The Water Budget

Eighteen gauged rivers, a few hundred ungauged brooks and the rain that falls on the surface go in. One river comes out — the Richelieu, north into Quebec, past a gauge at Fryers Rapids that has been counting since 1937. None of these figures is taken from a published source; all of them are computed here, and the inflow and outflow are compared as a check.

—cubic kilometers of water, computed from the chart's own depth contours

—cubic meters a second out through the Richelieu, averaged over the last thirty years

—years for the lake to replace itself

—between the wettest month and the driest

Inflow and outflow balance

The eighteen rivers are measured and the outlet is measured; the residual is what is left over. If it had come out at twice the gauged runoff, or negative, something here would be wrong.

Residence time: 2.1 years

The figure usually quoted for Lake Champlain is three years. It is not obvious where that comes from; three years would need an outflow of about 275 cubic meters a second, and the gauge has averaged 362 over eighty-eight years and 395 over the last thirty.

Outflow by month

Mean outflow by month across eighty-eight years. Four fifths of a normal year's variation is snowmelt: outflow peaks in April and May and is lowest in September.

The lake’s water by segment

A residence time for the whole lake averages over very unequal water. Sixty per cent of the water is in one deep trough down the middle; the bays at the north-east corner hold almost none of it. Segments here are the monitoring program's own — the same names its cyanobacteria reports use — extended out from its shoreline stations only through the water, so a label cannot cross a headland the way a straight-line partition would.

segment	km²	km³	% of the water	mean depth

Volume and area: computed here from the chart bathymetry this site already carries — forty-one closed depth contours rasterised at 25 m (82 feet), nested rings resolved by exclusive-or so an island inside the lake stays an island, each cell taking the midpoint of the ten-foot (3 m) band that holds it. The surface area that results, km², is within one per cent of what the USGS National Hydrography Dataset gives for the same polygon; the two are computed from unrelated sources. Inflow: the eighteen gauged tributaries, USGS and Quebec daily discharge, mean of each river's complete years. Outflow: Environment and Climate Change Canada gauge 02OJ007, Rivière Richelieu aux Rapides Fryers, 1937 to 2025, via the GeoMet API. The residual is reported as a residual — ungauged runoff plus rain on the lake minus evaporation — and not split into parts this data cannot separate.

part of Lake Champlain Hydrodynamics · see also the blooms and the watershed