Watershed filtering of climate produces heterogeneity in stream thermal regimes

Background/Question/Methods

Thermal regimes are fundamental features of river systems that regulate many ecological processes.  A central challenge for river ecologists is to understand how thermal regimes are controlled by interactions between regional climatic conditions and the diverse geomorphic features of the landscapes they flow through. Regionally, streams express significant variation in thermal response to air temperature, yet the mechanisms that produce this variation are poorly quantified. Efforts to forecast river thermal characteristics under new climate regimes or land-use conditions require improvements in models that quantify relationships between thermal regimes and hydro/geomorphic features of rivers. We measured water temperatures among 50 streams ranging in size from 1^st order tributaries to 5^th order rivers draining large lakes across a single river basin in southwest Alaska.  We hypothesized that thermal variation among smaller streams would be controlled by the dominant sources of water in the watersheds they drain, while large rivers would be influenced by the circulation, stratification, and mixing characteristics of upstream lakes. We used hydrogen and oxygen isotopes of water to estimate the relative contribution of rain and snow to summer streamflows.  Second, we monitored the circulation conditions in lakes with thermal arrays paired with meteorological conditions on land.

Results/Conclusions

Headwater streams draining steep terrain were dominated by snowmelt, tended to be cooler, and relatively insensitive to variation in air temperature compared to warmer, rain dominated streams draining flat watersheds.  The ratio of rain to snow among streams explained ~70% of the variation in stream sensitivity to air temperature among smaller 1^st to 4^th order streams.  However, larger rivers draining lakes were not sensitive to daily changes in air temperature even though the surface water of the lakes they drained were thermally stratified. Instead, river outlets were sensitive to wind driven seiching events that spilled cold hypolimnetic water into rivers, creating short disturbances to thermal regimes dropping 6 to 10^oC following substantial wind events. We found a strong positive relationship between the surface area of lakes and the variability of downstream rivers, a result that is not consistent with prevailing theory regarding the effects of lakes on river thermal characteristics. These results highlight the importance of water source and geomorphic features, such as watershed elevation and slopes, and size of lakes in the landscape, for controlling thermal regimes across river basins. The ecological implications of differences in thermal response (including thermal stability) for the organisms inhabiting these landscapes remains unexplored.