Abstract:Little Kickapoo Creek (LKC), a low-gradient stream, mobilizes its streambed-fundamentally altering its near-surface hyporheic zone-more frequently than do higher-gradient mountain and karst streams. LKC streambed mobility was assessed through streambed surveys, sediment sampling, and theoretical calculations comparing basal shear stress ( b ) with critical shear stress ( c ). Baseflow b is capable of entraining a d 50 particle; bankfull flow could entrain a 51Ð2 mm particle. No particle that large occurs in the top 30 cm of the substrate, suggesting that the top 30 cm of the substrate is mobilized and redistributed during bankfull events. Bankfull events occur on average every 7Ð6 months; flows capable of entraining d 50 and d 85 particles occur on average every 0Ð85 and 2Ð1 months, respectively. Streambed surveys verify streambed mobility at conditions below bankfull. While higher gradient streams have higher potential energy than LKC, they achieve streambed-mobilization thresholds less frequently. Heterogeneous sediment redistribution creates an environment where substrate hydraulic conductivity (K) varies over four orders of magnitude. The frequency and magnitude of the substrate entrainment has implications on hyporheic zone function in fluid, solute and thermal transport models, interpretations of hyporheic zone stability, and understanding of LKC's aquatic ecosystem.

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