Dispersion Mechanisms of a Tidal River Junction in the Sacramento–San Joaquin Delta, California

Abstract

In branching channel networks, such as in the Sacramento-San Joaquin River Delta, junction flow dynamics contribute to dispersion of ecologically important entities such as fish, 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 in the Georgiana Slough junction, which is composed of the North and South Mokelumne rivers, Georgiana Slough, and the Mokelumne River, show that flow phasing differences between these rivers arise from operational, riverine, and tidal forcing. 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 intrajunction dynamics. We supplemented field results 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, we observed smallscale junction flow features 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. KEYWORDS Junction dispersion, flow phasing, tidal trapping, super-tidal time scales, Delta Cross Channel Hydrodynamic processes in the Sacramento-San Joaquin River Delta affect ecosystem function through the transport of salt, sediments, heat, contaminants including selenium and ammonium, and,

Recent Advances in Understanding Flow Dynamics and Transport of Water-Quality Constituents in the Sacramento–San Joaquin River Delta

“…Thus, numerical models of transport used to inform policy must correctly represent the mixing effects of junctions. The challenge of this task is demonstrated by the complex flow behavior seen at the Mokelumne River-Georgiana Slough junction described by Gleichauf et al (2014) (see also Gleichauf 2015). Tidal flows through this junction produce fronts (regions of separation) and secondary flows, both of which can significantly affect the paths taken by materials and organisms that pass through the junction (Figure 4).…”

“…From 1999 to 2010 there was a significant decreasing trend in total suspended solids in the Delta, possibly resulting from supply limitation or trapping by aquatic vegetation. Hestir diurnally averaged heating in summer) attributed to dispersion were found to be necessary to close the heat balances in the Sacramento Deep Water Ship Channel (SDWSC) at the Port of Stockton ) and the Delta as a whole (Gleichauf 2015), suggesting that flow-based heat transport may in fact be an important determinant of Delta temperatures.…”

“…Water temperatures in the Delta are generally determined by surface heat fluxes Gleichauf 2015) and thus can be well correlated with atmospheric temperatures . Inflow temperatures are generally lower than temperatures in the interior of the Delta (Figure 9), so inflows can affect the spatial gradient of water temperature.…”

“…Both observations (Gleichauf et al 2014) and numerical modeling of the Delta (Monsen et al 2007) show that gate operations significantly affect hydrodynamics and transport patterns in the Delta. When the DCC gate is closed, Sacramento River water reaches the pumps through Threemile Slough and through the confluence region (Monsen 2000;Monsen et al 2007).…”

“…This temperature stratification does appear to be dynamically significant, and is the result of turbidity-dependent solar heating and mixing provided by the wind and surface cooling, as well as by turbulence production near the bottom. The practical significance of this stratification is that, given that existing descriptions of the temperature field of the Delta are based on the existing network of sensors, which are largely nearsurface, it may be necessary to reconsider the nature of the effects of long-term temperature variability on fish habitat Gleichauf 2015). In particular, it may be possible that there are daytime refugia of colder water that temperature-sensitive fish such as Delta Smelt can use to avoid warmer surface waters.…”