1-D vertical flux dynamics in a low-gradient stream: An assessment of stage as a control of vertical hyporheic exchange

Abstract

This study characterized vertical one-dimensional (1-D) flux rates in the top 150 cm of the streambed and explored the relationship between stage and 1-D vertical flux rates in a low-gradient stream. Six multi-level samplers were installed along a 25 m stretch of the thalweg spaced at 5 m intervals. Each sampler recorded temperature at five separate depths (30, 60, 90, and 150 cm) from February 2009 to March 2010. Temperature and stage data were collected at 15 min intervals. For the midpoint between sensors, 1-D flux rates were calculated using the 1-D conduction-advection-dispersion equation utilized in the VFLUX MATLAB program. The dominant flux direction at a depth of 15 cm was downward (negative), while the average flux directions at depths of 45, 75, and 120 cm were upward (positive). Variable flux directions above 15 cm indicated hyporheic exchange with background upwelling groundwater between 15 to 120 cm. Flux rates approach zero in the summer, representing limited hyporheic exchange. Relationships between stage and flux at the near-surface streambed (15 cm) were weak, but statistically significant, with Spearman's rank correlations for all six multi-level samplers ranging from 0.032 to 0.369 with an average of 0.085.