Lonja field is the largest protected wetland in the entire Danube basin. It covers an area of 505.6 square kilometres (195.2 sq mi), 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 river Veliki Strug.
Wast pasture lands of Lonja field are feeding the horses, cows and pigs of nearby resitends, while whole wetland is home to large number of animals and plants. Inside the nature park, you can find around ten villages, which represent local cultural heritage. The the park is also an important habitat for birds (Important Birds Area – IBA).
Starting from 1993., Lonja field is protected by Ramsar convention.