Effects of Drought and the Emergency Drought Barrier on the Ecosystem of the California Delta

Wim Kimmerer,Shruti Khanna,Janet Thompson,Bryan Downing,Frances Wilkerson,Edward Gross,Richard Dugdale,Alexander Parker,Karen Kayfetz

doi:10.15447/sfews.2019v17iss3art2

Wim Kimmerer, Shruti Khanna + Show 7 more

Open Access

https://doi.org/10.15447/sfews.2019v17iss3art2

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Abstract

In 2015, the fourth year of the recent drought, the California Department of Water Resources installed a rock barrier across False River west of Franks Tract to limit salt intrusion into the Delta at minimal cost in freshwater. This Barrier blocked flow in False River, greatly reducing landward salt transport by decreasing tidal dispersion in Franks Tract. We investigated some ecological consequences of the Barrier, examining its effects on water circulation and exchange, on distributions of submerged aquatic vegetation (SAV) and bivalves, and on phytoplankton and zooplankton. The Barrier allowed SAV to spread to areas of Franks Tract that previously had been clear. The distributions of bivalves (Potamocorbula and Corbicula) responded to the changes in salinity at time–scales of months for newly settled individuals, to 1 or more years for adults, but the Barrier’s effect was confounded with that of the drought. Nutrients, phytoplankton biomass, and a Microcystis abundance index showed little response to the Barrier. Transport of copepods — determined using output from a particle-tracking model — indicated some intermediate-scale reduction with the Barrier in place, but monitoring data did not show a larger-scale response in abundance. These studies were conducted separately and synthesized after the fact, and relied on reference conditions that were not always suitable for identifying the Barrier’s effects. If barriers are considered in the future, we rcommend a modest program of investigation to replicate study elements, and to ensure suitable reference conditions are available to allow barrier effects to be distinguished unambiguously from other sources of variability.

Highlights

The California (Sacramento–San Joaquin) Delta (Figure 1) is the hub of much of California’s water supply, where water from reservoirs to the north is routed to farms and cities in and south of the Delta and to the San Francisco Estuary
In 2004, the central section of Franks Tract remained mostly clear of submerged aquatic vegetation (SAV), which we attribute to tidal flow entering Franks Tract from the “nozzle” of False River (Figure 1)
The northeast corner of Franks Tract, which gets a smaller influx from Old River (Figure 9A), stayed clear of SAV

Summary

INTRODUCTION

The California (Sacramento–San Joaquin) Delta (Figure 1) is the hub of much of California’s water supply, where water from reservoirs to the north is routed to farms and cities in and south of the Delta and to the San Francisco Estuary (the estuary). This process combines “tidal pumping” and “tidal trapping” (Fischer et al 1979) in a dispersive process that brings salt into Franks Tract, increasing salinity in the water that flows toward the export facilities in the Southern Delta Operable gates, such as the Delta Cross Channel (DCC) gates (Figure 1), and temporary barriers have been used to alter flow pathways in the Delta to improve water quality and water-supply reliability at the South Delta pumping plants and other locations. We set up a spatial box model that spanned the Delta and Suisun Bay, and used a particle-tracking model driven by hydrodynamic model results to determine elements of a matrix that showed the probability that a particle in one box would be in the same or a different box, or lost from the system, after 1 day This exchange matrix was used to calculate movement rates of copepods (see “Zooplankton”) for representative conditions with and without the Barrier. Spatial scope Entire estuary Central Delta, Suisun Bay Franks Tract, Big Break Entire Delta

Results and Discussion