Hydrodynamics Of Shallow Water Habitats In The Sacramento San Joaquin Delta

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.

An ecosystem in freefall, a shrinking water supply for cities and agriculture, an antiquated network of failure-prone levees—this is the Sacramento-San Joaquin Delta, the major hub of California's water system. Written by a team of independent water experts, this analysis of the latest data evaluates proposed solutions to the Delta's myriad problems. Through in-depth economic and ecological analysis, the authors find that the current policy of channeling water exports through the Delta is not sustainable for any interest. Employing a peripheral canal-conveying water around the Delta instead of through it—as part of a larger habitat and water management plan appears to be the best strategy to maintain both a high-quality water supply and at the same time improve conditions for native fish and wildlife. This important assessment includes integrated analysis of long term ecosystem and water management options and demonstrates how issues such as climate change and sustainability will shape the future. Published in cooperation with the Public Policy Institute of California