(Barbara Stalmachova, (from: ”Olza - od pramene po ujście”: „Historia rzeki”, published by: Region Silesia s.r.o. 2000) translated by D.French

      The Olza is the second longest river in the Silesian Beskid Mountains belonging to the Odra’s drainage basin after the Vistula. It has a length of 99 km, 16 km of which are in Poland and 83 of which are in the Czech Republic. Its average drop is 0.67% (in the Istebna section this figure rises to 2.5%) and it has 49 significant tributaries.
      The springs of which the Olza has seven on the western slopes of Gańczorka and Karolówka in the Silesia Beskid Mountains are at altitudes of between 840 and 880 m above sea-level. At the river-confluence with the Odra north of Bohumin its altitude is 195 m above sea-level. The river basin has an area of 1118 km², of which 479 km² are in Poland. The average flow at the point of the river-confluence is around 10 m³/s.
      In the upper course from Jablunkov to Třinec the Olza is the border between the Silesia Beskid Mountains and the Silesian Moravian Beskid Mountains. The Olza’s river channel forms the Czech-Polish border in the middle course from Cieszyn to Karvina and the lower course from Zavada to the Odra river-confluence.