The river develops from the confluence of the Great Jizera (Velká Jizera) in the Jizera Mountains (Jizerské hory) and the Little Jizera (Malá Jizera) in the Giant Mountains (Krkonoše), and flows for 164km into the Labe (Elbe) at the village of Káraný near Brandýs nad Labem-Stará Boleslav. On its way, it intersects the Ještěd-Kozákov Ridge. For the first 15 kilometers, the river constitute border between Poland and Czech Republic.