Where the Lake Floods
Lake Champlain's surface is flat, so one number describes the whole thing: its height above sea level. Through a normal summer it sits near 95 feet. Above 100 the National Weather Service calls it a flood. On 6 May 2011 it reached 103.2, the highest ever measured. USGS flew lidar over the shore and drew the water's edge at each of those levels. Drag the slider and watch it move.
Flooded area at each half-foot of lake level
Flooded ground against lake level, for the whole lake. It is not a straight line and it is not smooth: the water climbs a bank slowly, then reaches a flat river delta or a bay shore and takes several square kilometers at a step.
Most of the flooded ground is at the north end of the lake
The lake barely floods along most of its length. Six hundred miles (966 km) of shore are mostly rock, and the water comes up it and stops. Then in a few places the ground is flat and at lake level, and the water spreads inland for miles: the Winooski backs up over its own delta, and the bays and wetlands behind the island chain fill from the edges.
The datum, and the gauge the maps are keyed to
The mapped levels are published in one vertical datum and the lake gauges report in another, 0.43 feet apart. Taken at face value the stages look arbitrary — 99.6, 102.8, 104.6. Corrected, they are the report's own series of round half-feet, and the odd one out, 103.2, is not a round number because it is the 2011 crest, mapped for its own sake. Everything here is labeled in the same datum the lake gauges report in.
When the lake used to peak, and when it does now
This section is about the date of the annual peak, and it comes from a century of gauge readings this site does not hold. Myer and Gruendling (1979) report an observation of Barnett's from the Rouses Point record: over 1870–1893 the annual maximum fell on 10 May on average; from 1894 to 1976 it fell on 22 April. Eighteen days earlier, and the report offers no explanation — its words are that “the cause of this apparent abrupt shift has not been determined”.
How high will it go next spring?
Barnett found the spring maximum at Rouses Point could be predicted from the snow standing in the basin before the thaw. A model of that is rebuilt on every data run here and scored on springs it was not fitted on. It does not meet the skill threshold set for it, so no forecast is shown.