| Upstream (metres AOD) | The level of the river upstream of the hydro plant. AOD means ‘above ordnance datum’. The ordnance datum is a fixed reference level from which vertical heights on maps are measured. (AOD has replaced sea level as the standard reference.) |
| Downstream (metres AOD) | The level of the river downstream of the hydro plant. |
| Head (metres) | This is the difference between the upstream and downstream water levels, which is the distance the water falls going over the weir. The head needs to be high enough for the water falling through the turbines to have sufficient force to turn them and thus drive the electrical generators. |
| Above set level (metres) | This is the difference between the upstream level of the river and the present set level. This needs to be zero or above for the hydro plant to be allowed to operate. |
| Present set level (metres AOD) | We are allowed to operate the hydro plant only when the water upstream is above a threshold agreed by the Environment Agency. This is called the ‘set level’. If the upstream level gets close to the set level then power generation is reduced. If the upstream level falls below the set level, the hydro plant must stop operating. Only the set level at present is shown on the graph. Past changes to the set level are not shown. |
| Water flow turbine 1 or 2 (m3/s) | Flow through turbine 1 or 2 in cubic meters per second, estimated from the power delivered by each generator. |
| Flow at Reading Bridge (m3/s) | Total flow of the River Thames, measured by the Environment Agency at Reading Bridge. Only the last four weeks of measurements are shown on this graph. Longer term flow data can be found here on the Environment Agency website. |
