Date of this Version
Streambed hydraulic conductivity (K) is known to be spatially and temporally heterogeneous, but few attempts to understand the controls on temporal variability have been made. This study documents temporal K transience and demonstrates how hydraulic, geophysical, and sedimentological methods can be combined to understand the processes that give rise to changes in streambed K. Falling head permeameter tests and slug tests were conducted to determine vertical K (Kv) and K (slug test K), respectively. These tests were repeated three times over a twelve-week period on the same grid at a depth of 0.5 meters below the bed of the Loup River in east-central Nebraska during the summer of 2017. This grid included (1) a stationary braid bar where diagenetic pore clogging is expected to control K transience, and (2) mobile sediments of the adjacent stream channel where deposition and erosion are thought to be the dominant controls. Sediment samples were collected at the site of each hydraulic test to determine grain size distributions and estimate K. Ground penetrating radar surveys at 450 MHz and frequency domain electromagnetic geophysical surveys provided high resolution images of subsurface structure. Kv ranges between 0.1 and 45 meters/day, and K ranges between 15 and 55 meters/day. Kv and K changed significantly only between the second and third sampling events. K declined 14-20% in both environments while Kv declined 27% on the bar, but was unchanged in the channel. Despite evidence of scour and fill in the channel captured by GPR, deposition and erosion did not exert a dominant influence on K transience. The results of this study suggest that processes other than physical sediment transport, such as bioclogging or gas ebullition, were responsible for the decrease in K.
Advisor: Jesse T. Korus