Lake Outflow and Hillslope Lateral Inflows Dictate Thermal Regimes of Forested Streams Draining Small Lakes

Abstract

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.Plain Language Summary Climate change and forest disturbance are altering stream thermal regimes and this impacts ecologically, culturally, and economically important fish species, such as salmon and trout. Decades of research have provided important insights on how stream temperatures respond to environmental change; however, more insight is needed, particularly for complex stream network structures, such as lake-stream systems. We collected detailed field measurements at a forested stream that drains a lake in northern Sweden to quantify the energy exchanges controlling stream temperature below a lake. We found that warm water coming from the lake, cool water from the hillslopes, and stream water exchanged with the streambed sediments had the most influence on stream temperature. The lake delays and moderates runoff from the catchment upstream of the lake; therefore, the lake and hillslopes deliver water to the stream at different times, creating a dynamic thermal environment below lakes. Such information is highly relevant for building better models to predict how stream temperatures will respond to environmental change and management interventions, particularly for the extensive lake-dominated landscapes in northern regions.LEACH ET AL.

Effects of lake‐groundwater interaction on the thermal regime of a sub‐alpine headwater stream

“…Previous studies have examined the distinct thermal characteristics of lake‐headed stream temperatures (Dripps & Granger, 2013; Garrett, 2010; Leach et al, 2021; Mellina et al, 2002) and the thermal effects of lakes within lake‐stream networks (Carlson & Poole, 2021). This study presents an example of an indirectly lake‐headed stream, where the interaction of groundwater and lake water, and the hydraulic gradient determine the resulting stream temperature.…”

Do headwater lakes moderate downstream temperature response to forest harvesting? Illustrating opportunities and obstacles associated with virtual experiments

“…Due to these modelling results and the uncertainty in parameterizing the hyporheic exchange term for the virtual experiments conducted in this study, we also ran scenarios with q HTS = 0.002 L s −1 m −1 . This value was selected for comparison because it is within the range of values from the literature for small streams (Edwardson et al, 2003; Story et al, 2003) and also produced reasonable predictions of downstream observed stream temperature (Leach et al, 2021; root mean square error = 0.83°C, mean error = 0.03°C and Nash‐Sutcliffe efficiency = 0.87). Considering both values for the hyporheic exchange term and comparing post‐harvest stream temperature responses and energy flux estimates highlights model sensitivity to this uncertainty and helps diagnose potential issues and implications of using a calibrated model for conditions outside of those for which the model was calibrated, such as simulating streamside harvesting.…”

“…Hourly stream discharge from July 1 to October 10, 2016, was measured at hydrometric stations bounding the main study reach (C5 and C6). In addition, we also drew upon data collected at the C1 hydrometric station, which drains a lakeless watershed located within 2 km of the C5–C6 study reach (Leach et al, 2021; Leach & Laudon, 2019). The catchment area upstream of the C1 stream gauge is 48 ha, covered primarily by forest (98%) with a minor component of mire (2%).…”

“…We have previously applied this stream temperature modelling framework to the C5-C6 study reach using detailed field measurements collected during summer and autumn 2016 (Leach et al, 2021).…”

Groundwater, Soil, and Vegetation Interactions at Discrete Riparian Inflow Points (DRIPs) and Implications for Boreal Streams

“…DRIPs are not only important for source-transport mechanisms from terrestrial to aquatic ecosystems, but also have an important biogeochemical role for streams. DRIPs create variable streamflow conditions along the reach (Lupon et al, 2019;Gómez-Gener et al, 2020), and control stream temperature dynamics (Leach et al, 2021). Further, Lupon et al (2019) demonstrated that directly downstream of DRIPs, the biological uptake of DOC increased, leading to enhanced emissions of methane and carbon dioxide across flow conditions.…”