Monthly river temperature trends across the US confound annual changes.

Area of Science:

• Environmental Science
• Hydrology
• Ecology

Background:

• Climate change and human activities are altering global water cycles and impacting surface water energy exchanges.
• River temperatures are critical indicators of aquatic ecosystem health and water quality.
• Existing studies on river temperature trends often lack large spatial extents and seasonal analyses.

Purpose of the Study:

• To analyze seasonal trends in river temperatures across the contiguous United States.
• To investigate the spatial organization of monthly temperature anomalies.
• To determine the influence of human impact on river temperature patterns.

Main Methods:

• Conducted annual and monthly trend analyses of river temperatures.
• Utilized data from 138 sites for 20-year periods and 40 sites for 40-year periods.
• Examined spatial patterns of temperature anomalies at both reference and human-impacted sites.

Main Results:

• Annual warming trends often obscured significant sub-annual cooling trends at many sites.
• Monthly temperature anomalies exhibited spatially organized and persistent regional patterns.
• Human-impacted sites showed the largest warming and cooling anomalies, particularly during summer months.

Conclusions:

• River temperature trends display significant monthly variability that is masked by annual analyses.
• Human activities play a substantial role in driving observed river temperature patterns.
• Effective management of lotic networks requires consideration of sub-annual river temperature dynamics.