https://doi.org/10.71548/79

Szeitz, A. J. (2026) Advancing understanding of stream temperature controls in rapidly changing sub-Arctic environments, McMaster University, https://doi.org/10.71548/79

Climate warming has occurred at much greater rates in northern latitudes than elsewhere globally, and northern regions are experiencing substantial hydroclimatic and environmental changes in response to warming. Precipitation regimes are shifting with an increasing proportion of precipitation as rain, shrub vegetation is expanding in density and distribution across the sub-Arctic, and permafrost is undergoing widespread degradation; all of these changes have impacts on northern hydrological processes including stream temperature. However, stream temperature in northern latitudes has received substantially less attention than temperate regions, so there is a lack of knowledge of the distribution and nature of northern stream thermal regimes, and uncertainty as to how well stream temperature process understanding transfers to northern environments with their distinct hydrological and environmental conditions, e.g., permafrost. In this thesis, a data set of regional hydrometeorological and catchment characteristics and two field observation-driven simulations of changing environmental processes are used to advance our understanding of the current state of northern stream temperatures and how they may respond to further environmental changes. The first assessment conducted of stream thermal regimes and thermal sensitivities of headwater and meso-scale streams across Yukon Territory indicates substantial variability in thermal regimes across catchment scales. A linear regression analysis was conducted on a data set compiled for 57 catchments in Yukon, ranging in area from 5.4 to 86,500 km2 , and with catchment mean permafrost probabilities ranging from 0.0 to 0.99. Stream thermal sensitivity is a measure of how responsive stream temperature is to changes in available energy, and the analysis identified thermal sensitivities ranging from 0.14 to 0.84 ºC ºC-1 . Multiple regression and redundancy analyses identified that the greatest single environmental characteristic explaining the variance in thermal sensitivity was catchment topography (9 % variance explained); however, 39 % of the variance in thermal sensitivity was jointly explained by catchment physiography, land cover, and permafrost presence indicators, suggesting thermal sensitivity is the result of multiple interacting controls. These results indicate that catchment permafrost extent does not directly impact thermal sensitivity, but rather influences catchment runoff and groundwater processes which in turn directly influence thermal sensitivity. The northern riverine thermalscape may shift towards one influenced by increasing groundwater contributions which may buffer thermal sensitivity against warming climatic conditions. Results of the first field-driven simulation of the impact of riparian shrubification on stream temperature show that the widespread advance of shrubification across northern latitudes suggest the change in mean hourly incident radiation received by streams may range from −250.2 to +17.1 W m−2 , with a mean of −41.6 W m−2 , across a range of stream widths (up to 35 m wide), orientations, and gradients. This alteration in incident radiation will alter stream radiative balances and is largely expected to reduce stream temperatures through a combination of reduced incoming radiation and suppressed turbulent heat exchange, with two complimentary temperature metrics suggesting cooling of ~1-2ºC may occur for headwater streams less than 5 m wide. However, this thesis highlighted the need for additional, detailed investigations in the future as it was shown that the stage ofvi shrub expansion along the spectrum of ‘absent’ to ‘maximum extent’ could result in either a cooling or warming impact on stream temperature due to the inter-related responses of altered riparian shrub canopy, incident radiation, and the responses of above-stream micrometeorology (i.e., air temperature, relative humidity, and wind speed). These complexities were further demonstrated through the results of the process-based stream temperature model simulations that assessed stream temperature responses to changing environmental conditions anticipated due to climate warming. Certain process changes, such as increased lateral inflows to streams, were found to cool or warm streams depending on the permafrost setting of the streams, with continuous permafrost settings resulting in warming from increased lateral inflow while discontinuous permafrost settings predicted cooling. Across the breadth of environmental change scenarios simulated, there were contrasting responses within depending on the permafrost setting and on the process altered, with temperature responses ranging from +1.25 to -0.66ºC. These results indicate that the net stream temperature response to future conditions must involve careful assessment and accurate representation of local environmental conditions and a fuller understanding of the complexities and inter-relationships present between changing environmental conditions and hydrological responses which are known to
impact stream temperature.

https://macsphere.mcmaster.ca/items/80ab6de1-6eee-4dbc-afbe-1066ec3ee1ea/request-a-copy?bitstream=d0710674-11cc-487b-b43b-4c77ac00f1fa