Okanagan Lake — the physical model
An interactive model of how the lake is shaped and how it moves — the bathymetry, the basins and sills, the inflows and the dam, and the physics that carry water and heat through the system. Drag to rotate, scroll to zoom.
Why the seiche matters
On 25 August 2024, Ironman Canada cancelled the swim leg of its final Penticton race barely half an hour before the start. Okanagan Lake's surface had cooled by more than 10 °C in under a day, to roughly 13–14 °C — below the 12 °C floor that World Triathlon and Triathlon BC set for open-water swimming. Organizers first shortened the swim to 900 m with mandatory wetsuits, then dropped it entirely and sent athletes straight to the bike. That kind of abrupt surface cooling is the signature of wind-driven upwelling: a strong along-lake wind pushes the warm surface layer downwind and tilts the thermocline, so cold deep water rises to the surface at the windward end — the same internal (baroclinic) seiche this model represents.
The seiche does more than chill a swim. Because it tilts and sloshes the thermocline back and forth along a 90‑km lake, it repeatedly lifts cold, nutrient-rich, lower-oxygen deep water toward the surface at one end while depressing it at the other. That upwelling can fertilize nearshore algae, briefly drop dissolved oxygen at depth-fed intakes, and swing the temperature drawn by water-supply and cooling intakes as the thermocline tilts past their depth. It reshuffles the cold-water refuge that kokanee and other fish hold below the thermocline, moving that habitat up and down through the season. It carries and disperses creek plumes and any pollutants along the lake rather than letting them settle, and its surface (barotropic) counterpart nudges water levels and currents at the Vernon and Penticton ends, felt at docks, moorings, and the outlet. The stronger and more persistent the wind — largest in the exposed reaches and tied to season — the larger these swings.
Along-lake depths from the BC Fish & Wildlife bathymetric survey (1966, FIDQ), verified against Hay & Company's H3D section (Stronach 2010); planform and across-lake bowl approximate; vertical exaggerated for visibility; white contour lines mark depth bands. Length ~91.6 km; maximum 242m — the deep spine is central-north (near Grant Island); the south basin (Summerland) reaches ~137 m and shoals toward the Penticton outlet. The Kelowna floating bridge crosses ~55 m of water on 690 m of pontoons that draft 5–8 m — a constriction on the surface layer at the narrows.