The account
Every flow, in millimetres of lake
One Water converts every Okanagan Lake flow — inflow, evaporation, the dam release, and what each community withdraws and returns — into a single common unit, millimetres of lake level, so the net effect of community water consumption can be seen directly against the lake level people actually watch.
Okanagan Lake's water balance is normally written in acre-feet, a unit almost nobody can picture. This page writes every flow the same way the lake itself reports: as millimetres of lake level. Rain, evaporation, what communities take, what they give back, and what runs out of the dam are all on one scale, so they can finally be compared with each other — and with the water line people can see from the beach.
One millimetre of Okanagan Lake is 344 million litres. That is the conversion everything on this site rests on, and it is worth holding onto: a flow that sounds enormous in litres is often a few millimetres of lake, and a few millimetres of lake is a great deal of water.
Which months take the water away
The lake gains and loses on a strong seasonal rhythm. Below is the current water year month by month, against what a normal year does. Bars above the line are months the lake rose; bars below are months it fell. The grey ghost behind each bar is the 1944–2025 median for that month.
This is the part of the account that contains no model at all. It is two gauges: the lake level at Kelowna and the outflow at Penticton Dam. The change in level each month is the net inflow minus the release, and that is an identity, not an estimate.
What makes up the net inflow
Net inflow is a single measured number that already nets everything: the creeks, the rain landing on the surface, the evaporation coming off it, and every litre withdrawn and returned. Splitting it back into those parts takes a model, so this second layer is less certain than the one above — and one of its terms is not a measurement at all.
What the level means
A level in metres is not an answer either. Okanagan Lake has a ladder of published levels, each of which means something concrete — an operating limit, a design threshold, a record, or a physical floor. Here is the whole ladder, with where the lake sits today.