Temporal Pattern and Spatial Variability of Streambed Exchange Flux in a Braided River

The streambed flux is variable in space and over time; the spatial variability results in part from bedforms and the temporal variability is commonly driven by stream stage change associated with hydrological events. Heat as a tracer to delineate the streambed flux pattern has been widely adopted in numerous fields, whereas if it is of applicability in strongly abrupt varying bedforms is still in doubt. Here a braided reach was selected as study site. A 10-day monitoring campaign including the temperature and water level of stream and streambed was conducted, during which three precipitation events occurred. One-dimensional (1-D) analytical method based on the amplitude attenuation (A_r) and 1-D numerical method were used to interpret the temperature time series data for any sensor pair. This study summarizes two physical mechanisms driving hyporheic exchange: hydraulic pumping and pressure head on the streambed; the former was distributed in narrow channels between braided bars with coarse-grained and poorly sorting sediments and resulted in a negative relationship of stage change to vertical streambed fluxes, but in contrast, the latter occurring in wider channels with finer sediments was driven by bedforms and showed a positive correlation. Moreover,a total of five bedform-driven flux patterns were found: (1) downward flow driven by the head difference between groundwater and stream, (2) downward flow related to a meter-scale pool, (3) a transition from upward to downward flow associated with a centimeter-scale riffle, (4) horizontal flow in braided bars and (5) upward flow driven by vegetation roots. Apparently, the yielded fluxes in braided bars (the fourth scenario) were not realistic, so to accurately describe the streambed flux, simultaneous temperature and head monitoring is necessary. Overall, multiple physical mechanisms together contributed to the complex streambed flow system, which reflected great challenges for the scaling up of point-in-space seepage flux.