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].
Past experience suggests that the Red Flag will continue while the river remains more than 6" above normal.
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.
Data in numerical form (since Nov 2006)