Lake Tahoe is an oligotrophic alpine lake located on the border of California and Nevada. It was formed over 2 million years ago and is renowned for its deep blue waters and forested shoreline. However, human activities over the past century have accelerated the natural eutrophication process and reduced water clarity. Increased development, pollution from urban runoff, and the introduction of invasive species have all negatively impacted the lake's ecology. Ongoing restoration efforts focus on improving water quality and clarity through projects that reduce sediment and nutrient loading into the lake. The long-term goal is to restore Lake Tahoe's clarity to the historical depth of 97 feet.

1. Lake Tahoe:
Social Nature in Vacation
Paradise

4. Lake Tahoe facts
• 2 million years old
• 6,225 feet in elevation
• 1,645 feet deep
• America’s largest alpine lake

6. Eutrophic or Oligotrophic?
• Eutrophic (well-nourished) lakes have plenty of nutrients
for aquatic plants and algae, which supply oxygen for
fish. Algal blooms can deplete oxygen supplies, resulting
in fish kills.
• Oligotrophic (poorly nourished) lakes have few nutrients,
less life and very clear waters. They typically support fish
like trout that need cold, well-oxygenated waters. Such
lakes are most common in cold regions underlain by
resistant igneous rocks like granite.
• Eutrophication is a slow process in nature. Accelerated
eutrophication results from human activities that enrich
nutrient content, depleting oxygen supplies.

8. Comstock Lode
• Silver ore discovered in western Nevada in
1859, 15 miles east of Lake Tahoe
• Rush of prospectors
• Industrial-scale development generated
immense fortunes in Virginia City, San
Francisco
• Square set timbering devastated Lake Tahoe’s
forests: by 1874, mines declined but Tahoe
forests had almost all been clear cut

10. Samuel Clemens (Mark Twain)
• Visited Lake Tahoe near present-day Tahoe
City in 1861 to stake a timber claim
• “As it lay there with the shadows of the
mountains brilliantly photographed upon its
still surface, I thought it must surely be the
fairest picture the whole earth affords.”
• Discovered “yellow pine timber land – a dense
forest of trees a hundred feet high and from
one to five feet through at the butt.”

12. Today’s forest is second growth

13. Tourist Destination beginning in 1850’s

14. Tahoe National Park?
• Efforts to create Lake Tahoe National Park in
1912, 1913, 1918 failed
• Much of the land in the Tahoe Basin was
privately owned and had already been
developed or logged.
• It seemed too late to preserve the lake, so the
Tahoe basin was left to private development

17. Tampering with Nature
• Introduced sport fish
– Rainbow, Brook & Brown trout (1800s)
– Lake trout (1912)
– Kokanee salmon (1948)
• Dwindling food supply, so opossum shrimp
introduced in the 1960s as food for salmon
• Shrimp fed on the lake's zooplankton
(Daphnia), competing with juvenile fish for
food

18. Tampering with Eden

19. Squaw Valley Winter Olympics, 1960

20. Population Boom
• Winter Olympics, I-80 access, building boom
• 1960
– permanent population 10,000
– summer population 10,000
• 1980
– permanent population 50,000
– Summer population 90,000

23. Tahoe Keys, late 1960s
• “Swamp” at the south end of the lake
• Developers built houses & boat canals
• Lawns & urban runoff added nutrients
• “Developed” wetlands can’t filter out dirt and
pollutants before runoff flows to the lake
• Additional shoreline developments planned

24. Wetland Alteration

25. Keep Tahoe Blue
• The League to Save Lake Tahoe (“Keep Tahoe
Blue”) founded in 1957 to fight a proposal to
build a 4-lane highway around the lake
• Embraced “responsible and diversified use of
the Lake's resources while protecting and
restoring its natural attributes.”

26. Tahoe Regional Planning Agency
• By late 1960’s a city the size of SF was planned
for Tahoe Basin, w/ freeways ringing the Lake
• Bi-state compact in 1969 created TRPA
– environmental quality thresholds
– ordinances to achieve thresholds
• Federal building moratorium 1984-1987
• 1987 Regional Plan kept development slow

28. … but problems continue
• Lake clarity declining due to erosion, sediment
delivery by urban runoff
• Sewage exported but nutrient loading from
lawns, golf courses & roads promotes algal
growth (up 4-fold since 1959), reduces clarity
• Native fish declining due to overfishing and
introduction of non-native species
• Destruction of wetlands reduces ecosystem’s
ability to purify water

29. Declining Clarity (Secchi disk visibility)

30. Accelerated Eutrophication
• 20 feet of clarity lost since 1968
• Goal is to restore clarity to depth of 97 feet
(currently, about 70 feet)
• Could be possible within 20-30 years
• Wintertime improvement because of wetland
restoration & storm water projects.
• Summertime problems due to climate change,
warmer water, more algae.

32. Loving Lake Tahoe to Death?
• Clearest water of any large lake in the U.S.
• 3 million visitors/year arrive by motor vehicle,
many engage in motorized recreation
• Visitors cause air pollution & water pollution
• Visitor facilities (golf courses, ski slopes,
parking lots, vacation homes) all contribute to
nutrient loading
• Visitors continue to introduce exotic species

33. Can you spot the problem?
• What environmental harm might result from
the activities shown in the following pictures?

39. • Tahoe's clarity reached an all time low in
1997: Secchi disk could only be seen 64 feet
below the surface.
• Recent environmental projects have
prevented more than 268,500 pounds of fine
sediment particles from flowing into the lake,
equal to about 70 dump truck loads of
sediment.
• Storm water runoff from roads and urban
areas are the greatest sources of pollution.
• By 2016, clarity had increased to 73 feet.
• The next target is 78 feet by 2026.

41. February 2019