Greater Blue Skimmer (Orthetrum melania, 异色灰蜻). Male.
I see them throughout the summer and autumn by the lakes and in the forest, but never (yet) by the river. There are other powder blue dragonflies about which bear a casual resemblance, but these greater blue skimmers are noticeable, even at a distance, for their size and in particular the great thickness of their tails.
