That's exactly what I am trying to say. Temperature is just one of the factors. Fast flowing river water contains a lot of organics (particulate and dissolved). There are several scientific papers showing the importance of those organics when it comes to supporting aquatic plant life (primarily production of CO2, maintaining decent oxygen levels, possibly continuous supply of most nutrients needed by plants etc.).This is most probably not related to temperatures but proves that in a close environment plant don't necessarily behave like in their natural habitat which is 10 fold more complex.
Even though the tds is low but the water is extremely rich in those organics.