Science a decisive factor in restoring Tahoe clarity

W settlers first discovered Lake Tahoe in 1844, when General John Fremont and his tired group of cavalry gazed upon its cobalt waters from a mountaintop to the southwest. For the next century, stagecoaches brought steadily increasing summer visitors. When Mark Twain first saw Tahoe he was so impressed by its blue waters that he described it as the “fairest picture the whole earth affords.” The 1857 discovery of gold and silver in the Comstock Lode at Virginia City created the first major disturbance of the Lake Tahoe Basin. Loggers clear-cut most of the basin’s timber to shore up the mines of the Comstock; when the mines ran out of silver most of the old-growth timber was also gone. White fir and brush grew back in dense, overcrowded stands, which in recent years have created a major fire hazard. This revegetation was important, however, in slowing the high soil-erosion rates that characterized the peak logging period. The high losses of soil, chronicled in lake sediments, dropped back to less than one-quarter of those that occurred during the lumbering activity. Lakes are reservoirs of history. Their bottom sediments are an indelible record of what has occurred on land, air and water. The sediments of the Tahoe Basin are thought to contain a continuous 1-million-year record of climate, one of the longest on the continent. Fossil remains of invertebrates and fish scales portray the postglacial history of Lake Tahoe, beginning about 11,000 years ago. More recent sediment layers preserve sawdust from the Glenbrook sawmills of the Gold Rush, chronicle the introduction of tetra ethyl lead in gasoline in 1948, and record the appearance of mercury from industrial atmospheric pollution. When the Comstock mining ended, forests and brush cover returned to the basin within about 20 years, and with it Tahoe recovered its pristine quality as one of the clearest large lakes in the world. In 1887, John Le Conte measured the lake’s transparency at over 100 feet, a revelation that provides hope that the lake can once again recover from the recent period of high development activity. Over 70% of the Tahoe Basin is U.S. Forest Service land under the control of the federal government. After World War II, developers built roads and structures using flatland technology unsuited to the steep slopes, fragile soils and limited vegetation cover of this subalpine area. In the late 1950s, when the value of wetlands was not well understood, approval was given for Dillingham Corporation to develop a marina on Pope Marsh, the single largest wetland in the Sierra Nevada. This became the extensive Tahoe Keys at the south end of the lake, and the important filtering capacity of Pope Marsh was lost forever. To make things worse, the major tributary to the lake, the upper Truckee River, was relocated along the east side of the Tahoe Keys and now delivers nutrients and sediment directly to the lake without the filtering benefits of the former wetland. (Scientists have proposed that it be diverted into the narrow remaining marsh known as Cove East.) Tahoe Keys contains warmer water than that of the lake, and it has provided habitat for a number of invasive plants and animals. These invasive species, now exemplified by the spread of the notorious waterweed Eurasian watermilfoil, have gradually moved from the Keys to other areas around the lake. Warm-water and aquarium fish introduced to the Keys have moved to the new, warmer microenvironments that the waterweeds have created. Other invasive fish, particularly the coldwater-tolerant smallmouth bass, may eventually threaten the very existence of the native minnow, trout and kokanee salmon populations. The invasion of these exotic organisms will be further aided by the gradual warming of the lake. Tahoe’s enormous volume of 39 trillion galScience a decisive factor in restoring Tahoe clarity Editorial overview