In August 1988, researchers released green and red dyes at the centre and shoreline of Twin West Lake to uncover its hidden circulation; wind above helped drive the hidden movement below

A lake can look still while water below the surface moves in different directions. Scientists got a rare glimpse of that hidden movement in August 1988, when they released green and red dyes into Twin West Lake in British Columbia. The experiment showed that wind across the surface did not simply push the whole lake in one direction. Instead, it helped create a layered circulation, with the central dye moving one way near the surface and being carried back in the opposite direction lower down.According to a study published in Limnology and Oceanography, researchers carried out dye-release experiments in Twin West Lake on August 2, 18 and 24, 1988. Sodium fluorescein, which appeared green, was released near the centre of the lake, while Rhodamine WT, which appeared red, was released from the shoreline. Typically, researchers slowly poured about two litres of a 20% dye solution into the water. The concentrated dye solution was initially slightly denser than the surrounding lake water, causing it to sink. As it mixed with the surrounding water, however, it became diluted. The thermocline limited its downward movement, keeping the dye concentrated in the lake’s upper layer.Tracing how substances spread through a lakeThe experiment was designed to show how substances spread through a small lake. Twin West Lake lies about 75 km east of Vancouver, at an elevation of roughly 500 metres. It is a small, oligotrophic lake, meaning its waters contain relatively low nutrient concentrations. During summer, the lake develops a warm upper layer, known as the epilimnion, above a much colder layer called the hypolimnion. Between them is a sharp thermocline, located only about 3 metres below the surface. This layering was important because it meant that water could move in different directions at different depths.To capture those movements, researchers photographed and videotaped the spreading dye from a vantage point overlooking the lake. A camera recorded images every two minutes, while instruments at the centre of the lake simultaneously measured wind speed and direction. Fourteen marker buoys arranged on a 60-metre grid provided fixed reference points, allowing researchers to reconstruct how the coloured patches changed position over time. The observations therefore showed not only where the dye travelled, but also how its movement varied within the stratified upper layer.The August 24 experiment provided the clearest example. Limnology and Oceanography reported that the central green dye responded to the wind-driven circulation, but its movement differed with depth. At the surface, the plume travelled in the direction of the wind at an average speed of about 2.8 cm per second. Lower in the epilimnion, however, the water carried the dye in the opposite direction at roughly 1 cm per second. This contrast revealed that the wind was not simply pushing the entire upper layer forward. Instead, it was associated with a circulation in which water moved back beneath the surface flow. As the wind changed direction, the dye cloud stretched and distorted, further exposing differences between surface and deeper currents. The researchers concluded that horizontal spreading was influenced by vertical differences in water velocity, changes in wind direction and turbulent mixing.What dye reveals about lake circulationThe coloured patches were more than a visual demonstration. By processing the video images, researchers could track how the dye clouds expanded over time and use that information to estimate the lake’s horizontal dispersion rate. Similar tracer experiments have shown why such measurements matter. According to a study published in Water Resources Research, researchers using Rhodamine WT in a stratified lake found that boundary mixing produced a distinct dye intrusion that travelled more than 200 metres offshore. They also recorded elevated eddy diffusivity near the lake boundary, showing how circulation and mixing can influence the transport and spreading of dissolved material in stratified water.This matters because substances introduced into a lake do not always stay where they were first released. Nutrients, pollutants, and other dissolved substances may get transported by currents before they are eventually distributed over a larger area as a result of turbulence. That is why it is important to understand how quickly and in what direction material moves for uses such as lake fertilisation and pollution control.Making hidden water movement visible Modern tracer experiments use more advanced instruments to follow the same basic process. A study in the Journal of Environmental Engineering introduced Rhodamine WT into an arm of California’s Keswick Reservoir and monitored the dye for four days using moored water-quality sondes. Researchers combined those observations with surface drifters and the Delft3D hydrodynamic model to examine how the tracer moved through the reservoir. The measurements and modelling revealed water velocities of about 1-5 cm per second across much of the study area, while the model was used to examine how mixing developed as the dyed water travelled downstream.The approach was more sophisticated than the work carried out at Twin West Lake in 1988, but the underlying principle was similar: follow a visible tracer to reveal otherwise difficult-to-see water movement. At Twin West Lake, researchers released sodium fluorescein near the centre of the lake and tracked its changing position with time. The dye exposed contrasting currents within the epilimnion, with surface water moving with the wind while deeper water moved in the opposite direction. Rather than showing a single current carrying the dye uniformly across the lake, the experiment revealed a layered circulation in which water movement varied with depth.Catch the latest World News and Live updates. Download the TOI app.