In the ever-changing landscape of the North American 

continent, there are few medium to large-scale rivers 

that remain largely undisturbed.

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2005-3135

December 2005

Introduction

In 1962, Luna B. Leopold, then Chief Hydrologist of the 

U.S. Geological Survey (USGS), proposed the establishment 

of a network of “hydrologic benchmarks” on the nation’s 

rivers (Leopold, 1962). The main purpose of the Hydrologic 

Benchmark Network (HBN) is to provide a long-term database 

to track changes in the flow and water quality of undisturbed 

streams and rivers (rivers draining undeveloped lands), and to 

serve as a reference for discerning natural from human-induced 

changes in river ecosystems. In the ever-changing landscape of 

the North American continent, there are few medium- to largescale watersheds that remain largely undisturbed. The HBN 

is the only nationwide network of environmental monitoring 

stations that tracks the health of rivers draining medium sized, 

undisturbed basins in the United States. HBN watersheds 

range in size from 2 mi2 to 254 mi2 though one watershed 

has an area close to 2,000 mi2. HBN watersheds are larger 

than typical research watersheds in which most ecosystem 

research is conducted, but are small enough to be responsive 

to anthropogenic atmospheric inputs and climate change. The 

HBN thus provides a frame of reference to evaluate changes 

in river chemistry and flow patterns in large or developed 

watersheds, such as those commonly sampled as part of State 

and Federal monitoring programs (for example USGS National 

Water-Quality Assessment Program (NAWQA; http://water.

usgs.gov/nawqa/), U.S. Environmental Protection Agency 305-

B program; http://www.epa.gov/305b/). 

History of HBN

The HBN was started in 1963 and gradually grew to include 

57 river-gaging stations and 1 lake-stage station in 39 States 

by 1990 (fig. 1). Most of the stations were at the outlet of 

watersheds that were virtually free of human activities, located 

in places such as in national parks and forests, wilderness 

areas, or nature preserves. Streamflow was monitored 

continuously at each station, and samples were collected every 

month for water-quality analyses. The frequency of water 

sampling at HBN stations was decreased to quarterly in 1986 

because of budgetary restrictions. Sampling was discontinued 

in October 1997, except for a small study in the eastern United 

States that focused on the initial response of rivers to decreases 

in industrial emissions mandated by the Clean Air Act 

Amendments of 1990 (http://www.epa.gov/oar/oaq_caa.html/). 

All HBN watersheds were evaluated in 2002 to determine 

whether upstream development had made them unsuitable as 

reference watersheds. The 36 sites that best met the network 

criteria were selected for continued streamflow monitoring and 

water sampling was reinitiated at 15 of those 36 sites (fig. 1); 

additional sites are to be added in coming years by developing 

monitoring partnerships with private and public sector 

cooperators. Dr. Leopold’s vision of a benchmark network 

for the United States has thus been re-established, with the 

promise of a new era of improved surveillance over how our 

actions affect the world in which we live, and the water on 

which we depend.

The U.S. Geological Survey 

Hydrologic Benchmark Network
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Minam River, Oregon

Vallecito Creek, Colorado

Red Butte Creek, Utah

Previous Studies that used the HBN

The more than 40 years of HBN’s existence 

have resulted in the publication of more than 40 

scientific papers that have relied on HBN data 

to tell their story. These papers include:

• the first descriptions of 57 HBN basins and 

the types of data collected at each site (Cobb 

and Biesecker, 1971);

• a comparison of water chemistry from HBN 

basins with similar but developed watersheds 

(Biesecker and Leifeste, 1975);

• an analysis of acid-precipitation-induced 

trends in pH, alkalinity, and sulfate for 10 to 

15 years of record at 47 HBN stations (Smith 

and Alexander, 1983);

• a principal-components analysis to discern 

whether patterns in sulfate concentrations 

from 30 HBN basins could be used as 

analogs for regional patterns of atmospheric 

sulfate deposition (Lins, 1986);

• a compilation of streamflow characteristics 

for 57 HBN basins, including an analysis of 

streamflow trends for periods ranging from 

13 to 65 years (Lawrence, 1987). 

In 1990, the USGS reevaluated the 

environmental characteristics of HBN 

watersheds to ascertain the suitability of each 

site according to the original design criteria. 

The result of this evaluation was four reports 

that describe the environmental characteristics 

and water quality of 56 HBN basins in the 

eastern, midwestern, west-central, and western 

regions of the United States (http://water.usgs.

gov/pubs/circ/circ1173/). HBN water-quality 

data is much easier to evaluate with the aid of 

these detailed compilations. 

In the late 1990s, Clow and Mast (1999) 

reassessed the effect of acid rain on trends in 

stream chemistry at five HBN basins in the 

northeastern United States. Results indicated 

that sulfate concentrations in stream water 

were decreasing at all five HBN stations 

that paralleled a downward trend in sulfate 

concentrations in precipitation. The decline 

in streamwater sulfate concentration was 

accompanied by a decline in calcium and 

magnesium concentrations. This unexpected 

result was attributed to a depletion of soil 

buffering capacity caused by long-term 

exposure to acid rain. 

Improved Methods of Trend Detection

Environmental scientists must provide 

resource managers with temporal trends 

in stream water quality so those managers 

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Active water-quality stations

Discontinued stations 

EXPLANATION

Streamflow-only stations

Figure 1. Approximate locations of the Hydrologic Benchmark Network monitoring stations, United States.

can make informed land management decisions. The 

HBN provides water-quality data to identify trends for 

rivers draining undisturbed, “benchmark” watersheds (fig. 

2). Measures of river health from HBN watersheds can 

be compared to those from watersheds affected by human 

activity, and thus the effect of that activity on water quality 

can be assessed (fig. 3). Decades of data have typically been 

required to accurately detect trends in water quality caused by 

atmospheric pollution however resource managers cannot wait 

decades to make land management decisions. Scientists need 

methods to detect trends in water quality for much shorter time 

periods than have been required in the past.

Data from selected HBN stations were used during the late 

1990’s to identify and implement a strategy for early detection 

of water-quality trends. The frequency of water sampling 

was increased to include the annual range of flow conditions 

at each site. The study concluded that if sufficient samples 

were collected to identify a statistically significant annual 

correlation between river flow and water quality, trends at 

high, median, and low flow could be detected for the HBN 

rivers. The stronger the trends, the shorter the period of record 

needed for trend detection. The study further showed that 

the effects of acidic deposition can be detected in HBN-scale 

rivers in the northeastern United States, and that changes in 

sulfate deposition rates during the past 15 years were first 

detectable in the rivers at high flow (Murdoch and Shanley, in 

press) (fig. 4). The HBN is providing data for early detection 

of water-quality trends in 15 rivers draining undeveloped 

watersheds across the United States.

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Neversink River, New York

Sopchoppy River, Florida

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Creating Attainable Water-Quality 

Goals for Developed Watersheds 

Undisturbed basins contain many of the 

chemicals in low concentrations that are used 

in concentrated forms for agricultural and 

industrial activities. Regulations have been 

adopted that set stream water concentration 

thresholds on many naturally occurring 

and man-made chemicals. Fines or costly 

cleanup requirements are imposed where those 

thresholds are exceeded, but in some areas 

natural concentrations of those same chemicals 

are present and no clean-up strategy will 

completely remove them from the environment. 

In other cases a certain amount of a pollutant or 

landscape development could be accomodated 

without causing ecosystem degradation (fig. 

3). Knowledge of background concentrations 

for environmental pollutants, and the thresholds 

of environmental disturbance above which 

environmental degradation occurs, is critical 

for establishing rational and cost-effective 

mitigation objectives.

References Cited

Biesecker, J.E., and Leifeste, D.K., 1975, Water quality 

of hydrologic bench marks: An indicator of water 

quality in the natural environment: U.S. Geological 

Survey Circular, 460-E, 21 p.

Clow, D.W., and M.A. Mast, 1999, Long-term trends 

in streamwater and precipitation chemistry at five 

headwater basins in the northeastern United States: 

Water Resources Research, v. 35, no. 2, p. 541-554.

Cobb, E.D., and Biesecker, J.E., 1971, National 

Hydrologic Bench-mark Network: U.S. Geological 

Survey Circular 460-D, 38 p.

Fischer, J.M., Riva-Murray, K., Hickman, R.E., 

Chichester, D.C., Brightbill, R.A., Romanok, 

K.M. and Bilger, M.D., 2004, Water Quality in 

the Delaware River Basin, Pennsylvania, New 

Jersey, New York, and Delaware, 1998-2001: U.S. 

Geological Survey Circular 1227, 38 p.

Lawrence, C.L., 1987, Streamflow characteristics at 

hydrologic bench-mark stations: U.S. Geological 

Survey Circular 941, 123 p.

Leopold, L.B., 1962, A national network of hydrologic 

bench marks: U.S. Geological Survey Circular 

460-B, 4p.

Lins, H.F., 1986, Recent patterns of sulfate variability in 

pristine streams: Atmospheric Environment, v.20, no. 

2, p. 367-375.

Murdoch, P.S., and Shanley, J.B., 2005, Flow-specific 

trends in river-water quality resulting from the effects 

of the Clean Air Act in three mesoscale, forested 

river basins in the northeastern United States through 

2002: Environmental Monitoring and Assessment, 

in press. 

Smith, R.A., and Alexander, R.B., 1983, Evidence for 

acid-precipitation-induced trends in stream chemistry 

at hydrologic bench-mark stations: U.S. Geological 

Survey Circular 910, 30 p.

Kawishiwi River, Minnesota

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Figure 3. Effect of urbanization on stream macroinvertebrate populations in 

streams of the Delaware River Basin. EPT (Ephemeroptera, Plecoptera, and 

Trichoptera) richness is an index of river health based on the populations 

of mayfly, stonefly, and caddisfly, respectively (modified from Fischer and 

others, 2004).

Nitrate

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Alkalinity

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200

300

400

Calcium plus Magnesium

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100

200

300

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500

Samples

Trend Line

Sulfate

1965 1970 1975 1980 1985 1990 1995 2000 2005

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150

200

250

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Young Woman's Creek

MICRO-EQUIVALENTS PER LITER

PERCENT WATERSHED WITH URBAN LAND USE

0 20 40 60 80 100

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Delaware Basin stream survey 

Neversink River HBN station 

PERCENT EPT OF TOTAL SPECIES

Biscuit Brook

1984 1988 1992 1996 2000 2004

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High Flow

Moderate Flow

Low Flow

SULFATE CONCENTRATION,

IN MICRO-EQUIVALENTS PER LITER

Figure 4. Trends in sulfate concentrations during high flow, 

moderate flow,and low flow conditions for the Biscuit 

Brook watershed in the Catskill Mountains in southeastern 

N.Y. High flow shows as upturn in sulfate concentrations 

during the last 2 years of record that is not apparent at 

low flow.

Figure 2. Concentration and temporal trends of calcium plus magnesium, acid-neutralizing capacity, nitrate, and sulfate for 

Young Woman’s Creek in north-central Pennsylvania from 1968 to 2004. 

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By Peter S. Murdoch, Michael R. McHale, M. Alisa Mast, and David W. Clow

HBN in 2005

The HBN of 2005 has fewer stations than the original network of Dr. Leopold, but modern 

sampling strategies and equipment allow a far more detailed understanding of how the Nation’s 

streams and rivers are changing than was feasible in the 1960s when the HBN began. In 2003, 15 

of the original HBN stations were equipped with refrigerated, automated samplers and telemetry 

systems that allow program coordinators to monitor stream conditions and adjust sampling frequency 

and capture unique stream conditions or special sampling needs. The automated sampling system is 

designed to collect samples through a wide range of flow conditions and to transmit data by satellite. 

About 25 water samples are collected annually at each HBN water-quality station and refrigerated 

on site until retrieved by field personnel who visit the sites regularly. Flow-based automatic 

sampling assures that samples are collected through the range of flow conditions at each site during 

each season of the year. Samples are analyzed for major anions and cations, pH, dissolved organic 

carbon (DOC), dissolved organic nitrogen (DON), acid neutralizing capacity (ANC), silica, and 

aluminum. HBN basins are often used for investigations of local concern that supplement HBN data 

collection, provide additional water-quality data such as pathogens, pesticides, stable isotopes, and 

phosphorus for periods of months to years, and offers the opportunity to collaborate with other local, 

State, and Federal agencies as well as academia and the private sector. This flexible monitoring 

capability allows a baseline of long-term data to be maintained while supporting regional and national 

investigations of emerging environmental issues.

For more information contact:

Michael McHale, 

USGS Eastern Program Coordinator

Phone: 518-285-5675

Email: mmchale@usgs.gov

Alisa Mast/ David Clow, 

USGS Western Program Coordinators

Phone: 303-236-4882 ext. 314

Email: mamast@usgs.gov

On-line source for HBN data, 

publications and site information is

http://water.usgs.gov/hbn 

Printed on recycled paper

The HBN places a priority on building collaborative monitoring 

strategies with other agencies and maintaining a network of streams 

draining undisturbed watersheds at which ecosystem change can 

be monitored through time. By developing partnerships with other 

government agencies, academia, and the private sector HBN sites 

could become a focus for monitoring atmospheric deposition, soil 

chemistry, forest health, and wildlife populations to create the 

first network of undisturbed, medium-sized watersheds in which 

the interactions of multiple components of the environment could 

be tracked in an integrated manner. Partnerships with Federal 

agencies such as the U.S. Forest Service and the National Park 

Service, and local agencies such as the New York City Department 

of Environmental Protection, have already been established. The 

Hydrologic Benchmark Network provides the first crucial building 

block for the long-term surveillance of the Nation’s undeveloped 

ecosystems by providing flow and water quality data from rivers that 

drain undisturbed, medium-sized watersheds.

A Vision for HBN

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