Water and Heat Fluxes in a Channel Bar Compared with Adjacent Streambed and Streambank
AuthorShope, Christopher Lee
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Channel bars are a dominant in-stream geomorphic feature present throughout the western United States in a large range of river classes. A study area in the Truckee River in northwestern Nevada was used to quantitatively examine the influence of channel bars primarily through heat-based estimates of water fluxes in the bed sediments. These channel bar water fluxes were then compared to and contrasted with the adjacent streambed and streambanks over equivalent lengths, to develop a bar, bed, bank relative magnitude relation for this representative site.A variety of field methods were used, which included the installation of monitoring wells and piezometers, instrumentation of the piezometers with pressure transducers and temperature thermistors, and slug tests to estimate hydraulic conductivity. The potentiometric surface throughout the study site was monitored over time and the temperature thermistors were used to estimate transport using heat as a tracer. Horizontal and vertical water fluxes were calculated from the field data. Estimated results were between 3 x 10<super>-8</super> and 9 x 10<super>-6</super> m s<super>-1</super> for the horizontal water fluxes and between 1 x 10<super>-7</super> and 1 x 10<super>-5</super> m s<super>-1</super> for the vertical water fluxes. To better quantify the hydraulic conductivity values for water flux estimates, heat-based numerical simulations were completed. Both one- and three-dimensional numerical models were constructed to aid in the interpretation of these water fluxes and the relationship between the channel bar, streambed, and streambank. Inverse estimation of the hydraulic conductivity was completed by minimizing the difference between observed and simulated temperatures at a variety of locations. The numerical results indicated that using the heat-based hydraulic conductivity values, median vertical water flux for the channel bar (3 x 10<super>-7</super> m s<super>-1</super>) was five times less than the streambed median vertical water flux of 1 x 10<super>-6</super> m s<super>-1</super>. Water fluxes were predominately downward throughout the study area and the vertical flux magnitude ranged between 6 x 10<super>-10</super> to 1 x 10<super>-5</super> m s<super>-1</super>. Lateral flow patterns appeared to be significant and multi-dimensional simulations were completed. Fluid flux results estimated by the multi-dimensional simulations indicated a range between 4 x 10<super>-10</super> and 3 x 10<super>-6</super> m s<super>-1</super>, throughout the study area. Lateral water fluxes between the channel bar and the stream were found to be an order of magnitude greater than between the streambank and the stream. Downstream channel bar fluxes were an order of magnitude greater than upstream fluxes and the stream/channel bar fluxes were at least 2 times greater than fluxes within the channel bar. This research provided the channel bar, streambed, and streambank hydraulic physical framework to form a foundation for assessing critical water quality issues, such as biogeochemical interpretation of stream exchanges with sediment in a channel bar environment. Many biogeochemical interactions occur in the near stream region affected by these interactions. The results suggested that the impact of this channel bar increased local exchange 6 times relative to vertical streambed exchange during the study period. It was suggested that without the increased groundwater and stream exchange of the channel bar, near stream biogeochemical reactions, vegetation, and habitat structure would be severely impacted.