COVER PHOTOGRAPH: Glaciers near Mount Shuksan and Nooksack Cirque, Washington.

Photograph 86R1-054, taken on September 5, 1986, by the U.S. Geological Survey.
A Strategy for Monitoring Glaciers

By Andrew G. Fountain, Robert M. KrimmeI, and

Dennis C. Trabant

U.S. GEOLOGICAL SURVEY CIRCULAR 1132
U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Gordon P. Eaton, Director

The use of firm, trade, and brand names in this report is for identification purposes only and

does not constitute endorsement by the U.S. Government

U.S. GOVERNMENT PRINTING OFFICE : 1997

Free on application to the

U.S. Geological Survey

Branch of Information Services

Box 25286

Denver, CO 80225-0286

Library of Congress Cataloging-in-Publications Data

Fountain, Andrew G.

A strategy for monitoring glaciers / by Andrew G. Fountain, Robert M. Krimmel, and Dennis C. Trabant.

P. cm. -- (U.S. Geological Survey circular ; 1132)

Includes bibliographical references (p. - ).

Supt. of Docs. no.: I 19.4/2: 1132

1. Glaciers--United States. I. Krimmel, Robert M. II. Trabant, Dennis. III. Title. IV. Series.

GB2415.F68 1997

551.31’2 --dc21

96-51837

CIP

ISBN 0-607-86638-l
CONTENTS

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...*..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...* . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Goals ...................................................................................................................................................................................... 3

Previous Efforts of the U.S. Geological Survey .................................................................................................................... 3

Methods of Assessing Glacier Mass Balance ........................................................................................................................ 6

Monitoring Strategy ............................................................................................................................................................... 7

Benchmark Glacier Network ................................................................................................................................................. 8

Primary Measurements ................................................................................................................................................ 9

Mass Balance ..................................................................................................................................................... 9

Geometry ........................................................................................................................................................... 11

Meteorology and S treamflow ............................................................................................................................ 12

Supplementary Measurements..................................................................................................................................... 12

Observation Summary ................................................................................................................................................. 13

Reports ......................................................................................................................................................................... 13

Secondary Glacier Network ................................................................................................................................................... 14

Primary Measurements ................................................................................................................................................ 14

Observation Summary ................................................................................................................................................. 15

Reports ......................................................................................................................................................................... 15

Remote-Sensing Inventory .................................................................................................................................................... 15

Primary Measurements ................................................................................................................................................ 15

Supplementary Data .................................................................................................................................................... 15

Observation Summary ................................................................................................................................................. 16

Reports ......................................................................................................................................................................... 16

Summary ................................................................................................................................................................................ 16

References Cited .................................................................................................................................................................... 16

FIGURES

l-2. Graphs showing:

1. Effect of glacier cover on runoff variability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2. Effect of the melting of alpine glaciers worldwide on global sea level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

3. Diagram showing processes that link meteorology with glacier mass balance and their effect on landscape.. . . . . . . . . . 2

4. Map showing major glacierized regions of the United States . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

5. Diagram showing correspondence between physical processes that determine glacier change and the

processes measured by the different surveillance levels. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

6. Map showing idealized monitoring strategy for a glacierized region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

TABLE ;

1. Frequency of aerial photographic surveys for glacierized mountain regions of the United States . . . . . . . . . . . . . . . . . . . . . . . . . 16

Contents III
CONVERSION FACTORS

Multiply BY

kilometer (km) 0.6214

millimeter (mm) 0.03937

square kilometer (km2) 0.3861

To obtain

mile

inch

square mile

Other abbreviations used in this report:

millibar (mb)

Sea level: In this report “sea level” refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929)-a geodetic datum derived

from a general adjustment of the first-order level nets of both the United States and Canada, formerly called Sea Level Datum of 1929.

IV Contents
A Strategy for Monitoring Glaciers

Andrew G. Fountain, Robert M. Krimmel, and Dennis C. Trabant

Abstract

Glaciers are important features in the

hydrologic cycle and affect the volume, variability,

and water quality of runoff. Assessing and predicting

the effect of glaciers on water resources require

a monitoring program to provide basic data for this

understanding. The monitoring program of the

U.S. Geological Survey employs a nested approach

whereby an intensively studied glacier is surrounded

by less intensively studied glaciers and those

monitored solely by remote sensing. Ideally, each

glacierized region of the United States would have

such a network of glaciers. The intensively studied

glacier provides a detailed understanding of the

physical processes and their temporal changes that

control the mass exchange of the glaciers in that

region. The less intensively studied glaciers are used

to assess the variability of such processes within the

region.

Glaciers are an important water resource of

Alaska and a number of Western States because they

are naturally regulated reservoirs that reduce the

runoff variability by increasing flow during hot, dry

periods and by storing water as ice and snow during

cool, wet periods (fig. 1) (Meier and Tangborn, 1961;

Meier, 1962; Krimmel and Tangborn, 1974; Fountain

and Tangborn, 1985). The mass wastage of glaciers

during this century has diminished the volume of

water stored as ice in the natural reservoirs and has

reduced the capacity of runoff from the glaciers to

limit annual variations in streamflow. As demands on

water resources continue to increase, the effect of

glaciers on streamflow needs to be defined better.

Reduction of the water in storage as glacier ice has

0.4 1 I

EXPLANATION

0 Washington

A Southeast Alaska

@ Mean

- Predicted coefficient of variation _

h

8 -

0 0 10 20 30 40 50 60 70 80 90 100

PERCENT OF BASIN COVERED BY GLACIERS

Figure 1. Effect of glacier cover on runoff variability. The

coefficient of variation is the standard deviation divided by

the mean.

contributed to the rise in global sea level (fig. 2)

(Meier, 1984; Oerlemans and Fortuin, 1992). Glaciers

also present special hydrologic hazards, such as floods

(Post and Mayo, 1971; Driedger and Fountain, 1989;

Mayo, 1989; O’Connor and Costa, 1993) and debris

flows (Osterkamp and others, 1986; Walder and

Driedger, 1994). In addition, glaciers are recognized

as sensitive indicators of climate change because their

size changes in accordance with variations in mass

gain or loss (Johannesson and others, 1989; Meier and

Roots, 1982; Moss and Lins, 1989; Paterson, 1981).

However, the relation

response of an individ

between climate change and

ual glacier is only generally

understood and not clearly known for large glacierized

regions.

Understanding the effects of glaciers on runoff

requires a long-term program (Benson and others,

1986; Fountain and others, 1991) to define the natural

variability of the processes that control glacier mass

change (fig. 3). A long-term program also captures

short-duration catastrophic events, such as floods and

debris flows, which are difficult to predict. Furthermore, a long-term program is required to define the

Abstract 1
100

EXPLANATION

Sea level

Sea level corrected for

thermal expansion

Glacier melt

Uncertainty of glacial melt

1920

YEAR

relation between climate change and glacier

response. Long-term programs are best maintained by

a Federal agency because the stability in funding and

personnel ensures consistency in goals and methods,

thus resulting in long-term, high-quality data. The

U.S. Geological Survey (USGS) has a fundamental

responsibility for assessing the Nation’s water

resources and has internationally recognized expertise

in glacier research. Together, these reasons provide

the justification for the USGS to assume lead responsibility for monitoring glacier change in the United

States.

The purpose of the USGS Glacier Monitoring

Program is to document changes in glacier mass and

extent, water runoff, and local climatic variables

to increase our understanding of the relation between

these variables and to evaluate the effect of glaciers

on water resources and hydrologic hazards. These

objectives address two mission goals of the USGS

(1986):

“Assesses water resources and develops an understanding of...natural phenomena on hydrologic

systems.”

“Evaluates hazards.. .and

hazard prediction.”

develops methods for

Furthermore, studies on glacier mass changes

provide climatic information in remote regions, help

interpret ice cores, and provide boundary conditions

g Program is pa

in glacierized

basins, which constitutes an international effort to

for modeling glacier movement.

The USGS Glacier Monitorin

of a global network of observations

2 A Strategy for Monitoring Glaciers

rt

Figure 2. Effect of the melting of alpine

glaciers worldwide on global sea level

(from Meier, 1984).

assess the effects of glaciers on the global hydrologic

cycle. The global network 1s coordinated by the

World Glacier Monitoring Service, which is located in

Zurich, Switzerland, and is sponsored by the Intemational Commission on Snow and Ice under the aegis of

the International Association of Hydrological

Sciences. The USGS Glacier Monitoring Program has

collaborated with the National Hydrology Research

Institute, Environment Canada, to refine the approach

to measuring glacier mass balance and to evaluate

glacier network in North America. Circumpolar

the

efforts with Russia and Scandinavia and cooperative

ET MASS BALANC

Figure 3. Processes that link meteorology with glacier

mass balance and their effect on landscape (modified from

Meier, 1965).
studies southward along the Western Cordillera to the

Andes Mountain Range of South America are being

investigated.

This document outlines a strategy for

monitoring glaciers in the United States. Three levels

of surveillance are proposed. One glacier in each

glacierized region is measured in detail to define the

seasonal mass-balance processes, meteorological

environment, and water runoff. The next level of

surveillance is to measure several glaciers in each

region for seasonal mass balance only. This information helps to extrapolate understanding of processes at

the glacier measured in detail to other glaciers in the

region. The third level of surveillance is remote

sensing of glaciers by using aerial photography and

satellite images to define the changing area1 extent and

pattern of snow and ice exposure on a large number of

glaciers. The remote-sensing approach allows extrapolation to a large number of glaciers and provides a

large enough sample to define the change of ice mass

in a region and its effect on water resources. For each

level of surveillance, the basic measurements,

analysis, and reporting standards are summarized.

These standards are intended neither to limit the scope

of collected information nor to obstruct improvements

in methodology. Rather, a consistent set of basic

measurements, analytical methods, and reports is

required to increase the ability to compare glacier

variations areally and temporally (Fountain and

others, 1991).

GOALS

This glacier monitoring strategy addresses the

need to evaluate the changing water-resource and

hazard potential of glaciers in the United States. The

strategy is motivated by a number of fundamental

research questions and supported by a comprehensive

data-collection effort. The goals of the monitoring

strategy are as follows:

Quantify the magnitude

change and its effect

of regional glac

on streamflow.

ier mass

l Define the relation between mass change in regional

glacier cover and mesoscale weather/climate

variations.

l Monitor and investigate potentially hazardous

situations to develop a predictive capability.

PREVIOUS EFFORTS OF THE

U.S. GEOLOGICAL SURVEY

The USGS began a program of measurements

on glaciers in the late 1950’s and has continued this

commitment. For a comprehensive reference list of

glacier studies, see Snyder (1996). The program is

focused on the water-resource potential of glaciers, the

relation between glaciers and climate, and glacierhazard assessment. Fundamental to each of these

broad topic areas is glacier mass balance, which

determines, in large part, the effect on water resources,

the link between climate change and glacier response,

and the potential for hazards. Because of the slow

response of glaciers to yearly changes in mass, a longterm program was established.

Glacier mass-balance studies were in their

infancy in the mid-20th century. A few glaciers were

measured in Europe, and none were measured in

North America. In 1958, the USGS began a study at

South Cascade Glacier, Washington, to measure the

mass balance directly by using surface-based methods

(Meier, 1958). In the mid-1960’s, three more glaciers

were added-Gulkana in the Alaska Range,

Wolverine on the Kenai Peninsula, Alaska, and

Maclure Glacier in the Sierra Nevada of California,

which is no longer monitored. These glaciers (fig. 4)

formed part of a north-south transect of glaciers

studied for heat and mass exchange during the

International Hydrological Decade (IHD, 1966-75)

(UNESCO/IASH, 1970). In North America, the

USGS collaborated in this effort with Canadian and

American universities and governmental agencies.

During the start of the IHD, there was little

agreement on mass-balance terminology and methodology. To reduce the confusion and occasional

contradictions in terminology, a standardization of

mass-balance terms was proposed by the USGS

(Meier, 1962; Mayo and others, 1972). The methods

of directly measuring mass balance at points on the

glacier surface, outlined during the IHD

(UNESCO/IASH, 1970), were based, in large part, on

the work at South Cascade Glacier (Meier, 1962).

Campbell (1969) suggested a statistical approach to

determining the relation between the number of point

measurements of mass balance and the resulting error

in the final value of total mass balance. Fountain and

Vecchia (1992) showed that, by assuming a general

relation between mass balance and elevation, the error

in the total mass balance can be determined, and the

effect of changing the number of point measurements

GOALS 3
Range

- Southeast Panhandle

r South Cascade

Sierra

UTAH I

ARIZONA

Figure 4. Major glacierized regions of the United States. The glacierized regions are shaded and enclosed by

solid lines.

4 A Strategy for Monitoring Glaciers
can be evaluated. In addition to surface-based

measurements, Trabant and Mayo (1985) concluded

that subsurface measurements were required to

account for an apparent mass loss from the seasonal

snowpack, whereby meltwater refreezes in the firn.

Furthermore, subsurface mass loss, which is caused by

flowing meltwater (Mayo, 1992), may be significant in

some situations.

It quickly became clear that direct measurements of mass balance, which require intensive and

costly field programs, could not be used to assess the

mass changes of a large glacierized region and that

indirect means were required. Aerial photography

provided the means to cover large regions in a short

time (LaChapelle, 1962; Meier and Post, 1962). From

the fraction of glacier area covered by snow at the end

of the summer, the mass balance of the glaciers was

inferred (Meier and Post, 1962). Glacier-surfaceelevation changes, which were derived from aerial

photographs, also could be used to infer mass changes

for the whole glacier, but not to determine point values

of mass balance (Meier, 1966). Elevation change

results from mass change and glacier movement, and

to calculate point values, one also needs to know the

vector of glacier movement. Meier (1966) showed

contrary changes in two neighboring glaciers-North

Klawatti and South Klawatti Glaciers. Tangborn and

others (1990) concluded that such differences may

result from differences in distribution of glacier area

with elevation rather than from local complications in

mesoscale climate.

Indirect methods of determining glacier mass

balance also were in use during the 1960's, yet their

accuracy was doubted. Tangbom and others (1975)

compared three methods of assessing glacier mass

balance-direct measurements on the glacier surface,

estimates based on elevation changes of the glacier,

and estimates from differences in the precipitation

input to the glacier and stream runoff from the glacier.

They found that direct surface measurements of mass

changes compared well with the results of volume

change based on the elevation changes of the glacier

surface. Krimmel (1989) later showed that the volume

change from elevation changes was an important

check on the direct measurements of mass balance.

Measurements of water input to the glacier (precipitation and glacier melt) greatly differed from runoff

from the glacier, which indicated significant water

storage in glaciers.

The use of aerial photogrammetry was further

advanced during the USGS study of Columbia

Glacier, near Valdez, Alaska (Meier and others, 1985).

Because of its large area (about 1,100 km2), photogrammetry was utilized extensively to extend the few

surface-based measurements. Rasmussen (1988) was

able to infer point measurements of mass balance

by applying the continuity equation to photogrammetrically determined surface velocity and

elevation changes. Assuming a vertically integrated

velocity in the glacier, the divergence of that velocity

at a point minus the surface-elevation change is equal

to the mass balance at that point. This method has

proved useful on the large, fast-moving tidewater

glaciers of southeastern Alaska. Photogrammetric

analysis of aerial (Brown and others, 1982) and

ground-based (Krimmel and Rasmussen, 1986)

photographs is used to determine the component of

mass loss from the calving of ice from glacier termini.

By using data from the benchmark glacier

network, which included Gulkana, Wolverine, and

South Cascade Glaciers, the USGS has been able to

examine the link between climate change and glacier

response and the effect of glaciers on water resources.

The conceptual linkage between climate, as it affects

glacier mass balance and glacier change, was

illustrated by Meier (1965) (modified in fig. 3). On

the basis of analysis of aerial photography, Meier

and Post (1962) were able to quantify regional

variations and to report that the advance/retreat

activity of glaciers in Washington State was not in

phase with glaciers in Alaska. Walters and Meier

(1989) examined the mass-balance data, which were

derived from the USGS glacier and the Canadian

networks, in relation to climate variability. They

confirmed the out-of-phase relation between glaciers

in Alaska and those in southern British Columbia and

Washington and demonstrated the effect of the

Aleutian low-pressure system in steering storm tracks

toward either Alaska or Washington. McCabe and

Fountain (1995) documented changes in the 700 mb

circulation pattern and the decreased mass balance at

South Cascade Glacier in Washington and the

increased mass balance of Wolverine Glacier in

south-central Alaska. Mayo and Trabant (1984)

demonstrated that the mass of some glaciers in

southern Alaska increases rather than decreases with

rising winter air temperature. The temperature

increase is associated with moisture-bearing storms

from the Pacific Ocean.

PREVIOUS EFFORTS OF THE U.S. GEOLOGICAL SURVEY 5
Investigation of the effect of changes in glacier 1969) and tidewater (Post, 1975), respond strongly to

mass on water resources has been a continuous effort

by the USGS. The distinctive characteristics of runoff

from glaciers were explained by Meier and Tangborn

(1961). The causes for these characteristics were

explored by Meier (1969), Tangborn and others

(1975), and Fountain (1992). Glaciers also have a

buffering effect on streamflow that attenuates

variations in runoff (Meier, 1969; Krimmel and

Tangborn, 1974). Further investigations by Fountain

and Tangborn (1985) explained the delay of seasonal

peak flow through the summer and presented a

theoretical prediction of the attenuation for basins with

different fractions of glacier cover (fig. 1). A number

of studies have examined the physical processes of

water flow in a glacier to understand how they control

the discharge of water from a glacier. These studies

include examination of surface processes that affect

the rate of water input to the glacier’s interior

(Fountain, 1989, 1994) and the processes that control

the rate of movement through and under the glacier

(Krimmel and others, 1973; Hodge, 1976, 1979;

Fountain, 1992, 1993, 1994).

nonclimatic factors.

METHODS OF ASSESSING GLACIER

MASS BALANCE

The observations needed to determine glacier

mass balance depend on the processes that control the

mass exchange, the information desired about the

mass change and, consequently, the approach to

assessing that change. This section briefly

summarizes the different mass-balance-assessment

methods. Later sections describe the application of

these methods to different levels of surveillance.

The concept behind measuring glacier mass

balance is quite simple-sum the mass losses and

gains; however, in practice, it becomes complex.

Generally, the main source of mass input is snow

accumulation from snowfall and avalanches. Other

sources include the freezing of rain within the snow,

condensation of water vapor, and rockfall events.

Processes of ablation (all forms of mass loss) include

melting from the surface and interior of the glacier,

evaporation, and‘calving of ice from the glacier

margin. Another consideration is the redistribution of

mass, such as the refreezing of surface meltwater in

the cold layers below the snow surface.

The mass wastage of glaciers contributes flow

to streams and rivers that would otherwise not be

present if the glaciers did not exist or were in equilibrium. Tangbom (1980) showed that the mass loss

from glaciers in the Columbia River Basin supplied

$18 million in generated electricity. The effect of

glacier mass balance is not limited to local glacierized

basins. The mass wastage of alpine glaciers worldwide has contributed a significant fraction to sea-level

rise in the past century (fig. 2) (Meier, 1984).

There are thousands of glaciers in the United

States for which no mass-balance data or other

surface-based information are available. The USGS

has been documenting the condition of many of these

glaciers by aerial photography. This effort has

cataloged a collection of more than 50,000 negatives

of glaciers of western North America, with a few

glaciers represented nearly every year since 1960.

This record makes available a history of transient

snowlines and terminus positions in areas where no

glacier balance data exist. As previously mentioned,

transient snow-lines may be used to estimate glacier

mass balance (Meier and Post, 1962), and terminus

changes commonly can be related to long-term climate

change. Observation of a large number of glaciers

also has resulted in the understanding that certain

types of glaciers, such as surging (Meier and Post,

Methods of determining glacier mass balance

include direct measurements of mass change

(UNESCO/IASH, 1970; Meier and others, 1971;

Paterson, 1981; Østrem and Brugman, 1991), calculation of flux divergence based on glacier velocity and

surface height change (Rasmussen, 1988), and

calculation of volume change based on elevation

changes of the glacier surface (Mayo and others, 1985;

Haakensen, 1986; Mayo and Trabant, 1986; Krimmel,

1989). Mass change also can be inferred from the

elevation of the snowline or the ratio of snow-covered

area to the total area of the glacier at the end of the

summer (Meier and Post, 1962). The hydrologic

method, which is based on a comparison of precipitation and runoff, does not provide results consistent

with the other methods (Tangbom and others, 1975)

and is considered to be inadequate as a technique for

estimating mass balance (Fountain and others, 1991).

Direct measurement of glacier mass-balance

components provide the most-detailed information

about the processes that control mass balance, spatial

differences, and changes during time. The specific

mass balance (mass per unit area) is calculated from

A Strategy for Monitoring Glaciers
surface-based measurements of glacier-surface height

changes and the density of the surface layers of ice or

snow. By using the area-elevation distribution of the

glacier surface, point measurements of mass balance

are extrapolated to all parts of the glacier to determine

total mass change. In addition, the amounts of water

refrozen in the firn (Trabant and Mayo, 1985) and

melted in the body of the ice (Mayo, 1992) are

calculated. Measurements of ablation owing to

calving are not made directly because the calving face

is unstable. Instead, calving losses are calculated from

photographs (Brown and others, 1982; Krimmel and

Rasmussen, 1986).

sheets. Furthermore, it is quite useful for checking

surface-based measurements of mass balance

(Haakensen, 1986; Krimmel, 1989; Fountain and

others, 1991). The average mass-balance change with

elevation can be determined if the total mass change of

two glaciers is known and the same mass-balance/

elevation relation is assumed for both glaciers

(Tangborn and others, 1990).

Flux divergence measurements rely on the

continuity equation to infer mass balance (Rasmussen,

1988). The loss or gain of mass at a point is calculated

from the divergence of the glacier velocity, which is

integrated over the thickness of the glacier and the

surface height change. This technique has been

applied to points derived from photogrammetric

analysis of surface velocity and surface height.

Although this method may not be as accurate as direct

measurements because of the necessary assumptions

and accuracy of the photogrammetry, it may be the

only method applicable on very crevassed surfaces or

where logistic costs are prohibitive. The mass balance

can be inferred anywhere on the glacier where surface

targets can be repeatedly identified. The time-resolution of this method is limited by the interval between

photographs, which can be no shorter than the time

required to displace surface targets a distance greater

than the error in the photogrammetric analysis.

The yearly mass balance of a glacier is linked to

the elevation of the equilibrium line (dividing line

between areas of net mass gain and loss), which is

approximated by the snowline at the end of the

summer, and to the accumulation-area ratio (fraction

of the glacier area covered by snow) (Meier and Post,

1962). Although the quantitative accuracy of this

method is limited, it provides a means of rapidly

assessing net gain or loss of glacier mass in a region

by means of remote sensing.

MONITORING STRATEGY

To document the processes that link glacier

changes to climate and to predict the long-term effect

of glacier mass change on streamflow, detailed

measurements are required. Because of the vast

number of glaciers and their wide distribution, we also

need to sample a large number of glaciers to define

variations within and between regions. However, it is

not feasible to acquire detailed measurements on a

large number of glaciers; instead, a small number of

glaciers will be monitored at different levels of

intensity.

To determine mass balance from volume

change, an assumption is made about the density of

The glaciers of the United States can be divided

the glacier, and the density is multiplied by the

into nine major regional groups (fig. 4). This division

integrated surface height change over the glacier area

is based on a qualitative understanding of the relation

(Haakensen, 1986; Krimmel, 1989). This method is

between weather patterns, local climate, and the

very useful for determining the total mass balance of a

glaciers. Mass-balance variations in one region

glacier over long periods of time. The surface-height

probably cannot be used to ’predict those in another

change may be determined from either surface-based

region. Within each glacierized region, the glaciers

will be monitored by using a nested approach that

measurements of glacier elevation or photogrammetri- consists of three hierarchical levels of documentacally determined elevations. This method cannot be tion. This approach is philosophically similar to that

used to determine spatial variations of mass balance proposed for monitoring sediment in streams and

on the glacier, such as elevation gradient, because rivers (Osterkamp and Parker, 1991). One intensively a

component of the glacier velocity is normal to th e studied glacier, called a Benchmark Glacier, will be

glacier surface, and although the surface elevation ha located in each glacierized region. Secondary glaciers s

not changed, a significant mass flux may have passe will be chosen throughout the region to define spatial d

through that surface (Meier, 1966). This method i strends and variability in mass balance, but will not be

useful for determining mass balance in regions o fstudied as intensively as the Benchmark Glaciers.

difficult terrain or on extremely large glaciers or ic Additional glaciers in the region will be monitored by e

MONITORING STRATEGY 7
using only remote sensing. Knowledge of the physical

processes that control mass exchange on the

intensively studied Benchmark Glaciers will be used

to understand the mass changes on the less intensively

studied glaciers. In turn, changes in the less

intensively studied glaciers will provide information

on the range of glacier variation within the region.

The attributes of each group of glaciers are briefly

summarized.

The Benchmark Glacier Network will consist of

one glacier in each glacierized region on which

detailed direct measurements of seasonal glacier mass

changes, meteorological environment, and streamflow variations are collected. These data will be used

to examine the processes and develop the models that

link meteorological changes to glacier mass and

streamflow variation within each glacierized region.

Because the cycle of mass change is annual and

climatic cycles, such as El Niño-Southern Oscillation

events, have larger periods, long-term data bases that

span decades are required. Because of the expense of

maintaining each site, the number of sites will be

restricted to one Benchmark Glacier in each glacierized region. The information gained from the

Benchmark Glacier Network is fundamental to

understanding the cause of glacier change and impact

on water resources. For this reason, the Benchmark

Glacier Network shares the philosophy of the USGS

National Hydrologic Benchmark Network (Cobb and

Biesecker, 1971): “Data will be collected indefinitely.. . will provide valuable information on time

trends...” and will be “used to study cause-and-effect

relationships between various physiographic, meteorologic, and hydrologic variables.” One Benchmark

Glacier in each glacierized region is considered to be

adequate to define the salient physical processes that

govern glacier evolution and streamflow in that

region.

The Secondary Glacier Network comprises

three to five glaciers in each glacierized region where

yearly (as opposed to seasonal) measurements of

glacier mass balance will be made. No other direct

measurements are intended for this network. Based on

the processes elucidated at the Benchmark Glaciers,

yearly mass-balance data collected from these glaciers

will aid in adapting mass-balance models to the range

of glacial conditions in the region.

The Remote-Sensing Inventory, which includes

aerial photographic surveys and satellite images,

provides a fast, inexpensive survey of glacier change

in a region. Glacier characteristics that can be

8 A Strategy for Monitoring Glaciers

monitored remotely include area, terminus and

snowline position, accumulation-area ratio , and

surface elevation, as well as glacier events, such as

surging (sudden accelerations) or the filling or

draining of ice-dammed lakes. This information,

particularly accumulation-area ratio, can be used to

infer mass balance and, thereby, extend and test massbalance modeling. From the large number of glaciers

monitored by remote methods, spatial trends will be

documented, and degree of variation will be quantified. Benchmark and secondary glaciers will be

included in the Remote-Sensing Inventory because

they provide the ground-based informati on needed to

interpret the images. The number of glaciers

monitored solely by remote sensing will depend on the

size of the region and number of glaciers; however,

20 to 40 glaciers are desirable.

These three levels of documentation are

mutually supporting and satisfy the simultaneous need

for detailed studies and broad coverages in a costeffective manner. A summary of surveillance levels

and physical processes measured is depicted in

figure 5, and an idealized implementation is depicted

in figure 6. This strategy outlines the purpose and

scope of the USGS Glacier Monitoring Program and

provides a framework to cooperate efficiently with

other agencies and institutions.

The Benchmark Glacier Network is a set of

intensively studied glaciers, one in each glacierized

mountain region of the United States. The purpose of

this network is to collect and provide data needed to

understand the effects of climate on glacier mass

changes and the resulting effect on streamflow. These

glaciers are index sites for the surrounding glacierized

region, whereby the understanding gained from these

sites will be used to predict how the glaciers in the

region respond to climate and affect streamflow. This

understanding depends, in part, on information gained

from the Secondary Glacier Network and the RemoteSensing Inventory

A number of important factors, which parallel

those of the IHD (UNESCO/IASH, 1970), need to be

considered in the selection of a Benchmark Glacier.

The drainage basin should be well defined hydrologically, and the gaged portion should be at least

30-percent glacier covered. Topographic characteristics (area, orientation, and elevation range) and mode
hange in geometr

BENCHMARK

NETWORK

SECONDARY

NETWORK

REMOTE-SENSING

NETWORK

Figure 5. Correspondence between physical processes

that determine glacier change and the processes measured

by the different surveillance levels. The main processes

are shown in the large figure. The processes monitored at

each surveillance level are indicated at the bottom by

shading the corresponding ellipse. Light shading indicates

detailed surface mass-balance measurements and

occasional energy-balance measurements.

of snow accumulation should be typical of the glaciers

in the region. Access to the glacier should be simple

to reduce transportation costs, and most of the glacier

surface should be accessible to facilitate data

gathering. However, the glacier should be sufficiently

remote to reduce possible tampering with instruments

and installations. Prior information about the glacier,

such as its advance and retreat activity, its mass

balance, and its meteorological environment, also is

beneficial. Currently ( 1996), three glaciers compose

the USGS Benchmark Glacier Network-Gulkana in

the Alaska Range, Wolverine in the Kenai Mountains,

Alaska, and South Cascade in the North Cascade

Range, Washington (fig. 4).

Repeated, systematic measurements of

Benchmark Glaciers are the primary method for

determining how glaciers respond to climatic

variations and affect water resources. Measuremen

are divided into two categories. Primary measurements are considered to be fundamental and are

ts

required, if possible, for every Benchmark Glacier.

Secondary measurements are considered to be

important and significantly enhance the usefulness of

the monitoring program, b

to assessing mass change.

ut are not considered critical

Primary Measurements

Mass Balance

Documenting glacier mass changes is

fundamental to understanding the response of glaciers

to changes in climate and the effect of glaciers on

water resources. For these reasons, mass balance is

one of the primary measurements of the Benchmark

Glacier Network. Mass balance is measured directly

through detailed surface-based measurements to gain

an understanding of how the processes that control

mass balance vary across the glacier and through time.

These data will be used to develop and test models that

predict glacier mass balance and glacier-derived

streamflow.

The mass balance is evaluated by measuring the

addition and loss of snow and ice mass at points on the

glacier surface and extrapolating the point data to the

whole glacier surface (UNESCO/IASH, 1970; Meier

and others, 1971). The gain or loss of mass within the

glacier, which is caused by freezing of water (Trabant

and Mayo, 1985) or melting of ice (Röthlisberger,

1972; Mayo, 1992), can be estimated. The quantities

of ice melt commonly are small compared with the

exchange of mass at the surface. At this time, there is

no standard procedure that governs the number of

measurement points of snow and ice accumulation and

ablation on a glacier, but the rule-of-thumb approach

is to make enough point measurements of mass

balance to sample adequately the range of variation on

the glacier. The number of points may be reduced

once the spatial pattern of mass balance has been

determined.

For a calving glacier, the mass loss from calving

may be significant and can be evaluated by using

photography (Brown and others, 1982; Meier and

others, 1985). To evaluate mass balance of a calving

glacier properly, in addition to quantifying the mass

balance of the surface of the glacier, changes in

terminus position and velocity of ice flow to the

terminus also must be measured to determine the rate

of calving.

The important mass-balance values are seasonal

and yearly balances and the balance change with

BENCHMARK GLACIER NETWORK 9

elevation. Seasonal balance measurements are defined

during periods when the glacier is dominated by

accumulation or ablation processes (Paterson, 198 1)

The length of each season varies among glacierized

regions and from year to year. For glaciers with large

elevation ranges, winter conditions of snow accumulation may prevail at high elevations, while summer

conditions of icemelt and snowmelt prevail at lower

elevations. In these situations, the net mass balance of

the whole glacier determines the division between

summer and winter. Seasonal balances are important

because yearly balance alone does not indicate the

magnitude of accumulation or ablation. From a

climatic perspective, the weather in one season is not

necessarily correlated with the next; a winter of large

snow accumulation does not portend a summer of

cool, rainy weather (Walters and Meier, 1989).

Modeling the effect of climate on glaciers depends, in

part, on correctly predicting seasonal influences.

A yearly value of mass balance needs to be

calculated because it is the link between the weather

influences on the glacier, the resulting change of

glacier geometry, and the effect on landscape as

illustrated in figure 3. Mass balance is reported as an

annual value determined between fixed calendar dates

or as a net change between successive minimums in

mass balance (Mayo and others, 1972). To exploit the

advantages of both approaches, the USGS has adopted

a combined system of measurements (Meier and

others, 1971; Mayo and others, 1972); both values are

calculated and reported. Because glacier mass balance

is calculated from the extrapolation of point measurements, which are subject to errors, the mass balance

should be periodically compared with changes in ice

volume (Krimmel, 1989).

The change in mass balance with elevation is an

important relation because, in addition to the distribution of glacier area with elevation, it determines if a

glacier is in equilibrium or whether continued advance

or retreat may occur. This relation provides a physical

basis for mass-balance modeling and has been used to

explain the mass exchange during previous glaciations

(Pierce, 1979; Porter and others, 1983). Changes in

the mass balance/elevation gradient from year to year

indicate the effect of weather variations, and gradient

differences among glaciers reflect influences of the

climatic environment and topographic characteristics.

The annual equilibrium line is the demarcation

between the accumulation and the ablation zones at

the end of the summer melt season (UNESCO/IASH,

1970). The position of the annual equilibrium line

relative to the distribution of glacier area indicates

whether the glacier has gained or lost mass. The

elevation of the annual equilibrium line determined

from aerial photography or satellite images can be

used to assess quickly whether the glaciers of a region

are gaining or losing mass (LaChapelle, 1962; Meier

and Post, 1962). Also, former mean elevations of the

equilibrium lines can be estimated from the geologic

record, thus providing information about glacier

characteristics long before current glaciological

observations were made (Porter, 1977; Waythomas,

1990).

The position of the annual equilibrium line will

be determined everv vear and plotted on a map of the

glacier. Determination of the position of the annual

equilibrium line can be made by the following

methods: sketching the position on a map, surveying,

a time-lapse camera, aerial photographs, or satellite

images (Williams and others, 1991); however, these

techniques require almost daily observations near the

end of the ablation season. Therefore, the position is

usually inferred retroactively from mass-balance and

meteorological data.

Another measure, which is closely related to the

equilibrium line, i sS the accumulation-area ratio, the

fraction of accumulation-area ratio to total gl acierr area

(Meier and Post, 1962). The ratio is important

because it can be easily determined from aerial

photographs and is one measure that can be used to

relate the Remote-Sensing Inventory to the

Benchmark and Secondary Glacier Networks.

Geometry

The geometry of a glacier is constantly

changing in response to mass changes and glacier

movement and is an important link between dynamics

and mass change. Furthermore, changes in geometry,

specifically area, terminus position, and surface

elevation, are important for evaluating the effect of

glaciers on water resources and landscape and to

verify surface mass-balance measurements. The

fraction of the basin covered by a glacier directly

affects the runoff variation from the basin (Krimmel

and Tangborn, 1974; Fountain and Tangborn, 1985;

Braithwaite and Olsen, 1988). Changes in glacier area

largely result from changes in terminus position. The

terminus position also defines glacier length, which is

one of the variables used to evaluate the effect of

climate change (Huybrechts and others, 1989;

BENCHMARK GLACIER NETWORK 11
Johannesson and others, 1989). Furthermore,

terminus position can be reconstructed from the

geologic record and used to infer past climate (Porter,

1977; Meierding, 1982; Rodbell, 1992). Therefore,

terminus position should be monitored and mapped

near the end of the ablation season by either

photogrammetry or geodetic surveying methods if

appreciable change has occurred. It is proposed that a

Benchmark Glacier be remapped when the glacier

length changes by 0.5 percent since the last mapped

position.

The surface elevation of a study glacier should

be mapped to document changes in the elevation

distribution of glacier area and for determining

volume change. The distribution of area with

elevation is an important relation that determines, in

addition to the mass balance/elevation gradient,

whether the glacier is in climatic equilibrium (Furbish

and Andrews, 1984; Tangborn and others, 1990). The

glacier volume change, which is derived from changes

in surface elevation, provides an important and

independent check on mass-balance measurements

(Haakensen, 1986; Krimmel, 1989). Mapping

intervals depend on the rate of change, which may

vary widely. For example, a calving tidewater glacier

in catastrophic retreat may require mapping every

2 years; conversely, for slowly changing glaciers, such

as polar glaciers, the mapping interval may be 15 to

20 years or more. In the absence of information about

surface-elevation changes, mapping intervals should

not exceed 10 years. Topographic maps can be

derived from photogrammetry, with independent

geodetic information to provide an estimate of

accuracy, or by geodetic methods alone. If funding

precludes mapping at necessary intervals, then

mapping-quality aerial photographs should be

acquired and stored until maps can be constructed. If

mapping-quality aerial photographs cannot be

obtained, then surface elevations should be measured

during the interval between aerial photoflights by

measuring the elevation of the glacier surface at index

sites located at fixed x-y locations.

Meteorology and Streamflow

The meteorological environment of the

-Benchmark Glacier will be monitored to examine the

links between climate, glacier response, and streamflow. The variables to be measured are precipitation,

air temperature, humidity, windspeed and direction,

solar radiation, and stream stage. The meteorological

12 A Strategy for Monitoring Glaciers

instruments are best located at the elevation of the

long-term equilibrium line. Stream discharge will be

measured because it defines the hydrologic effects of

glaciers on the watershed. Precipitation, particularly

snowfall, is difficult to measure accurately in windy,

mountainous environments. Furthermore, snow is

redistributed in alpine basins by wind and

avalanching. Precipitation-gage catch may be

improved by using a windshield, such as a Nipher

Shield (Goodison and Metcalfe, 1992). The efficiency

of this type of shield is a function of wind speed, such

that knowing the wind speed, the measured catch can

be adjusted to the true precipitation value (Goodison

and Metcalfe, 1992). This factor is important in the

windy environments common to glaciers. The storage

precipitation gage should be charged with an

antifreeze solution so that new snow melts on contact

with the fluid. A sonic transducer, located near the

gage, may be used to determine when new snow is

falling and help approximate the precipitation during

times when the orifice of the gage is clogged. Air

temperature and humidity sensors must be shielded

from radiation and ventilated to reduce the effect of

solar heating. Standard USGS practices for measuring

stream stage, making discharge measurements, and

developing a stage/discharge rating curve will be

employed.

The meteorological and streamstage measurements should be recorded at intervals of no longer than

1 hour. These data should be telemetered, in part, to

determine whether the data-collection system is

working. Generally, the sites are remote, and access

incurs a significant travel expense. Telemetered data

indicate when the sites need repair and reduce the

magnitude of missing records and the cost of unnecessary field visits.

Supplementary Measurements

Although not vital for assessing mass balance

and changes in glacier geometry, several additional

observations are extremely useful. Glacier-bed

topography, which is a one time measurement, is

important for determining absolute volume changes

from the relative volume change calculated from

surface elevations. It also is a significant parameter

for predicting glacier movement, estimating the

hydraulics of basal water flow, and modeling glacier

dynamics. Bed topography can be determined by

using ice radar (Watts and Wright, 1981; Bogorodsky

and others, 1985).
Another observation is the rate of ice

i-movement, which links mass balance with glacier

geometry. Ice movement is detected by repeated

surveying of targets on the glacier (Meier and

Tangborn, 1965; Paterson, 198 1) or by photogrammetric analysis of natural (crevasses) or artificial

targets on a glacier (Meier and others, 1985). For a

long-term program, where the mass and volume of a

glacier are expected to change, a coincident data set of

flow information is useful for determining the

dynamic response of glaciers to changes in mass input.

Water-quality measurements, which include

sediment transport and electrical conductivity, are

indicative of glacial erosion and subglacial water

processes (Fountain, 1992). The measurements are

recorded at intervals no longer than 1 hour and telemetered, if possible. Hourly measurements are important

to record the diurnal variation in water quality.

Telemetered data are a practical consideration for

these remote field sites to help determine the schedule

of visits to maintain the measurement equipment and

reduce gaps in the data record.

Observation Summary

The observations and calculations associated

with glaciers in the Benchmark Network are

summarized as follows:

Mass balance The location of the data-collection sites will be

0 Point measurements/calculations of mass balance on

the glacier

l Seasonal mass balance, summer and winter

l Net and annual mass balance

l Mass balance as a function of elevation

l Equilibrium-line elevation

l Accumulation-area ratio

Geometry

l Glacier area

l Terminus position

l Surface elevation

Meteorology and streamflow

l Precipitation

l Air temperature and humidity

l Wind speed and direction

l Solar radiation

l Stream discharge

Supplementary data

l Glacier-bed topography

l Glacier velocity

l Water-quality data-suspended sediment, electrical

conductivity

Reports

All the glaciological, meteorological, and

streamflow data that are part of the Benchmark Glacier

Program and the data that are used to calculate the

mass-balance quantities, will be published annually in

the report series of the U.S. Geological Survey. The

data also will be archived on magnetic or optical

media to facilitate computer access and be submitted

to the World Data Center for Glaciology, University of

Colorado, Boulder, Colo. Results of the mass-balance

data will be made available to the World Glacier

Monitoring Service to continue the international

cooperation in glacier observations.

A brief narrative summary of field activities at

the glacier for the year will be written. The narrative

should include the number of visits and accomplishments during each visit, particularly instrument repairs

or location change. Special events, such as outburst

floods or other uncommon events, will be noted.

indicated on a base map. All data collected by digital

or analog recorders will be reported in tables, plotted

as daily averages, and archived in a digital data base in

the original time base. Exceptions are solar-radiation,

precipitation, and stream sediment and discharge data,

which will be presented in tables and plots as daily

sums rather than averages. Summary statistics, which

include monthly averages and totals, will be included

in the tables. If possible, plots of the data in the

original time base will be included to illustrate the

details of diurnal variations. The data will be archived

in the original and daily time intervals on magnetic or

optical media

The positions of point measurements of mass

balance will be plotted on a map of the glacier. The

data from each position will be listed in tables and will

include the position (x,y,z); dates of measurements;

and all data, either measured or inferred, used to

calculate the mass balance at that point. Mass balance

will be expressed in water-equivalent terms. Relevant

plots (for example, snow density with depth, stake

BENCHMARK GLACIER NETWORK 13
movement) also will be included. The position change

of stakes used to calculate glacier flow velocity will

include the x,y,z positions of the stake for every

measurement, as well as the calculated magnitude of

the horizontal and vertical components of velocity.

The elevation of the equilibrium line will be

indicated and if the position across the glacier is

known, then it should be plotted on a map. If the

position was inferred, then the method of inference

should be included. If the terminus position and

surface elevations are measured, then the results and

changes from the previous measurements also will be

included.

The method of calculating mass balance will be

presented with the assumptions explained and the

fundamental equations included. Annual, net, winter,

and summer mass balances will be reported. In

addition, the mass-balance values, glacier area, and

area-integrated mass-balance values will be plotted as

a function of elevation and summarized in a table.

If a method used in a continuing program is

changed, then the new method will first be verified and

reviewed by USGS colleagues in the context of a

USGS report or in a peer-reviewed journal. One

example of this process is the paper by Mayo (1972),

which described a self-mixing antifreeze solution for

recording precipitation gages. All methods also will

be reviewed annually, in the context of annual reports

from each site, to maintain quality and to seek

improvements continually.

.

SECONDARY GLACIER NETWORK

The Secondary Glacier Network will comprise

glaciers that are monitored with a reduced set of

measurements compared with the Benchmark Glacier

Network. These glaciers provide information on the

variability of glacier mass balance across a region, in

contrast to detailed measurements of mass balance,

runoff, and meteorol ogical environment at a single

glacier. Documenting the variation of mass balance

across a region is essential for the development of

models of regional glacier variation and their effect on

runoff. Variations may be caused by differences in

local climate and topography. Glaciers in the

Secondary Network will be candidates for benchn

status in regions where the initially selected

Benchmark Glacier may have to be abandoned

because of changes implemented by the landmanagement agency. This arrangem.ent provides for

a number of different glaciers to be monitored at a low

cost and provides important background information

for potential inclusion in the Benchmark Glacier

Network.

Although maintaining data collection at

secondary glaciers indefinitely is desirable, it is more

practical to consider these glaciers a flexible part of

the monitoring program. For example, if two

secondary glaciers exhibit mass-balance variations

that are virtually identical or are similar to the

Benchmark Glacier, then observations on one of the

glaciers could be discontinued. Also, the Secondary

Glacier Network provides opportunities for cooperation with other agencies, such as the National Park

Service, or university research programs, that may

wish to develop glacier-monitoring programs without

requiring a comprehensive program of measurements,

such as that of the Benchmark Glacier Network.

Primary Measurements

At minimum, annual or net mass balance will be

calculated for each secondary glacier. Although

surface-based measurements are desirable because of

the detail provided, alternative methods to determine

mass balance, such as flux-divergence or volume

methods, are appropriate. A combination of methods

also may be used. In addition, the elevation of the

equilibrium line and the accumulation-area ratio need

to be measured. These data will be compared with the

mass-balance values from the Benchmark Glacier to

define the trends within each glacierized region better.

Furthermore, the accumulation-area ratio will be

compared with those ratios determined from the

Remote-Sensing Inventory to infer more precisely

mass changes for the region.

The glacier area and, hence, terminus position

must be known to estimate a glacier’s mass balance.

This information will be compared with the mass and

the glacier-area changes obtained from the RemoteSensing Inventory.

Several additional calculations are

recommended and include seasonal mass balance,

mass-balance change with elevation, and surface

elevation. Each of these calculations significantly

increases the understanding of the geographic

variability of the processes that control glacier mass

balance. Other data outlined in the Benchmark

Glacier Network, such as subglacial topography,

meteorology, and water runoff, also would be useful to

14 A Strategy for Monitoring Glaciers
expand understanding of the regional variability in

these variables. This knowledge would improve the

ability to predict regional mass-balance variations and

effect on runoff.

Observation Summary

Mass Balance

l Net mass balance

l Equilibrium-line elevation

0 Accumulation-area ratio

Geometry

l Glacier area

l Terminus position

Supplementary data

l Seasonal mass balance

l Mass balance with elevation

l Surface elevation

Reports

Annual reporting of the Secondary Glacier

Network data should maintain the standards set for the

Benchmark Glacier Network. For example, the

method used to estimate mass balance should be

described with all the fundamental measurements

included in tables and graphs.

REMOTE-SENSING INVENTORY

Aerial photographs and satellite images of

glacierized regions provide the most efficient means

for monitoring regional glacier activity. For practical

reasons, such as time and available resources, few

glaciers can be observed on the ground, whereas

hundreds can be observed from aircraft and thousands

from satellites. Remote imaging provides direct

information on changes in glacier area, terminus

position, snowline location, and accumulation-area

ratio. Furthermore, remote images document unusual

activity, such as surging, or development of potential

hazards, such as ice-dammed lakes. Such imagery

enables the extension of ground observations, which

typically are limited to a few basins, to broad regions.

When properly annotated and stored, photographic or

digital images are the most complete record of glacier

conditions available. Although aerial photographs are

specifically discussed because of long experience with

their use, the resolution of satellite images is

improving, and advances in interpretation of these

images have brought the technology ever closer to the

utility of aerial photographs. For these reasons, in the

following discussion, “aerial photography” also may

be read as “satellite images.”

Aerial photographic surveys should be made of

each glacierized region of the United States. Within

each region, specific glaciers will be identified for

aerial survey. The selected glaciers will represent the

range of glaciers that characterize the region and

include those that present hazards, such as icedammed lakes, or those of glaciological interest, such

as surging glaciers. The inclusion of Benchmark and

secondary glaciers in the aerial surveys will link the

known processes controlling mass balance and

resulting variations with glacier variations observed

across the region. Aerial surveys should be flown late

in the summer so the terminus will be in its most

retracted position; the transient snowline nearest its

final, seasonal position; and the accumulation-area

ratio at its seasonal minimum. The interval between

photographic surveys will depend on the rates and

magnitude of glacier changes in each region and the

importance of coverage. For example, the terminus

position of a tidewater glacier undergoing catastrophic

retreat can change by a kilometer or more per year

(Post, 1975), whereas the termini of small cirque

glaciers, such as those in Colorado or Montana, might

change only a few meters each year. Suggested

intervals between aerial photographic surveys are

provided in table 1.

Primary Measurements

Specific attributes, which include glacier area,

terminus position, and snowline, will be identified on

selected glaciers of each region. From these data, the

accumulation-area ratio will be calculated. This

information will be used to develop and evaluate the

relation of variations of the Benchmark and secondary

glaciers to regional glacier variations. The

Benchmark and secondary glaciers then can be used as

predictors of regional glacier change for quantifying

the effect of glaciers on water resources.

Supplementary Data

If photographs

then it is desirable to

or images are of suitable quality,

determine the surface elevation

REMOTE-SENSING INVENTORY 15
Table 1. Frequency of aerial photographic surveys.

glacierized mountain regions of the United States

Geographic region

(fig. 4)

Flight interval

(Years)

Alaska

Southeast Panhandle

Wrangell-S t. Elias Range

Chugach and Kenai Ranges

Alaska Range

Brooks Range

Washington and Oregon:

Olympic and Cascade Ranges

California

1

1

1

2-3

10

2-3

Sierra Nevada

Colorado

Front Range

Montana and Wyoming:

Glacier National Park, Beartooth Mountains,

Wind River Range

10

10

10

of a number of glaciers (in addition to the Benchmark

or secondary networks of glaciers) for calculating

volume change. This additional information greatly

increases the usefulness of the data by providing an

independent check on the mass changes inferred from

the accumulation-area ratio.

Satellite images of entire glacierized regions

should be obtained once every decade with more

frequent coverage of regions in which changes are

rapid, such as the tidewater glaciers of southern

coastal Alaska. Imagery should have sufficient resolution to show the small glaciers in the region. Like

aerial photographs, the images should be obtained late

in the summer near the time when the greatest area of

snow-free ice is exposed. These images should be

analyzed for glacier area and snowline position.

Observation Summary

Mass Balance

l Accumulation-area ratio

Geometry

Reports

Reports on the Remote-Sensing Inventory will

include a brief flight summary of the mapped flight

lines, a list of glaciers photographed, camera position

(vertical or oblique), and part of each glacier

photographed if coverage is incomplete. The glaciologic data for each selected glacier within each region

will be included in tables.

Glacier change is an important environmental

variable that affects global sea level and terrestrial

water resources and poses potentially hazardous

situations. For these reasons, it is important to

document changes in glacier cover. Because of the

number and wide distribution of glaciers in the

Western States, a practical monitoring plan must

encompass sufficient detail and broad coverage. To

accomplish this goal, the glaciers of the United States

were divided into regional groups. Each region should

contain a Benchmark Glacier on which the objective is

to collect data to define the relations between local

climate, glacier mass balance, and glacier extent and

basin runoff. Extrapolating the information gained

from a Benchmark Glacier to other glaciers in the

region with different physiographic characteristics

requires that mass-balance data be collected on a few,

less intensively studied secondary glaciers. These

glaciers may be monitored by the USGS or cooperating institutions. Estimating the degree of variability

of glacier changes across a region and identifying

potentially hazardous situations will be accomplished

by remote sensing. This nested approach to glacier

studies will provide sufficiently detailed data to

understand the physical processes important in each

region and to provide information on how such

processes may vary across the region. This strategy

will provide the basic data to understand and predict

the effects of glaciers on global sea level, water

resources, and hydrologic hazards.

l Glacier area

l Terminus position

Supplementary data

l Glacier surface elevation

l Satellite images covering the entire region

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