are considered parts of historical Silesia. Those districts, along with Poland's Lower Silesian Voivodeship and parts of Lubusz Voivodeship, make up the geographic region of Lower Silesia. Silesia has undergone a similar notional extension at its eastern extreme. Historically, it extended only as far as the Brynica River, which separates it from Zagłębie Dąbrowskie in the Lesser Poland region. However, to many Poles today, Silesia () is understood to cover all of the area around Katowice, including Zagłębie. This interpretation is given official sanction in the use of the name Silesian Voivodeship () for the province covering this area. In fact, the word in Polish (when used without qualification) now commonly refers exclusively to this area (also called or Upper Silesia). As well as the Katowice area, historical Upper Silesia also includes the Opole region (Poland's Opole Voivodeship) and Czech Silesia. Czech Silesia consists of a part of the Moravian-Silesian Region and the Jeseník District in the Olomouc Region. Natural resources Silesia is a resource-rich and populous region. Since the middle of the 18th century, coal has been mined. The industry had grown while Silesia was part of Germany, and peaked in the 1970s under the People's Republic of Poland. During this period, Silesia became one of the world's largest producers of coal, with a record tonnage in 1979. Coal mining declined during the next two decades, but has increased again following the end of Communist rule. The 41 coal mines in Silesia are mostly part of the Upper Silesian Coal Basin, which lies in