Investigating Upwelling Events in Multi-Arm Fjord-Type Lakes: A Case Study of Quesnel Lake

Lakes are an important part of the environment, being influenced by their inflows and influencing their outflows. The physical properties of lake water, such as temperature and dissolved oxygen concentration, affect downstream habitats and ecosystems. Limnological processes, such as upwelling, can rapidly change downstream river properties. Upwelling introduces a sudden influx of cold water downstream, potentially disrupting the riverine ecosystem, including salmon migration, which is closely tied to water temperature.

Upwelling occurs in thermally stratified lakes when wind forces cause cold water from the hypolimnion to rise to the warm lake surface. For this to happen, wind must have sufficient amplitude and fetch (Wedderburn number close to one) and persist for sufficient duration (exceeding one-quarter of the lake's fundamental seiche period). Upwelling typically occurs during periods of weak stratification, mainly at the beginning or end of the stratification season, and is more pronounced near lake boundaries, where outflows originate.

One example of such a lake-river system is Quesnel Lake, a fjord-type lake in British Columbia, Canada. It is the source of the Quesnel River, which feeds into the Fraser River and one of the world's most productive salmon-bearing systems. Quesnel Lake is a three-armed, Y-shaped lake with West, North, and East Arms. The West Arm is divided by a shallow sill (maximum depth of 35 m) and contraction into the West Basin and the Main Basin, which includes the North and East Arms. The Main Basin reaches a maximum depth of 511 m and includes 97.7% of the lake volume. The Quesnel River originates from the western end of the West Basin and is thus affected by upwelling in the West Basin. The lake’s complex geometry (i.e. multiple arms and basins), combined with complex surrounding topography that creates local wind patterns, complicates the upwelling process

This study uses nine years (2016-2024) of mooring and meteorological data to investigate upwelling in the West Basin of Quesnel Lake and how it affects temperature in the Quesnel River. Data were collected using moorings and meteorological stations in all three Arms to capture the spatial variability of wind and temperature. Temperature loggers, ADCPs, wind speed and direction were used to identify upwelling in the West Basin, as well as the wind patterns that generate upwelling and the river’s response.

During each year of observation, upwelling in the West Basin occurs multiple times during summer stratification, despite the strong thermal stratification. Upwelling is correlated with winds aligned with the lake’s thalweg, exceeding the 80th percentile of wind speed, and lasts 3-6 days, causing temperature drops of over 5°C in the Quesnel River.  The most pronounced lake surface temperature drops occur at the Quesnel River mouth and moorings in the West Basin. Within the West Basin, the narrowing and shallowing of the lake toward the river mouth further contributes to the temperature decrease in the river. Therefore, the lake’s geometry not only complicates the occurrence of upwelling but also amplifies its downstream impacts.