My first impression is of geothermal activity, as well.
I would think that the only type of man-made activity that could cause the temperature of a river to rise to these levels is venting of nuclear generator cooling tanks. I would be interested in learning what type of industrial buildings are adjacent to this river.
By the way, what exactly does a heat supply company do? I am not familiar with the term.
I'm also curious as to why the 'local authorities' are so convinced that this is man-made.
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Just add onions. May be a geothermal connection like magma rising?