Water levels at Welney Causeway have risen enough for the A1101 to be closed, leaving motorists with a 30-mile diversion.
At 12.15pm today (Monday) the water measured 2.63m, with the typical range being between zero and two metres.
An Environment Agency report said Wash Road at Welney starts to flood at 2.35m and locals tend to use the 2.40m red mark on the level gauge.
Welney Causeway floods as part of an essential safety feature of the Ouse Washes flood defence system. The washes provides storage for floodwater so the River Great Ouse does not break its banks. The water remains until tides and river flows allow it back into the river and then the sea. This can take a few days or several weeks.