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 2023 r. poz. 1478, z późn. zm.)