Seasonal aquatic macrophytes reduce water temperatures via a riverine canopy in a spring-fed stream

Maximum water temperatures in streams throughout the western USA typically occur in late summer and early autumn, coinciding with low stream flow. However, in the spring-fed Big Springs Creek in northern California, where constant-temperature groundwater springs provide relatively stable stream flow throughout the year, peak water temperatures and maximum diurnal variability occur in spring. We attribute this anomaly to the riverine canopy provided by emergent aquatic macrophytes (e.g., Polygonum hydropiperoides and Nasturtium officinale), which mimics the shade function of a riparian canopy. Macrophyte biomass increased 264% between January and August 2011. This increase coincided with a 111% reduction in flow velocity and a 53% increase in stream depth. Solar radiation was reduced by an average of 88% in patches of macrophytes that covered ~50% of the water surface during the summer. Decreased solar radiation reduced rates of stream heating, maximum temperatures, and temperature variability. We tested the riverine canopy hypothesis analytically based on a 2-dimensional hydrodynamic and water-temperature model. The model predicted that emergent aquatic macrophytes reduce maximum water temperatures by an average of 5.1°C (p < 0.001) during late summer, when water temperatures in northern California streams typically increase. Our study shows the influence of a riverine canopy on naturally occurring temporal patterns of water temperature in a spring-fed stream. Our results could inform basin-scale management or regulatory strategies to address water-temperature conditions.