In 1945 the border between Poland and Czechoslovakia was set at the 1920 line. Polish troops then occupied northern Orava and Spiš on 17 July 1945. There were armed clashes and fatalities in some villages over the following two years. Slovaks from the Polish part of Spiš settled mainly in the newly created industrial town of Svit near Poprad, Kežmarok, Poprad, and in depopulated German villages  near Kežmarok. Slovaks from the Polish part of Orava settled mainly in Czech Silesia, and in depopulated German villages in the Czech lands . On 10 March 1947 a treaty guaranteeing basic rights for Slovaks in Poland was signed between Czechoslovakia and Poland. As a result, 41 Slovak basic schools and 1 high school were opened in Poland. Most of these however were shut down in the early sixties because of lack of Slovakian teachers. On June 13, 1958, in Warsaw, the two countries signed a treaty confirming the border at the line of January 1, 1938 , and since then there have been no conflicts regarding this matter. In March 1975 Czechoslovakia and Poland modified their border along the Dunajec to permit Poland to construct a dam in the Czorsztyn region, southeast of Kraków.. Why Poland wanted to modify its border with  Czechoslovakia?