Effects of Topographic Resolution and Geologic Setting on Spatial Statistical River Temperature Models

Abstract

River temperature exerts a critical control on habitat for aquatic biota. As the climate warms in eastern Canada, threats to habitats of cold-water species will increase, underpinning the necessity to develop an understanding of landscape-scale, thermal regimes of flowing waters. We assessed the performance of spatial statistical network (SSN) models of river temperature using high-resolution thermal infrared imagery (0.6 m) and LiDAR (1 m) compared to NASA's Shuttle Radar Topography Mission (SRTM-30 m) topographic data and interrogate LiDAR derived fine-scale models (3 ha) to describe groundwater connectivity to surface waters in catchments with shallow overburden and varied bedrock geology. LiDAR improved model performance in a catchment underlain by a homogeneous, high hydraulic conductance bedrock (Cains River) but did not improve model performance in a catchment with heterogeneous bedrock and variable hydraulic conductance (North Pole Stream). We hypothesize that differences in bedrock conductance modified topographic controls on subsurface flows and discharge patterns to the rivers and thus produced the mixed performance of the SSN models. At finer scales, river reaches in steep valleys incising high conductance bedrock produced groundwater discharge, which was absent in incised valleys with low conductance bedrock. These findings indicate that while topography exerts an important control on landscape-scale hydrological processes, geologic setting is a similarly important influence on hydrological processes. We suggest the inclusion of a third dimension of spatial autocorrelation, representative of the vertical plane that captures the geologic setting, would broaden the geographic applicability of spatial statistical models for river temperature studies.

Salmonid thermal habitat contraction in a hydrogeologically complex setting

“…In the Miramichi, the majority of wetlands are located in upland areas. It is likely that these upland wetlands are tightly coupled with climatic and solar radiative forcing, thereby providing heat to the rivers that flow through them in summer (e.g., Monk et al 2013, O'Sullivan et al 2020a. Precipitation was negatively correlated with temperature in all MLR and SSN models and was the least important parameter in the RF models (Tables 4 and 5).…”

“…Landscape structure and form are highly influential on the spatial variability of river temperatures within the Miramichi catchment (Monk et al 2013, O'Sullivan et al 2019a. In particular, elevation and valley wall slope are suggested to have a significant role in river temperature regimes in the catchment (O'Sullivan et al 2020a). Elevation was calculated as average elevation delimited by the catchment area upriver of a sampling location.…”

“…It is becoming increasingly apparent that surface water-groundwater interactions can significantly modify river thermal regimes (e.g., Constantz 1998, Briggs et al 2017. O'Sullivan et al (2020a) demonstrated that bedrock hydraulic conductance influenced river temperature in the Miramichi, hypothesizing that both bedrock geology and topography influence how groundwater impacts river temperature. Haitjema and Mitchell-Bruker (2005) introduced the concept of a water table ratio (WTR).…”

“…The advent of spatial statistical models for river networks has allowed for unprecedented insights into the spatial variability of river thermal regimes at regional scales (e.g., Turschwell et al 2016, Isaak et al 2017, Jackson et al 2017). However, recent work has suggested that while current spatial statistical models can account for spatial autocorrelation of surface water dominant river networks, they are less effective in regions where surface water has strong connections to groundwater, particularly water with weak links to surface conditions, for example, bedrockderived or deep-sourced groundwater (Tague et al 2007, O'Sullivan et al 2020a). We found that a RF approach generated more accurate river temperature models.…”

“…Valley incision has been shown to be influential to river thermal regimes (see Dugdale et al 2015, Briggs et al 2020). However, this influence is suggested to be governed by hydraulic conductance of the underlying geologic material, where incisions in areas with low hydraulic conductance geomaterial can lack significant groundwater influence and are more influenced by climatic and solar radiative forcing (e.g., O'Sullivan et al 2020a). The negative correlation between valley wall slope and WTR and the increased importance in both parameters during maximum temperature events may therefore be linked to surface water-groundwater interactions.…”