Rivers and lakes
About 62% of the territory covered by the branching river network belongs to the Black Sea catchment basin. The longest Croatian rivers, the River Sava (562 km) and River Drava (505 km) also belong to this catchment basin, as does the Danube, into which they both flow. These three rivers to a large extent form natural borders.
The main tributaries of the Sava are the Sutla, Krapina, Kupa (the longest river whose entire course is inside Croatia), Lonja and Una. The main tributaries of the Drava are the Mura, Bednja and Karašica, while the River Vuka flows into the Danube. Most rivers have a high water table in winter and a low one in summer, with the exceptions of the Drava and Mura. The main navigable waterway is the Danube. The Drava is navigable by larger vessels as far as Osijek, and the Sava as far as Sisak.
In the Adriatic catchment basin region which covers 38% of the territory, due to the predominant limestone formations, the hydrographic network is less diversified, and rivers spring from copious sources, run more steeply downstream and have shorter courses. The larger among them are the Mirna, Zrmanja, Krka and Cetina, while the largest is the Neretva, although it flows for only 20 km through Croatia, and is navigable at that point. The karst underground streams of the Lika and Gacka also belong to the Adriatic catchment basin.
There are lakes in all parts of the country, but most of them have small surface areas. The largest is Lake Vransko (30.7 km²), a natural lake near Biograd. The world famous, picturesque Plitvice Lakes are in Lika. Artifical lakes built for hydroelectric plants include Lake Dubrava (17.1 km²) and Lake Varaždin (10.1 km²) on the River Drava, and Lake Peruća (13 km²) on the River Cetina.
In terms of the proportion of surface and underground water reserves in the country, Croatia ranks near the top globally, while in terms of the size of its per capita water reserves, it is the third in Europe, behind Iceland and Norway.