The plot shows the level of the River Thames (known locally as 'The Isis') just above Folly Bridge, as judged against the steps on the left bank just below the footbridge, on Friars Wharf [Map].
Below 6", the river level is not a reliable indicator of flow so periods of Amber and Blue flag are just plotted as 4" and 2" respectively to show the duration of these periods.
The EA plot of the river level at the upstream end of [Iffley Lock] is usually a good indication of how things have changed overnight, i.e., whether the flow is increasing or decreasing, and therefore the likely direction of any flag change during mid-morning.
The actual flow rate is related to the difference between the level at the downstream end of [Osney Lock] and the upstream end of Iffley Lock. Zero flow corresponds to 2.14m (the offset between the two gauges). The red/amber transition seems to be around 2.8m, and the amber/blue about 2.7m.
Past experience suggests that the Red Flag will continue while the river remains more than 6" above normal.