Olza (river)

The Olza (Czech: Olše, German: Olsa) is a river in the Czech Republic and Poland, a right tributary of the Oder River. It flows through the Silesian Voivodeship in Poland and through the Moravian-Silesian Region in the Czech Republic. It is 89.1 km (55.4 mi) long. The river forms a significant part of the Czech-Polish state border.

The name is derived from the Proto-Slavic word oliga, meaning "a river rich in water". The origin of the name was demonstrated in 1900 by Czech linguist and writer Vincenc Prasek and the revelation was confirmed by various etymological studies in the 20th century. There was also a theory that the name is a derivative of the Germanic Aliza, meaning 'flow'.

The oldest written mention of the Olza is in a letter written by Duke Mieszko in 1290. The river was then mentioned in a written document in 1611 as the Oldza. At the end of the 19th century, with the rise of mass nationalism, both Polish and Czech activists claimed the name Olza to be not Polish enough, on the one hand, and insufficiently Czech, on the other.

Local people always used the Olza name, regardless of their national or ethnic origin. However, the central administration in Prague saw Olza as a Polish name and when most of the river became a part of Czechoslovakia in 1920, it tried to change its name to the Czech form, Olše. However, a degree of dualism in the naming persisted until the 1960s, when the Central State Administration of Geodesy and Cartography ruled that the only official form in the Czech Republic was Olše. This modern Czech name literally means 'alder' in Czech.

The Olza originates in the territory of Gmina Istebna in the Silesian Beskids at an elevation of 842 m (2,762 ft) and flows to the Czech-Polish border in Bohumín/Gorzyce, where it merges with the Oder River at an elevation of 190 m (620 ft). Its drainage basin has an area of 1,107 km² (427 sq mi), of which 636.1 km² (245.6 sq mi) is in the Czech Republic. The average discharge at its mouth is 15.0 m³/s (530 cu ft/s).

The Olza forms two sections of the Czech-Polish state border with a total length of 25.3 km (15.7 mi). Length figures vary by source. According to the newest official measurements, the Czech part of the river (including the Czech-Polish state border) is 73.1 km (45.4 mi) long. The length of the Polish section of the river to the first crossing of the state border is usually stated as 16 km (10 mi), which means that according to the latest measurements, the river has a total length of 89.1 km (55.4 mi). However, based on older measurements, the total length of the river is stated as 86.2 km (53.6 mi), 83 km (52 mi) or even 99 km (62 mi).

The longest tributaries of the Olza are:

The river flows through the territory of Istebna in Poland, then crosses the Czech- Polish border and flows through territories of Bukovec, Písek, Jablunkov, Návsí, Hrádek, Bystřice, Vendryně, Třinec and Český Těšín. Here it begins to form the state border with Cieszyn, Pogwizdów and Kaczyce on the Polish side and Chotěbuz on the Czech side. The river then continues through the territories of Karviná, Dětmarovice and Petrovice u Karviné before it begins to form the state border again, which lasts until its mouth. In this section, it flows along the territories of Godów and Gorzyce in Poland and Dolní Lutyně and Bohumín in the Czech Republic.