Hosszúrét (Long meadow) Brook, aka Stone Brook (Kő-ér) is the only natural watercourse of New Buda. Its names come from the vast open land near its source and the borough called “Kőérberek” it crosses. Its source is in Torbágy Forest. It feeds the Lake of Törökbálint and crossing District 11 and Budafok it flows into the Danube at the tip of Csepel Island. It has tributaries from Budakeszi, Budaörs, Törökbálint and Diósd.

The brook has a watershed of 114 km², collecting water mainly from the Budakeszi and Budaörs areas. Although this small size of watershed area makes the brook mostly lack water, heavy rainfalls may cause floods, just like in the 2000s requiring temporary evacuations.

The brook runs through various habitats both in its native bed and in a concrete canal bordered by retaining walls. Entering Budapest the brook widens and forms a lake called Kána Lake. It was established by damming the brook and today it is a popular fishing lake.

Wetland flora is represented by species like cutleaf teasel, marsh thistle, hedge-nettle, purple loosestrife, hairy willowherb, water dock and marsh horsetail. The most common animal species here include grey herons and mallards as well as frogs and toads. From Kőérberek downwards the stream bed narrows and follows a concrete canal.