Lake Outflow And Hillslope Lateral Inflows Dictate Thermal Regimes Of Forested Streams Draining Small Lakes

Empirical studies have highlighted the important influence of lakes on stream temperature at landscape scales, even when lakes comprise just a small fraction of the catchment area. However, only a few studies have focused on the hydrologic and thermal processes underpinning these landscape patterns. We collected detailed field measurements at a boreal stream that drains a headwater lake and used these data within a process-based stream temperature model to, (a) document the downstream extent of lake influences at both seasonal and event-based timescales, (b) assess the hydrologic and thermal processes that control the observed downstream variability, and (c) compare downstream temperature for streams with and without a headwater lake. Summer and autumn lake outlet temperatures were elevated compared to hillslope lateral inflow temperatures. During periods of low lake outflow, stream temperatures decreased rapidly downstream as local energy fluxes, primarily lateral inflows from the hillslopes and hyporheic exchange, overwhelmed the lake effects. The lake influence on downstream temperature was the greatest during periods of high lake outflow and persisted at least 1.4 km downstream. Since lakes can moderate and delay upstream rainfall runoff response, periods of high lake outflow and high hillslope inflow rates were generally out-of-phase. This difference in timing of warm lake outlet and cool hillslope water creates a dynamic thermal environment downstream of the small lake. Such lakes are ubiquitous in northern landscapes, and accounting for the competing influence of lake and hillslope contributions on downstream water temperature is critical for predicting how network-scale thermal regimes will respond to environmental change.