By William Fleenor, Amber Manfree, and Megan Nguyen

Delta water diversions have significant effects on flows and water quality within the Delta. Diversions can re-direct river flows and draw salt water inland from the sea, impacting water quality and the environment. Episode 3 explores how water diversion quantity affects in-Delta flow directions and quantities. This episode also looks at how in-Delta gates and barriers are used to improve in-Delta flows and water quality.

Some lessons:

• The two biggest in-Delta diversions are the State Water Project (SWP) and the Federal Central Valley Project (CVP) near Tracy. CVP exports are fairly constant during the irrigation season and draw water from south Delta Channels, while the nearby SWP exports occur in big gulps, taking water on high tides into Clifton Court Forebay.
• The Delta Cross Channel (DCC) near Walnut Grove helps move Sacramento River water to pumps in the south Delta. Open DCC gates direct additional flow southward into the central Delta, moving fresh water from the northern to southern Delta, supplying fresh water to the pumping plants. Large flows of Sacramento River water toward these diversion pumps reverse net flows in Old and Middle Rivers in the southern Delta.
• Gates and flow barriers in the southern Delta keep lower-quality San Joaquin River water away from the pumps and improve south Delta water quality.
• Heavy pumping in the southern Delta can lower water elevations in local channels so much that farmers cannot siphon water onto their fields, forcing farmers to pump water. Agricultural barriers during higher pumping months keep the water elevation high enough for siphoning in some Delta channels (Old River, Grant Line Canal and Middle River).
• The Head of Old River Fish Barrier, on the San Joaquin River, directs migrating fish downstream rather than toward the pumps and improves export water quality by reducing flows of lower-quality San Joaquin River water to the pumps.
• Agricultural barriers and the fish barrier along the San Joaquin River increase net negative flows in Old and Middle Rivers.

Flows change in the north Delta due to the Cross Channel and in the south Delta due to barriers and the Cross Channel. The overall effect does not change net outflow, but shifts net outflow contributions from the Sacramento River to the San Joaquin River. Unnatural water circulation from north to south entrains fish in the pumps and the southern Delta, and disrupts natural gradients between the rivers and ocean that cue fish movements.

William Fleenor is a senior researcher who specializes in hydrodynamics and hydraulic modeling at the UC Davis Center for Watershed Sciences. Amber Manfree is a postdoctoral researcher with the UC Davis Center for Watershed Sciences. Megan Nguyen is a GIS researcher at the Center for Watershed Sciences.

Further Reading

A Tale of Two Deltas: A Comparison of Transport Processes in the Predevelopment and Contemporary Delta (Jon Burau, as summarized by Maven, 2016)

Episode 1: “Unraveling the Knot” – Water movement in the Sacramento-San Joaquin Delta – Introduction

Episode 2: “Unraveling the Knot” – Water movement in the Sacramento-San Joaquin Delta – Tidal Forces

Episode 3: “Unraveling the Knot” – Water movement in the Sacramento-San Joaquin Delta – Managing Flows