Cle Elum Lake in the Cascade Mountains of Washington state, USA.  The lake level is controlled by a dam to irrigate farmland in the Yakima valley - when this photo was taken, the reservoir was near-100% full, thus trees on the shore are partially submerged.
<a href="//commons.wikimedia.org/wiki/User:ECTran71" title="User:ECTran71">ECTran71</a>
<span class="int-own-work" lang="en">Own work</span>