> ... modelling suggests the system will affect the average temperature of the river by less than 0.1C.

Okay, so that clears up the question I had, then. Not enough to make any appreciable difference.

There used to be a coal-fired power station on the east coast of Scotland, a little south of Edinburgh, Cockenzie, where the cooling loops dumped a huge plume of warm water into the sea. It was well-known as a local fishing spot, with surprisingly clean water flow detectable even a mile or so out from shore. That was several degrees warmer and definitely had a (possibly positive) influence on the ecology of the area - there were certainly a lot of interesting things swimming around there.