By William Fleenor, Amber Manfree, and Megan Nguyen

Tides are the biggest driver of Delta flows, and in Episode 2 we look at their impacts in different locations under a variety of inflow conditions.  Tides have a twice-daily cycle in the region, with a range of about six feet at Martinez.  In the first part of the animation, we remove all in-Delta controls and diversions and fix inflows at a common moderate early summer level to isolate effects of tidal forces from those of inflows, gates, and export diversions.  When the moon and sun are more aligned (full and new moon periods), tidal magnitude is greater.  Distances to the moon and sun influence tidal magnitude as do winds and barometric pressure.  Winds and barometric pressure are fixed in this animation.

The main lessons are:

• Tidal ‘sloshing’ greatly exceeds Delta outflows. Tidal flows can be 400,000 to 600,000 cubic feet per second (cfs) at Martinez, while net outflows are often a few thousand cfs.
• A given amount of tidal force exists at the Golden Gate, and a change in one location in the estuary has effects throughout.
• Sacramento and Stockton Deep Water Ship channels have been deepened and straightened with dredging, which increases tidal flows up these channels and decreases tidal flows (and mixing) in other Delta channels.
• Higher Spring tides, occurring when the sun and moon are more aligned, add volume to the Delta and by themselves can produce brief net negative flows on Old and Middle Rivers.

The second part of the animation varies Delta inflows to demonstrate how inflows and tidal forces interact, again without major Delta diversions.  We look at inflows lower than those previously shown, representing late summer, and two higher inflow levels.  Finally, we show effects of a flood pulse moving through the Yolo Bypass. The main lessons are:

• Lower inflows increase tidally driven negative flows through Old and Middle Rivers.
• As inflows increase, tidal influence diminishes from the upstream direction and net negative flows from the tides cease in Old and Middle Rivers.
• Flood flows through the Yolo Bypass greatly reduce tidal influences.

Modeling produces a better understanding of natural and anthropogenic influences on Delta flows, which can help improve planning and policy-making for the Delta.

Coming next in Episode 3 is an examination of flow and salinity effects of major water diversions from the Delta.

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