Qc threshold departs from theoretical Qc in urban watersheds: The role of streambed mobility data in managing the urban disturbance regime

The threshold discharge (Qc) for streambed mobilization is both biologically and geomorphically relevant to stream ecosystems. Excess streambed mobilization can disturb benthic organisms and initiate cycles of channel instability. The mechanistic relevance of Qc gives it great utility for aquatic ecosystem studies, stormwater management, and stream restoration design. However, field and laboratory data document considerable variability in Qc across hydrogeomorphic settings, underscoring the importance of using field data to calibrate the Qc estimate for a given stream or region. This paper shows how both high- and low-tech monitoring protocols can be used to constrain a Qc estimate, depending on monitoring program goals and budgets. Data from 3 hydrogeomorphically distinct settings in the USA and Australia show that the departure of Qc from theoretical estimates increases with watershed imperviousness. Although Qc estimates derived from conventional critical Shields stress values tend to be a reasonable and conservative starting point for stormwater management in streams that lack site-specific or regional data, streambed mobility monitoring is recommended to calibrate and validate Qc estimates for a stream or region prior to making large investments in stormwater interventions aimed at mitigating the urban streambed disturbance regime.