A repaired dam will only benefit the canoe and rowing club. Let’s not say that the only way to canoe on the Thames and have people ‘return to the river’ is with a repaired dam. The number of people that canoe the Thames are substantially less than those who walk it banks or drive their cars over the number of bridges in the city. Those people unfortunately are not as vocal as the small number of canoeist. The notion that people don’t care about the river because they can’t go down and appreciate it via canoeing or rowing is nonsense. I fish the Thames regularly and I would say almost every time I do I come across someone canoeing (with smiles and a friendly wave)
Finally to say that one dam on a short stress of water will not affect things is short sighted. By this same logic one shouldn’t bother recycling every piece of paper or choose organic options. Why bother when the majority don’t. Please don’t consider yourself an environmentalist if choose the line of thought.