Lake Tahoe, March 1995; northward view of Incline Village area, from near Sand Harbor,

Nevada. Photograph by Timothy G. Rowe, U.S. Geological Survey.

 Lake Tahoe has long been admired for its alpine setting and the clarity 

of its water. During the last half-century, however, human activity in the 

lake basin has increased while the lake has been losing water clarity at a 

rate of about 1 foot (ft) per year. The Tahoe Regional Planning Agency 

(TRPA), the U.S. Geological Survey (USGS), and the Tahoe Research 

Group of the University of California, Davis (TRG) are monitoring loads 

of sediment and important nutrients flowing into the lake from the streams 

and ground-water 

aquifers in the basin. 

This fact sheet 

provides an overview 

of that monitoring 

program and 

summarizes some of 

the results regarding 

loads of sediment and 

nutrients to the lake.

Basin Geography

 The basin is 

surrounded by 

mountain peaks of 

the Sierra Nevada to 

the west and the 

Carson Range to the 

east (fig. 1). The lake 

is renowned for its 

deep, clear water 

which, on sunny 

days, appears to be 

cobalt-blue. The Lake 

Tahoe Basin was 

formed by downward 

block faulting during 

the uplift of the Sierra Nevada 2-3 million years ago, which resulted in 

dramatic topographic relief. Mountain peaks, snow capped nearly yearround, rise to altitudes above 10,000 ft above sea level. Lake Tahoe, 1,646 

ft deep, is the second deepest lake in the United States and tenth deepest 

in the world. It has an average lake-surface altitude of about 6,225 ft.

 The Lake Tahoe Basin is 506 square miles (mi2). The surface area of 

the Lake is 192 mi2, and the watershed area is 314 mi2. Most of the land 

in the basin is mountainous, limiting development mainly to relatively flatlying areas along tributary streams, such as the southern part of the basin 

within the Upper Truckee River and Trout Creek Basins. About 78 

percent of the basin is at altitudes from about 6,500 ft to greater than 

10,000 ft. This altitude range, combined with other factors such as 

prevailing storm systems from the Pacific Ocean, causes an unequal 

distribution of precipitation throughout the basin. More than 80 inches per 

year (in/yr) of precipitation, mostly as snow, falls on the western side of 

the basin, whereas about 30 in/yr falls on the eastern side. 

 Since 1874, the outflow of Lake Tahoe into the Truckee River has 

been regulated by a dam at Tahoe City, Calif. The current dam was built 

by the Bureau of Reclamation in 1913 to provide irrigation water for the 

Newlands Project in the Fallon, Nev., area. The upper 6 ft of the lake 

forms the largest storage reservoir in the Truckee River Basin, with an 

effective capacity of 745,000 acre-feet (acre-ft), about 0.6 percent of the 

estimated 122 million acre-ft in the lake. The dam is operated by the 

U.S. District Court Water Master under a complex set of legal agreements 

and operating rules to maintain levels between a maximum altitude of 

6,229.1 ft and the altitude of the natural rim (6,223 ft). During droughts the 

lake level can fall 

below the rim, and 

during wet years 

the lake level can 

rise higher than the 

legal maximum. 

Since 1987, the 

lake levels have 

fluctuated from 

6,220.26 ft (about 

3 ft below the rim), 

during a prolonged 

drought in 1992, to 

6,229.39 ft (about 

0.2 ft above the 

legal maximum), 

during the flood of 

January 1997.

 The Lake Tahoe 

Basin is divided 

by the NevadaCalifornia State 

line, with about 

one-third of the 

basin in Nevada 

and two-thirds in 

California. The 

location of the basin, about 150 miles (mi) from the San Francisco Bay 

area and 90 mi from the Sacramento Valley, makes a wide variety of 

recreational opportunities available to a population of about 8 million. 

Major recreational activities within the basin include casino gaming in 

Nevada, alpine and cross-country skiing, golfing, water sports, hiking, 

fishing, camping, and bicycling.

History of Environmental Regulation 

 Until its “discovery” in 1844 by General John C. Fremont, the basin 

was occupied by the Washoe Tribe who had hunted and fished there for 

centuries. Upon discovery of gold in the South Fork of the American River 

in 1848, thousands of west-bound gold seekers passed near the basin on 

their way to the gold fields. “White-man’s” civilization first made its mark 

in the Lake Tahoe Basin with the 1858 discovery of the Comstock Lode, 

just 15 mi to the east in Virginia City, Nev. From 1858 until about 30 

years later, logging in the basin supplied large timbers to shore up the 

underground workings of the Comstock mines. The logging was so 

extensive that almost all of the native forest was cut. In 1864, Tahoe City 

was founded as a resort community for Virginia City, the first recognition 

of the basin’s potential as a destination resort area.

U.S. Department of the Interior—U.S. Geological Survey

Stream and Ground-Water Monitoring Program,

Lake Tahoe Basin, Nevada and California

TAHOE

REGIONAL

PLANNING

AGENCY 
2

 Public appreciation of the Tahoe Basin grew, and during the 1912, 

1913, and 1918 congressional sessions, unsuccessful efforts were made to 

designate the basin as a national park. During the first half of this century, 

development around the lake consisted of a few vacation homes. The postWorld War II population and building boom, followed by construction of 

gambling casinos in the Nevada part of the basin during the mid-1950’s, 

and completion of the interstate highway links for the 1960 Squaw Valley 

Olympics, resulted in a dramatic increase in development within the basin. 

From 1960 to 1980, the permanent resident population increased from 

about 10,000 to greater than 50,000, and the summer population grew 

from about 10,000 to about 90,000. 

 Increased development included urbanization of wetland areas that 

had formerly served as zones for retention of sediments and nutrients 

(nitrogen, phosphorus, and iron); development on steep mountain sides 

with consequent sediment erosion; discharge of septic and sewage systems 

within the basin; and increased airborne nutrients from automobile 

emissions and wood-burning stoves.

 By the 1950’s, evidence was mounting that the clarity of the lake 

was decreasing. Concerns about the effects of sewage effluent and septicsystem leakage on stream and lake quality led to formation of the Lake 

Tahoe Area Council (LTAC) which, in a historic decision, acted to 

develop a basin-wide sewage-collection system by which the effluent 

would be exported from the basin, primarily to other areas in Douglas 

County, Nev., and in Alpine County, Calif. During this time, researchers 

of TRG documented increases in algal growth and decreases in lake 

clarity. It was suspected that development was increasing transport of 

nutrients to the lake, thus stimulating growth of algae.

 In 1969, at the joint request of the States of California and Nevada, 

TRPA was chartered by Federal law under an Interstate Compact. TRPA

was formed as a bi-state agency to better manage and regulate land use 

and development to protect the lake and the natural resources of the basin.

The first two decades of TRPA's management focused on development 

and application of land-use regulations. In the early 1990's, the agency 

shifted focus from regulation to science-based environmental management 

and decision making.

LTIMP Cooperative Monitoring Program

 In 1978, the Lake Tahoe Interagency Monitoring Program (LTIMP) 

was formed. This group included collaborative monitoring and research 

efforts among TRPA, USGS, TRG, U.S. Forest Service, California State 

Water Resources Control Board (CSWRCB), Lahontan Regional Water 

Quality Control Board, California Department of Water Resources 

(DWR), California Department of Transportation, California Air 

Resources Board, California Department of Fish and Game, Nevada 

Department of Environmental Protection, and U.S. Environmental 

Protection Agency (EPA). The combined resources of LTIMP have 

contributed significantly to the body of literature and hydrological and 

limnological data available for the Lake Tahoe Basin.

 Although early concerns focused on suspended-sediment and nutrient 

transport to the lake by streams, potential nutrient contribution by ground 

water became recognized in the late 1980’s. Suspected sources included 

abandoned septic systems, golf courses, organic-rich stream deposits, and 

contaminated surface-water infiltration into ground water. 

 In 1982, TRPA adopted Resolution No. 82-11, which includes 

environmental thresholds for the Lake Tahoe Basin. Among those 

thresholds is “Water Quality 4,” which establishes standards for total 

nitrogen, soluble inorganic nitrogen, total phosphorus, soluble phosphorus, 

total iron, and suspended sediment in tributary streams. 

 TRPA also adopted “Water Quality 6,” a threshold that establishes 

standards for total nitrogen, total phosphorus, total iron, turbidity, and 

grease and oil in surface discharge to ground water.

 These thresholds provide the basis for the current program for stream 

and ground-water monitoring operated cooperatively by TRPA, USGS, 

and TRG. Since 1988, funding for this program has been shared equally by 

TRPA and USGS, with additional support and services provided by TRG.

 The California part of Lake Tahoe is designated by EPA as an 

Outstanding Natural Resource Water, which provides that no further 

degradation of Lake Tahoe can be allowed. All reasonable, cost-effective, 

best-management practices for nonpoint source control are required. Under 

Sampling water quality at Incline Creek, January 1993.

Photograph by Rita Whitney, U.S. Geological Survey.

Lake Tahoe, September 1996; eastward view from Rubicon Point,

California. Photograph by Timothy G. Rowe, U.S. Geological Survey.
3

Figure 1. Geographic setting, hydrologic basins, bathymetry, and surface-water and ground-water

monitoring sites in the Lake Tahoe Basin.

200

600

1000

1000

1500

1500

1400

1200

200

600

1000

1400

1200

800

1400

1550

1600

1646

50

395

395

50

50

88

89

431

89

267

80

207

28

Glenbrook

Incline

Village

Tahoe

City

Stateline

Meyers

South Lake

Tahoe

Trout

Upper Truckee

River

Creek

Lake

Tahoe

Glenbrook

Incline

Village

Tahoe

City

Stateline

South Lake

Tahoe

0 10 MILES

0 10 KILOMETERS

Glenbrook

Carson

City

Incline

Village

Tahoe

City

Truckee

Stateline

Meyers

Minden

Gardnerville

South Lake

Tahoe

NEVADA

CALIFORNIA

OREGON IDAHO

Area

of map San

Francisco

Los

Angeles

Reno

Las

Vegas

PLACER

EL DORADO

ALPINE

DOUGLAS

CARSON

CITY

WASHOE

NEVADA

Trout

Upper Truckee

River

Creek

River

Truckee

Lake

Tahoe

River

Carson

Washoe

Lake

NEVADA

CALIFORNIA

39° 15'

120° 15' 120° 00' 119° 45'

39° 00'

38° 45'

Base from U.S. Geological Survey digital data, 1:24,000 and 1:100,000,

1969-85. Universal Transverse Mercator projection, Zone 11

Bathymetric contours from Rush, 1973. Compiled from

soundings made by the U.S. Coast and Geodetic Survey (1923)

SIERRA

NEVADA

CARSON

RANGE

EXPLANATION

Selected hydrologic basin used

 in this study

Boundary of Lake Tahoe Basin

Surface-water site

Bathymetric contour, in feet below highest

legal lake-surface altitude (6,229.1 feet above

U.S. Bureau of Reclamation datum of 1929) 

Ground-water site Boundary of subbasin

1200

Incline

Creek

Wood

Creek

Second

Creek First

Creek

Third

Creek

Ward

Creek

Blackwood

Creek

General

Creek

Upper

Truckee

River

Trout

Creek

Glenbrook

Creek

Logan House

Creek

North Logan

House Creek

Edgewood

Creek
4

Nevada Pollution Control Regulations, the Nevada part of Lake Tahoe has 

a designated beneficial use as a water of extraordinary ecological and/or 

aesthetic value, which is also a nondegradation standard (Adelle Basham, 

Nevada Department of Environmental Protection, oral commun., 1997). 

Although no specific monitoring program has been implemented within 

the basin to meet all the requirements of these policies, regulatory agencies 

rely upon LTIMP data in implementation of their programs.

Stream Monitoring Network

 In 1979, the LTIMP stream-sampling network began as a cooperative 

effort of the CSWRCB, DWR, USGS, and TRG. The objectives of this 

network are to acquire and disseminate the water-quality information 

necessary to support science-based environmental planning and decision 

making in the basin. Seven major tributary streams were monitored for 

streamflow and suspended-sediment and nutrient contribution to Lake 

Tahoe. By 1987, decreases in funding had reduced the network to four 

streams in California: General, Blackwood, and Ward Creeks, and Upper 

Truckee River.

 In 1987, TRPA and USGS provided funding to expand the program 

by adding four Nevada tributaries. By 1993, the LTIMP network 

had expanded to 32 sites in 14 basins (fig. 1). Of the 32 sites, 20 have 

recording streamflow gages. The 14 basins total 157 mi2, or about half of 

the entire basin tributary to Lake Tahoe. The largest basin monitored is the 

Upper Truckee River Basin (56.5 mi2, 18 percent of the total drainage to 

the lake); the smallest is the First Creek Basin (1.08 mi2). 

 Routine and storm-based monitoring is done to provide data for 

comparisons between spring runoff, storm-generated runoff, and baseflow, and for estimating suspended-sediment and nutrient transport. 

Comparisons can be made among the 14 monitored basins. Monitoring 

stations at the mouths of the basins measure loads to the lake and provide 

a basis for comparisons of the effects of the differing geology, soils, and 

land uses on those loads. Eight of the basins also have internal sampling 

stations to allow for comparisons of the effects of upstream and downstream land uses. 

 The greatest transport of sediment and some associated nutrients occurs 

during high flows caused by storms and snowmelt. To quantify transport 

during such events, individual samples must be collected as the streamflows rise, peak, and recede. A timely and steady field presence in the 

basin during storms is required to accomplish this. During runoff 

monitoring, USGS field crews frequently collect hydrologic data late into 

the night and on weekends and holidays to meet the program objectives.

 Tributary monitoring includes field measurement of streamflows, 

temperature, pH, dissolved oxygen, and specific conductance; and 

laboratory measurement of major nutrients (dissolved nitrate and nitrite, 

dissolved ammonia, total ammonia and organic nitrogen, dissolved 

orthophosphorus, total phosphorus, and total iron) and suspended 

sediment. These measurements are necessary to determine whether the 

environmental thresholds for the basin are being exceeded and to provide 

long-term data that can be used to determine suspended-sediment and 

nutrient loads to the lake. 

 Depth-integrating and equal-width-increment sampling techniques are 

used for suspended-sediment and nutrient sampling. Suspended-sediment 

analyses are made by the USGS California sediment laboratory in Salinas, 

Calif. Nutrient analyses are done at TRG laboratories in Tahoe City and 

Davis, Calif. The streamflow gages are operated by USGS personnel from 

the Carson City, Nev., and Carnelian Bay, Calif., field offices. LTIMP data 

are entered in national USGS data bases and published every year in 

USGS California and Nevada Water Data Reports.

Ground-Water Monitoring Network

 In 1990, USGS and TRPA established a ground-water monitoring 

network with 32 sampling sites (fig. 1) to provide a long-term data base

on ground water. Previous ground-water studies found concentrations of 

nitrogen, phosphorus, and iron to be greater in ground water than in the 

lake. These studies indicated the need to better describe ground-water 

quality and rates of ground-water flow into the lake.

 Field measurements of water from wells include temperature, pH, 

dissolved oxygen, specific conductance, and water level. Laboratory 

measurements of dissolved nutrients, including iron, are made. Groundwater samples are obtained by pumping long enough to remove stagnant 

well water before sampling.

Monitoring Results

 The monitoring provides scientific data on stream discharge and quality 

and ground-water levels, quality, and flow paths. Selected results are 

described below.

 The hydrograph of daily mean discharge for Incline Creek (fig. 2) for 

1996 shows a seasonal pattern that is typical of streams in the Lake Tahoe 

Basin. Most runoff is during the April through June snowmelt period. 

Sharp peaks represent fall rains, rain-on-snow storms, and summer thunder 

storms.

1988 1989 1990 1991 1992 1993 1994 1995 1996 1997

WATER YEAR

200

1

5

.5

10

50

100

DISCHARGE, IN CUBIC FEET PER SECOND

Figure 3. Mean daily discharge for Incline Creek, 1988-97 water

 years, representing years of drought and above-normal runoff.

Annual

mean

0

20

40

60

80

DISCHARGE, IN CUBIC FEET PER SECOND

OCT NOV DEC JAN FEB MAR APR MAY

1995 1996

JUNE JULY AUG SEPT

Figure 2. Daily mean discharge for Incline Creek during

1996 water year, representative of streams in the Lake Tahoe Basin.
5

50

395

395

50

50

88

89

431

89

267

80

207

28

Glenbrook

Incline

Village

Tahoe

City

Stateline

Meyers

South Lake

Tahoe Upper

Lake

Tahoe

Glenbrook

Incline

Village

Tahoe

City

Stateline

South Lake

Tahoe

Glenbrook

Carson

City

Incline

Village

Tahoe

City

Truckee

Stateline

Meyers

Gardnerville

South Lake

Tahoe Upper

Truckee

River

Lake

Tahoe

Washoe

Lake

NEVADA

CALIFORNIA

39° 15'

120° 15' 120° 00' 119° 45'

39° 00'

38°45'

Base from U.S. Geological Survey digital data, 1:24,000 and 1:100,000, 1969-85

Universal Transverse Mercator projection, Zone 11

0 10 MILES

0 10 KILOMETERS

0.01

0.1

1

10

100

0.01

0.1

1

10

100

1,000

10,000

Blackwood Creek

152 150 150 150 149

Edgewood Creek

110 111 111 111 109

0.01

0.1

1

10

100

0.01

0.1

10

100

1,000

10,000

Glenbrook Creek

228 269 274 274 170

0.01

0.1

1

10

100

0.01

0.1

1

10

100

1,000

10,000

0.01

0.1

1

10

100

0.01

0.1

1

10

100

1,000

10,000

Third Creek

320 363 366 368 236

0.01

0.1

1

10

100

0.01

0.1

1

10

100

1,000

10,000

Logan House Creek

206 237 240 242 140

0.01

0.1

1

10

100

0.01

0.1

1

10

100

1,000

10,000

Incline Creek

307 351 358 357 228

0.01

0.1

1

10

100

0.01

0.1

1

10

100

1,000

10,000

General Creek

140 138 138 138 137

0.01

0.1

1

10

100

0.01

0.1

1

10

100

1,000

10,000

Upper Truckee River

135 130 131 131 127

0.01

0.1

1

10

100

0.01

0.1

1

10

100

1,000

10,000

Ward Creek

Number of analyses

154 150 151 151 150

Instantaneous suspended-sediment load,

in tons per day

Instantaneous nutrient load,

in pounds per day

Suspended sediment

Total ammonia and 

organic nitrogen

Dissolved nitrate and

nitrite nitrogen

Total phosphorus

Total iron

0.01

0.1

1

10

100

0.01

0.1

1

10

100

1,000

10,000

Trout Creek

121 115 116 116 115

EXPLANATION

Boundary of Lake Tahoe Basin

Boundary of hydrologic basin—

 Selected streams shown for

 study basins

90th Percentile

75th Percentile

Median

25th Percentile

10th Percentile

Figure 4. Instantaneous suspended-sediment and nutrient loads depicted by box plots for

selected surface-water monitoring sites in the Lake Tahoe Basin. 
6 June 1997 Fact Sheet FS-100-97

Public Information Assistant

U.S. Geological Survey

333 W. Nye Lane, Rm 203

Carson City, NV 89706

tel.: (702) 887-7649

fax: (702) 887-7629

email: usgsinfo_nv@usgs.gov

URL: http://wwwnv.wr.usgs.gov

For more information please contact:

 The longer term hydrograph (fig. 3) for Incline Creek for the 9-year 

period of record clearly shows the effects of drought (water years 1988-92 

and 1994) as compared to years in which runoff was above normal (1993, 

1995, and 1996). The mean daily discharge for the 9 years is 6.38 ft3/s. 

The highest mean daily discharge (15.4 ft3/s) was in 1995 and the lowest 

(2.51 ft3/s) in 1992. The average annual runoff for the period of record is 

5,160 acre-ft. Among the 14 basins monitored, the Upper Truckee River 

has the highest average runoff (101,500 acre-ft) and Logan House Creek 

(330 acre-ft) the lowest. Instantaneous suspended-sediment and nutrient 

concentrations are highest during summer thunderstorms and rain-on-snow 

storms, but overall loads are greater during spring runoff. 

 Calculations for measured loads of suspended sediment and nutrients 

are shown for 10 tributary watersheds in figure 4. As many as 368 analyses 

for a given variable are included. For each basin, “boxplots” are shown 

summarizing sampled loads for five selected constituents. For each 

constituent, the box shows the range in load for 25-75 percent of the 

samples. The median value (half the samples were less than this value 

and half were more) is indicated by the horizontal line through the box. 

The vertical lines above and below the box extend from the 10th percentile 

(only 10 percent of the samples were lower) to the 90th percentile (only 10 

percent had higher values). For example, of the 307 suspended-sediment 

samples from the Incline Creek site, half (154) had loads between 0.11 ton 

per day (ton/d), the 25th percentile, and 3.1 ton/d, the 75th percentile. The 

range from 0.032 to 14 ton/d represented 80 percent of all samples (10th to 

90th percentile). The median value was 0.61 ton/d.

 The Upper Truckee River had the largest load of suspended sediment 

and all nutrients. This is because the Upper Truckee River Basin is the 

largest basin and contributes the most flow. The Logan House Creek Basin 

contributes the smallest sediment and nutrient loads. Watersheds on the 

western side of the basin (California) of the lake have higher loads of 

sediment and nutrients than the sites on the eastern side (Nevada) due to 

smaller drainage areas and less precipitation on the eastern side.

Summary and Conclusions

 Lake Tahoe has long been admired for the clarity of its water and 

majestic mountain setting. Human activity in the basin has accelerated the 

decline in clarity and quality of this pristine lake. Resource-management 

agencies, such as TRPA, need long-term water-quality data to assess the 

effectiveness of both current and new projects and regulations. Since 1987, 

the LTIMP has been monitoring the water quality of surface-water and 

ground-water flow tributary to Lake Tahoe. Additional data are necessary 

to provide the basis for reliable quantification of nutrient loads to the lake 

from ground water. Additional scientific data and interpretation are 

essential for water managers to prioritize their efforts for the most 

effective protection of Lake Tahoe.

—Carol J. Boughton, Timothy G. Rowe, Kip K. Allander, and Armando

R. Robledo

Selected References

Bauer, D.J., Foster, B.J., Joyner, J.D., and Swanson, R.A., 1996, Water 

resources data, Nevada, water year 1995: U.S. Geological Survey 

Water-Data Report NV–95–1, 734 p.

California Department of Water Resources, 1991, Truckee River atlas: 

Sacramento, Calif., 128 p.

California Regional Water Quality Control Board, 1995, Water quality 

control plan for the Lahontan Region (basin plan): South Lake Tahoe, 

Calif., 6 chapters, 5 appendixes.

Cartier, K.D., Peltz, L.A., and Long, Kati, 1995, Hydrologic basins and 

hydrologic-monitoring sites of Lake Tahoe Basin, California and 

Nevada: U.S. Geological Survey Open-File Report 95–316, 1 sheet.

Crippen, J.R., and Pavelka, B.R., 1970, The Lake Tahoe Basin, 

California–Nevada: U.S. Geological Survey Water-Supply Paper 1972, 

56 p.

Farquhar, F.P., 1965, History of the Sierra Nevada: Berkeley, Calif., 

University of California Press, 262 p.

Goldman, C.R., 1988, Primary productivity, nutrients, and transparency 

during the early onset of eutrophication in ultra-oligotrophic Lake 

Tahoe, California-Nevada: Limnology and Oceanography, v. 33, no. 6, 

part 1, p. 1321–1333.

Goldman, C.R., and Byron, E.R., 1986, Changing water quality at Lake 

Tahoe—The first five years of the Lake Tahoe Interagency Monitoring 

Program, Tahoe: Tahoe Research Group, Institute of Ecology, 

University of California, Davis, 12 p.

Rush, F.E., 1973, Bathymetric reconnaissance of Lake Tahoe, Nevada and 

California: Nevada Division of Water Resources, Information Report 

17, 1 sheet.

Thodal, C.E., 1995, Hydrogeologic setting and ground-water quality of 

areas tributary to Lake Tahoe in Douglas County and Carson City, 

Nevada, through 1987: U.S Geological Survey Water-Resources 

Investigations Report 94–4079, 31 p.

Thodal, C.E, 1997, Hydrogeology of Lake Tahoe Basin, California and 

Nevada, and results of a ground-water quality monitoring network, 

water years 1990–92: U.S. Geological Survey Water-Resources 

Investigations Report 97–4072, 53 p.

—Publications support by Carol A. Myers and Angelia M. Thacker

Measuring streamflow at Third Creek, May 1993. 

Photograph by Timothy G. Rowe, U.S. Geological Survey.