It is separate and significant part of the surface water, so called uniform part of the surface water.
Water reservoir is:
a) lake or other natural water reservoir,
b) unnatural water reservoir,
c) stream, water jet, river, water channel and it's parts,
d) sea-water, transitional and coastal water.
Act of 20 July 2017, New Water Law
Place of publication: (Dz. U. z 2021 r. poz. 624, z późn. zm.)