Poor water quality, for example from sewage releases, can affect the ecology of rivers and streams, damaging habitats and harming wildlife. However, during periods of low flows and high temperatures, as we have seen in London this summer, it will be very difficult to attribute impacts to any one given cause. All these influences will affect the resilience of our rivers. High temperatures reduce the ability of water to hold oxygen and low flows exacerbate this by there being less water in total.
The Environment Agency oversees water quality issues, which in turn indicates the health of wildlife. They monitor dissolved oxygen in the Tidal Thames linked to the use of oxygenation needs as a proxy for ecology health. This information is used to apply oxygenation dosing to the tidal Thames if needed, as it was this summer.
The Environment Agency also respond to information from members for the public where fish have been reported in distress. Aeration (water agitation via fountains etc.) was deployed on London rivers this summer, although water quality fell, no dead fish were recorded.