Temperature and tidal river junction dynamics in the Sacramento-San Joaquin Delta, California [electronic resource]
Keywords: Civil and Environmental Engineering.

The Sacramento-San Joaquin River Delta is the complex of tidally-forced rivers stemming from the Sacramento and San Joaquin Rivers that flow into San Francisco Bay. The dynamics within the Delta, determined by the balance between tides, freshwater inflows, atmospheric forcing, and bathymetric forcing, determine the viability of the water for local ecology, biology, and for two-thirds of California water-users. The hydrodynamics of the Sacramento-San Joaquin River Delta were examined over super-tidal, tidal, tidally-averaged, spring-neap, and month-long timescales using observations from deployed instruments and an array of public data. Two processes were studied: super-tidal and subtidal dynamics at tidal river junctions and subtidal temperature dynamics over the Delta. In the first part of the dissertation, junction flow dynamics are explored because they contribute to dispersion of ecologically important entities such as fish and their larvae, pollutants, nutrients, salt, sediment, and phytoplankton. Flow transport through a junction largely arises from velocity phasing in the form of divergent flow between junction channels for a portion of the tidal cycle. Field observations at several tidal junctions show that flow phasing differences between junction branches arise from operational, riverine, and tidal forcing. At the Georgiana Slough junction, composed of the North and South Mokelumne Rivers, Georgiana Slough, and the Mokelumne River, a combination of Acoustic Doppler Current Profile (ADCP) boat transecting and moored ADCPs over a spring--neap tidal cycle (May to June 2012) monitored the variability of spatial and temporal velocity, respectively. Two complementary drifter studies enabled assessment of local transport through the junction to identify small-scale intra-junction dynamics. Field results were supplemented with numerical simulations using the SUNTANS model to demonstrate the importance of phasing offsets for junction transport and dispersion. Different phasing of inflows to the junction resulted in scalar patchiness that is characteristic of MacVean and Stacey's (2011) advective tidal trapping. Furthermore, small-scale junction flow features were observed including a recirculation zone and shear layer, which play an important role in intra-junction mixing over time scales shorter than the tidal cycle (i.e., super-tidal time scales). The study period spanned open- and closed-gate operations at the Delta Cross Channel. Synthesis of field observations and modeling efforts suggest that management operations related to the Delta Cross Channel can strongly affect transport in the Delta by modifying the relative contributions of tidal and riverine flows, thereby changing the junction flow phasing. In the second part of the dissertation, the dynamics of subtidal water temperatures within the Sacramento-San Joaquin Delta were examined during a summer containing high flows (2011) and low flows (2014). Significantly warmer temperatures were observed in 2014 throughout the Delta with disparities reaching 6°C in the southern Delta. Differences in Delta water temperature were linked to fluctuations in freshwater inflows, tidal dispersion, and atmospheric forcing. First, the Sacramento and San Joaquin along-channel temperatures and reservoir flows were compared for 2011 and 2014, finding that large reservoir and tributary flows provided cool entry temperatures to the Delta and vice versa for 2014. Next, a heat balance was performed over the Delta to estimate the volume and time averaged heat transport due to residual advection, atmospheric heating, evaporation, exports, tidal dispersion, and Stokes drift. While atmospheric heating transferred the most energy by an order of magnitude, what dictated the overall heat transport was the interplay between the water and the atmosphere. In particular, in 2014 less heat was transported to the Delta despite warmer air temperatures because water temperatures were warmer than air temperatures, which accelerated the latent heat flux cooling. Ultimately, inflows largely determined Delta water temperatures because it dictated the water's exposure time to atmospheric heating and changed the strength of the tidal dispersion.