Lake Outflow and Hillslope Lateral Inflows Dictate Thermal Regimes of Forested Streams Draining Small Lakes. Issue 6 (10th June 2021)

• Record Type: Journal Article
• Authors: Leach, Jason A.; Neilson, Bethany T.; Buahin, Caleb A.; Moore, R. Dan; Laudon, Hjalmar
• Is Part Of: Water resources research. Volume 57:Issue 6(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-06-10
• Subjects: energy balance -- forest hydrology -- hyporheic -- lake‐stream -- lateral inflow -- stream temperature; Hydrology -- Periodicals; 333.91
• DOI: 10.1029/2020WR028136
• Languages: English
• ISSNs: 0043-1397

Abstract

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. Key Points: Lake influence on downstream temperature varies in space and time, primarily as a function of lake outflow Lakes can moderate streamflow and elevate water temperature which both influence downstream thermal regimes Differences in timing of warm lake outflows and cool lateral hillslope inflows to the stream reach creates a dynamic thermal regime