5 – Looking Deeper – Why is Lake Tahoe clarity declining?

By Sage Alexander

Lake Tahoe is often referred to as the gem of California. It is North America’s largest alpine lake and is famed for beautifully clear waters. The geology of the basin provides a uniquely strong natural filtration system, which leads to a crystalline quality of waters unmatched in California. The clarity of the lake, however, has been declining for the last few decades. The water that runs off urban surfaces (roads, sidewalks, roofs, anything that water cannot soak into) collects tiny particles and pollutants that are delivered into the lake every time it rains. These tiny particles, mostly between 0.5 and 5 microns, are the reason clarity has dropped reached to a record low in 2017. How are particles too small to see with the naked eye having such a catastrophic impact on Lake Tahoe? Human development and urbanization, concentrated here out of attachment to the natural environment, is the source of clarity loss that threatens to change Tahoe as we know it. This is a common tale of wild places; smothered or tamed because of our innate desire to be surrounded by undeveloped land. As someone who grew up in South Tahoe, I see the particulars of this area trampled by overuse, and one is the steady decline in lake clarity and health. For those who come to the Lake on the weekends, I hope to instill some understanding of how a body of water can be so greatly impacted even with so many interested in protecting it.

As of now, stormwater is not treated in this basin. This is in a region of paranoia about algae growth; all treated sewage water is not introduced back into the local water cycle unlike most municipalities. We pump it over the hill to reduce the amount of nitrogen and phosphorus deposited into the lake. Stormwater, on the other hand, is usually directed into gutters and pipes and dumped right into the lake or one of its tributaries. The reason the tiny particles are so impactful is simply the light refracting. They stay in solution for decades, and can be resuspended quite easily. Clarity isn’t really a measure of the ecological health of a body of water, all it tells us is how far a white disk can be seen underwater. Stormwater includes not only those tiny particles of ground inorganic matter, but also pretty much everything that touches the streets. Runoff from lawns treated with pesticides and fertilizer, the various fluids that drip from cars, garbage, metals, salt and sand from road safety measures, all coming together to create a dark grey sludge that mixes with the otherwise unadulterated snow melt that makes up the streams. In order to measure the change in clarity, scientists use a white disk called a secchi. This disk is lowered off a boat using a rope, and is measured at the point you can no longer see it. When clarity began to be recorded in the sixties, light could be seen hitting the secchi at 100 feet. The lake during 2017, the worst year in recorded history, had a clarity of 59.7 feet. This number fluctuates with season; according to the U.C Davis State of the Lake Report, the gains in winter months are usually offset by the summer. Groups working to curb clarity loss often cite secchi measurements during a single winter to display successes, but long-term trends show that without drastic change in infrastructure clarity will continue declining.

Wetlands along the edges of the lake are crucial ecological filtration systems, cleaning the water before it is introduced into Tahoe. The slow-moving water must pass through miles of organic matter before it emerges in the lake sediment free. Plants stabilize soil and ensure a slower water flow. The destruction of these marshes has caused significant trauma to the natural water purification that is responsible for Tahoe’s transparency. Nearshore mouths such as the Upper Truckee River have been heavily impacted, replacing the meadows with new development that does little to halt the discharge of sediment into Lake Tahoe. Once a meandering river, the Upper Truckee’s mouth has been channeled into a straight line to allow for the ‘Tahoe Keys,’ a system of algae rich man-made channels that form a housing development. Birds in particular have lost a large portion of their habitat in these construction projects. The most visible change that comes as a result of urbanization is the recent uptick in algae. The majority of nitrogen is deposited in the lake from the air, and about 20 percent of the total phosphorus load comes from the urban environment (according to the 2017 State of the Lake report). The warming of the lake from climate change along with the disruption of the delicate balance of chemicals introduced through stormwater has caused significant algal growth. Beaches that were once common swimming areas are now too grimy for many to jump in; Lakeview commons looks more like a science experiment than a community beach. The aspect of this that is most essential is how these changes have impacted the ecological health of the lake. The Mysis shrimp, an invasive species that was introduced to Tahoe in the 60s, mysteriously disappeared from iconic Emerald bay in 2011. The clarity strangely increased by 40 feet, reversing as soon as the shrimp returned. The connection between clarity and ecological health of Tahoe is probably more apparent than what was once thought, but the extent is somewhat of a mystery.

Stormwater management has been attempted in a variety of ways around the basin. Installing curb and gutter does not treat storm water before it is introduced into the lake, as it is not afforded the time and surface material to pass through layers of sediment to catch the tiny particles. Another method has been installation of filter systems. These will clog in extreme winter conditions and are quite expensive to install and maintain. In my view, filters are band-aid measures; no infrastructure changes other than something added onto the end of a broken system. When humans find ourselves lost over a problem caused by our meddling with nature, we must look again to natural processes to make things right. If natural water systems can retain clarity simply by allowing water to absorb into the ground, we can stop clarity loss by mimicking these systems and allowing processes to take place that have been halted for decades. BMPs, or Best Management Practices, are an example of allowing some natural filtration to take place. This works by letting water build up in depressions in the ground, and as the water soaks into the ground the sediment and fine pollutants are left behind. These structures can be as simple as a large open hole, or as complex as an underground chamber tucked away beneath a parking lot. The first priority in this is to give the water plenty of time to run along surfaces that allow percolation. The hurdle is finding urban spaces that can accommodate appropriate amounts of stormwater; as a rule, development is never curbed to allow for more natural spaces.

People are quite interested in keeping Tahoe blue. The Lake Tahoe Restoration Act is a source of funding for many projects around the basin; Senator Dianne Feinstein was especially instrumental in creating and extending the 2000 legislation that has provided over a billion dollars to improve the health of Tahoe. A portion of the storm water slice has ironically been spent on developing new curb and gutter. This unfortunate reality is the result of misunderstanding of science and agencies competing for limited funding to continue operations.

The League to Save Lake Tahoe developed a citizen science Pipe Keeper program to monitor the storm drain runoff pipes around the basin. People brave bone chilling rain and blackened water from pipes out of love for Tahoe; they collect samples of storm water and do the kind of ground work that is necessary when a natural feature is being threatened. This provides an updating status that compliments the research from U.C Davis for a greater understanding of Lake Tahoe. I visited the local Department of Transportation and learned about the salt and sand that is applied to the road to protect people from sliding on the seasonal ice. There are huge garages filled to the brim with salt and sand, and apparently there’s noticeable differences in which you choose to use. Russel Wigart works tirelessly to reduce the sediment load of these measures and to improve clarity in general. Since his employment, they have replaced the standard sanding material with local decomposed granite and salt with liquid brine, both of which have measurably improved what is deposited into the lake. In addition to this, the city recently bought up an area of development that was built up on a creek bed that saw catastrophic flooding seasonally and put into place stormwater treatment basins and a real creek for the water to flow through. Jason Burke, the South Tahoe Public Utilities District Stormwater program coordinator, is approaching the management of stormwater to match natural systems. There are more and more individuals learning how stormwater must be treated in this special basin to correct the mistakes of the past.

Lake Tahoe is still seeing a reduction in clarity, and it seems to me now this is what comes when humans want cars and roads and ski resorts and a functioning economy in a place that everyone wants to be. It’s not reasonable for me to nostalgically compare Crater Lake, OR, a basin that hasn’t seen development, to a place where thousands live full time. Tahoe was not federally protected and is fairly developed, but this doesn’t mean its fate is set. We can and are mitigating damage, and I am very proud to be in a community that contains people working hard to shift the balance.