Lake Tahoe isn’t the world’s deepest lake, but it’s in the top twenty. 1645 feet deep at its lowest point, it averages 1,000 feet over its 191 square-mile surface. Surrounded by the wooded slopes of the Sierra Nevada, it is a treasure nestled between California and Nevada, right where their shared straight line borders form a roughly 130 degree angle. Lake Tahoe is famously blue. It used to be bluer. Or should that be “more bluer?” The visibility of Tahoe’s Sierran waters surpasses that of any other large lake in the world. But the clarity of the high mountain lake has dropped significantly from the level it was when first scientifically measured in 1968. At that time, a device called a Secchi disk was lowered into the lake. This is pretty straightforward stuff—the Secchi disk looks like nothing so much as a white dinner plate on a string—but it’s a simple and accurate measurement of clarity: How far down can you lower the disk and still see it? In 1968, that was 102.4 feet. Who knows what the depth of visibility might have been back in 1873, when concerns were first heard about the effects of logging and its attendant forest fires on the Tahoe Basin? Still, the data compiled by such readings since 1968 represent the longest unbroken recording of lake water clarity in the world.
Tahoe’s waters shift in clarity seasonally, affected not only by sediment but by algae. These tiny photosynthetic organisms have their growth cycles, with getting more in the way of light penetration in warmer months, when they are greater in number. Unfortunately, however, the annual rise and fall in visibility began to fluctuate to ever shallower depths as development around the lake’s shoreline, growing since the 1950s, increased into the 1970s. More development meant more disturbed soil, which meant more rainwater runoff carrying sediment and nutrients. These feed the growth of algae, as do things like the nitrogen fertilizers used on the lawns of the newly-built houses and condos and hotels. It began to look like the golden goose of Lake Tahoe, the gorgeous deep blue hue that drew so many to want to visit or live there, was being killed. The setting of Lake Tahoe itself is magnificent, but if the trend continued, its waters would just be another big green lake.
A concerted effort by government agencies to buy up and preserve undeveloped land in the Tahoe Basin, as well as environmental activism and regulations imposed on building, helped slow the rampant growth that was threatening the lake’s clarity. The worst clarity readings to date were recorded in 1997, when 64.1 feet was as deep as you could see. Since then the depths have varied, but not gotten markedly shallower, so the worrisome decline has eased. Readings in winter 2016 were the clearest since 2012. So local efforts to curtail excessive runoff have been effective, but Tahoe now faces new pressures—the same ones being felt worldwide. Climate change has steadily raised lake temperature levels. Spring comes many days earlier, and the warm, algae-friendly waters not only persist longer in the fall, but they tend to stratify the water. Tahoe’s clarity depends upon the mixing of its very deep bottom water with surface waters, a turnover that takes about four years to complete. Water held immobile at depth loses its oxygen. When the oxygen’s gone, phosphorus in lake floor sediments will be released to the surface, fueling the growth of even more algae. Though clarity readings are not declining on a yearly average, summer season readings for 2016 were 16.7 feet shallower than in 2015, so tourists visiting in the high season for water activity are encountering less depth of visibility, and a less deep blue.
The groups that focus on Tahoe’s environmental health have set themselves a target of 97.4 feet of clarity, on average, by 2076, and at this point in time readings are running slightly ahead of schedule. But the lake is a vastly complicated system, involving a myriad of moving parts, some acting synergystically. Who knows how sediment, algae, forest fires, drought, heavy rain, heat, and many other influences will combine to affect Tahoe’s future clarity? The challenge to save Lake Tahoe’s stunning cobalt-hued richness will assuredly continue, as the negative effects of both humankind and climate keep coming.
Want to see that amazing blue water for yourself? Take a road trip to Lake Tahoe, guided by a copy of Mad Maps’ Sierra Nevada, available from Maps.com:
caption: Bounded by Sierran peaks and ridges, Tahoe’s deep blue is evident even from space.
source: Wikimedia Commons: http://www.terraprints.com (CC by 2.5 Generic)
source: Flickr: Biological Sciences Department Kent State University (CC by 2.0)
source: Wikimedia Commons: Tahoepipeclub (CC by SA 3.0)
source: Wikimedia Commons: NASA (Public domain)
source: Wikimedia Commons: Michael (CC by 2.0)