Influence of an Observed Decadal Decline in Wind Speed on Turbidity in the San Francisco Estuary

Aaron J Bever,David K Fullerton,Michael L Macwilliams

doi:10.1007/s12237-018-0403-x

Abstract

Turbidity is an important habitat component in estuaries for many fishes and affects a range of other ecological functions. Decadal timescale declines in turbidity have been observed in the San Francisco Estuary (Estuary), with the declines generally attributed to a reduction in sediment supply to the Estuary and changes to the erodible sediment pool in the Estuary. However, we analyzed hourly wind data from 1995 through 2015 and found statistically significant declines of 13 to 48% in wind speed around the Estuary. This study applied a 3-D hydrodynamic, wave, and sediment transport model to evaluate the effects of the observed decrease in wind speed on turbidity in the Estuary. The reduction in wind speed over the past 20 years was predicted to result in a decrease in turbidity of 14 to 55% in Suisun Bay from October through January. These results highlight that the observed declines in both wind speed and sediment supply over the past 20 years have resulted in reduced turbidity in the San Francisco Estuary from October through January. This decline in turbidity in Suisun Bay potentially has negative effects on habitat for fish like the endangered Delta Smelt which are more commonly caught in relatively turbid water.

Highlights

Motivation and BackgroundWater column turbidity is an important habitat component in many estuarine systems, and changes to turbidity can have significant management implications
The root mean square (RMS) wind speed trends are not discussed because they showed similar trends to the average wind speed, which are discussed in detail
Analyses from this study demonstrate that the observed long-term reduction in wind speed has resulted in a reduction in turbidity in the Estuary

Summary

Introduction

Motivation and BackgroundWater column turbidity is an important habitat component in many estuarine systems, and changes to turbidity can have significant management implications. Metropolitan Water District of Southern California, 1121 L Street, Sacramento, CA 95814, USA observed turbidity in the San Francisco Estuary are of concern due to the potential importance of elevated turbidity to habitat for endangered fishes such as Delta Smelt. Turbidity is a metric based on the optical properties of a water sample that act to scatter light at a specific angle emitted by an instrument (Davies-Colley and Smith 2001; Gray and Gartner 2009), and turbidity is correlated to the amount of sediment suspended in the water column in many coastal, estuarine, and riverine environments. Turbidity is used as a surrogate for SSC because it is easy to observe and the emitted light is scattered primarily by sediment suspended within the water column (Davies-Colley and Smith 2001; Gray and Gartner 2009). There is a direct relationship between turbidity (nephelometric turbidity units, NTU) and SSC (mg/L), as long as the sediment size

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