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Delta Breezes Are Dying and That Could Be Bad News for an Imperiled Fish

Research has found that wind is on the decline in the Sacramento-San Joaquin Delta, which subsequently impacts the turbidity of the water. That in turn has an impact on critically endangered delta smelt, writes Metropolitan Water District’s Tom Philp.

Written by Tom Philp Published on Read time Approx. 3 minutes
A windsurfer takes advantage of gusty winds in 2014 at Sherman Island County Park in the Sacramento-San Joaquin River Delta.Florence Low / California Department of Water Resources

As a 27-year Sacramentan, it’s pretty easy to detect that there are fewer winter moments of dense, bone-penetrating valley fog than before. Winter used to mean days on end without seeing a sun in the sky. But how many of us are just as aware that the blessed Delta breeze also isn’t what it used to be? (It sure abandoned us in July.) Or how this ebbing of the wind has shifted the ecology of the Sacramento-San Joaquin Delta itself?

Earlier this year, David Fullerton, a researcher for the Metropolitan Water District of Southern California and a 17-year resident of Sacramento, was a co-author of a study published in the peer-reviewed journal Estuaries and Coasts, which looked at wind in the Delta. Monitoring stations have been measuring wind in the Delta for years, and by examining their records from 1995–2015, Fullerton and co-authors Aaron Bever and Michael MacWilliams found wind declines ranging from 13 percent to 48 percent throughout the estuary.

The Delta breeze, in short, has been waning.

In the summer months of June through September, for example, the winds decreased 42 percent at Rio Vista. They dropped 20 percent at Sacramento Executive Airport. In the fall and early winter (October to January), they decreased nearly 48 percent. Everywhere around the Delta, there was less wind.

Miles away in Lake Tahoe, the latest State of the Lake report by researchers raised the fear that higher temperatures will steadily lead to a clouding of the lake, fading its signature blue color. Yet if the Delta breezes continue to ebb, precisely the opposite will happen. The Delta, as the winds have subsided, has become clearer.

The wind does more than provide a sense of human relief from the heat. The wind stirs things up, literally.

Wind is a primary driver of turbidity in the waters of the Delta. And what this research did was to apply sophisticated modeling tools to estimate how much clearer the Delta has become as a result of the decreased winds.

Take Suisun Bay as an example. There, winds can suspend sediment in the shallower channels in places like Grizzly and Honker bays. The extraordinary power of the tides then helps to distribute the turbid water into the deeper channels throughout the area.

Fall/winter winds in Grizzly Bay decreased by 50 percent over the 20 years and 32 percent in Honker Bay. This is the period, October through January, that the reduction in wind has been the most pronounced, although the north Delta has experienced less wind in summer months at all monitoring stations.

Based on the modeling in this study, the decrease in wind had the effect of decreasing turbidity in Suisun Bay between 14 percent and 55 percent.

Why does that matter? Take, for example, the endangered delta smelt. Turbidity is a documented factor in the whereabouts of the much-studied fish species. It seems to like turbid water. Monitoring is far more likely to detect smelt in turbid water than clear stretches of the Delta. Presumably the smelt like to hide as they migrate from summering grounds such as Suisun Bay.

Less wind means less turbidity, which means less cover for a fish such as the delta smelt. This species is under stress for many reasons. Add wind, or the lack of it, to the list.

The study found less wind, but did not answer why. It concluded with this: “Future work examining the cause of the long-term declines in observed wind speed would provide a greater understanding of whether the decline in wind speed over the past 20 years is due to cyclical processes and will increase in the future, or if wind speed is expected to remain low or decrease further in the future.” At this point, the future of the Delta breeze is unclear.

So far this year, Fullerton says that it has been less windy overall in the Delta than the year before. It is just a snapshot in time. But it is consistent with what for many of us is a hidden weather trend.

For both human and broader environmental reasons, it is time to pay more attention to the Delta wind, or what is left of it.

The views expressed in this article belong to the author and do not necessarily reflect the editorial policy of Water Deeply.


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