^USGS

science for a changing world

U.S. Department of the Interior 

U.S. Geological Survey

Mercury and Suspended Sediment, 

Carson River Basin, Nevada Loads To and 

From Lahontan Reservoir in Flood Year 1997 

and Deposition in Reservoir Prior to 1983

During the flood on January 3, 1997, about 200,000 tons of 

sediment and 3,000 pounds of total mercury flowed past the 

Carson River streamflow gage near Fort Churchill.

During January through September 1997:

 About 600,000 tons of sediment and 10,000 pounds 

of total mercury flowed past the Fort Churchill site.

 About 2,000 pounds of total mercury was discharged 

from Lahontan Reservoir.

 Lahontan Reservoir retained about 90 percent of the 

sediment and 80 percent of the total mercury that 

flowed past the Fort Churchill site.

 Concentrations of total mercury and total methylmercury ranged from 28,000 to 260 ng/L, and 5.24 to 

1.44 ng/L at the Fort Churchill site, respectively.

A sedimentation rate of about 0.8 in/yr was measured in a 

core sample of bottom sediment collected from Lahontan 

Reservoir in April 1982.

By late December 1996, a huge snow pack (more than 180 

percent of normal in some places) had been deposited on the highaltitude eastern slopes of the Sierra Nevada. A carpet of snow had 

fallen on many of the low-lying valleys in west-central Nevada as 

well. Coincidently, near the Hawaiian Islands, a subtropical lowpressure system was developing over warm Pacific Ocean waters. 

Moving rapidly northeastward, the weather system dumped large 

quantities of unseasonably warm rain on the snow-covered slopes 

and valleys. During December 30, 1996-January 3, 1997, the rapid 

release of snowmelt delivered a torrent of water down both main 

forks of the Carson River in California and Nevada, causing a combined flow that inundated the main channel and flood plain with a 

deluge of turbulent, debris-carrying water the likes of which few 

have seen. On January 3, the catastrophic flood wave peaked at 

22,300 ft3/s at the Carson River streamflow-gaging station near Fort 

Churchill, Nev., about 10 mi upstream from Lahontan Reservoir 

(Thomas and Williams, 1997). At that gaging station (figs. 1 and 2), 

the flow exceeded the 100-year peak discharge (16,800 ftVs), a flow 

that, statistically, has a 1 -percent chance of happening in any given 

year (Garcia, 1997). For comparison, the long-term (1911-96) 

maximum mean flow for January is about 1,500 ft3/s.

But a flood represents more than just an abundance of water; it 

also represents an abundance of "things" in water, such as mineral 

sediment and certain trace elements typically attached to sediment 

particles mercury, for example. Inorganic mercury, commonly 

known as quicksilver, is a metal and in the environment is a potential contaminant hazard to fish and wildlife, and to humans.

Inorganic mercury and its organic compounds, such as methylmercury, have no known biological function, and are potentially 

toxic to living organisms. Methylmercury (MeHg) is formed by 

bacteria from inorganic mercury in the aquatic environment.

/ DaytonCarson J River

fSffif Fort Cjjtirchill

T

EXPLANATION 

A Sampling site

0 20 MILES

20 KILOMETERS

Figure 1. Location of sampling sites, Carson River Basin, 

California and Nevada.

The most toxic of the naturally formed mercury compounds, MeHg 

is known to bioaccumulate and to biomagnify. Bioaccumulation is 

a biological process wherein an organism accumulates a contaminant in its body at a greater rate than the contaminant is excreted. 

Biomagnification is a process wherein a contaminant concentration 

increases incrementally at each level of the food chain, including 

humans. A local consequence of biomagnification of MeHg is the 

public-health warnings issued by the State of Nevada for the consumption of game fish from Lahontan Reservoir (Nevada Division 

of Wildlife, written commun., 1997) and of shoveler ducks from 

Carson Lake southeast of Fallen, Nev. (Nevada State Health 

Division, written commun., 1989).

Figure 2. Sampling site at U.S. Highway 95-A bridge crossing 

Carson River near Fort Churchill during January 1997 flood.
During the 1800's. large amounts of inorganic mercury were 

imported from California to the Carson River Basin and used in 

the milling of gold and silver ore from the Comstock Lode near 

Dayton, Nev. (fig. 1). About 7,500 tons of the imported mercury 

were lost from the milling process and became potentially available 

by erosion to the Carson River system (Smith, 1943, p. 257).

Elevated concentrations of inorganic mercury in the Carson 

River Basin were first reported by the U.S. Geological Survey 

(USGS) in 1973 (Van Denburgh, 1973). That study showed concentrations of mercury up to 200-times background concentrations in 

samples of whole-water (unfiltered) and bottom sediment downstream from areas where inorganic mercury was used during the 

milling process. Subsequent investigations by researchers at the 

University of Nevada-Reno have further documented the extent and 

severity of this human-caused contamination problem (for example, 

see Gustin and others, 1994). Prior to completion of Lahontan Dam 

in 1915, much of the mercury stored in mill tailings along stream 

flood plains was flushed downstream to the Carson Desert wetlands 

by episodic floods. After 1915, Lahontan Reservoir provided a 

partial trap for fluvial sediments and adsorbed mercury that settled 

on the reservoir bottom. In August 1990, the U.S. Environmental 

Protection Agency (USEPA) listed the Carson River as a Superfund 

site for possible remediation of mercury contamination from just 

upstream from Dayton to the river's terminus in the Carson Desert.

What, Where, Why

Streamflow conditions were hazardous near the Fort Churchill 

gage in the first week of January 1997 as USGS hydrologists collected water samples during the rising and falling stages of the 

flood wave. Samples also were collected periodically from February through September 1997. All these samples were subsequently 

analyzed for mercury and suspended-sediment concentrations in the 

laboratory. These data, and those from samples collected at another 

gaging station on the Carson River about 1.5 mi downstream from 

Lahontan Reservoir during January-September 1997, were used to 

calculate the monthly discharge of suspended sediment (in tons per 

day) and mercury (in pounds per day) to and from the reservoir. 

Only water from the Carson River entered the reservoir during the 

flood. Flow from the Truckee Canal (a diversion from the Truckee 

River Basin) to Lahontan Reservoir was discontinued during 

January-September 1997.

The purpose of this paper is (1) to present a contemporary and 

quantitative perspective on the hydrologic role of a major flood to 

mobilize and transport large quantities of sediment and associated 

mercury (as measured just upstream from Lahontan Reservoir) 

from the active channel and flood plain of the Carson River; (2) to 

estimate the load of sediment and mercury moving to and from 

Lahontan Reservoir during January-September 1997: and (3) to 

provide a historical glimpse of the rate of sediment accumulation 

and on the concentration of mercury in core samples from bottom 

sediment in Lahontan Reservoir prior to 1983.

Collection of Data on Surface-Water Quality

Water samples were collected near the Fort Churchill gaging 

station during rising flood water near the peak flow on January 3, 

1997, by grab sampling at three nearly equidistant locations across 

the channel. Grab samples were collected because emergency conditions existed at the time. These samples were combined and 

thoroughly mixed to obtain representative subsamples of 

suspended-sediment and mercury concentrations. Subsequent 

sample collection was done by the depth-integration method at 

several verticals across the channel. This method of collection 

more accurately represents the discharge-weighted sample concentration in each vertical. Water samples from each vertical were 

combined and thoroughly mixed for obtaining subsamples for 

chemical analysis. A separate set of depth-integrated water samples was collected for suspended-sediment analysis. Obtaining 

whole-water samples representative of the river is especially 

important for high-velocity flows containing appreciable concentrations of suspended particles that are typically poorly mixed 

within the stream channel.

Water samples for mercury analysis were chilled at 39°F 

or less, sent overnight to the chemical laboratory, and analyzed 

according to procedures described by Bloom (1989, 1995). The 

determination of suspended-sediment concentration in whole- . 

water samples was done in the sediment laboratory according to 

procedures described by Guy (1969, p. 11-18). External laboratory quality-control samples for mercury comprised equipment 

blanks and field blanks for the flood-peak samples and for subsequent data collection. Results of those analyses, in light of the 

high environmental concentrations, showed that rigorous qualitycontrol objectives were met. Water samples were collected more 

frequently at the Fort Churchill site where flow variability is 

greater compared to the site below Lahontan Reservoir where 

water release (flow) is controlled. At the downstream site, water 

samples were collected near-monthly. Load estimates below 

Lahontan Reservoir were calculated using time-weighted averages of streamflow multiplied by sample concentrations of suspended sediment, total mercurya (THg), and total methylmercury 

(TMeHg).

Load estimates were developed for the Fort Churchill site 

using linear regression equations of suspended-sediment concentration with streamflow (fig. 3), and of THg concentrations with 

streamflow (fig. 4). Data used to develop the equations were for 

the period of USGS record (1975-97). The time-weighted average 

method was used for estimating loads of TMeHg.

4,000

3,500

DC

' K 3,000 

z-i

p E 2,500

C^Hi 2,000

1,500

1 1 I I T

 Observed data

 Equation for line: SS = (0.186)Q + 121.9 * 

r2 = 0.85

5 1,000

500 -

0 6,000 12,000 18,000 

STREAMFLOW (Q), IN CUBIC FEET PER SECOND

Figure 3. Linear regression of suspended sediment and 

streamflow for Carson River near Fort Churchill period of 

USGS record. Symbol: r2, statistical coefficient indicating 

decimal percentage of total variation in suspended sediment 

that can be attributed to streamflow.

a Total mercury and total methylmercury refer to all extractable forms 

of each chemical for a whole-water (unfiltered) sample.
Equation for line: THg = 1 n(°-754l°9Q + 1 - 241 ) 

r2 = 0.72

i___i

0.5 1 1.5 2 2.5 3 3.5 4 4.5 

LOG10 STREAMFLOW (Q), IN CUBIC FEET PER SECOND

Figure 4. Linear regression of Iog10 total mercury and Iog10 

streamflow for Carson River near Fort Churchill period of USGS 

record. Symbol: r2 , statistical coefficient indicating decimal percentage of total variation in total mercury that can be attributed 

to streamflow.

Results of Surface-Water Data Collection

Streamflow

Daily mean streamflow during January-September 1997 for 

the gages near Fort Churchill and below Lahontan Reservoir is 

shown in figure 5. The flow at the Fort Churchill gage represents 

nearly natural runoff conditions because of the lack of storage 

reservoirs upstream. The flood wave during the first week of 

January dominates the 9-month hydrograph (fig. 5). The maximum instantaneous streamflow near Fort Churchill on January 3 

was 22,300 ft3/s. A secondary peak (4,000 ftVs) occurred in late 

January owing to runoff from local thundershower activity. The 

increase in streamflow from mid-April through June represents 

spring snowmelt runoff. Lahontan Reservoir was at about 70 percent maximum capacity on December 31. Precautionary releases 

for flood control from the reservoir were begun at noon January 2 

according to Bureau of Reclamation flood control criteria (Bureau 

of Reclamation, written commun., 1997) and continued to midMarch. From mid-March (fig. 5) to mid-April, the flows were 

reduced. For the remainder of the period, flows were released for 

irrigation in the Fallen area.

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 Carson R

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L Carson R ver below Lahontan

! Reservoir (station 10312150)

s.

-

-

-

 T--- 

JAN FEB MAR APR MAY JUNE JULY AUG SEPT 

1997

Figure 5. Streamflow at Carson River near Fort Churchill and 

below Lahontan Reservoir, 1997

Concentration of Total Mercury and Total 

Methylmercury

Concentrations of THg in water samples collected periodically 

at both sites from January through September are shown in figure 6. 

For the Fort Churchill site, THg concentrations generally correlate 

positively with flow. The largest concentrations in 1997 were measured near the peak of the flood wave. The 28,000 ng/Lb (28 ug/L) 

maximum value shown in figure 6 represents the mean of three discrete samples collected just prior to the peak. After the flood peak 

passed the Fort Churchill site on January 3-4, the concentrations of 

measured THg dramatically declined with reduced streamflow. Concentrations ranged from 1,000 to 4,200 ng/L from mid-January to 

mid-April. Concentrations of THg again increased with increasing 

flows from spring snowmelt runoff in April. The large concentration 

of 10.000 ng/L was measured on April 23 during the "first flush" of 

the snowmelt runoff. Thereafter, throughout the snowmelt runoff 

period, the concentration of THg varied positively with flow, ranging in concentration from about 2,200 to 6,600 ng/L. During the 

low-flow period of July through September, the THg concentrations 

decreased substantially, ranging from about 260 to 960 ng/L 

throughout the summer. The increase in concentration of THg at the 

end of September coincided with a slight increase in flow (not 

readily apparent in figure 5).

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 Carson River near Fort Churchill

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 Carson River below Lahontan

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1997

Micrograms per liter (|ig/L) are equivalent to "parts per billion:" nanograms per liter (ng/L), "parts per trillion;" and milligrams per liter (mg/L) and 

micrograms per gram (u,g/g), "parts per million."

Figure 6. Concentration of total mercury at Carson River near 

Fort Churchill and below Lahontan Reservoir, 1997

At the sampling site below Lahontan Reservoir, concentrations 

of THg increased from below the analytical reporting limit (100 

ng/L) on January 7 to 2,500 ng/L on January 16, about 2 weeks after 

the flood peak passed the Fort Churchill site. The January 7 sample 

represents pre-flood water. Subsequent samples for THg were measured at a minimum reporting limit of 0.1 ng/L. Concentrations of 

THg stayed near 2,000 ng/L to the beginning of March. However, 

from the limited data available, during the winter and spring 

months, THg concentrations typically were less at the site below 

Lahontan Reservoir than at the upstream sampling site.

To put the above concentrations of THg in perspective, even 

though the Carson River is not a source of drinking water (at least 

not for humans), the Nevada standard for THg in public water supplies is 2,000 ng/L (2 |J.g/L). Upstream from the main source of contamination, THg concentrations in water samples from the Carson 

River generally are less than 100 ng/L (0.1 |Ug/L).
Samples for total methylmercury (TMeHg) concentrations 

(fig. 7) were collected at the Fort Churchill site during spring 

snowmelt runoff and through the summer of 1997. The data were 

highly variable throughout the sampling period, with the 11 values 

ranging in concentration from 5.24 to 1.44 ng/L. The limited 

TMeHg data for samples collected at the site below Lahontan 

Reservoir showed an increase in concentration from 0.52 ng/L 

to 1.70 ng/L from mid-June to mid-September, respectively.

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LL O

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Carson River near Fort Churchill

(station 10312000) 

Carson River below Lahontan 

Reservoir (station 10312150)

1997

Figure 7. Concentraton of total methylmercury at Carson River 

near Fort Churchill and below Lahontan Reservoir, 1997

Overall, the Carson River TMeHg concentrations are relatively high. For example, water samples from the Sudbury River, 

Mass., a stream contaminated with mercury from a nearby Superfund site, contained concentrations of TMeHg ranging from 0.03 

to 0.75 ng/L (J.A. Colman, USGS, oral commun., 1997). Surfacewater samples from the Sacramento River Basin, Calif., affected 

by non-point sources of mercury, contained concentrations of 

TMeHg ranging from 0.065 to 1.3 ng/L (J.L. Domagalski, USGS, 

written commun., 1997). Alternatively, Bloom (1995) suggests 

TMeHg concentrations in natural surface water not contaminated 

with mercury typically range from 0.01 to I ng/L. Thus, the elevated TMeHg concentrations in the Carson River system have 

important implications for potential adverse effects on the aquatic 

ecosystem. For the present work, the minimum reporting limit for 

TMeHg was less than 0.05 ng/L.

Load of Sediment and Mercury

Because of uncertainties associated with measuring streamflow, including other data-collection and data-analysis activities, 

for extreme high flows, the summary loading estimates shown in 

tables 1 and 2 are rounded to one significant figure. The loads of 

sediment for both sampling sites in terms of monthly values from 

January to September are shown in figure 8. The total sediment 

loads for selected periods are given in table 1.

For the Fort Churchill site, the data in table 1 show that about 

70 percent of the sediment load for the 9-month period occurred 

during January alone. Of the January load, 50 percent flowed past 

the site on a single day January 3, the day of the flood peak.

Table 1. Estimated load of suspended sediment for selected periods 

at two sampling sites on Carson River, Nev., 1997 _____

Suspended sediment 

(tons, rounded to one significant figure)

Sampling site

Near Fort Churchill

(above Lahontan

Reservoir)

Below Lahontan

Reservoir

January 3 

(flood)

200,000

-

January 

1-31

400,000

10,000

AprilJune 

(snowmelt)

100,000

10,000

JanuarySeptember

600,000

40,000

Indeed, about 30 percent of the total sediment for the 9-month 

period occurred on a single, but significant day. For comparison, 

the 200,000-ton estimate for January 3, is equivalent to about 

13,000 standard (six-wheel) dump trucks filled with moist sand.

The data also show that the sediment load below Lahontan 

Reservoir was only about 10 percent of that measured upstream 

from the reservoir at the Fort Churchill site for the same 9-month 

period.

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250,000 -

200,000 -

150,000 -

100,000 -

50,000 -

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I I I I I I I I

 Carson River near Fort Churchill 

(station 10312000)

 Carson River below Lahontan

Reservoir (station 10312150)

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JAN FEB MAR APR MAY JUNE JULY AUG SEPT 

1997

Figure 8. Estimated load of suspended sediment at Carson River 

near Fort Churchill and below Lahontan Reservoir, 1997.

The loads of THg, in terms of monthly values, from January 

through September 1997 for both sites are shown in figure 9. The 

load of THg for selected time periods is shown in table 2. The data 

in table 2 indicate that about 70 percent of the THg load for the 9- 

month period (10,000 pounds) occurred during January. Of the 

January load, about 40 percent flowed past the Fort Churchill site 

on January 3. For the site below the reservoir, about 2,000 pounds 

of THg was released from the reservoir for the 9-month period of 

January through September, or 20 percent of that at the upstream 

site.

Table 2. Estimated load of total mercury for selected periods at two 

sampling sites on Carson River, Nev., 1997

Total mercury 

(pounds, rounded to one significant figure)

Sampling site

Near Fort Churchill

(above Lahontan

Reservoir)

Below Lahontan

Reservoir

January 3 

(flood 

peak)

3,000

-

January 

1-31

7,000

400

AprilJune 

(snowmelt)

3,000

300

JanuarySeptember

10,000

2,000
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j| (station 1031 2000) 

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Reservoir (station 10312150)

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JAN FEE MAR APR MAY JUNE JULY AUG SEPT

1997

Figure 9. Estimated load of total mercury at Carson River near 

Fort Churchill and below Lahontan Reservoir, 1997.

Monthly loads of TMeHg (fig. 10) at the Fort Churchill site 

ranged from 0.3 to 0.7 pound per month, whereas at the site below 

Lahontan Reservoir, the TMeHg loads ranged from 0.1 to 0.2 

pound per month.

0.7

0.6 

CC

0.4

0.3

OZ 0.2 

Q

0.1

Carson River near Fort Churchill

(station 10312000) 

Carson River below Lahontan

Reservoir (station 10312150) 

No data «

APR MAY JUNE JULY 

1997

AUG SEPT

Figure 10. Estimated load of total methylmercury at Carson 

River near Fort Churchill and below Lahontan Reservoir, 1997.

Mercury Concentrations and Sedimentation 

Rates From Sediment Cores, April 1982

Vertical core samples of bottom sediments in a reservoir can 

reveal patterns of natural and human-caused activities in the river's 

watershed during the last century or more. With the purpose of collecting data on the vertical disposition of mercury and other waterquality variables in bottom sediments of Lahontan Reservoir, core 

samples were collected April 1982. The samples were collected 

about 0.5 mi south of the dam using a gravity corer from a water 

depth of about 80 ft on or near the pre-reservoir channel. To agedate the sediments, three 39.4-in. cores were each sliced horizontally at 3.9-in. intervals from the top of the core to 27.6 in.; the 8th 

subsample comprised the remainder of the core. Each of the respective subsample intervals was then combined for measurement of the

radioisotopes cesium-137 (Cs-137) and lead-210 (Pb-210) in the 

laboratory. A fourth 39.4-in. core was sliced at 2-in. intervals for 

measurement of concentrations of mercury.

All cores appeared uniform throughout their length, except 

about the top 1 in. of brown organic matter, which is typical of the 

oxygenated conditions observed at the water-sediment contact at the 

time of collection. The remainder was dark grey with the texture of 

clay or silt, strong hydrogen sulfide (rotten egg) smell, and no benthic invertebrates nor their burrows seen. Activities of Cs-137 and 

concentrations of mercury (total recoverable) in bottom sediment 

are reported in terms of dry weight.

Cesium-137 is a by-product of atmospheric nuclear weapons 

testing following World War II. Cs-137 is not produced naturally, 

has a half-life of about 30 years, and is rapidly and almost irreversibly adsorbed onto fine-grained sediments exposed to atmospheric 

fallout. Over time, sediment particles with adsorbed Cs-137 that 

enter a reservoir tend to sink and accumulate on the bottom sediment. Measurable concentrations of Cs-137 were first detected in 

the atmosphere in the early 1950's and peaked at about 1963-64. 

Atmospheric fallout of this manmade isotope provides a useful agedating tool for assessing sediment inputs to a reservoir (or a lake) 

from its watershed (Krishnaswasmi and Lai, 1978, p. 153-177).

The deepest occurrence of measurable Cs-137 along the core 

composite was in the 23.6- to 27.6-in. subsample and the maximum 

Cs-137 content was in the 11.8- to 15.7-in. subsample. None of the 

cores penetrated to pre-reservoir sediments, which were initially 

covered by water in 1916. Thus, the 25.6-in. core-composite depth 

was assigned year 1952, and the 13.8-in. depth was assigned year 

1964 (fig. 11). The top of the core composite was assigned 1982, 

the date of collection.

On the basis of analysis for Cs-137, normalized for compaction (P. Van Metre, USGS, written commun., 1997), the average sedimentation rates for 1952-64 and 1965-82 are 0.4 g/in2/yr and 0.2 

g/in2/yr, respectively, at the coring location. The average linear rates 

for the same two periods are 0.9 in/yr and 0.6 in/yr, respectively, or 

an overall average rate of 0.8 in/yr. This overall rate is comparable 

to the 0.7-0.9 in/yr overall average linear sedimentation rate determined by analysis for Pb-210 from the same core composite (A. 

Yang, USGS, written commun., 1982). Pb-210 is a naturally occurring radioisotope with a half-life of about 22 years. This isotope is 

useful for dating sediments less than 150 years old. The reasonable 

agreement between the two independent dating measurements provides a degree of confidence to accompany the Cs-137 rate. The 

greater sediment rate during 1952-64 (13 years) than during 1965-82 

(18 years), may be due, in part, to the greater number of recorded 

peak runoffs at the Fort Churchill gage during the earlier period 

(eight peaks exceeding 8,000 ft3/s) compared to the later period 

(zero peaks exceeding 8,000 ftVs). The bankfull capacity of the 

river near Dayton is about 8,000 ftVs.

The mercury profile in bottom sediment near the site cored for 

the Cs-137 and Pb-210 data, is shown in figure 11. With the exception of the abrupt peak concentration of 15 u,g/g at the 21.7- to 25.6- 

in. core interval, the overall concentrations of mercury at the sampled 

site tend to decrease slightly with time, ranging from about 6-8 u.g/g 

near the bottom of the core to 4 |ug/g near the top of core. This
CESIUM-137, 

(picocuries per gram) 

0 0.4 0.8 1.2

o

10

20

30

40

 1982-

TOTAL MERCURY

(micrograms per gram)

0 5 10 15 20

 1964-

 1952-

9 

* 

*

20 ^u

40

60

80

100

Figure 11. Cesium-137 activities and mercury (total recoverable) 

concentrations (dry weight) in bottom-sediment cores from 

Lahontan Reservoir, Nev., April 1982.

overall decrease may be due, in part, to the continued flushing, 

hence depletion, of human-caused mercury pollution from upstream 

source areas. The random peak concentrations of mercury concentration along the core probably are due to episodic floods in the 

basin. The maximum peak mercury concentration of 15 (J,g/g in the 

core sample lies just above the 1952 marker and probably corresponds to the flood of late December 1955 with its probable high 

load of mercury to the reservoir. The 1955 flood had a peak flow 

of 9,680 ftVs at the Fort Churchill site.

Concluding Comments

The reconstruction of historic mercury and sediment deposition 

in Lahontan Reservoir at the coring site (albeit with limited data) 

coupled with measurement and analysis of the recent (January 1997) 

flood flow and its aftermath to and from the reservoir, suggests that 

episodic floods infrequent as they may be play a major hydrologic role in the erosion, transport, and deposition of fluvial sediments and associated mercury in the Carson River. For example, 

about 200,000 tons of sediment and 3,000 pounds of total mercury 

moved past the sampling site near Fort Churchill when the Carson 

River peaked at 22,300 ftVs on January 3. Future floods (and spring 

snowmelt runoff) will again accelerate the flushing of human-caused 

mercury contamination from the affected reach of the Carson River 

to Lahontan Reservoir. However, future winter floods having similar 

physical characteristics as the January 1997 flood also may allow 

noteworthy discharge of mercury from the reservoir. Thus, under 

certain hydrologic conditions, the reservoir acts as an imperfect trap 

for mercury.

Acknowledgments

The authors wish to thank Douglas D. Hutchinson, R. Nyle 

Pennington, Karen A. Thomas, Sonya L.Vasquez, and Rita Whitney 

of the USGS for their assistance in the field with the collection of 

hydrologic data during and after the flood of January 1997. The 

USGS National Water-Quality Assessment Program provided funds

for the chemical analysis of flood-water samples. The USEPA 

Superfund Program and the Department of the Interior National 

Irrigation Water Quality Program funded hydrologic data collection following the flood. The USGS Office of Surface Water 

provided funds for publishing this fact sheet.

^ By Ray J. Hoffman and R. Lynn Taylor

References Cited

Bloom, N.S., 1989, Determination ofpicogram levels of

methyimercury by aqueous phase ethyiation, followed by 

cryogenic gas chromatography with cold vapor atomic 

fluorescence detection: Canadian Journal of Fisheries and 

Aquatic Sciences, v. 46, p. 1131-1140.

 1995, Ultra-clean sample handling: Environmental Lab,

March/April 1995, p. 20-25.

Garcia, K.T., 1997, January 1997flooding in northern Nevada Was 

this a 100-year flood?: U.S. Geological Survey Fact Sheet 

FS-077-97, 4 p.

Gustin, M.S., Taylor, G.E., Jr., and Leonard, T.L., 1994, High levels of 

mercury contamination in multiple media of the Carson River 

drainage basin of Nevada Implications for risk assessment: 

Environmental Health Perspective, v. 102, no. 9, p. 722-778.

Guy, H.P., 1969, Laboratory theory and methods for sediment 

analysis: U.S. Geological Survey Techniques of WaterResources Investigations, Book 5, Chapter Cl, 58 p.

Krishnaswami, S., and Lai, D., 1978, Radionuclide limnochronology, 

in Lerman, Abraham, ed., Lakes Chemistry, geology, physics: 

New York, Springer-Verlag, p. 153-177.

Smith, G.H., 1943, The history of the Comstock Lode, 1850-1920: 

Nevada University Bulletin, v. 37, no. 3, 305 p.

Thomas, K.A., and Williams, R.P., 1997, Flood of January 1997 in 

the Carson River Basin, California and Nevada: U.S. Geological 

Survey Fact Sheet FS-183-97, 2 p.

Van Denburgh, A.S., 1973, Mercury in the Carson and Truckee River 

Basins of Nevada: U.S. Geological Survey Open-File Report 

73-352, 15 p.

For More Information

For more information on water resources in Nevada 

please contact:

Public Information Assistant 

U.S. Geological Survey 

333 W Nye Lane 

Carson City, NV 89706 

tel. (702) 887-7649 

fax (702) 887-7629 

usgsinfo_nv@usgs.gov

Publications assistance by 

Robin L. Sweet and Graham A. Patterson

6 January 1998 F«:t Sheet FS-001-98