All the rivers and streams in the commune of Brenna lie within the draining divide of the Vistula river and the Baltic Sea catchment area. The main watercourses of the area include: the Vistula, the Brennica and the Jasienica. The Vistula, which flows through the north-western part of the village of Górki Wielkie and which receives its tributary, the Brennica, is the largest river in the commune. The source of the Brennica is situated on the slopes of the Beskidek at the altitude of 790 m above the sea level, under the Karkoszczonka pass . The Brennica river begins after the two streams, Węgierski and Połaczny, join. The biggest tributaries of the Brennica are separated by the Stary Groń mountain range (the streams of Leśnica and Hołcyna). The abundance of water in springs in the commune is determined by the relative altitudes, flysch rock formations, humid climate and considerable forestation. The fluctuations of the water level in the Brennica are quite large – as much as about 1.8 m in Górki Wielkie. The level of the Brennica decreases by 30 m per 1 km of its length. According to the statistical yearbook of 1990, the waters of the Brennica are classed as 2nd class of purity.
Nearly along its whole length, the flow of the Brennica river is regulated. There are cascades in it thanks to which tourists can have a swim in summer.
In 1999 regulation work was started on the Jatna stream, which was completed in 2001. The Leśnica stream was also regulated in subsequent years.
There are dams with marinas on the Hołcyna and Leśnica streams.