A National Network of 

Hydrologic Bench Marks

GEOLOGICAL SURVEY 

CIRCULAR 460-B
Part B

A National Network of 

Hydrologic Bench Marks

By Luna B. Leopold

CONSERVATION NETWORKS

GEOLOGICAL SURVEY CIRCULAR 460-B

This volume complete in three parts

Washington 1962
United States Department of the Interior

STEWART L. UDALL, Secretary

Geological Survey

William T. Pecora, Director

First printing 1 962 

Second printing 1967

Free on application to the U.S. Geological Survey, Washington, D.C. 20242
GEOLOGICAL SURVEY CIRCULAR 460-B 

CONSERVATION NETWORKS

PART B

A National Network of Hydrologic Bench Marks

By Luna B. Leopold

We are engaged in great national programs 

of water control and development. An expanding population demands ever-increasing supplies of the natural resources which are to be 

found in or upon the landscape soil, water, 

minerals, food, timber, and fiber. By his 

works, by his extractions, man's mark upon 

his environment becomes ever deeper, his 

effects more indelible.

We often read that water tables are falling, 

that floods are increasing, that springs go dry 

more often now than in grandfather's time, 

or that rivers are muddier than before. Such 

changes, if true, are troublesome but water 

is a fluctuating resource, responding over 

time to changes in the environment. A recurring question of our times, and one that we 

anticipate will be increasingly vexing to posterity, is to know how much of the change in 

our environment is caused by man and how 

much is natural. In trying to answer this question we immediately face the insurmountable 

fact that changes must be measured relative 

to some standard base or datum. What can 

we compare against?

The most pervasive and probably the most 

important of the slow and subtle changes 

result directly or indirectly from variations 

in climate. Over a shorter or longer period 

of time, pulsations in precipitation and temperature change the amounts of water that are 

evaporated or transpired by the soil and vegetation, the amount of water that replenishes 

soil water, the quantity of water for recharge 

to ground water and for riverflow. Climatic 

variations also cause changes in the pattern 

of erosion, of which some spectacular consequences can be observed in the arid zones. 

Changes in climatic pattern, through their 

effects on the hydrologic cycle, on soil, and 

on vegetation, can produce results remarkably similar to those effected by the works 

of man.

Oraibi Wash, Navajo County, Ariz., July 1960.

About 20 miles upstream from Oraibi Village. The gully is 

about 20 feet deep, 30 feet wide, and tens of miles in length. 

Like many others in the Southwest, this gully was formed in the 

1880's.

These are direct practical reasons for our 

concern. The division of water of highly developed streams between bordering States is 

often based on the so-called virgin flow. What 

constitutes the flow unvexed by the hand of man 

cannot be determined at the council tables of 

negotiation, or even in the halls of justice, 

without essential hydrologic facts.

It therefore becomes a matter of practical 

concern, as well as a matter of scientific 

interest, that the specific causes of given observed effects be identified. The chain of 

events the linkage of interrelated factors 

must be known in far mo re detail than is possible at present. Attainment of this objective 

requires a means for dissociating the direct 

and indirect effects of man's use from similar 

effects which are brought about by natural 

variations in climatic factors.

Determination of these changes is often 

sought in the analysis of records of streamflow, but the answer usually is neither simple 

nor unequivocal. Natural daily, seasonal, and 

secular changes in precipitation cause changes
Tennessee River ot Chattanooga, Tenn.

-1.0

1875 1885 1895 1905 1915 1925 1935 1945 1955

Example of downward trend in a record of riverflow.

in the rate of replenishment to streams, to the 

soil, and to ground-water storage which do 

not show up immediately, but are mingled with 

the effects of previous changes. Water levels, 

soil moisture, and streamflow are constantly 

changing; add the complication of manmade 

changes and the problem of resolution of the 

separable effects becomes exceedingly complex.

BENCH MARKS

There is a vital need for a series of measurements of water discharge, ground-water 

levels, and other hydrologic parameters at 

selected places representing various environments, unchanged by and permanently protected from the effects of man's activities.

Itshouldbeno surprise that, out of the 7,000 

stream -gaging stations and the thousands of 

wells for observation of the levels of ground 

water now maintained in the United States, 

only a few fit the requirement. As Langbein 

and Hoyt1 show, the vast national program of 

hydrologic measurements is motivated by the 

occurrence' of hydrologic events water shortage, flood control, and river regulation, to 

name a few. Hydrologic measurements, such 

as streamflow, ground-water levels, and 

water quality have been made in those areas 

where there is maximum potential for development and control of water, and for this reason, such information has least potential for 

observing the long-term operation of natural 

processes apart from the artificial.

A lengthening of the period of record can 

be of little help. When the total period of record of streamflow and other hydrologic elements becomes much longer, say twice its

Facts for the Nation's Future, The Ronald Press,

present 60-odd years, statistical variance 

could be better defined. Meanwhile, man's 

effects and pressures on the enviroment will 

grow, and so the records become what hydrologists call "inconsistent." The early part of 

the record measures conditions which differ 

from those in the latter part. Consequently, 

increasingly long records can never, by themselves, be analyzed hydrologically or statistically to demonstrate what part of the measured variations in hydrologic conditions is 

natural and what part is induced by man.

If an answer is to be found it must come 

from a planned network of observations that 

include measurements of the natural hydrologic regimen: the flow of streams, the quality of water, and the levels and temperature 

of ground water, unaffected by man.

For long-term observations of the natural 

hydrologic regimen, the Geological Survey 

proposes to establish a network of "hydrologic bench marks " in areas which are as free 

as possible from man-induced changes.

SCOPE OF THE BENCH MARK NETWORK

Obviously, the establishment and operation 

of a nationwide network of hydrologic bench 

.marks entails many difficult scientific and 

technologic problems. The right kinds of observations must be made at the right places. 

The areas selected must be representative 

of different climates and terrains. The areas 

must be as safe from development and disturbance as this generation can assure. Wilderness areas will be especially suited. Because they can help us to understand and to 

.measure the quantities and qualities of water 

in their native state, wilderness areas to this 

extent will have direct value to society as 

hydrologic bench marks. For the same reasons, National and State parks would rank 

high in the selection of sites for hydrologic 

bench marks.

Some of the bench marks should be arranged in the pattern of a transect across 

various geologic and climatic provinces as 

well as a wide variety in vegetation and soils. 

One transect would extend east to west across 

the continent to include mountains and plains. 

Another would extend north to south through 

the central regions where the terrain is relatively flat, thus giving a representation of the
effects of latitude uninfluenced by major differences in topography. The ranges of landscapes that can be included will be severely 

limited at first because funds now available 

for this purpose are small. However, we 

hope that as many as a hundred hydrologic 

bench marks can be established among the 

50 States of the Union.

KINDS OF OBSERVATIONS

Several kinds of observations would be carried out at the hydrologic bench marks. At 

most sites, autographic records will meassure runoff the flow from a watershed in a 

stream and observations may be made of the 

movement of salts and sediments transported 

in runoff. Measurements may be made of 

fluctuations in the level and temperature of 

ground water, and precipitation and other 

climatologic phenomena may be observed. 

Because channel stability is a problem of 

growing concern, consideration will be given 

to the inclusion of observations of changes in 

cross-sectional shapes of streams. There is 

also the possibility of installing traps for the 

collection of windblown pollen useful because 

the species composition of pollen is a clue to 

vegetal changes in an area.

Apart from these kinds of observations, 

which are made periodically or seasonally, 

a survey woul<4 be made of the drainage basin 

above the bench-mark measuring points to 

fecord the soil,geology,topography,and vegetation, in some areas, particularly in the arid 

zones, quadrants or transects will be established to give quantitative estimates of gradual 

changes, if any, in plant growth or species.

Light rainstorms Heavy rainstorms

Change in character of rainfall at Las Cruces, N. Mex.

Light rains promote vegetal growth; heavy rains produce runoff 

and create erosion. Hence, there was in the 1850-80 period a 

climatologic condition that led to a modern epicycle of erosion 

that began with the 1880's. Climatologic conditions in the 1931- 

42 period lessened the erosion.

HOW HYDRQLOGIC BENCH MARKS ARE TO BE USED

The powerful tools of statistical inference 

cannot do what bench-mark observations can 

do distinguish man's effects from those of 

nature in the records. Bench marks, as simple measures of the time trend in the secular 

march of hydrologic events unaffected by the 

works of man, have been mentioned already. 

Direct comparison with other records is a 

more sophisticated use. For example, if the 

record of flow at a regular gaging station shows 

a downward trend relative to that at a nearby 

hydrologic bench mark, there is direct evidence of depletion of streamflow. Hydrologic 

bench marks can resolve the interrelations 

between climatic and hydrologic variables 

with confidence that the results are not clouded 

by the effects of the works of man. The 

interrelationships thus demonstrated at proven hydrologic bench marks can provide the 

base or datum for separating natural from 

artificial effects.

THE VIGIL OBSERVATION AREAS

A second kind of observation is needed for 

the study of time trends. We must also begin 

now to observe changes in various phenomena 

related to land-water relations that change 

with time. The established network of gaging 

stations and temperature stations furnishes a 

record of variations of streamflow and rainfall through time. Both streamflow and (to a 

lesser extent) water temperature may be affected by man and his activities. Even precipitation may be indirectly affected by, for 

example, changes in the carbon dioxide content of the atmosphere and by atmospheric 

pollutants. Records of streamflow, temperature, and precipitation indicate time trends, 

whether caused by natural processes or induced by man's activities. No comparable 

records are being kept on the shape and character of river channels, the density of vegetation and its species composition, the amounts 

and character of dust, pollen, and other particulate matter carried by and deposited from 

the air.

In addition, then, to the establishment of 

bench marks which will provide a record of 

observations of hydrologic phenomena uninfluenced by man's activities, observations of 

phenomena which might be affected by man's 

activities should be extended to include many 

other parameters not now being measured.
The network of such observations might be 

thought of as observational areas or points 

where the hydrologic phenomena as they 

change with time would be recorded. This is 

the nature of the Vigil Observation Network I 

earlier proposed a*s one aspect of our international program for hydrologic investigations. 2

INTERNATIONAL COOPERATION

The program of the Geological Survey for 

a network of hydrologic bench marks and the

Vigil network will become part of a proposed 

international program of cooperation in hydrologic research. By this means, American 

scientists will have direct access to information on a far wider range of variations of 

soils, vegetation, and climate than are represented in this country. These measurements 

on diverse types of landscape in different 

parts of the world will greatly enhance our 

opportunity to distinguish between the effects 

of man and of nature on the environment.

2 Leopold, L. B., 1961, The man and the hill: opening address 

to the Symposium on climatic change, UNESCO and WMO, Rome, 

Italy, Oct. 3, 1961; U.S. Geol. Survey Circ. 460-A. 1962.

*U. S. GOVERNMENT PRINTING OFFICE : 1967 O - 274-075