Lake Zygmunt August <a href="https://en.wikipedia.org/wiki/Sigismund_II_Augustus" class="extiw" title="en:Sigismund II Augustus">Sigismund II Augustus</a>, Podlaskie Voivodeship, Poland. Although it's called lake in fact it's reservoir. It was created circa 1559 and is the oldest reservoir in Poland.
<a href="//commons.wikimedia.org/wiki/User:Simpledot" title="User:Simpledot">Simpledot</a>
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<span class="int-own-work" lang="en">Own work</span>