In the wind

Project UPWELL

TERC research diver Katie Senft deploys a platform of temperature, pressure and current instruments as part of the UPWELL project.

TERC research diver Katie Senft deploys a platform of temperature, pressure and current instruments as part of the UPWELL project.

COURTESY/Brant Allen

Strong spring winds and Lake Tahoe’s deeper, colder waters are again under scrutiny by scientists.

When combined, researchers wonder, do those things help make for the slippery, algae-covered rocks many say are increasingly being encountered along Tahoe’s shore?

That’s the fundamental question being asked by scientists at UC Davis’ Tahoe Environmental Research Center. Project UPWELL, is one of the latest attempts to examine the complex dynamics of Tahoe’s ecology and noticeable changes taking place at this national treasure.

For decades, much attention has been focused on the middle part of Lake Tahoe and why the lake’s famous clarity has steadily declined.

The problem—primarily associated with the introduction of fine, suspended sediments washing into the lake from Tahoe’s roads and urban centers—was meticulously documented. The good news, experts have said, is that the steady decline in mid-lake clarity appears to have stabilized, largely due to projects designed to halt the flushing of sediments into Tahoe’s waters.

Now, attention is shifting to what’s happening closest to Tahoe’s shorelines. Using submerged monitoring stations installed near Tahoe’s shore in recent years, researchers monitored regular events during which high winds common in late May and early June were apparently associated with the “upwelling” of waters from as deep as 1,000 feet to the lake’s surface.

“One of the distinct patterns seen are these upwelling events,” said Derek Roberts, a graduate student and lead researcher for Project UPWELL. “We’ve seen this pattern occur over several years.” These deep waters drawn to the surface are cold and rich in nitrates, which could help nourish algae growing on Tahoe’s shoreline rocks.

“We decided we want to address the question of ’Do these upwelling events really matter?’” Roberts said. “Our goal is to understand on a really basic level if this upwelling really plays a significant role in that algae growth.”

To learn more, Roberts—joined by researchers from the Bodega Bay Marine Laboratory, Stanford University and the University of British Columbia—installed a 1.5-mile-wide “measurement curtain,” stretching from the shore on Tahoe’s west side toward the middle of the lake.

Nearly 100 instruments were installed at seven moorings along the curtain—the shallowest at seven feet and the deepest at nearly 900 feet below the surface. Current velocity, temperature and dissolved oxygen levels were recorded every 30 seconds, and water samples were taken before, during and after upwelling events. An autonomous underwater glider was deployed to examine variable upwelling events along the west shore.

Data collected over two months in the spring of 2018 is still being analyzed. So far, the experiment documented two major and two minor upwelling events. During the big ones—May 31 and June 9—water from as deep as 500 feet rose to the surface in just a few hours. Water temperatures at the west shore dropped to a frigid 40 degrees while temperatures on the east shore remained at 57 degrees.

After winds died, cold waters rapidly sank into the depths of the west shore and warm water from the east shore rushed westward across Tahoe’s surface at speeds in excess of two feet per second.

Further research should shed more light on what’s happening close to Tahoe’s shore, Roberts said.