U.S. Department of the Interior 

U.S. Geological Survey

Hydrologic Aspects of the 1998-99 

Drought in the Delaware River Basin

Water-Resources Investigations Report 00-4112

science for a changing world

U.S. Department of the Interior 

U.S. Geological Survey

HYDROLOGIC ASPECTS OF THE 1998-99 

DROUGHT IN THE DELAWARE RIVER BASIN

by Gary N. Paulachok, Bruce E. Krejmas, and Heidi L. Soden 

Water-Resources Investigations Report 00-4112

Milford, Pennsylvania 

2000
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Charles G. Groat, Director

The States of Delaware and New Jersey, the Commonwealth of Pennsylvania, and the City of 

New York, through their funding of the Office of the Delaware River Master, provided financial 

support for this report.

For additional information Copies of this report may be

write to: purchased from:

Delaware River Master U.S. Geological Survey

U.S. Geological Survey Branch of Information Services

Milford Professional Park Box 25286, Federal Center

Building 3, Suite 304 Denver, CO 80225-0286

10 Buist Road Telephone: 1-888-ASK-USGS

Milford, PA 18337-9311
CONTENTS

Page

Abstract............................................................................. 1

Introduction.......................................................................... 1

Purpose and scope ................................................................ 1

Location of study area............................................................. 1

Concept of drought ...............................................................3

Climatological aspects of the 1998-99 drought ..............................................4

General atmospheric circulation .....................................................4

Circulation patterns during the drought................................................4

Hydrologic conditions associated with the 1998-99 drought....................................4

Occurrence of precipitation.........................................................4

Precipitation during the drought .....................................................5

Streamflow......................................................................6

Drought effects on Streamflow.......................................................7

Reservoir storage................................................................ 15

Ground-water occurrence and levels .................................................20

Drought effects on ground-water levels...............................................21

Institutional response to the 1998-99 drought ..............................................26

Summary...........................................................................28

References cited .....................................................................29

ILLUSTRATIONS

Page

Figure 1. Map showing the Delaware River Basin and location of index streamflowgaging stations. ......................................................2

2. Sketch showing propagation of precipitation deficiencies through major

components of the hydrologic cycle.......................................3

3. Graphs of actual and normal precipitation, and cumulative precipitation deficit in

the Delaware River Basin above Trenton, N.J., July 1998 to September 1999. .... .6
ILLUSTRATIONS

Page

4-5. Hydrographs of:

4. Daily mean discharge at index gaging stations in the Delaware River Basin 

during water year 1999 and water year of minimum annual mean 

discharge.......................................................9

5. Daily mean discharge at index gaging stations in the Delaware River Basin 

during water year 1999 and minimum and median daily mean 

discharges for period of record prior to water year 1999. ................10

6. Graphs of monthly mean discharge for four selected drought years and

period of record prior to water year 1999, Flat Brook near Flatbrookville, N.J..... 11

7. Sketch showing features of a standard boxplot.................................11

8. Boxplots showing streamflow distributions for index gaging stations in the

Delaware River Basin. ................................................ 12

9. Graphs of streamflow trends at index gaging stations in the Delaware River Basin.... .14

10. Plots of flow-duration curves for three selected index gaging stations in the

Delaware River Basin................................................. 15

11. Maps showing streamflow conditions in the Delaware River Basin before and after

Tropical Storm Floyd. ................................................ 16

12-13. Graphs of:

12. Actual contents and operating curves for New York City reservoirs

in the Delaware River Basin, January 1, 1998, to December 31, 1999...... .18

13. Actual contents and operating curves for New York City reservoirs

in the Delaware River Basin, June 1, 1984, to December 31, 1985.. .......19

14. Map showing location of selected observation wells operated by the U.S.

Geological Survey in the Pennsylvania part of the Delaware River Basin.........21

15-16. Hydrographs of:

15. Ground-water levels in three observation wells in the Delaware River Basin

in Pennsylvania, 1995-99. ........................................23

16. Ground-water levels in observation well CH-10, Chester County, Pennsylvania,

1961-63 and 1997-99. .......................................... .23

17. Boxplots showing distribution of ground-water levels in observation well CH-10,

Chester County, Pennsylvania, 1963 and 1999..............................24

18. Maps showing ground-water levels in observation wells in the Pennsylvania part of

the Delaware River Basin for six selected months during the 1998-99 drought..... .25
TABLES

Table 1. Monthly and normal precipitation for selected locations in the Delaware River 

Basin, 1998-99. ...............................................

Page

2. Ranking of selected minimum annual mean streamflows for index gaging stations

in the Delaware River Basin.............................................7

3. Summary data on low streamflows during period of record prior to water year 1999

and water year 1999 for index gaging stations in the Delaware River Basin.......17

4. Summary data on annual n-day minimum streamflows during the 1998-99 drought 

and record low n-consecutive day values at index gaging stations in the 

Delaware River Basin................................................. 18

5. Summary data on combined storage for New York City reservoirs in the Delaware

River Basin, June 1998 - December 1999..................................20

6. Comparison of record low ground-water levels with low levels during the 1998-99

drought. ...........................................................22

CONVERSION FACTORS AND ABBREVIATIONS

Multiply By To obtain

inch (in.) 

foot (ft) 

mile (mi)

millimeter

meter

kilometer

square mile (mi )

million gallons (Mgal) 

million gallons (Mgal) 

billion gallons (Bgal) 

cubic foot per second-day (ft3/s-d)

Area 

2.590

Volume

3,785

1.547

3.785

0.002447

square kilometer

cubic meter

cubic foot per second-day 

cubic hectometer 

cubic hectometer

billion gallons per day (Bgal/d) 

cubic foot per second (ft3/s) 

cubic foot per second 

per square mile [(ft3/s)/mi2]

Flow rate 

43.81 

0.02832 

0.011

cubic meter per second

cubic meter per second

cubic meter per second

per square kilometer

HYDROLOGIC ASPECTS OF THE 1998-99 DROUGHT 

IN THE DELAWARE RIVER BASIN

by Gary N. Paulachok, Bruce E. Krejmas, and Heidi L Soden

ABSTRACT

A notable drought in the Delaware River Basin 

during late 1998 and most of 1999 had a major 

effect on surface and subsurface components of the 

hydrologic system. The drought conditions 

resulted from anomalous patterns in the general 

atmospheric circulation that diverted Gulf and 

subtropical Atlantic moisture away from the basin. 

From September 1998 to August 1999, the accumulated precipitation deficiency was greater than 

12 inches in the part of the basin above Trenton, 

N.J. Flows in some streams, mainly in the middle 

and lower parts of the basin, decreased to levels 

near or less than those measured during the 

drought of the 1960's, the most severe drought of 

record in the basin. On several dates in August 

1999, combined storage in three New York City 

water-supply reservoirs in the upper Delaware 

River Basin decreased by more than 2 billion 

gallons per day. The drought had a pronounced 

effect on ground-water levels, as the combination 

of below-normal recharge and elevated rates of 

evapotranspiration produced abnormal water-level 

declines and record low water levels in much of the 

basin. The drought was broken in mid-September 

1999 when the remnants of Tropical Storm Floyd 

delivered drenching rains throughout the basin.

INTRODUCTION

Exceptionally dry conditions dominated the 

Middle Atlantic region and the northeastern United 

States during late 1998 and most of 1999. In the 

Delaware River Basin, as in other parts of the 

region, the hydrologic system showed a 

pronounced response to the precipitation deficiency. Below-normal amounts of rainfall, snowfall, and runoff, and unusually low streamflows and 

ground-water levels, were prevalent. In summer 

1999, the dry ness was compounded by excessively 

high temperatures, which intensified the drought 

conditions. In September 1999, the remnants of 

Tropical Storm Floyd moved across the region and 

delivered drenching rains to the basin, breaking the

drought conditions. Whether these rains ended the 

drought or merely interrupted a longer-term event 

will depend upon future weather conditions.

Purpose and Scope

The drought of 1998-99 affected, in varying 

degrees, a broad area from Virginia to Maine. The 

Delaware River Basin, which occupies part of that 

area, was particularly hard hit and is the focus of 

this report. The report documents the precipitation 

deficiency associated with the drought and examines the response of the hydrologic system 

namely, reductions in streamflow and reservoir 

storage, and declines in ground-water levels to 

the deficiency. The report does not address 

water-management and non-hydrologic aspects of 

the drought, including those concerned with agricultural, sociological, and economic issues and 

impacts.

Location of Study Area

The Delaware River Basin comprises a land

SJ

area of 12, 967 mi in New York, Pennsylvania, 

New Jersey, Delaware, and a small part of northeastern Maryland (fig. 1). The river originates in 

the Catskill Mountains of New York, as the East 

and West Branches of the Delaware River, and 

flows for about 370 mi on a generally southsoutheasterly course to its mouth at Delaware Bay. 

The basin lies between the Susquehanna River 

Basin to the west and the Hudson, Passaic, and 

Raritan River Basins to the east. The Mongaup and 

Neversink Rivers in New York; the Lackawaxen, 

Lehigh, and Schuylkill Rivers in Pennsylvania; the 

Maurice and Musconetcong Rivers and Rancocas 

Creek in New Jersey; and the Christina River in 

Delaware are the major tributaries of the Delaware 

River. The localities of Port Jervis, New York; 

Easton, Philadelphia, Chester, Allentown, and 

Reading, Pa.; Trenton and Camden, N.J.; and 

Wilmington, Del., are the major cities in the Delaware River Basin.
DELAWARE RIVER/ 

BASIN

Figure 1. The Delaware River Basin and location of index streamflow-gaging stations.
The Delaware River provides water for municipal, industrial, and other uses for more than 

7 million basin residents. In addition, a 1954 

Supreme Court Decree authorized diversions of 

water from the Delaware River Basin to New York 

City and out-of-basin parts of New Jersey; these 

diversions provide water to nearly 10 million 

people who live outside the basin. New York City, 

the largest importer of Delaware River water, 

obtains about one-half of its total water supply 

from three large reservoirs Pepacton. Cannonsville, and Neversink located in the upper Delaware River Basin (fig. 1).

Concept of Drought

Drought is a natural, recurring, and temporary 

feature of climate. Although drought can be 

defined from several discipline-based perspectives 

including meteorology, hydrology, and agriculture, the central theme in the definitions is the 

concept of a water deficiency. Drought is difficult 

to define precisely, however, because of the need to 

identify the specific components of the hydrologic 

cycle affected by the deficiency and the time period 

associated with it. Figure 2 illustrates the propagation of the effects of two hypothetical precipitation 

deficiencies through the surface-runoff, soil-moisture, streamflow, and ground-water components of 

the hydrologic cycle. From this perspective, precipitation can be considered the carrier of the drought 

signal, and streamflow and ground-water levels can 

be considered the last indicators of drought occurrence (Hare, 1987; Klemes, 1987).

This report is concerned with aspects of hydrologic drought in the Delaware River Basin. 

According to the World Meteorological Organization, a hydrologic drought is defined in terms of 

reduction of streamflows, reduction in lake or 

reservoir storage, and lowering of ground-water 

levels (Subrahmanyam. 1967). Because the subject 

matter of this report is bounded by this definition, 

the report describes only several aspects of the 

complex and wide-ranging phenomenon of 

drought.

Historically, the Delaware River Basin has 

been affected by numerous episodes of drought. 

Severity and duration are the main factors that 

distinguish one drought from another. The most 

serious drought affected the northeastern United

 DROUGHT SIGNALSURFACE RUNOFF

1

TIME, IN YEARS

EXPLANATION

PRECIPITATION

 i ABOVE NORMAL

CH BELOW NORMAL (Deficiency)

Figure 2. Propagation of precipitation deficiencies 

through major components of the hydrologic cycle 

(Modified from Changnon, 1987),

States, including the Delaware River Basin, in the 

1960's. The 1961-67 drought in the Northeast was 

the longest and most severe in the recorded history 

of the region, and it affected human activities 

mainly by its impact on water resources related to 

agriculture and water supplies (Barksdale and 

others, 1966). The Water Resources Council 

(1966) estimated that in the Northeast, a drought of 

this duration occurs, on the average, once in about 

160 years. Although drought conditions eased at 

different times in different parts of the region, the 

drought was not completely broken until spring 

1967. The Delaware River Basin also was affected 

by droughts in 1980-82 and 1984-85, and by 

several transitory droughts in the 1990's. The 

drought of 1998-99. characterized by a substantial 

precipitation shortfall and intense heat, was one of 

the most severe short-term droughts of the 20th 

century.
CLIMATOLOGICAL ASPECTS OF THE 

1998-99 DROUGHT

General Atmospheric Circulation

The weather in the Delaware River Basin is 

characterized by seasonal changes typical of a 

temperate humid climate. The Atlantic Ocean to 

the east moderates temperature extremes to a 

certain extent, but this effect is not always evident 

because prevailing winds are westerly. Storms 

moving parallel to the Atlantic coast from the Gulf 

of Mexico are important precipitation-producing 

systems during the winter months. In summer, 

warm, moist air is circulated into the area by the 

Bermuda high and scattered convective showers 

contribute the bulk of the precipitation. Occasionally, tropical storms move through the area. The 

large rainfall totals associated with these storms 

interrupt the otherwise generally even distribution 

of precipitation throughout the year.

Circulation Patterns During the Drought

Droughts are associated with persistent or 

persistently recurring atmospheric circulation 

patterns (Namias, 1985). These patterns, which 

produce little or no precipitation, are not associated 

with any discernibly unique feature of individual 

daily circulation patterns. When averaged for a 

month or more during a drought, however, the 

atmospheric circulation patterns begin to reveal 

anomalies when compared to long-term average 

patterns.

The progressive development of drought 

conditions in the Delaware River Basin from 

summer 1998 to fall 1999 resulted from anomalous 

patterns in the general atmospheric circulation. 

During this period, no single factor was responsible for the reduction of atmospheric moisture. 

Rather, two factors northerly displacement of the 

jet stream and westerly displacement of Gulf and 

subtropical Atlantic moisture played important 

roles in diverting atmospheric moisture away from 

the basin.

By late spring 1999, the jet stream, which 

forms a boundary between cool polar air and warm 

tropical air, was located farther north than normal, 

steering most precipitation systems to the north of 

the Delaware River Basin. Occasionally during the

drought period, the jet stream split into two distinct 

streams, causing storms to pass to the north and 

south of the basin.

The lack of Gulf and subtropical Atlantic 

moisture entering the region was a major reason for 

the lack of significant rainfall. During the drought, 

the flow of moisture from lower latitudes often was 

weak and migrated further west than normal. 

During summer 1999, many parts of the Delaware 

River Basin received little rain as Gulf and subtropical moisture moved into areas west of the Appalachian Mountains.

By September 1999, the jet stream shifted into 

a southwest to northeast orientation and allowed 

tropical storms to move inland and deliver 

much-needed rain. Without question, hydrologic 

conditions in the Delaware River Basin would have 

continued to deteriorate if Tropical Storm Floyd 

had not delivered unusually large amounts of rain 

to the basin in mid-September.

HYDROLOGIC CONDITIONS ASSOCIATED 

WITH THE 1998-99 DROUGHT

As a result of the precipitation deficiency associated with the 1998-99 drought, water resources in 

surface and subsurface systems came under considerable stress. This section documents the precipitation shortfall and examines the resulting effects on 

streamflows, reservoir storage, and ground-water 

levels.

Occurrence of Precipitation

Precipitation is the source of all fresh water in 

the Delaware River Basin. Mean annual precipitation ranges from less than 40 in. in some southern 

and western parts of the basin to more than 50 in. 

in some northern parts, where terrain effects have 

an important local influence on precipitation delivery. During the warmer months, much of the 

precipitation input is returned to the atmosphere 

through evapotranspiration while during the cooler 

months, water loss is greatly reduced. Precipitation 

delivered as snow may be stored temporarily in 

snowpack. Water in frozen form may be returned to 

the atmosphere directly through sublimation or 

may eventually melt, providing seasonal runoff to 

streams and recharge to ground-water reservoirs.
Precipitation During the Drought

Precipitation data discussed in this report are 

for the part of the Delaware River Basin located 

north of Trenton, N.J. (fig. 1). The values given are 

arithmetic means of amounts measured at several 

recording and non-recording stations in New York, 

New Jersey, and Pennsylvania. No attempt has 

been made to estimate areal mean precipitation by 

use of weighting factors or other computational 

techniques. These data, along with point-precipitation data for recording stations at Allentown, Pa., 

Philadelphia, Pa., and Wilmington, Del., are 

presented in table 1. In general, precipitation totals 

during 1998-99 were greater in the northern and 

southern parts of the basin than in the central part.

In the Delaware River Basin, generally dry 

conditions prevailed during fall and winter 1998, 

setting the stage for drought conditions during

summer and fall 1999. Antecedent conditions 

reflected a precipitation surplus, however, as 

precipitation from August 1997 to July 1998 was 

nearly 6 in. greater than normal (in this report, the 

term "normal", in the context of precipitation, 

refers to average precipitation during 1961-90), 

and from April to July 1998 precipitation was more 

than 4 in. above normal. The deficient precipitation 

trend began in July 1998 (fig. 3), as monthly 

precipitation totals dropped below normal and 

remained at below-normal levels through December 1998. An exception to this trend was the October total, when precipitation was slightly above 

normal. In December 1998, precipitation in the 

basin was 32 percent of normal. Monthly precipitation amounts in November and December 1998 

were small, and the total for the two months was 

only 2.78 in.

Table 1. Monthly and normal precipitation for selected locations in the Delaware River Basin, 1998-99

[All figures reported in inches. Normals for all locations are for 1961-90 except above Montague, N.J., which is for 

1941-94. Data for all locations except above Montague from National Weather Service]

Date

1998

Jan.

Feb.

Mar.

Apr.

May

June

July

Aug.

Sept.

Oct.

Nov.

Dec.

Total

1999

Jan.

Feb.

Mar.

Apr.

May

June

July

Aug.

Sept.

Oct.

Nov.

Dec.

Total

Total, 98 + 99

Multi-station 

average above 

Montague, N.J.

Total

3.73

2.96

4.04

4.67

4.82

7.52

3.62

1.42

2.97

3.36

1.35

1.32

41.78

5.33

1.60

4.37

2.22

3.17

2.02

1.71

3.05

9.08

2.72

2.98

1.63

39.88

81.66

Normal

2.88

2.69

3.30

3.77

4.20

3.92

4.09

3.98

3.72

3.39

3.86

3.38

43.18

2.88

2.69

3.30

3.77

4.20

3.92

4.09

3.98

3.72

3.39

3.86

3.38

43.18

Allentown, Pa.

Total

3.42

3.71

4.00

5.22

4.58

4.95

1.18

3.13

2.99

4.50

1.39

.64

39.71

5.42

2.67

3.07

1.41

5.06

.98

.33

3.82

11.57

2.52

2.43

2.45

41.73

81.44

Normal

3.16

2.95

3.28

3.52

4.20

3.75

4.14

4.28

3.93

2.94

3.88

3.49

43.52

3.16

2.95

3.28

3.52

4.20

3.75

4.14

4.28

3.93

2.94

3.88

3.49

43.52

Multi-station 

average above 

Trenton, N.J.

Total

4.15

3.68

4.28

4.97

5.43

6.90

2.90

2.35

2.72

3.89

1.64

1.14

44.05

6.44

1.90

4.55

2.46

3.20

1.85

1.64

3.67

10.01

2.66

3.31

2.01

43.70

87.75

Normal

3.20

2.83

3.74

3.89

3.71

3.71

3.99

4.28

3.95

3.52

3.83

3.58

44.23

3.20

2.82

3.74

3.89

3.71

3.71

3.99

4.28

3.95

3.52

3.83

3.58

44.23

Philadelphia, Pa.

Total

4.24

3.25

3.93

2.70

3.87

4.92

1.79

1.26

1.86

1.84

1.18

.82

31.66

4.89

2.95

4.02

4.19

2.82

1.16

1.22

5.32

13.07

3.55

2.31

2.99

48.49

80.15

Normal

3.18

2.81

3.86

3.47

3.18

3.92

3.88

4.10

3.42

2.83

3.32

3.45

41.42

3.18

2.81

3.86

3.47

3.18

3.92

3.88

4.10

3.42

2.83

3.32

3.45

41.42

Wilmington, Del.

Total

4.51

2.87

5.23

2.94

4.13

4.66

2.18

3.14

1.78

2.81

1.27

1.04

36.56

5.42

3.51

3.96

3.37

3.58

1.61

.89

4.25

12.68

3.45

2.11

1.85

46.68

83.24

Normal

3.11

2.99

3.87

3.39

3.23

3.51

3.90

4.03

3.59

2.89

3.33

3.54

41.38

3.11

2.99

3.87

3.39

3.23

3.51

3.90

4.03

3.59

2.89

3.33

3.54

41.38
 NORMAL (1961-90) 

E ACTUAL

Figure 3. Actual and normal precipitation, and cumulative 

precipitation deficit in the Delaware River Basin above Trenton, N.J., 

July 1998 to September 1999.

Precipitation was above normal in January and 

March 1999, but in February 1999 and from April 

to August 1999, monthly totals were below normal. 

As a result of these many shortfalls, the accumulated precipitation deficiency increased quickly. In 

August 1998. precipitation during the previous 

12 months was several inches greater than normal. 

By August 1999. however, precipitation during the 

previous 12 months was 12.14 in. less than normal. 

For the period April 1999 to August 1999, the 

accumulated precipitation deficiency was 6.76 in. 

Despite the soaking rains delivered by the 

remnants of Tropical Storm Floyd on September 

16, 1999, all 12-previous month accumulated 

precipitation totals for the months from September 

1999 to April 2000 remained in the deficient range.

The amount of snow in the upper Delaware 

River Basin is an important factor in the water 

balance. The snowpack acts as a natural reservoir, 

storing water in winter and releasing it in spring. 

Depending on the amount of snow and the timing 

of melting, snowmelt can be an important source of 

runoff to streams and recharge to ground-water 

reservoirs.

During most of winter 1998-99, the 

snowpack in the upper Delaware River 

Basin was below normal. Snowfall was 

greatest in January 1999 but the snow 

melted shortly thereafter. Because the 

snowpack stored relatively little water 

and released the water quickly, the 

water-supply benefits of the snowmelt 

were marginal and transitory. Like the 

below-normal amounts of liquid precipitation, the below-normal snowpack 

was a precursor of the drought conditions that would intensify later in the 

year.

The drought of 1998-99 was 

preceded by a period of above-normal 

precipitation and was broken by Tropical Storm Floyd. which delivered about 

5 in. of rain to the northern part of the 

Delaware River Basin and about 10 in. 

to the southern part. Because of the 

timing of wet and dry periods, precipitation totals for 1998 and 1999 fall in 

the normal range if interpreted on a 

calendar-year basis. Consequently, the calendar-year totals mask the true magnitude of the 

precipitation shortfall during the drought period.

Streamflow

Streamflow in the Delaware River Basin is 

derived from precipitation that falls within the 

basin boundaries. Within the basin, about one-half 

of the average annual precipitation, or nearly 

5,000 Bgal, becomes Streamflow. An important 

component of Streamflow is baseflow or dryweather flow, which consists mainly of groundwater discharge or releases from surface storage.

The following discussions, illustrations, and 

tables describe and summarize low streamflows 

during the drought of 1998-99. Two reference time 

periods are used in the analysis of the Streamflow 

data. They are the water year, which is the interval 

from October 1 to September 30, and the climatic 

year, which is the interval from April 1 to March 

31. Customarily, the climatic year is used for 

low-streamflow analysis in order to incorporate the 

entire low-flow season into a single 365-day 

period.
Discharge records for nine long-term U.S. 

Geological Survey (USGS) streamflow-gaging 

stations ("index gaging stations") in New York, 

New Jersey, Pennsylvania, and Delaware were 

used in the analysis of low-flow conditions during 

the drought period. Figure 1 shows the location of 

these stations. Except for the two stations in Delaware, the index stations are on streams that are 

unregulated or subject to only minor regulation. 

Streamflow at White Clay Creek near Newark. Del. 

(site number 8 on figure 1) is affected by upstream 

operations of the City of Newark water plant. Low 

streamflow at Red Clay Creek at Wooddale, Del. 

(site number 9 on figure 1) is affected at times by 

flow-augmentation releases from Hoopes Reservoir. In identifying historical records for low-flow 

analysis, the objective was to select those that, to 

the extent possible, reflect mainly the effects of 

climate and natural conditions on streamflow.

Drought Effects on Streamflow

In the context of streamflow, the drought of 

1998-99 in the Delaware River Basin was noteworthy for several reasons. First, it was one of the most

severe short-duration droughts of the 20th century. 

In a relatively short time, minimum daily mean 

discharges at many gaging stations decreased to 

levels near or less than those measured during the 

drought of the 1960's. the most severe drought of 

record in the basin. On a water-year basis, annual 

mean streamflow was the 4th lowest of record at 

one index gaging station and the 5th lowest of 

record at two stations. At four other stations, 

annual mean streamflows ranged from the 8th 

lowest to the 15th lowest of record (table 2). In 

addition, in summer 1999, extreme heat 

compounded the effects on low streamflow, as 

persistently high temperatures resulted in increased 

demand for water and greater loss of water by 

evapotranspiration. Finally, the drought had its 

greatest effect on streamflow in late summer, as 

flows receded to minimum levels in August and 

early September. Although there are exceptions, in 

the Delaware River Basin, streamflow generally is 

lowest in early fall.

Table 2. Ranking of selected minimum annual mean streamflows for index gaging stations in the Delaware River Basin

.3 .3 

[Format is /cfsm where 1 , cubic I'eel per second; cfsm, cubic.feel per second per square rnile; wy, water year; >, greater than]

S (wy)

Site 

number USGS station 

on figure number 

1

1

2

3

4

5

6

7

8

9

01413500

01420500

01423000

01435000

01439500

01440000

01467000

01479000

01480000

Station name

East Branch

Delaware River at

Margaretville, N.Y.

Beaver Kill at

Cooks Falls, N.Y.

West Branch

Delaware River at

Walton, N.Y.

Neversink River

near

Claryville, N.Y.

Bush Kill at

Shoemakers, Pa.

Flat Brook near

Flatbrookville, N.J.

North Branch

Rancocas Creek at

Pemberton, N.J.

White Clay Creek

near Newark, Del.

Red Clay Creek at

Wooddale, Del.

Drainage 

area 

(square 

miles)

163

241

332

66.6

117

64.0

118

89.1

47.0

Rank of annual mean streamflow

1

138/0.85

(1965)

277/1.15

(1965)

263/0.79

(1965)

100/1.51

(1965)

95.4/0.82

(1965)

43.4/0.68

(1965)

92.3/0.78

(1995)

55.9/0.63

(1966)

32.3/0.69

(1995)

2

170/1.04

(1985)

333/1.38

(1985)

340/1.02

(1985)

116/1.75

(1985)

115/0.98

(1985)

55.6/0.87

(1966)

92.7/0.78

(1985)

60.8/0.68

(1965)

32.3/0.69

(1981)

3

183/1.12

(1962)

358/1.49

(1941)

362/1.09

(1995)

121/1.82

(1962)

132/1.13

(1966)

61.8/0.96

(1985)

98.8/0.84

(1966)

60.9/0.68

(1995)

34.4/0.73

(1966)

4

198/1.21

(1944)

361/150

(1962)

391/1.18

(1963)

132/1.99

(1966)

135/1.15

(1999)

64.7/1.01

(1932)

103/0.87

(1981)

61.9/0.69

(1932)

36.3/0.77

(1992)

5

199/1.22

(1966)

376/1.56

(1944)

400/1.20

(1999)

135/2.03

(1995)

138/1.18

(1932)

71.0/1.11

(1999)

107/0.91

(1931)

64.7/0.73

(1969)

38.9/0.83

(1965)

Rank of WY 1999 

annual mean 

streamflow

>5

249/1.53

Rank = 14

446/1.85

Rank = 15

147/2.21

Rank = 8

125/1.06

Rank = 14

98.6/1.11

Rank = 24

51.3/1.09

Rank = 21
The severity of the 1998-99 drought can be 

assessed by comparing observed streamflows with 

streamflows during droughts of known severity and 

with certain statistical measures of streamflow. 

Various comparisons for the index gaging stations 

are presented in the figures and tables that follow.

Figure 4 shows a station-by-station comparison of daily mean discharge during water year 

1999 with daily mean discharge recorded during 

the water year of minimum annual mean discharge. 

Although daily mean discharges generally were 

greater during water year 1999, discharges from 

late December 1998 to early January 1999 and 

during part or all of the period from July to 

mid-September 1999 generally were less than the 

corresponding discharges during the water year of 

record low annual mean discharge.

The hydrographs in figure 5 present a comparison of daily mean discharge during water year 

1999 with minimum and median daily mean 

discharges for the index gaging stations. The minimum daily mean discharge is the smallest mean 

value of discharge for a specific date, computed for 

this report using data for the period beginning with 

the first complete water year of record and ending 

with water year 1998. The median daily mean 

discharge is the middle value of the arrayed (that is, 

arranged in increasing order) discharge data for a 

specific date, such that one-half of the values are 

less than the median value and one-half are greater 

than this value.

Inspection of hydrographs of daily mean 

discharge during water year 1999 and minimum 

and median daily mean discharge for the abbreviated periods of record reveals two basic patterns of 

streamflow during the water year: (1) a pattern in 

which the daily mean discharges generally fell 

within the region bounded by the graphs of minimum and daily median discharge; and (2) a pattern 

in which the daily mean discharges, particularly 

during December 1998 and late summer 1999, 

converged upon and dropped below the long-term 

minimum daily mean discharges. Water year 1999 

hydrographs for the four index gaging stations in 

New York show patterns of the first type while 

hydrographs for the five stations in Pennsylvania, 

New Jersey, and Delaware show patterns of the 

second type. The differences in patterns of low

streamflows result mainly from differences in the 

geologic environments of the drainage basins, and 

to a lesser extent, areal variations in precipitation. 

Deposits of alluvium and glacial outwash, common 

in major stream valleys of the northern Delaware 

River Basin, can store very large volumes of water. 

The gradual release of this water to stream channels and other surface-water features helped to 

moderate flows in local streams and to maintain 

baseflows during the drought period at relatively 

higher levels.

Figure 6 shows monthly mean discharges for 

Flat Brook near Flatbrookville, N.J., for the 1999 

water year, three previous drought years, and the 

period of record prior to water year 1999. At this 

gaging station, annual mean discharge was the 

lowest of record during the 1965 water year, the 

third lowest of record during the 1985 water year, 

and the fourth lowest of record during the 1932 

water year. Annual mean discharge during the 

1999 water year was the fifth lowest of record 

(table 2). Flows during the fall months in all of 

these years were unusually low. In water year 

1999, new record minimum discharges were 

recorded during December, July, and August. 

Annual mean discharges for the water years immediately preceding these four drought years do not 

show a consistent relation to mean discharge 

during the drought year. For example, water years 

1932 and 1965 followed water years in which 

mean discharge was 72 and 65 percent, respectively, of long-term annual mean discharge. In 

contrast, water years 1985 and 1999 followed 

water years in which mean discharge was 137 and 

109 percent of long-term annual mean discharge.

In addition to comparison of hydrographs, the 

distribution of streamflow can be compared 

through the use of boxplots. Boxplots are graphical 

displays that summarize the distribution of a data 

set. More importantly, boxplots can be used to 

compare selected statistical attributes of several 

data sets. These attributes are:

1. The middle value of the data set (the median 

the center line of the box)

2. The variation or spread (the inter-quartile 

range the height of the box)

3. The skewness (the quartile skew the relative 

size of box halves)
910,000

Xo en o

WATER YEAR 1965 

WATER YEAR 1999

EAST BRANCH DELAWARE RIVER AT MARGARETVILLE, NY (01413500)

g_ 10,000

o oUJ e/j£ 1,000 0.ti LLJ

CD Z

100

10

WATER YEAR 1965

WATER YEAR 1999

NEVERSINK RIVER NEAR CLARYVILLE.NY (01435000)

100,000

10,000

1,000

100

10

10,000

1,000

100

10

WATER YEAR 1965 

WATER YEAR 1999

BEAVER KILL AT COOKS FALLS, NY (01420500)

WATER YEAR 1965

WATER YEAR 1999

BUSH KILL AT SHOEMAKERS, PA (01439500)

10,000

1,000

100

10

10,000

1,000

100

10

WATER YEAR 1965

WATER YEAR 1999

WEST BRANCH DELAWARE RIVERV AT WALTON, NY (01423000)

WATER YEAR 1965 

WATER YEAR 1999

FLAT BROOK NEAR FLATBROOKVILLE, NJ (01440000)

§10,0008UJ 03 CC

1.000

OCO1 100UJO 

cc <

o w

Q 10

WATER YEAR 1995 

WATER YEAR 1999

NORTH BRANCH RANCOCAS CREEK AT PEMBERTON, NJ (01467000)

10,000

1,000

WATER YEAR 1966 

WATER YEAR 1999

WHITE CLAY CREEK NEAR NEWARK, DE (01479000)

ONDJFMAM JAS OND FM AS

10,000

1,000

100

10

1

WATER YEAR 1995 

WATER YEAR 1999

RED CLAY CREEK AT WOODDALE, DE (01480000)

ONDJFMAMJJAS

Figure 4. Daily mean discharge at index gaging stations in the Delaware River Basin during water year 1999 and water year of minimum annual mean discharge.
100,000

10,000

1,000

100

10

i 1g 10,000UJ 

0)

a:wt 1.000Bl o

CD3 100

10

<nQ 1

10,000

1,000

100

10

MEDIAN MINIMUM 

WATER YEAR 1999

EAST BRANCH DELAWARE RIVER AT 

MARGARETVILLE, NY (01413500)

NEVERSINK RIVER NEAR CLARYVILLE, NY 

(01435000)NORTH BRANCH RANCOCAS CREEK AT PEMBERTON, NJ (01467000)

100,000

10,000

1,000

100

10

10,000

1,000

100

10

1

10,000

1,000

100

10

I BEAVER KILL AT COOKS FALLS, NY 

(01420500)

BUSH KILL AT SHOEMAKERS, PA 

(01439500)

WHITE CLAY CREEK NEAR NEWARK, DE 

(01479000)

100,000

10,000

1,000

100

10

10,000

1,000

100

10

1

10,000

1,000

100

10

WEST BRANCH DELAWARE 

RIVER AT WALTON, NY (01423000)

FLAT BROOK NEAR FLATBROOKVILLE, NJ 

(01440000)RED CLAY CREEK AT WOODDALE, DE 

(01480000)

ONDJFMAMJJAS ONDJFMAMJJAS ONDJFMAMJJAS

Figure 5. Daily mean discharge at index gaging stations in the Delaware River Basin during water year 1999 and minimum and median daily mean discharges for 

period of record prior to water year 1999.
220

200

uJQ 180

O Z

CE O 160

< o

I UJ

O "5 

M cr

140

120

100

80

60

40

20

WATER YEAR 

.......... 1932

 1965

 1985

 1999

 1924-98

OCT. NOV DEC. JAN. FEB. MAR. APR. MAY JUNE JULY AUG. SEPT

Figure 6. Monthly mean discharge for four selected drought years and 

period of record prior to water year 1999, Flat Brook near 

Flatbrookville, N.J.

4. The presence or absence of unusual values 

(the outlying values).

Figure 7 shows the features of a (sample) standard boxplot. In this example and this report 

section, the data sets under consideration consist of 

values of discharge. Later, boxplots are used to 

depict distributions of ground-water levels.

In this report, because the focus is on low 

streamflows. the "whisker" above the box has been 

shortened to extend only to the last observation 

within one step beyond the end of the box. A step 

is equivalent to 1.5 times the height or interquartile range of the box. Values of discharge greater 

than 1 step above the box are intentionally not 

shown on the boxplots for the index gaging 

stations.

Boxplots of discharge for the index gaging 

stations are presented in figure 8. The boxplots 

illustrate the distribution of streamflows during 

water year 1999, the water year of lowest annual 

mean streamflow. and the period of record prior to 

water year 1999. A common characteristic of the 

boxplots is the longer upper box halves and whiskers, which indicate a typical, right-skewed distribution of the streamflow data.

Inspection of boxplots for the index-station 

data sets reveals two basic distributions of low 

streamflow: (1) a distribution in which the minimum. 25th percentile, and median 

values of discharge during water year 

1999 are measurably greater than the 

corresponding values for the water year 

of lowest annual mean streamflow: and 

(2) a distribution in which the minimum, 25th percentile. and median 

values of discharge during water year 

1999 are approximately equal to the 

corresponding values for the water year 

of lowest annual mean streamflow. 

Distributions of the first type are indicated for the four index gaging stations 

in New York whereas distributions of 

the second type are indicated for the 

five stations in Pennsylvania. New 

Jersey, and Delaware. As noted previously, differences in the distributions 

of low streamflows are primarily a 

result of differences in the geologic 

environments of the drainage basins.

Overall, the boxplots show that the distributions of low streamflows during water year 1999 in 

the Pennsylvania, New Jersey, and Delaware parts 

of the Delaware River Basin strongly resembled 

the distributions observed during the water years of 

lowest annual mean streamflows. In contrast, 

except for flows in the Neversink River near Claryville, water year 1999 streamflows in the New 

York part of the basin, although under stress, did 

not recede to levels approaching the record low 

levels measured during the drought of the 1960's.

DISCHARGE, CUBIC FEET PERSECOND IN 

r\3 -t^coen o 

0000

o

i

lnterQuartile - 

Range

-

LARGEST POINT WITHIN 1 STEP 

(1 .5 TIMES INTERQUARTILE 

RANGE) ABOVE BOX

L 75th PERCENTILE

- MEDIAN (50th PERCENTILE) 

- 25th PERCENTILE

- MINIMUM VALUE

Figure 7. Features of a standard boxplot.

11.
1,000 

o 

§ 900

co 800 

§ 700

600 -

= 500 -

400 - 

300 - 

200 

100

EASTB 

-AT MAP 

(0141C

RANCH [ 

GARETV 

500)

5ELAWAR 

ILLE, NY

J

E RIVER BEAVER KILL AT COOKS 

-FALLS, NY 

(01420500) 11,394

11,048

WESTt 

-RIVER 

(0142:

3RANCH [ 

W WALTC 

JOOO)

1,18.

)ELAWAR 

)N,NY

E 

1,593

NEVERSINK RIVER NEAR 

-CLARYVILLE, NY 

(01435000)

1999 1965 1937-98

Water Water Water

Year Year Years

(lowest

mean

annual

streamflow)

Year

1999 1965 1914-98 

Water Water Water 

Year Years 

(lowest 

mean 

annual 

streamflow)

1999 

Water 

Year

1965 1950-98 

Water Water

Year Years 

(lowest

mean

annual

streamflow)

1999 1965 1951-98

Water Water Water

Year Year Years

(lowest

mean

annual

streamflow)

Q

O

aCO

cc

HI 

0.bLU

LL

O

CO

D

O

z

1LL

1CC

I.UUU

900

800

700

600

500

400

300

200

100 

n

BUSH KILL AT

.SHOEMAKERS, PA

(01439500)

-

.

-

-

_

-

-

FLAT BROOK NEAR 

.FLATBROOKVILLE, NJ 

(01440000)

NORTH BRANCH RANCOCAS 

-CREEK AT PEMBERTON, NJ 

(01467000)

WHITE CLAY CREEK NEAR 

.NEWARK, DE 

(01479000)

1999 1965 1908-98

Water Water Water

Year Year Years

(lowest

mean

annual

streamflow)

1999 

Water 

Year

1965 1923-98 

Water Water

Year Years 

(lowest

mean

annual

streamflow)

1999 

Water 

Year

1995 1921-98 

Water Water

Year Years 

(lowest 

mean 

annual

streamflow)

1999 1966 1959-98

Water Water Water

Year Year Years

(lowest

mean

annual

streamflow)

1,000

900

RED CLAY CREEK AT 

-WOODDALE, DE 

(01480000)

800

700

600

O

55 500

400 

300 

200 

100 -

Figure 8. Streamflow distributions for index gaging stations in the

Delaware River Basin.

1999 1995 1943-98

Water Water Water

Year Year Years

(lowest

mean

annual

streamflow)

12-
Trends in streamflow during most of 1998 and 

1999 at the index gaging stations are shown in 

figure 9. The graphs depict accumulated departures 

from normal streamflow by month. A downward-sloping line during any period indicates deficient flow in that period. The 13-to-17 month 

overall periods extend from the first month of deficient streamflow in spring or summer 1998 to 

September 1999, when streamflow surpluses 

resulting from Tropical Storm Floyd were recorded 

at all the stations. The lowest point on each graph 

represents the maximum streamflow deficiency. 

For 8 of the 9 stations, the maximum deficiency 

was in August 1999.

The accumulated deficiencies are equivalent to 

the normal streamflow for periods ranging from 

0.28 to 0.55 years. The shortest periods are for 

streams in the New York part of the basin, where 

flows were supported at moderate levels by greater 

ground-water inflow, precipitation, and some 

snowmelt. The longest period is for the Red Clay 

Creek at Wooddale, Del., where streamflow was 

deficient in all months from April 1998 to August 

1999. Because they begin at arbitrary points in 

time, the graphs in figure 9 should not be interpreted to mean that the total monthly accumulated 

departures from normal streamflow must rise to 

zero deficiency before streamflows in these basins 

return to normal.

An alternative tool for evaluating low-streamflow data is the flow-duration curve, which shows 

the percentage of time during which specified 

discharges were equaled or exceeded in a given 

time period. Flow-duration curves for three index 

gaging stations, constructed on the basis of 

daily-mean discharge values, are shown in 

figure 10. These stations are in the upper, middle, 

and lower parts of the Delaware River Basin. For 

each station, three curves are shown one for 

water year 1999, one for the water year of lowest 

mean annual flow, and one for the period of record 

beginning with the first complete water year of 

record and ending with water year 1998. Curves 

for individual water years represent the distributions of streamflow in specific one-year periods 

whereas curves for the abbreviated periods of 

record represent long-term average distributions of 

flow during the periods. The flow-duration curves 

are presented to illustrate how streamflow during

water year 1999 compares with record-low and 

long-term average flow frequencies at representative gaging stations, and how low-flow characteristics differ with location. The flow-duration curves 

have not been normalized on the basis of drainage 

area or average flow and therefore, direct comparison of curves between stations is not appropriate. 

Also, because the data used to generate the curves 

are not necessarily statistically independent, estimates of return periods for low-flow events have 

not been made directly from the curves.

The distribution of low streamflows during 

water year 1999 at Beaver Kill at Cooks Falls, N.Y. 

(fig. 10A) an upper basin stream resembles the 

long-term average distribution of low flows during 

water years 1913-98. The flow-duration curve for 

water year 1999 has the same general shape as the 

curve for water year 1965 the water year of minimum annual mean flow although flows during 

water year 1965, for equivalent values of 

frequency, are considerably smaller than flows 

during water year 1999. The low-streamflow distribution during water year 1999 at Bush Kill at 

Shoemakers, Pa. (fig. 10B) a middle basin 

stream closely resembles the distribution during 

water year 1965. The flow-duration curve for water 

year 1999 differs significantly from the curve for 

the 1908-98 reference period. At some levels of 

frequency, streamflows during water year 1999 

were smaller than the corresponding flows during 

water year 1965. The duration curve of low streamflows during water year 1999 for White Clay Creek 

near Newark, Del. (fig. 10C) a lower basin 

stream lies approximately midway between the 

curves for water year 1966 and water years 

1932-98. However, water-supply withdrawals by 

the City of Newark, upstream of this gaging 

station, have a definite but unknown effect on 

low-flow frequency. Further, withdrawals were 

discontinued for 56 days between June 29 and 

September 14,1999 because of extremely low 

streamflows (Talley, 2000). These perturbations of 

low streamflow need to be considered when interpreting the low-flow frequency characteristics for 

water year 1999.

Table 3 presents summary data on low streamflows during the abbreviated periods of record and 

water year 1999. Also tabulated is the 7-day, 

10-year low-flow statistic (Q7,10), which is the

13.
10

-10

-20

-30

-40

The maximum accumulated deficiency for the 13- 

month period from August 1998 to August 1999 

is equivalent to the normal Streamflow for 0.33 years. 

Streamflow was below normal for 10 of the 13 months.

EAST BRANCH DELAWARE 

RIVER AT MARGARETVILLE, 

N.Y. (01413500)

1998 1999

5rr

. The maximum accumulated deficiency for the 13- 

. month period from August 1998 to August 1999

is equivalent to the normal Streamflow for 0.33 years.

Streamflow was below normal for 11 of the 13 months.

-5

-10

-15

-20

-25

" NEVERSINK RIVER NEAR 

. CLARYVILLE, N.Y. 

(01435000)

If* < rn

10

-10

-20

-30

-40

-50

-60

-5

-10

-15

-20

-25

The maximum accumulated deficiency for the 13- 

month period from August 1998 to August 1999 

is equivalent to the normal Streamflow for 0.28 years. 

->Streamflow was below normal for 11 of the 13 months.

BEAVER KILL AT COOKS 

FALLS, N.Y. 

(01420500)

I I I I

20

-20

-40

-60

-80

-100

The maximum accumulated deficiency for the 13- 

month period from August 1998 to August 1999 

is equivalent to the normal Streamflow for 0.38 years. 

Streamflow was below normal for 11 of the 13 months.

WEST BRANCH DELAWARE RIVER 

AT WALTON, N.Y. 

(01423000)

1998 1999

1 %&% o

1998 1999

The maximum accumulated deficiency for the 14- 

month period from July 1998 to August 1999 

is equivalent to the normal streamffow for 0.50 years. 

Streamflow was below normal for 13 of the 14 months.

FLAT BROOK AT FLATBROOKVILLE, N.J. 

(01440000)

10

 K > O C .Q

-10

-20

-30

-40

-50

The maximum accumulated deficiency for the 15- 

month period from June 1998 to August 1999 

is equivalent to the normal Streamflow for 0.53 years. 

Streamflow was below normal for 13 of the 15 months.

BUSH KILL AT SHOEMAKERS, PA. 

(01439500)

mtSJ^S!*!

1998 1999 1998 1999 1998 1999

-5

-10

-15

-20

-25

-30

The maximum accumulated deficiency for the 14- 

month period from July 1998 to August 1999 

is equivalent to the normal streamffow for 0.40 years. 

Streamflow was below normal for 13 of the 14 months;

. NORTH BRANCH RANCOCAS CREEK 

AT PEMBERTON, N.J. 

(01467000)

?ti>S.B 3 -3 ;? <» O ~3 *"- (f)

1998

i-8 S s.^Sti»S. TO vu ^: rr~ -- « _^ i n^

1999

-5

-10

-15

-20

The maximum accumulated deficiency for the 14- 

month period from July 1998 to August 1999 

is equivalent to the normal streamffow for 0.46 years. 

Streamflow was below normal for 13 of the 14 months..

 WHITE CLAY CREEK NEAR 

' NEWARK, DEL. 

(01479000)

1998

^u.2<5^-.< s*j£|Hs>£« 

1999

2

0

-2

-4

-6

-8

-10

-12

-14

The maximum accumulated deficiency for the 17- 

month period from April 1998 to August 1999 

is equivalent to the normal Streamflow for 0.55 years. 

Streamflow was below normal for 17 of the 17 months.

RED CLAY CREEK AT WOODDALE, DEL. 

(01480000)

C 3 X1 QL 3 =, 3 <D -» < in

1998 1999

Figure 9. Streamflow trends at index gaging stations in the Delaware River Basin.

14
10,000

o 

o

UJ 

CO

1,000

55 100

5

z

O 10

WATER YEARS

 1913-98

 1965 

1999

BEAVER KILL AT COOKS FALLS, NY 

(01420500) A

WATER YEARS

 1909-98 

1965

 1999

BUSH KILL AT SHOEMAKERS, PA 

(01439500) B

WATER YEARS

 1932-98

 1966

 1999

WHITE CLAY CREEK NEAR NEWARK, 

DE (01479000) C

05 2 5 102030 50 7080909598 99.5 0.5 2 5 102030 50 7080909598 99.5 0.5 2 5 102030 50 7080909598 99.5

Figure 10. Flow-duration curves for three selected index gaging stations in the Delaware River Basin.

average minimum streamflow that can be expected 

for 7 consecutive days once every 10 years. This 

statistic commonly is used by regulatory agencies to 

determine assimilation capacities and permissible 

waste-loading rates for streams. During water year 

1999. minimum daily mean flows at the four index 

gaging stations in New York came close to. but did 

not fall below, the respective Q7.10 flows. In 

contrast, minimum daily mean flows at the index 

stations in Pennsylvania, New Jersey, and Delaware 

were less than the Q7,10 flows on one or more days 

during the water year.

Despite their value in comparing streamflow 

distributions for various periods, flow-duration 

curves do not reveal the sequence of low flows or 

indicate whether the low flows occurred consecutively over several weeks or were distributed more 

uniformly throughout the year. The analysis of flow 

duration can be made more useful, however, by 

determining the flows over a given period of 

consecutive days. The results of such an analysis are 

presented in table 4, which gives summary data on 

annual n-day minimum mean streamflows during 

the 1998-99 drought and the period of record 1,7, 

30, 60, and 90-consecutive day minimum mean 

flows for the index gaging stations.

Streamflow conditions at USGS streamflowgaging stations before and after the passage 

of Tropical Storm Floyd on September 16. 

1999, are illustrated on the maps in figure 11. 

These maps were obtained from the USGS 

Real-Time Water Data website at 

http://water.usgs.gov/realtime.html.

The symbols represent discharge as a percentile. which is computed from the historical record 

for each date shown. Only stations with at least 

30 years of record are used in the computation. On 

September 14, streamflows were deficient, with 

most flows in the less than 10th percentile and the 

10th - 25th percentile ranges (fig. 11 A). By September 17, drought effects on streamflows were virtually eliminated, as runoff from heavy rain caused 

most flows to surge to record high levels (fig. 1 IB).

Reservoir Storage

Several large reservoirs have been constructed 

in the upper and lower parts of the Delaware River 

Basin. Three water-supply reservoirs in the upper 

basin Pepacton, Cannonsville, and Neversink 

are owned and operated by the City of New York. 

With a combined capacity of nearly 271 Bgal, these 

reservoirs account for about 75 percent of available 

reservoir storage in the basin. Water from these 

reservoirs is diverted to New York City and northern 

New Jersey for public supply and released into the 

Delaware and Neversink Rivers to maintain legally 

stipulated streamflows at the USGS gaging station 

at Montague, NJ.(fig. 1). The remaining 25 percent 

of storage is split about evenly between hydropower 

reservoirs in the upper part of the basin and four 

reservoirs in the lower part. The hydropower reservoirs are Lake Wallenpaupack in Pennsylvania and 

the Mongaup River system in New York. The lower 

basin reservoirs Francis E. Walter, Beltzville. 

Blue Marsh, and Nockamixon are in Pennsylvania 

and can be used to augment flows in the Lehigh, 

Schuylkill, and Delaware Rivers (fig. 1).

-15-
EXPLANATION

DAILY STREAMFLOW CONDITIONS

 NEW RECORD HIGH FOR DAY 

9 >90TH PERCENTILE 

Q 75TH-89TH PERCENTILE 

O 25TH-74TH PERCENTILE 

Q 10TH-24TH PERCENTILE 

© <10TH PERCENTILE

 NEW RECORD LOW FOR DAY

Blue Hanh r-v f '

RtSnoir- V^ Reading

A - September 14, 1999 B - September 17, 1999 

Figure 11. Streamflow conditions in the Delaware River Basin before and after Tropical Storm Floyd.

.16-
Table 3. Summary data on low streamflows during period of record prior to water year 1999 and water year 1999 for index 

gaging stations in the Delaware River Basin

[ft3/s, cubic feet per second; low-flow statistic computed from Log-Pearson distribution of streamflow data]

Period of Record Prior to Water Year 1 999 Water Year 1 999

Site USGS 

number on station Station name 

figure 1 number

1 01413500 East Branch Delaware

River at Margaretville,

N.Y.

2 01420500 Beaver Kill at

Cooks Falls, N.Y.

3 01423000 West Branch Delaware

River at Walton, N.Y.

4 01435000 Neversink River near

Claryville, N.Y.

5 01439500 Bush Kill at

Shoemakers, Pa.

6 01440000 Flat Brook near

Flatbrookville, N.J.

7 01467000 North Branch Rancocas

Creek at Pemberton, N.J.

8 01479000 White Clay Creek near

Newark, Del.

9 01480000 Red Clay Creek

at Wooddale, Del.

Drainage 

area Period of 

(square record 

miles)

163 Feb. 1937 to

current year

241 July 1913 to

current year

332 Oct. 1950 to

current year

66.6 July 1951 to

current year

117 Oct. 1908 to

current year

64.0 July 1923 to

current year

118 Sept. 1921 to

current year

89.1 Oct. 1931 to

Sept. 1936;

June 1943 to

Sept. 1957;

Oct. 1959 to

current year

47.0 Apr. 1943 to

current year

Minimum Minimum 90 

daily annual percent 

mean flow mean flow, of flows 

(ft3/s) and (ftVs) and exceed 

date water year (frVs)

6.0;

09/25/64

23;

09/14/13

13;

09/24/64

7.5;

09/25/64

2.6;

09/25/64

4.1;

09/11/66

9.0;

09/29/32

5.0;

09/10/66

4.5;

09/4/66

138; 29

1965

277; 84

1965

263; 56

1965

100; 32

1965

95.4; 27

1965

43.4; 17

1965

92.3; 62

1995

55.9; 33

1966

32.3; 19

1995

7-day, .... .... , 90 

... ' Minimum daiy Annual 10-year .. ' percent 

. ,. mean flow mean , ., OW-fOW ..o. . . .. Of fOWS

statistic (ft ft and ,"§T exceed 

(ft'/s) date (ft%) (ft'/s)

10 13; 249

10/6-7/98

36 39; 446

09/5/99

22 24; 409

09/5/99

12 15; 147

10/6-7/98

7.2 4.4; 135

08/12/99

7.5 5.5; 71

10/3-4/98 &

08/9-13/99

36 25; 125

08/3/99

15 9.3; 98.6

08/19/99

9.9 7.0; 50.1

08/25/99

29

60

40

24

8.2

8.3

39

20

14

On June 1, 1998, the three upper Delaware 

River Basin water-supply reservoirs were spilling 

(fig. 12). Beginning in July 1998, below-normal 

precipitation coupled with normal diversions to 

New York City and New Jersey and releases to 

meet the streamflow objective at Montague caused 

storage to decrease rapidly. By December 31, 

1998, the combined capacity of the reservoirs had 

decreased to 99 Bgal, or about 37 percent of total 

capacity. The declining trend continued until January 9, 1999, when the combined contents stood at 

97 Bgal, or 36 percent of capacity.

Above-normal precipitation, mainly during 

early to mid-January 1999, caused the storage situation to improve considerably and, by January 31, 

combined reservoir storage was slightly more than 

162 Bgal, or 60 percent of total capacity. Storage in 

the New York City reservoirs in the Delaware 

River Basin increased steadily during the 

late-winter to spring refill period. By late April 

1999, the combined contents reached a seasonal

high of about 250 Bgal, or 21 Bgal less than full 

capacity, and remained around that level during 

May. Unlike 1998, the reservoirs did not spill at 

any time during 1999. The seasonal pattern of 

decline began in early June, and during June and 

early July, the rate of decline was moderate. Later 

in July, as a result of decreasing inflows and 

increasing releases, diversions, and evapotranspiration, combined storage began to decrease precipitously, and, on several dates in early August, the 

rate of depletion exceeded 2 Bgal per day. During 

September 9-15, immediately preceding Tropical 

Storm Floyd, combined storage decreased at about 

1.3 Bgal/d, and, on September 15, reached a level 

of 133 Bgal, or 49 percent of total capacity. 

Drenching rains in the upper Delaware River 

Basin, delivered by Tropical Storm Fioyd on 

September 16, resulted in a very rapid increase in 

inflow to Pepacton, CannonsviHe, and Neversink 

reservoirs. During the 3-day period immediately

17.
Table 4. Summary data on annual n-day 

n-consecutive day values at index gaging

[ft3/s, cubic feet per second]

minimum streamflows during the 1998-99 drought and record low 

stations in the Delaware River Basin

Site number USGS 

on station 

figure 1 number

1 01413500

2 01420500

3 01423000

4 01435000

5 01439500

6 01440000

7 01467000

8 01479000

9 01480000

Station name

East Branch Delaware

River at Margaretville,

N.Y.

Beaver Kill at

Cooks Falls, N.Y.

West Branch Delaware

River at Walton, N.Y.

Neversink River near

Claryville, N.Y.

Bush Kill at

Shoemakers, Pa.

Flat Brook near

Flatbrookville, N.J.

North Branch Rancocas

Creek at Pemberton, N.J.

White Clay Creek near

Newark, Del.

Red Clay Creek

at Wooddale, Del.

Minimum mean flow (ft3/s) from 

Draina9 e April 1 , 1998 to September 30, 

area Period of 1999, for indicated number of 

(square record consecutive days 

miles) -......._.... .... . ..,...._....

1 7 30 60 90

163 Feb. 1937 to 13 15 20 24 31

current year

241 July 1913 to 39 43 57 66 104

current year

332 Oct. 1950 to 24 27 39 45 57

current year

66.6 July 1951 to 15 17 20 25 37

current year

117 Oct. 1908 to 4.4 4.7 7.6 8.5 15

current year

64.0 July 1923 to 5.5 5.6 7.3 9.2 12

current year

118 Sept. 1921 to 25 27 30 42 53

current year

89.1 Oct. 1931 to 9.3 10 13 22 29

Sept. 1936;

June 1943 to

Sept. 1957;

Oct. 1959 to

current year

47.0 Apr. 1943to 7.0 8.6 12 16 18

current year

Minimum mean flow (ft3/s) for 

period of record for indicated 

number of consecutive days

1 7 30 60 90

6.0 6.8 8.3 8.6 8.9

23 26 30 31 33

13 13 15 15 16

7.5 8.9 10 11 12

2.6 2.7 4.0 6.0 6.1

4.1 5.3 6.0 8.4 8.7

9.0 27 31 36 41

5.0 5.7 9.0 16 17

4.5 4.9 6.8 9.5 12

300

5 250

rr

O 01

w n 200

g

O 

o

100

50

^PJy§lN_ErjJ^Af^rjJj^T_SJ^L_LmjJJVEL__________

- ACTUAL CONTENTS

DROUGHT 

i i i

J FMAMJ J ASOND|J FMAMJ J ASOND 

1998 1999

Figure 12. Actual contents and operating curves for New York City reservoirs in the

Delaware River Basin, January 1, 1998, to December 31, 1999.

.18-
after the storm, the combined contents increased by 

nearly 15 Bgal. Subsequently, storage in the 

reservoirs increased steadily through December 31, 

1999, when the combined contents stood at about 

186 Bgal (fig. 12). Table 5 presents a monthly 

summary of total, median, and minimum combined 

storage for the New York City Delaware River 

Basin reservoirs.

The impact of the 1998-99 drought on upper 

basin reservoir storage can be gauged by comparing observed storage with storage during previous 

notable droughts. The drought of 1984-85 in the 

Delaware River Basin was selected for this 

comparison, because it had major adverse effects 

on storage in the three New York City reservoirs in 

the upper basin. The following description of this 

earlier episode is modified from Harkness and 

others (1986).

From August 1984 to April 1995. precipitation 

in the upper Delaware River Basin was below 

normal. This deficiency resulted in 

well-below-normal streamflow for all months of 

this period except December. During the critical 

reservoir-filling months of November through 

May, streamflow was about 55 percent of normal. 

From late May to August, the upper basin received 

near-normal precipitation. During this period, 

however, streamflow remained below normal, 

except in July, when 

normal flows prevailed. 

The return to normal 

streamflow conditions 

was delayed because 

much of the precipitation in May and June 

contributed to easing 

the soil-moisture deficiency in the basin. By 

September 1985, 

streamflow conditions 

were well below 

normal. At the end of 

September, Hurricane 

Gloria and a frontal 

system moving 

through the area 

combined to provide 

about 5 in. of precipitation, resulting in

above-average precipitation and streamflow for the 

month. The hydrologic effects of this precipitation 

event carried over into October.

Figure 13 shows the effects of this precipitation (and consequently, streamflow) deficiency on 

the New York City reservoirs in the Delaware 

River Basin. From June to late-November 1985, 

combined storage declined continually and rapidly, 

reaching a level of about 110 Bgal, or 41 percent of 

total capacity. From January 1985 to late September 1985, storage remained at relatively low levels 

ranging from about 118 Bgal (44 percent of capacity) in February to about 167 Bgal (62 percent of 

capacity) in June. Heavy rain in late September 

1985 and above-normal precipitation in November 

1985 resulted in substantial increases in reservoir 

storage, and. by late December 1985. combined 

storage stood at 200 Bgal, or about 74 percent of 

capacity.

Comparison of upper basin reservoir storage 

during these two drought episodes shows that 

while storage was under stress during the 1998-99 

drought, particularly during summer 1999, 

low-storage conditions during 1984-85 were far 

more severe and persistent. Although the 1998-99 

drought had a significant effect on reservoir storage, the impact was not as great as impacts on 

other major components of the hydrologic system.

LU 

CD

o: O to

sg

£ =1

o <

> o

LLJ _l

CC =!

Q CO

O

o

300

250

200

150

100

50

n I I i | i i i i i i r 

COMBINED CAPACITY AT SPILLWAY LEVEL

DROUGHT

A S

1984

O N J J 

1985

Figure 13. Actual contents and operating curves for New York City reservoirs in the 

Delaware River Basin, June 1, 1984, to December 31, 1985.

19-
Table 5. Summary data on combined storage for New York City reservoirs in the 

Delaware River Basin, June 1998 - December 1999

[Storage reported in millions of gallons; actual, median, and minimum storage are for 

first day of month]

Combined storage 

for Pepacton, _ Median combined 

Date Cannonsville, Percentage of total ^ f 

andNeversink combined storage period of record 

Reservoirs

1998

June

July

Aug.

Sept.

Oct.

Nov.

Dec.

1999

Jan.

Feb.

Mar.

Apr.

May

June

July

Aug.

Sept.

Oct.

Nov.

Dec.

269,471

273,336

259,382

214,193

176,843

147,984

119,682

98,925

163,562

181,890

221,935

248,634

250,711

228,163

197,366

151,275

156,677

165,217

177,092

99

101

96

79

65

55

44

37

60

67

82

92

93

84

73

56

58

61

65

269,782

256,975

230,241

201,946

179,533

145,485

166,770

189,563

214,266

220,839

259,789

271,282

269,782

256,975

230,241

201,946

179,533

145,485

166,770

Percentage of 

median 

storage

100

106

113

106

99

102

72

52

76

82

85

92

93

89

86

75

87

114

106

Minimum combined 

storage for 

period of record

166,807

159,464

148,255

134,868

98,488

94,027

84,721

88,645

68,566

129,370

147,865

168,082

166,807

159,464

148,255

134,868

98,488

94,027

84,721

In summer 1999, supplemental releases of 

water from Beltzville, Blue Marsh, and Francis E. 

Walter Reservoirs were directed by the Delaware 

River Basin Commission, in order to meet flow 

objectives for the Delaware River at Trenton, N.J. 

and to control the upstream migration of brackish 

water in the Delaware estuary. Flow-augmentation 

releases were made from June 28 to September 15, 

and total directed-release quantities are as follows: 

2.5 Bgal from Beltzville Reservoir; 2.0 Bgal from 

Blue Marsh Reservoir; and 0.61 Bgal from Francis 

E. Walter Reservoir. In addition to the directed 

releases, conservation releases totaling about 

8 Bgal were made from these reservoirs for 

flow-augmentation and habitat-protection 

purposes.

Ground-Water Occurrence and Levels

Ground water in the Delaware River Basin is 

stored in, and moves through, water-bearing sediments and rocks called aquifers. The dry-weather 

flow of most streams in the basin is sustained by 

ground-water discharge (baseflow) from aquifers. 

Where aquifers with large capacity to store and 

transmit water are present, baseflow is sustained 

even through prolonged dry periods; where aquifers with limited capacity to store and transmit 

water are present, baseflow decreases fairly rapidly 

and may eventually cease, causing streams to dry 

up during rainless periods.

Ground-water levels are an important barometer of the state of ground-water systems and are 

used by many water-management agencies as a 

primary indicator of drought conditions. Fluctuations of ground-water levels are caused by imbalances in recharge to and discharge from the 

systems. In natural ground-water systems, the 

recharge-discharge regimen produces an annual 

cycle of fluctuations in which water levels typically 

are highest in spring and lowest in fall. Information

.20
on ground-water levels and fluctuations is needed 

to evaluate the status of ground-water supplies and 

to detect any changes or trends in those supplies. In 

this report, the term "ground-water level" refers to 

the depth to the water table or potentiometric 

surface, in feet (ft) below the land surface.

The USGS, in cooperation with various state 

and local agencies, operates continuing programs 

of observation-well networks in the Delaware 

River Basin. These networks are designed to 

monitor fluctuations of ground-water levels, which 

are controlled by geologic, climatic, and 

hydrologic factors, and by human activities. The 

networks provide basic information on how water 

levels respond to rainfall, ground-water pumping, 

and other stresses. Because of the relatively large 

number and long record length of USGS 

observation wells in the Pennsylvania part of the 

basin, water-level data for those wells are used in 

this report as the basis for evaluating the impact of 

the 1998-99 drought on ground-water systems. 

Figure 14 shows the location of these wells.

Drought Effects on Ground-Water Levels

The 1998-99 drought in the Delaware River 

Basin had a major effect on ground-water levels. 

In fact, many members of the water-resources 

community referred to this drought as a 

"ground-water drought." Although ground-water 

supplies generally were not deficient, the 

combination of below-normal recharge and 

elevated rates of evapotranspiration produced 

abnormal water-level declines and record or 

near-record low ground-water levels in much of 

the basin.

In the discussion and illustrations that follow, 

"normal" status signifies a mean water level 

between the 25th percentile and the 75th percentile of monthly mean water levels for all prior 

years of record for the month indicated. "Below 

normal" status signifies a mean water level less 

than the 25th percentile of historical monthly 

mean water levels. "Above normal" status signifies a mean water level greater than the 75th 

percentile of historical monthly mean water 

levels.

During the drought period, ground-water 

conditions elsewhere in the basin were generally 

similar to those in the Pennsylvania part. The 

general pattern of change in ground-water levels 

was one of decline from May to December 1998, 

and much less than the usual recovery in winter. 

Consequently, ground-water levels were below 

normal after winter 1998-99 and at the beginning 

of the 1999 growing season, and they declined 

steadily through summer 1999. In the Pennsylvania 

part of the basin, many new record monthly low 

levels were set in USGS network observation wells 

during 1999. In early January 1999, the lowest 

water levels of record, regardless of month, were 

measured in observation wells in Bucks and 

Schuylkill Counties. In early September 1999, the 

lowest water level of record was measured in a 

network well in Berks County. Water levels in 

these three wells have been measured continuously 

since 1975. Water levels in several other 

observation wells came very close to, but did not 

decline below, the lowest water levels of record 

(table 6). In neighboring Delaware, the lack of 

recharge resulted in near-record low ground-water 

levels throughout the state (Talley, 2000).

PENNSYLVANIA

LOCATION OF MAPPED AREA

EXPLANATION

fc296 LOCATION AND COUNTY 

9 IDENTIFICATION NUMBER 

OF OBSERVATION WELL

PHILADELPHIA

DELAWARE

Figure 14. Location of selected observation wells 

operated by the U.S. Geological Survey in the 

Pennsylvania part of the Delaware River Basin.

21
The hydrographs in figure 15 show water 

levels during 1995-99 in three USGS observation 

wells in the Pennsylvania part of the Delaware 

River Basin. For each well, the observed water 

levels are compared with the average of daily low 

water levels in the well during the period of continuous record prior to 1995. Hydrographs for 

1995-97 are included to illustrate antecedent 

conditions. During the period January 1998 to 

December 1999, water levels in the Monroe 

County and Chester County wells were below the 

long-term average low for 12 of 24 months. In 

contrast, water levels in the Berks County well 

were below the long-term average low for 21 of 24 

months. Even during the 1998-99 winter-to-spring 

recovery period and after the passage of Tropical 

Storm Floyd in mid-September 1999, water levels 

in the Berks County well did not rise above the 

long-term average low level.

The severity of the 1998-99 drought can be 

assessed by comparing observed ground-water 

levels with water levels measured during droughts 

of known severity. The 1961-67 event was the most 

severe drought ever recorded in the Delaware River 

Basin. Thus, ground-water levels measured during 

this historical episode provide a reference frame 

against which the effects of the 1998-99 drought 

can be measured. Water-level records for USGS 

observation well CH-10 in Chester County, Pennsylvania were selected for comparison because

records for this well date back to 1951 and water 

levels have been relatively unaffected by human 

activities during the entire period of record.

Hydrographs of water levels for observation 

well CH-10 for 1961-63 and 1997-99 are shown in 

figure 16. These periods were selected as the timeframes for the comparison because they include the 

intervals when ground-water levels declined to 

their lowest points during each drought. Hydrographs of water levels during 1963 and 1999 - the 

years with the greatest declines - are aligned on a 

common time scale to facilitate comparison of the 

lowest water levels during each drought.

In 1963, ground-water levels began the 

seasonal cycle of decline in March, decreased to a 

record low level of 16.22 ft below land surface in 

November 1963, and did not begin to recover 

substantially until January 1964. Water levels 

remained at about 16 ft below land surface during 

all of August, September, and October. In 1999, 

ground-water levels also began the seasonal cycle 

of decline in March, although water levels at the 

beginning of this cycle were about 2 ft higher than 

the corresponding levels in 1963. The decline 

continued steadily until September 1999 and was 

followed by a rapid recovery because of recharge 

from Tropical Storm Floyd. Water levels remained 

at their lowest point 15.27 ft below land 

surface for 2 days in August 1999. In addition to 

these differences in lowest water levels, 

ground-water levels remained at very low levels for

Table 6. Comparison of record low ground-water levels with low levels during the 1998-99 drought 

[Water levels reported in feet below land surface; well locations shown on figure 14]

Pennsylvania 

county

Berks

Bucks

Carbon

Chester

Lebanon

Lehigh

Monroe

Schuylkill

Wayne

USGS 

well 

number

BE-623

BK-1020

CB-104

CH-10

LB-372

LE-644

MO- 190

SC-296

WN-64

Year 

record 

began

1975

1975

1969

1951

1973

1971

1967

1975

1967

Lowest 

water level 

of record

141.20

41.68

90.58

16.22

11.32

93.42

16.87

56.03

32.98

Date of 

record low 

water level

9/5/99

1/2,3/99

1/31/81

11/3/63

1/23,30/81

2/6/71

10/24,25/80

1/1,2/99

11/9,10,11/91

Lowest water level 

measured during 

July 1998to 

December 1999

141.20

41.68

82.85

15.27

11.20

82.84

16.58

56.03

30.26

Date of lowest 

water level during 

July 1998to 

December 1 999

9/5/99

1/2,3/99

10/10/98

8/25,26/99

9/5/99

9/5/99

9/15/99

1/1,2/99

9/14,15,16/99

.22
115rTTT

Mean daily

low water level,

1968-94

10.79 feet

Figure 15. Ground-water levels in three 

observation wells in the Delaware River 

Basin in Pennsylvania, 1995-99.

1995 1996 1997 1998 I 1999

1961 . 1962 . 1963 

J FMAMJ JASON DJ FMAMJ JASON DJ FMAMJ JASOND

17

JFMAMJJASONDIJFMAMJJASONDlJFMAMJJASOND 

1997 1998 1999

Figure 16. Ground-water levels in observation well CH-10, Chester County, 

Pennsylvania, 1961-63 and 1997-99.

23.
a considerably longer period in 1963 than in 1999. 

Rates of water-level decline and recovery, however, 

were lower in 1963 than in 1999 (fig. 16).

Ground-water levels measured during the two 

droughts also can be compared through the use of 

boxplots. Figure 17 presents a comparison of 

boxplots of water levels in well CH-10 during 1963 

and 1999. Fewer water-level data were used in the 

preparation of the boxplot for 1963, because at that 

time, water levels were measured manually about 

two times per month. These water levels are 

considered instantaneous readings and do not 

necessarily represent the maximum (lowest) water 

level on the day of the measurement. In contrast, 

water levels were recorded on a continuous basis 

during 1999, and a data set consisting of daily 

maximum water levels was used to prepare the 

boxplot. Despite the differences in the number of 

data points, the boxplots can be used in a general 

way to compare ground-water levels during these 

two drought years.

Although ground-water levels were below 

normal in 1999, they did not exceed the historical 

low levels set in 1963. In 1963, 25 percent of all 

measured ground-water levels were above a level 

of about 14 ft; in 1999, 75 percent of ground-water 

levels were above 1.4 ft. Further, the median 

ground-water level for 1963 15.20 ft is 

approximately equal to the lowest level 

15.27 ft measured during all of 1999.

In terms of ground-water effects, the drought 

of 1998-99 had a major impact on ground-water 

levels in the Delaware River Basin, but it was not 

as great as the impact during the drought of the 

1960's. This is not unexpected, however, as severe 

depletion of ground-water reservoirs, with their 

large storage capacity, is unlikely during droughts 

of relatively short duration.

The areal extent and propagation of drought 

effects is illustrated by the series of monthly 

ground-water conditions maps presented in 

figure 18. These maps show the status of monthly 

mean ground-water levels in USGS observation 

wells in the Pennsylvania part of the Delaware 

River Basin. Maps showing ground-water conditions during June and December 1998, and March, 

June, August, and October 1999 are presented to 

illustrate the general patterns of change in 

ground-water levels before and during the drought, 

and after the passage of Tropical Storm Floyd. 

Monthly maps and water-level data for the period 

January 1997 to current month, including the 

months not shown on figure 18, are available on the 

USGS Pennsylvania District home page at 

http://pa.water.usgs.gov/gw_report.

In June 1998, ground-water levels in 12 of the 

13 USGS observation wells were in the normal to 

above-normal range. The water level in the Montgomery County observation well set a new record 

high for the month, although water levels in this

10 '

11 

LU

o

_r UL 12 

LLJ DC

> D

LU CO

-1 Q

S | 13 ~

| §

 O

i sj 14 ~

ID CD

O (-

QC LU 0 uj 15 _

Z

16 

 17 

b

^-

^^

Minimum level

/

 12.74

d^

13.90

 10.56

h 11.96

 12.41

-i- 14.08

15.20 _

 15.27

15.92 ^ 

^16.22 /

^^^- Maximum level

1963 1999

Figure 17. Distribution of ground-water levels in observation well CH-10, 

Chester County, Pennsylvania, 1963 and 1999.

.24
JUNE 1998

EXPLANATION

ABOVE NORMAL

NORMALBELOW NORMAL

NO DATA

/\ RECORD HIGH 

\/ RECORD LOW

Normal is defined as a water 

level between the 25th percentile and 75th percentile 

of monthly mean water levels 

for all previous years of record for this month.

Figure 18. Ground-water levels in observation wells in the Pennsylvania part of the Delaware River Basin for six selected months during the 1998-99 drought.
well have been rising steadily (an increasing trend) 

since the early 1990's in response to a reduction in 

ground-water pumping in the surrounding area. 

The water level in the Schuylkill County well was 

below normal during the month.

By December 1998, the status of ground-water 

levels had changed dramatically, and levels in 11 

observation wells were in the below-normal range. 

Water levels in observation wells in Bucks 

(2 wells), Carbon, Delaware, Pike, and Schuylkill 

Counties set new record lows for the month.

In March 1999, which roughly marked the end 

of the winter ground-water recharge period and the 

beginning of the growing season, ground-water 

levels were in the normal to above-normal range in 

5 observation wells and in the below-normal range 

in 8 wells. New record monthly lows were set in 

wells in Bucks (1 well), Delaware, Pike, and 

Wayne Counties.

Ground-water levels during June 1999 reflect 

deepening and spreading drought conditions. 

Except for normal to above-normal water levels in 

3 wells in southeastern Pennsylvania, groundwater levels were below normal in the rest of the 

Pennsylvania part of the Delaware River Basin. 

Water levels in 6 observation wells 3 in northeastern Pennsylvania and 3 in southeastern Pennsylvania set new record lows for the month.

August 1999 was the last full month of drought 

conditions, as rain associated with Tropical Storm 

Floyd arrived in the Delaware River Basin on 

September 16. During August, ground-water levels 

in 12 of 13 observation wells were in the

below-normal range. The lowest ground-water 

levels since July 1998 were recorded in observation 

well CH-10 (Chester County) on August 25 and 

26, 1999, and in early to mid-September in five 

other USGS observation wells (table 6). The lowest 

water levels of record in wells BK-1020 and 

SC-296 (Schuylkill County), regardless of month, 

were measured on January 2 and 3, 1999 and January 1 and 2, 1999, respectively. The lowest water 

level of record in well BE-623 (Berks County), was 

measured on September 5,1999. Records have 

been kept on these three wells since 1975.

By October 1999, rain from Tropical Storm 

Floyd replenished soil moisture and recharged 

ground-water reservoirs, and ground-water levels 

recovered significantly from their pre-storm levels. 

Although below-normal water levels persisted in 

several areas, water levels generally were in the 

normal to above-normal range throughout the 

Delaware River Basin.

INSTITUTIONAL RESPONSE TO THE 

1998-99 DROUGHT

Although this report is concerned with the 

hydrologic aspects of the 1998-99 drought in the 

Delaware River Basin, the following chronology is 

included to document the major institutional and 

management actions taken in response to the 

drought.

.26
Date, 1999 Action

June 10 Pennsylvania declares a drought warning in 47 counties, including all counties in the Delaware River Basin.

June 28 Delaware River Basin Commission (DRBC) begins directing releases from lower basin reservoirs in Pennsylvania.

July 14 DRBC requests use of one-half of the excess release quantity stipulated for the New York 

City Delaware River Basin reservoirs, to help meet the Trenton flow objective. DRBC 

develops the release schedule and the Delaware River Master administers the program.

July 20 Pennsylvania declares a drought emergency in 55 counties, including all counties in the Delaware River Basin.

July 21 DRBC and Parties to the 1954 Supreme Court Decree agree to reduce the Trenton flow objective to 2,700 ftVs, pending subsequent approval and ratification by the DRBC Commissioners.

July 23 Delaware declares a drought warning and urges voluntary water conservation.

August 5 New Jersey declares a statewide water emergency with mandatory restrictions on non-essential outdoor water use.

August 5 Delaware declares a drought emergency for the region north of the Chesapeake and Delaware Canal.

August 5 Army Corps of Engineers begins storing additional water in Francis E. Walter Reservoir. (Primary purpose of reservoir is flood control and recreation).

August 10 New York State issues a drought warning for counties in the Delaware River Basin.

August 16 Delaware River Master informs DRBC that all available excess release quantity has been 

used for meeting flow needs of the lower basin.

August 18 DRBC approves Resolution 99-18, which supports the drought actions of the basin states, 

authorizes use of water in power reservoirs and Nockamixon Reservoir to meet [lower 

basin] flow objectives, and allows DRBC to request contingency plans from users in excess 

of 1 Mgal/d if requested by a basin state.

September 16,17 Tropical Storm Floyd delivers heavy rain to the Delaware River Basin.

Source: Modified from Delaware River Basin Commission, written communication, March 31, 2000.

27.
SUMMARY

Unusually dry conditions prevailed in the 

Middle Atlantic region from July 1998 to September 1999. In the Delaware River Basin, as in other 

parts of the region, surface and subsurface components of the hydrologic system showed a pronounced response to the precipitation deficiency. 

This report documents the precipitation deficiency 

associated with the drought and examines hydrologic system response namely, reductions in 

streamflow and reservoir storage, and declines in 

ground-water levels to the shortfall.

The 1998-99 drought in the Delaware River 

Basin was caused by anomalous patterns in the 

general atmospheric circulation. Displacement of 

the jet stream and redirection of moisture from 

Gulf and Atlantic subtropical sources were major 

factors in diverting moisture away from the basin.

The deficient precipitation trend began in July 

1998 and, with few exceptions, monthly totals 

were below normal until September 1999. As a 

result of these many shortfalls, the accumulated 

precipitation deficiency increased quickly. By 

August 1999, precipitation during the previous 

12-month period in the part of the basin above 

Trenton, N.J. was more than 12 in. below normal. 

The precipitation deficiency also was reflected in 

below-normal snowpack during winter 1998-99. 

The drought was broken in mid-September 1999, 

when Tropical Storm Floyd delivered about 5 in. of 

rain to the northern part of the basin and about 

10 in. to the southern part. Despite these heavy 

rains, 12-previous month accumulated precipitation totals remained in the deficient range for at 

least 7 months after the storm. Thus, future 

weather conditions will determine whether this 

storm eliminated the drought or merely interrupted 

a longer term episode.

Discharge records for nine index gaging 

stations operated by the U.S. Geological Survey 

were selected for analysis of drought-affected low 

streamflows. These stations were chosen because 

of their distributed location and because they 

generally are on unregulated or minimally regulated streams. Various visual and statistical techniques, including hydrograph and boxplot 

comparisons, graphs of trends, flow-duration 

curves, and daily streamflow-conditions maps,

were used to analyze data on low streamflows. 

These analyses show that the 1998-99 drought in 

the Delaware River Basin was one of the most 

severe short-term droughts of the 20th century. In 

1999, streamflows receded to minimum levels in 

late summer instead of the more typical early fall 

timeframe. Extreme, persistent heat in summer 

1999 compounded the effects of the drought on 

low streamflows.

Three water-supply reservoirs in the upper 

Delaware River Basin, owned and operated by the 

City of New York, are used in-part to maintain 

legally stipulated flow rates in the Delaware River 

at Montague, N.J. These reservoirs have a 

combined capacity of nearly 271 Bgal and account 

for about 75 percent of available reservoir storage 

in the basin. From July 1998 to January 1999, storage in these reservoirs declined rapidly as a result 

of below-normal precipitation coupled with normal 

diversions and releases. The minimum storage 

level during the drought was reached on January 9, 

1999, when combined contents stood at 97 Bgal or 

36 percent of total capacity. On September 15, 

1999, immediately before Tropical Storm Floyd 

passed over the basin, combined contents stood at 

133 Bgal, or 49 percent of total capacity. On 

several days in early August 1999, at the height of 

the drought, depletion of these reservoirs exceeded 

2 Bgal/d.

The 1998-99 drought in the Delaware River 

Basin also had a major effect on ground-water 

levels. Below-normal recharge and elevated rates 

of evapotranspiration combined to produce abnormal water-level declines and record or near-record 

low ground-water levels in much of the basin. 

Records for USGS network observation wells in 

the Pennsylvania part of the basin were used as an 

indicator of drought impacts on ground-water 

levels. In 1999, many new record monthly low 

levels were reached in network wells, and the 

lowest water levels of record, regardless of month, 

were recorded in three network wells. Although 

ground-water levels were below normal in 1999, 

limited evidence suggests they did not exceed the 

historical low levels set in the early 1960's. By 

October 1999, rain from Tropical Storm Floyd 

replenished soil moisture and recharged 

ground-water reservoirs, and ground-water levels 

recovered significantly from their pre-storm levels.

.28
Although below-normal water levels persisted in 

several areas, water levels generally were in the 

normal to above-normal range throughout the 

Delaware River Basin.

REFERENCES CITED

Barksdale, H.C., O'Bryan, Deric, and Schneider, 

W.J., 1966, Effect of drought on water 

resources in the northeast: U.S. Geological 

Survey Hydrologic Atlas 243, 1 sheet.

Changnon, S.A., 1987, Detecting drought

conditions in Illinois: Illinois State Water 

Survey Circular 164-87, 36 p.

Hare, F.K., 1987, Drought and desiccation Twin 

hazards of a variable climate, in Whilhite, 

David, Eastering, William, and Wood, 

David, eds., Planning for drought: Boulder, 

Colo., Westview Press, p. 3-9.

Harkness, W.E., Lins, H.F., and Alley, W.M., 1986, 

Drought in the Delaware River Basin,

1984-85, in National Water Summary

1985 Hydrologic events and surface-water 

resources: U.S. Geological Survey 

Water-Supply Paper 2300, p. 29-34.

Klemes, V.E., 1987, Drought prediction A

hydrologic perspective, in Whilhite, David, 

Eastering, William, and Wood, David, eds., 

Planning for drought: Boulder, Colo., 

Westview Press, p. 81-94.

Namias, Jerome, 1985, Hydrologic aspects of 

drought, in Beran, M.A., and Rodier, J.A., 

eds., UNESCO-World Meteorological 

Organization, A Contribution to the 

International Hydrologic Programme: Paris, 

France, p. 27-64.

Water Resources Council, 1966, Drought in 

northeastern United States, a third 

appraisal: Water Resources Council Report 

to the President, 22 p.

Subrahmanyam, V.P., 1967, Incidence and Spread 

of'Continental drought: World 

Meteorological Organization/International 

Hydrological Decade Report no. 2, 51 p.

Talley, J.H., 2000, The drought and flood of 1999, 

in First State Geology: Delaware 

Geological Survey, vol. 18, no.l, p. 1-2.

29.
Paulachok, G.N., Krejmas, B.E., Hydrologic Aspects of the 1998-99 Drought in the Delaware River Basin US

and Soden, H.L.