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(male narrator) Cobalt blue, deeply clear, surrounded entirely by the mountains of the Sierra Nevada, Lake Tahoe is a national treasure. It's the largest alpine lake in North America. People from near and afar feel keenly passionate about its protection--

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now close to three million visitors a year. After the 1960 Winter Olympics at Squaw Valley, Tahoe became a major tourist attraction. Damage to the lake soon became noticeable. Decline in the lake's clarity raised the alarm. Dr. Charles Goldman of UC Davis used Secchi disks to quantify the problem. This catalyzed a movement to use science and regional planning to reduce human impacts on the lake. Contributing to this effort, the U.S. Geological Survey is delivering a range of science through its hydrologists, geologists, geographers, biologists, computer modeling experts, and others. The USGS provides a wide range of consistent, reliable, long-term data and maps that are crucial for evaluating and managing the lake and basin. Since the 1980s, in cooperation with its partners, the USGS has been doing repeated sampling of 10 of 63 streams entering the lake.

(female) One big concern up at Tahoe is what are the amounts of nutrients and suspended sediment entering the lake from the streams. By monitoring the ten streams, we're able to come up with estimates for the amount of nutrients and suspended sediment entering Lake Tahoe from all the streams.

(narrator) It's a long-term effort to understand exactly what's entering the lake. Samples are collected and quickly delivered to labs at the Tahoe Environmental Research Center for analysis. When it became clear that two-stroke jet skis were dumping polluting organic compounds into the lake, they were banned in 1999. Now, in cooperation with the Tahoe Regional Planning Agency, regular samples are collected during boating season and shipped to the USGS water analysis lab in Denver. They're analyzed for extremely low levels of boat fuel components.

(male) We're sampling for Benzene, all gasoline-related compounds that come off from boating. We're out here for the pre-boating season to give a baseline what's here during the off-boating season. We'll come six more times during the boating season and major holidays to see what's happening. We've been doing this since the jet ski ban in 1999 and following the concentrations of volatile organic compounds. They have come way down from readings before the ban. We see a slight increase during the major boating season. It comes down in October, and things are clean as can be.

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(narrator) Delivering round-the-clock data are USGS stream gauges on several streams entering Lake Tahoe, on the Truckee River leaving the lake, and on the lake itself. These are essential to water management in the Tahoe basin and downstream. The stream gauges record the vertical rise or drop of water levels and deliver their data via satellite. It has some intakes plumbed into the creek. It brings water from the creek into the gauge and stilling well. A float inside the well rises and falls with water level. The gauge records it every 15 minutes. This one's mainly used for the alert for any sort of flood issues. It also measures the water amount that's entering Truckee, so the Federal Water Master can gauge how much water to allocate. We must have USGS data to do our job. I am Chad Blanchard. I'm the Chief Deputy Water Master for the United States District Court Water Master's Office. From six in the morning till about midnight, I'm constantly looking at it. The Truckee is one of the most litigated rivers in the country, maybe the world. We enforce two federal court decrees that divvied up the water rights. We need USGS data to be able to meet those requirements.

(narrator) Many of the USGS stream gauges also measure and record water quality information-- things like turbidity, specific conductance, pH, and temperature. Combined, the stream gauge data keeps the region informed on its most precious and potentially hazardous resource--water.

(Honeywell) We have urban and suburban units here. The gauges gives us a good view of what's going on. Gauge readings in Incline Village are much different than the west and east shores. The timing and flows are different. We get a good view of what's coming into the lake.

(narrator) It's long-term data with a real and immediate impact. One of the great visual and scientific products developed over the past 15 years is the USGS map of the lake floor. Until this, no one knew what the lake bottom looked like. The bathymetry involved 60 million soundings, or individual depth measurements, of the deep parts of the lake. We used a system called a multibeam echosounder. In Lake Tahoe it was about every four seconds. As the boat moved, we sent out a ping and then listened. Four seconds later, we sent sound out again. We moved back and forth like you'd mow the lawn, making sure we overlapped, so we didn't have gaps. After ten days, the lake was mapped. We assemble it into a surface and begin to understand the processes happening on the lake floor. One great thing about multibeam bathymetry, or multibeam technology, is that it's three-dimensional, that you have latitude, longitude, and depth. With those three values, you can generate three-dimensional images on a computer screen, generate fly-throughs, interrogate the data. It's an analytical tool. No one expected what I found. What I found was this astounding landslide. Half of the lake's west side had caved in in the past.

(narrator) The blinders were suddenly removed, and there now is the lake bed in 3-D. More recently USGS scientists have spearheaded the effort to use aerial LIDAR to map the ground surface over the entire basin. Using a computer program, we can strip out that vegetative cover and reveal the bare earth. This is very clearly an active fault. For many years, these faults have been debated whether they even exist. Using this new LIDAR technology, we're able to clearly show that, yes, there are active faults on the west side of Lake Tahoe.

(narrator) Aerial photographs taken over the past century have been the focus of USGS geographers. Their work provides an instructive history lesson on Tahoe's past. Aerial photography gives you the power to go back in time. We took old aerial photos for 1940, 1969, and 1987. We can actually scroll through from 1940 to the present. You can see the data we created and see the changes from 1940 on.

(narrator) The dramatic data is easy to access for land managers and anybody. Google Earth used it as their first historical imagery sample. Then it was featured in the New York Times. That's a view of the past to present. What about the future? Over 80% of land in the basin is owned by the U.S. Forest Service and will never be developed. But what about the rest? There are 6,000 parcels available for some form of development or planned use. But what is the best use? USGS geographers have developed the novel New Tahoe Land Use Simulation Model for the Tahoe basin. It's being implemented by the Tahoe Regional Planning Agency. The model can project out fates of different land uses over time, whether there's a house, a hotel, whether there's a park, or open space. It can inform what the future's landscape might be. You can take that outcome and relate it to things like fire modeling or areas sensitive for water quality. You can do some post analysis from the output of the tool.

(narrator) It can project 20 years into the future-- results of choices made today. Interestingly, most of the private land ownership is closer to the lake, where things are more sensitive and the uses are more important, in terms of the overall concern for the Lake Tahoe Basin, the lake's water clarity. In the decision support tool, it's trying to adjust this balance of where is it okay to develop and where is it not so okay to develop.

(narrator) The goal? To make better choices for Tahoe's future. This future does hold real threats from the continued human presence in the basin and from looming factors such as climate change. In Tahoe, we're dependent upon Science as the driver for decisions we make for policy. Policy informed by science can really go a long way in protecting the quality of the environment we have.

(narrator) Science will remain an essential part of decision-making here. Whether it's stream gauging to assist with water choices or monitoring of the lake and streams to quantify change or mapping of the lake floor and mountains or modeling to better shape the future, USGS researchers are working to rapidly deliver the best science possible in the ongoing effort to protect, preserve, and restore the spectacular national treasure, Lake Tahoe.

Funding to purchase and make this educational production accessible was provided by the U.S. Department of Education:

PH: 1-800-USA-LEARN (V) or WEB: www.ed.gov.

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