Lonja is a river in central Croatia

Croatia

Croatia , officially the Republic of Croatia , is a unitary democratic parliamentary republic in Europe at the crossroads of the Mitteleuropa, the Balkans, and the Mediterranean. Its capital and largest city is Zagreb. The country is divided into 20 counties and the city of Zagreb. Croatia covers ...

, a left tributary of Sava. It is 133 kilometres (82.6 mi) long and its basin covers an area of 5944 square kilometres (2,295 sq mi).

Lonja rises in the Kalnik

Kalnik

Kalnik is a municipality in the Koprivnica-Križevci County in Croatia. According to the 2011 census, there are 1,361 inhabitants in the area.-References:...

 mountain in northern Croatia, southeast of Novi Marof

Novi Marof

Novi Marof is a town in north-western Croatia, located south of Varaždin and east of Ivanec, in the Varaždin county.The population of the municipality is 13,238 , distributed in the following settlements:* Bela, population 62...

, at 46.132404°N 16.371417°E. It flows westward until turning south near Breznički Hum

Breznicki Hum

Breznički is a village and municipality in Croatia in the Varaždin county. According to the 2001 census, there are 1,575 inhabitants, absolute majority which are Croats....

, passing east of Sveti Ivan Zelina

Sveti Ivan Zelina

-Geography:Sveti Ivan Zelina is north-east from Zagreb, connected: by A4 highway , then state road Sv.Helena - Sveti Ivan Zelina,-Population:...

, and turning southeast near Sveta Helena. East of Lupoglav it turns south again, passing through Ivanić-Grad and nearing the river Sava. It then flows in parallel to Sava for the rest of its course, and the nature park Lonjsko polje

Lonjsko Polje

Lonjsko Polje is the largest protected wetland in both Croatia and the entire Danube basin. It covers an area of , extending along the river Sava from the areas east of Sisak, the lower course of the river Lonja for which it is named, to the areas west of Nova Gradiška, along the course of the...

, a protected area, covers the remainder of the Lonja river basin. Near the end of its course, the river splits into Stara Lonja ("Old Lonja") that enters Sava at the eponymous village of Lonja; and Trebeš or Trebež that discharges into Sava some 5,5 km downstream in the eponymous village of Trebež at 45.348056°N 16.773333°E.

The main tributaries of Lonja are:

• Česma
  Česma
  Česma is a river in central Croatia, the left tributary of the river Lonja-Trebež.It is long, and its catchment area is .Created by merging streams Grđevica and Barna at near the village of Pavlovac in the municipality of Veliki Grđevac....
  
   (Čazma) (124 km long), which joins Lonja northeast of Sisak
  Sisak
  Sisak is a city in central Croatia. The city's population in 2011 was 33,049, with a total of 49,699 in the administrative region and it is also the administrative centre of the Sisak-Moslavina county...
  
  
• Ilova (85 km long), at Trebež, south of Kutina
  Kutina
  Kutina is a small city in central Croatia, the largest settlement in the hilly region of Moslavina, in the Sisak-Moslavina county. The town proper has a population of 14,814 , while the total municipality population is 22,815....
  
  
• Pakra, at Trebež, south of Kutina
• Zelina, south of Ivanić-Grad