Episodic Discharge Is the Dominant Correlate of Turbidity Variability in the Slave River Plume Zone of Great Slave Lake: Evidence from 40 Years of Satellite Observations

Lake surface temperature (LST) and NDTI-based optical turbidity are important indicators of water quality and ecosystem dynamics in northern lakes, where climate warming and river discharge strongly influence thermal structure and sediment transport. This study used multi-sensor satellite observations processed in Google Earth Engine to examine long-term (1984–2024) variability in LST and NDTI-based optical turbidity within the Slave River–Great Slave Lake system, Northwest Territories, Canada, using Landsat, MODIS-Aqua and Sentinel-2 imagery. Landsat (1984–2024) and MODIS Aqua (2002–2024) indicated open-water warming of approximately +0.06 and +0.07 °C yr−1, respectively; only the Landsat trend was statistically significant (p = 0.038 versus p = 0.073). The Normalized Difference Turbidity Index (NDTI), used as a relative optical proxy for turbidity, exhibited strong interannual variability that co-varied primarily with Slave River discharge (MODIS Aqua r = 0.91, n = 23, t = 9.80; Landsat 1984–2024 r = 0.72, n = 41, t = 6.41; both p < 0.0001). Principal Component Analysis revealed discharge and NDTI-based optical turbidity as the dominant modes of variability, explaining 65% of the total variance, while LST loaded mainly on a separate component, accounting for a further 32%. The 2020 high-flow event produced the most extensive NDTI-based optical turbidity episode within the 40-year study period, with elevated NDTI consistently detected across all satellite platforms, followed by a marked decline through 2024. Monthly analysis revealed a discharge-dependent NDTI-based optical turbidity lag, with peak NDTI-based optical turbidity occurring concurrently with or within one month of peak discharge during high-flow years but delayed by up to 3–4 months during low-flow years. Spatial analyses further demonstrated persistent NDTI-based optical turbidity intensification near the Slave River mouth and the southwestern basin. These findings indicate that although LST shows a positive but sensor-dependent warming tendency, NDTI variability within the Slave River Plume Zone (SRPZ) was primarily associated with episodic high-discharge conditions consistent with sediment transport processes dominating optical water quality. The study further highlights the value of multi-sensor satellite observations for long-term environmental monitoring in remote subarctic lake systems.