Much has been done to pinpoint the causes and sources of the decline in Lake Tahoe’s clarity.
Lake Tahoe’s famously clear water is not just at the center of the visual landscape it is at the center of debate. The debate concerns a longstanding and global issue: How to live in proximity to the water that is fundamental to life and livelihood while not degrading it? The unique vantage point Tahoe provides on this issue is giving way to innovative thinking and new breakthroughs on this global issue. But whether Tahoe will be able to apply new knowledge and reverse the decline in water quality remains in sway.
Few places in the world have water like that in Lake Tahoe. The Tahoe Basin filled with the water of melting glaciers during past ice ages and has been replenished with high sierra snowmelt and rainfall ever since. While Comstock-era logging had a significant negative impact on water quality, Tahoe otherwise has suffered relatively little impact from humans in its two million year history. Year-round residence and its accoutrements have been available to significant numbers of people only in the past half-century at Tahoe. Tahoe has very high quality water compared to most places in the world but with year-round inhabitation water quality has rapidly declined.
Water quality degradation at Tahoe has been the subject of much study. Scientists have kept a record of Lake Tahoe water clarity since 1968 using a Secchi disk. The Secchi disk is a white disk mounted on a rope that is lowered slowly in the water. The depth at which the disk is no long visible is taken as a measure of the transparency of the water. Water clarity data collection began approximately as people were settling at Tahoe in unprecedented numbers. The record shows that between the years 1968 and 2000 one-third of Lake Tahoe’s clarity was lost.
The Lake Tahoe transparency standard for Secchi depth—nearly 100 feet—is the annual average Secchi depth measured between 1968 and 1971. The long-term failure to meet the standard activated a program under the Clean Water Act called the Total Maximum Daily Load, or TMDL. A TMDL is a calculation of the maximum amount of a pollutant that a waterbody can receive and still meet water quality standards, and an allocation of that load among the various sources of that pollutant. The determination that Tahoe water clarity is “impaired” and requires a TMDL to address that impairment activated significant investment in the characterization of the problem and the development of a program to fix it. Many aspects of how to restore lake clarity have been thoroughly researched, vetted and developed as a result.
Scientists have pinpointed phosphorous, nitrogen and fine sediment particles as the causes of clarity decline. Nitrogen and phosphorous stimulate algae growth, which in turn absorbs light, reducing how far light can penetrate into the lake. Fine sediment particles decrease clarity by scattering light as the particles slowly settle through the water. Lake Tahoe has been losing clarity at a rate of approximately 9 inches a year since the late 1960s and fine sediment is the primary culprit. Roughly two-thirds of the clarity condition is attributed to the fine sediment particles in suspension.
Scientists and engineers have determined that most of the phosphorous, nitrogen and fine sediment particles that reach Lake Tahoe get there via four mechanisms: atmospheric deposition, stream channel deposition, forest runoff and urban runoff. The estimate is that 72% of the fine sediment contribution to the Lake comes from urban runoff. In other words, the biggest amount of the most-detrimental stuff comes from the urban landscape.
Tahoe’s chapter in the tome society has written about how to live in proximity to the water that is fundamental to life and livelihood while not degrading it is unique. Few places have had the privilege of starting with water that is of such high quality, or having a scientific record that originates so near in time to the start of decline, or having so many human and financial resources come to bear. Many contend that debate about how to reverse water clarity decline can be more focused now thanks to recent findings about the causes and sources of clarity decline and thanks to management tools that have been designed specifically for Tahoe.
At this juncture, answering how to finance and facilitate the transition to a new paradigm in water quality improvement is the matter at hand.
U.S. Environmental Protection Agency page: What is a TMDL?
U.C. Davis Tahoe Environmental Research Center: State of the Lake Report.
California Regional Water Quality Control Board Lahontan Region. Charting the Course to Clarity.
Project Description. Proposed Basin Plan Amendment: Lake Tahoe Total Maximum Daily Load
Nevada Division of Environmental Protection: Charting the Course to Clarity. (video)