Prepared in cooperation with the 

New York State Department of Environmental Conservation

Changes in Low-Flow Frequency from 1976–2006 at 

Selected Sites in New York, Excluding Long Island

Scientific Investigations Report 2011–5112

U.S. Department of the Interior

U.S. Geological Survey
Cover. Schroon River, Riverbank, NY, looking downstream from bridge near U.S. Geological Survey streamgage.
Changes in Low-Flow Frequency from 

1976–2006 at Selected Streamgages 

in New York, Excluding Long Island

By Thomas P. Suro and Christopher L. Gazoorian

Prepared in cooperation with the 

New York State Department of Environmental Conservation

Scientific Investigations Report 2011–5112

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2011

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Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Suro, T.P., and Gazoorian, C.L., 2011, Changes in low-flow frequency from 1976-2006 at selected streamgages in 

New York, excluding Long Island: U.S. Geological Survey Scientific Investigations Report 2011–5112, 21 p., at

http://pubs.usgs.gov/sir/2011/5112.
iii

Contents

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

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

Purpose and Scope ..............................................................................................................................2

Basins Studied................................................................................................................................................4

Selection of Streamgages............................................................................................................................4

Precipitation....................................................................................................................................................5

Land Use..........................................................................................................................................................5

Frequency Analysis .......................................................................................................................................6

Data 1976–2006 (31-year period) ......................................................................................................10

Data for the Entire Period of Record ...............................................................................................10

Trends ...........................................................................................................................................................10

Minimum Annual 7-Day Low Flow ...................................................................................................10

Precipitation.........................................................................................................................................11

Groundwater........................................................................................................................................11

Summary........................................................................................................................................................20

Selected References...................................................................................................................................21

Tables

1. Northeast Regional Climate Center station name, number, location, elevation, and

30-year average annual precipitation (1961–1990) for climate records analyzed in

this study ........................................................................................................................................6

2. Consolidation classes for U.S. Geological Survey (USGS) Geographic Information

Retrieval and Analysis System (GIRAS) and National Land Cover Database 2001

(NLCD 2001) classes used in this study.....................................................................................7

Figures

1. Map showing locations of physiographic provinces of New York State 

drainage basins, U.S. Geological Survey (USGS) streamgages, Northeast 

Regional Climate Center (NRCC) precipitation stations, and groundwater wells 

referred to in this study................................................................................................................3

2. Graphs showing minumum annual 7-day low flow discharges and Kendall tau

trend for eight streamgages in New York State ....................................................................12

3. Graphs showing annual precipitation from 1961 to 2006 and Kendall tau trend at

selected Northeast Regional Climate Center precipitation stations in New York ...........14

4. Graphs showing departure of total annual precipitation at selected Northeast

Regional Climate Center stations in New York State, from the 30-year average

(1961–90), in inches ....................................................................................................................16

5. Thirty-year average (1976–2006) hydrographs of weekly water levels at selected 

U.S. Geological Survey (USGS) groundwater observation wells in New York.................18
iv

3. Indicated percent change in land use or land cover from 1970 through 2003 as a 

percent of drainage basins for eight streamgages in New York (N.Y.)................................8

4. Estimated 7-day, 2-year and 10-year low-flow frequency discharges for eight

streamgages in New York (N.Y.) computed from log-Pearson type III analysis 

and compared with estimated low-flow frequency discharges published in 

Bulletin 74.......................................................................................................................................9

5. Selected data for three observation wells in the U.S. Geological Survey

observation-well network in New York State ........................................................................19

Conversion Factors, Datum, and Acronyms

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2

) 2.590 square kilometer (km2) 

Flow rate

cubic foot per second (ft3

/s) 0.02832 cubic meter per second (m3/s)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C=(°F-32)/1.8

Vertical coordinate information is referenced to North American Vertical Datum of 1988 

(NAVD 88). 

Horizontal coordinate information is referenced to North American Datum of 1983 (NAD 83).

Elevation, as used in this report, refers to distance above the vertical datum.
v

Acronyms Used in Report

CLIMOD Climate Information for Management and Operational Decisions

EROS Earth Resource Observation Systems

GIRAS USGS Geographic Information Retrieval and Analysis System

MA7CD2 or 7Q2 Minimum average 7-day 2-year discharge

MA7CD10 or 7Q10 Minimum average 7-day 10-year discharge

NEGA-RASA Northeast Glacial Aquifers Regional Aquifer-System Analysis project

NLCD National Land Cover Database

NRCC Northeast Regional Climate Center

NYSDEC New York State Department of Environmental Conservation

TMDL Total maximum daily loads

USGS U.S. Geological Survey

WMO World Meteorological Organization
vi This page has been left blank intentionally.
Changes in Low-Flow Frequency 1976–2006 at Selected 

Streamgages in New York, Excluding Long Island

By Thomas P. Suro and Christopher L. Gazoorian

Abstract

Many Federal, State, and local agencies use low-flow 

data to establish water-use policy and help determine the total 

maximum daily loads and effluent limits of point and nonpoint 

sources of contamination of surface water during periods of 

decreased streamflow. Low-flow magnitude and frequency are 

used often by water-supply planners, reservoir managers, and 

hydroelectric facilities to manage water availability for supply 

and power generation. 

Low-flow statistics for eight selected U.S. Geological 

Survey streamgages in New York State were calculated for the 

period from 1976 through 2006 and for the entire period of 

continuous streamflow record. The 7-day, 2-year and 10-year 

low flows were computed and compared with those low flows 

published in the1979 U.S. Geological Survey report, Lowflow frequency analysis of streams in New York, Bulletin 74. 

Observed changes in low-flow frequency at each gage were 

then examined and compared to changes in precipitation and 

land use to determine whether a relation between similar 

patterns could be identified.

A statewide U.S. Geological Survey study has not been 

done to develop equations for estimating low flows on rural 

unregulated streams in New York. Currently (2010) only one 

regional study developed for parts of the lower Hudson River 

Basin in 1986 is available to assist in estimating low flows on 

rural streams with unregulated streamflow in New York. Lowflow statistics published in the 1979 report need to be updated 

by using additional data collected since 1976 to determine 

current low-flow conditions across New York State. 

At-site low-flow statistics were updated for eight 

streamgages in New York by using continuous daily 

streamflow data through 2006 for the future development of 

a statewide research study. Selection of the eight streamgages 

used in this study identified a major deficiency in the number 

of available unregulated long-term U.S. Geological Survey 

streamgages needed for the development of regional low-flow 

equations in New York. A limited analysis of the changes 

in land use for the contributing drainage areas for each 

streamgage, changes in precipitation, and trends in the annual 

7-day minimum flow also are presented. The 7-day, 2-year 

low flow showed increases of 14 to 35 percent and the 7-day 

10-year low flow showed zero to 19 percent increases at rural 

streamgages with unregulated streamflows when statistics 

were computed by using data from 1976 through 2006 and 

compared with published data in Bulletin 74. When the entire 

period of record was used to compute low flow frequencies, 

the 7-day, 2-year low flows increased from about 6 to 

15 percent whereas the 7-day 10-year low flows showed zero 

to 5 percent increases. Streamgages affected by urbanization 

and regulation for water supply showed the most significant 

changes in the 7-day, 2-year and 10-year low-flow frequencies. 

These streamgages are included to help identify the effects of 

urbanization and regulation on streamflow at these locations. 

The 7-day 10-year low flow increased by 65 percent at the 

U.S. Geological Survey streamgage Hackensack River at West 

Nyack, N.Y., and increased 120 percent at the U.S. Geological 

Survey streamgage Neversink River at Godeffroy, N.Y., when 

statistics were computed by using data from 1976 through 

2006 and compared with the statistics for the regulated period 

computed in Bulletin 74. 

Introduction

Severe drought conditions in the southeastern United 

States have emphasized the need for improved estimates of 

low-flow frequency and magnitude. Drought watches and 

warnings have been issued for the New York City water supply 

three times since 1989. A Stage III Emergency was declared in 

1989 and the Chelsea Pumping Station was activated to draw 

water from the Hudson River (New York City Department of 

Environmental Protection, 2007). During the fall of 1995 the 

water supply in the New York City reservoir system dropped 

to 51 percent of capacity (Lumia, 1998). A Stage I Emergency 

was declared in 2002 when the supply system dropped to 

41.5 percent of storage capacity. 

An extended period of low flows on the Hudson River 

in 1995, 1999, and 2001, combined with other factors that 

allowed salt water to move upstream to Poughkeepsie more 

than 73 miles upstream from the mouth of the Hudson River, 

affecting drinking-water withdrawals from the river. Many 
2 Changes in Low-Flow Frequency 1976–2006 at Selected Streamgages in New York, Excluding Long Island

Federal, State and local permitting and regulatory agencies 

use low-flow data to establish policy on withdrawals from 

and effluent discharge into streams. These data also are vital 

for determining the total maximum daily loads (TMDL) and 

effluent limits of point and nonpoint sources of contamination 

of surface water during sustained periods of decreased 

streamflow. Water-supply planners and managers of reservoirs 

and hydroelectric facilities often use low-flow statistics to 

manage water availability for supply and generating power.

The U.S. Geological Survey (USGS) has been 

collecting surface-water data in New York State for more 

than 100 years, but the systematic use of low-flow partialrecord stations did not begin until the 1950s, and increased 

in number as a result of the drought during the 1960s. The 

USGS, in cooperation with the New York Department 

of Environmental Conservation (NYSDEC) completed a 

study in 1979 that computed at-site low-flow statistics for 

154 streamgages (146 in New York State) with unregulated 

streamflow and 124 streamgages (122 in New York State) 

with regulated streamflow in New York and the surrounding 

area (Eissler, 1979). This study also computed low-flow 

statistics for 648 low-flow partial-record stations in New 

York and the surrounding area. Low-flow statistics for the 

streamgages published in the 1979 report were computed 

directly from approved daily streamflow data, but the period 

of available record for many of these streamgages was less 

than 20 years, ranging from the mid-1950s (or early 1960s) 

through 1975. Since 1976, many of these streamgages have 

been discontinued, and the network of low-flow partial-record 

stations no longer exists. However, many streamgages that 

have been in continuous operation since the 1979 report now 

have an additional 31 years of data (through 2006) that if 

analyzed could provide updated low-flow statistics. Low-flow 

statistics published in the1979 report need to be updated by 

using these additional data since 1976 to determine current 

(2010) low-flow conditions throughout New York State.

The USGS has not done a statewide study to develop 

equations for estimating low flows on rural unregulated 

streams in New York. Only one outdated regional study is 

available for use in estimating low flows on rural streams 

with unregulated streamflow in New York. Barnes (1986) 

developed a method for estimating low-flow statistics for 

ungaged streams in parts of the Lower Hudson River basin. 

This method related the 7-day, 2-year and 10-year low flows 

to selected basin characteristics by multiple-regression 

analysis for 53 continuous and partial-record streamgages 

having watershed areas of less than 100 square miles (mi2

). A 

study of low flows in the Susquehanna River basin (Randall, 

2010) was done in the mid-1980s as part of the Northeast 

Glacial Aquifers Regional Aquifer-System Analysis project 

(NEGA-RASA). This study updated a previous study of 

the Susquehanna River basin (Ku and others, 1975). The 

1975 report presented at-site determinations of low-flow 

statistics but did not include methods for estimating lowflow magnitude and frequency at ungaged streams in the 

Susquehanna River basin.

A thorough analysis of the USGS streamgage network in 

New York State would help determine the effectiveness of the 

current (2010) network and whether the network is sufficient 

to adequately define regional equations to estimate low 

flows at rural unregulated streams in New York. A network 

analysis can help determine individual station contributions 

to defining regional low-flow equations and predicting lowflow statistics at ungaged streams. These data would help to 

determine whether the current network should be expanded 

by reactivating discontinued streamgages or partial-record 

stations that could provide more valuable data and improve the 

accuracy of regional low-flow equations.

In cooperation with NYSDEC, the USGS has updated 

at-site low-flow statistics for eight streamgages in New 

York by using continuous daily streamflow data through 

2006 (fig. 1). This report presents a limited analysis of the 

changes in land use for the contributing drainage areas for 

each streamgage; the changes in precipitation and the trends 

in the annual 7-day minimum flow are presented in this report 

to offer possible explanations for changes identified in the 

newly computed low-flow statistics. The eight streamgages 

have a contributing drainage area that lies in all or parts of six 

of the eight physiographic regions in New York, excluding 

Long Island. The minimum average 7-day, 2-year discharge 

(MA7CD2) and the minimum average 7-day 10-year discharge 

(MA7CD10) were calculated by using two datasets: data 

from the entire period of continuous streamflow record and 

data from 1976 through 2006. These low-flow statistics are 

commonly referred to as the 7-day, 2-year and 10-year low 

flows. This report abbreviates the MA7CD2 as the “7Q2” and 

the MA7CD10 as the “7Q10.” The newly computed low-flow 

frequency discharges were then compared with the statistics 

published in Bulletin 74 (Eissler, 1979). Changes in climate 

since the 1970s and the availability of 31 years of additional 

data at many locations emphasizes the need to update lowflow statistics published in Ku and others (1975) and Eissler 

(1979) to determine current (2010) low-flow conditions and 

to assist in developing a research study to create regional 

equations for estimating low flows on rural unregulated 

streams in New York.

Purpose and Scope

The purpose of this report is to provide updated statistics 

to help determine whether a statewide study that uses data 

through the 2006 climatic year (April 1 through March 31) 

would provide a better understanding of the current low-flow 

conditions and changes in low-flow frequency. This report also 

presents a means for inventorying the number of streamgages 

that remain active throughout New York State, excluding Long 

Island, with a complete period-of-record dataset since these 

stations were utilized in a low-flow study over 30 years ago. In 

1979 the USGS published “Low-flow Frequency Analysis of 

Streams in New York” (Eissler, 1979), commonly referred to 

as the Bulletin 74 report. This report evaluated data collected 
Introduction 

3

Suro_fig 01

APPALACHIAN PLATEAU

ADIRONDACK MOUNTAINS

CENTRAL LOWLANDS

NEW ENGLAND PROVINCE

ST. LAWRENCE VALLEY

TUG HILL PLATEAU

VALLEY AND RIDGE PROVINCE

LONG ISLAND COASTAL PLAIN PIEDMONT PROVINCE

Cy-7

Oe-151

Du-321

04256000

04234000

04232050

01437500

01420500

01376800

01372500

72° 74° 76° 78° 80°

44°

42°

Big Moose 3 SE

Ithaca Cornell Univ

Liberty 1 NE

Rochester Gtr Intl AP

Rock Hill 3 SW

Suffern

Millbrook

Poughkeepsie

Dutchess Co AP

Claryville

EXPLANATION

USGS streamgage and number

! Groundwater well and number"

Drainage basinPhysiographic provinceNRCC precipitation station

0 50 100 MILES0 50 100 KILOMETERS

Base from U.S. Geological Survey, Digital Line Graph, 1:2,000,000Universal Transverse Mercator projections, zone 18, NAD83

Pennsylvania

New York

VermontMassachusetts

Connecticut

New Jersey

Canada

04234000

Cy-7

Suffern

ork State, drainage basins, U.S. Geological Survey (USGS) streamgages, Northeast Regional Locations of physiographic provinces of New Y Figure 1.

Climate Center (NRCC) precipitation stations, and groundwater wells referred to in this study.
4 Changes in Low-Flow Frequency 1976–2006 at Selected Streamgages in New York, Excluding Long Island

prior to 1975 and provided data for 278 streamgages, of which 

154 were on unregulated streams (146 in New York State) 

and 124 (122 in New York State) were on regulated streams, 

data for 648 low-flow partial-record streamgages, and data 

for 2,384 miscellaneous sites. The main focus of Bulletin 74 

was to present statistical data for the 7-day, 2-year and 7-day 

10-year low flow at selected streamgages and at low-flow 

partial-record sites. The 7-day, 2-year and 7-day 10-year low 

flows are two of the most common low-flow statistics used 

by water-management agencies throughout New York State 

to assist in the planning and management of waste loads to 

streams, water-quality evaluations, water-supply design, 

groundwater management, and aquatic-habitat protection. 

Bulletin 74 has never been updated and regional equations 

for estimating low-flow statistics at unregulated streams 

throughout New York State have never been established.

This report is the first in a three-step process of 

developing methods to predict low-flow magnitude and 

frequency at rural unregulated sites in New York. The scope 

of this study was limited to the analysis of streamflow data 

at eight selected streamgages, and the presentation of data 

on land use, precipitation, and groundwater near these 

streamgages. The purpose of this first step is to investigate 

the hypothesis that low-flow statistics and the natural 

characteristics that strongly affect the statistics have changed 

throughout New York since the publication of Bulletin 74 

in 1979. This study further seeks to briefly examine the 

changes in the estimated low-flow magnitude and frequency 

at these streamgages by comparing statistics computed 

that used data collected from 1976 through 2006 with the 

statistics published in the Bulletin 74 report (Eissler, 1979). 

Step two would compute low-flow statistics statewide, 

analyze the effectiveness of the current network, and suggest 

improvements to the network. Step three would develop 

regional or statewide equations for estimating low flows 

on rural unregulated streams in New York, excluding Long 

Island. Currently no plan exists for the implementation of 

steps two and three.

Basins Studied

New York State occupies all or part of nine physiographic 

provinces that differ widely, from the high-relief areas of the 

Adirondack and Catskill Mountains to the low-relief areas 

along the Great Lakes and in the valleys of the Hudson, 

Mohawk, and St. Lawrence Rivers (fig. 1). Long Island, which 

is excluded from the study area of this report, is located in the 

Coastal Plain province. The eight streamgages in this study 

have a contributing drainage area that lies in all or parts of 

six of eight physiographic regions, excluding Long Island 

(fig. 1). Although the selection criteria restricted the ability 

to select basins from all eight physiographic provinces, the 

six provinces included in this study represent many of the 

physical characteristics (slope, shape, and others) of basins 

found throughout New York State. 

The St. Lawrence Valley province lies at the extreme 

northern border of New York and is primarily smooth 

glacial plain. The Adirondack province lies in the northern 

part of New York and covers an area of about 10,000 mi2

. 

The western half of the province is a plateau and has an 

abundance of lakes and ponds, whereas the eastern half is 

mostly mountainous. The northern part of western New York 

lies in the Central Lowlands whereas most of the central and 

southern areas of western New York are in the Appalachian 

Plateau province. The Appalachian Plateau province is mostly 

hilly terrain with the highest elevations being in the Allegheny 

and Catskill Mountains. The area of New York east of Lake 

Ontario and west of the Adirondack Mountains province is 

classified as the Tug Hill Plateau province. The provinces in 

southeastern New York are the Valley and Ridge province, the 

New England province, and the Piedmont province. 

Selection of Streamgages

This study initially considered most of the 146 

streamgages on unregulated streams and the 124 on regulated 

streams in New York presented in Bulletin 74 (Eissler, 1979) 

that had a continuous uninterrupted record. Only streamgages 

with continuous records collected at the same location for 

which low-flow statistics were computed for Bulletin 74 were 

considered. This criterion prohibited sampling drainage basins 

smaller than 29 mi2

. Additionally, the proximity of each basin 

to a precipitation station with a continuous record through 

the same study period (1976–2006) was assessed. Only eight 

streamgages were selected because of the limited scope of the 

study; five of the streamgages are in unregulated basins and 

three are affected by some degree of regulation. Ideally only 

unregulated basins are selected when computing statistical 

data; however, some regulated basins were included to reflect 

urban drainages, most of which are subject to some regulation. 

All urban streamgages included in Bulletin 74 (Eissler, 1979) 

with a period of continuous record through the 2006 climatic 

year are regulated. 

Only 53 of the 146 streamgages on unregulated streams 

in New York published in Bulletin 74 (Eissler, 1979) are 

still active with a continuous period of record. Geographic 

location, period of continuous record, percentage of drainage 

basin regulated, and urbanization were major factors in 

selecting the eight streamgages analyzed in this study. The 

Rondout Creek, Neversink River, and Beaver Kill streamgages 

are located in the Catskill Mountain region of New York. The 

Rondout Creek near Lowes Corners and Beaver Kill at Cooks 

Falls streamgages have long uninterrupted periods of record: 

69 years of data at Rondout Creek, and 93 years of data at 

Beaver Kill were used in the analysis. These basins on the 

eastern and western slopes of the Catskill Mountain region 

are in areas that remain predominately rural with minimal 
Land Use 5

development in the past 30 years. The Neversink River at 

Godeffroy streamgage also has a long period of record, but 

the record was split into regulated and unregulated when the 

Neversink Reservoir began storage operations in 1953. This 

station allows for the unique ability to compare 16 years of 

unregulated record and 22 years of regulated data published 

in Bulletin 74 (Eissler, 1979) with the conditions in 2006 to 

illustrate the effects of regulation on the streamflow. The Fall 

Creek and Allen Creek streamgages are in the Appalachian 

Plateau and Central Lowlands regions of western New 

York. The Fall Creek streamgage has an 81-year period of 

continuous record and is located on an unregulated stream. 

Allen Creek near Rochester has 45-year period of record, the 

shortest of the sites studied, and is located in an urbanized 

area. The Independence River at Donnattsburg streamgage 

is at the western border of the Adirondack Mountain region 

adjacent to the Tug Hill Plateau, and the Wappinger Creek 

and Hackensack River streamgages are in the New England 

province of New York. The Wappinger Creek streamgage has 

a 77-year period of unregulated streamflow and a contributing 

drainage basin that is sparsely urbanized. The Hackensack 

River at West Nyack streamgage is in a highly urbanized 

area and has regulated streamflows from Deforest Lake with 

diversions from the streamgage pool for municipal supply. 

These streamgages were included for analysis to allow for the 

comparison with statistics computed from Bulletin 74 (Eissler, 

1979) to examine trends in urban areas and changes in patterns 

of regulation. Urbanization and development generally 

increase the area of impermeable land surface, the tunneling 

of small creeks and streams and the demand for water supply. 

In general, increases in the area of impermeable land surface 

and the increased demand for water supply can lead to larger 

spikes in runoff during wet periods and have a greater effect 

on low flows during dry periods. 

Precipitation

The climate of New York State is classified as humid 

continental, the prevailing climate type in the northeastern 

United States. General climatic conditions in New York are 

controlled by the usual path of air mass movement in relation 

to the location of the State. Generally, cool and dry air masses 

move eastward across New York throughout the year. Warm 

and humid tropical air masses usually move northeastward 

from the south during the summer months. 

The locations of the National Weather Service 

precipitation stations used in this report are shown in figure 1. 

Table 1 lists the name and site number, location, elevation, and 

30-year (1961–1990) average precipitation for each station. 

The criteria for selecting these stations were continuity and 

length of record, minimal amount of estimated daily data 

by Climate Information for Management and Operational 

Decisions (CLIMOD), and proximity to one of the USGS 

streamgages analyzed in this study. Precipitation data for two 

precipitation stations in Dutchess County were combined 

to produce a continuous dataset for analysis. The record 

for Millbrook was used through 2000, when the station 

was discontinued. Data from 2001 through 2006 for the 

Poughkeepsie Dutchess County Airport was substituted for 

the missing Millbrook record to produce a single 30-year 

period of data. These two stations are fairly well correlated 

(R2 = 0.69 for the period 1949–1992) and are in close 

proximity and close to the Wappinger Creek at Wappingers 

Falls streamgage. The 30-year mean annual precipitation for 

Millbrook was computed by using the combined dataset.

The 30-year average annual precipitation for the period 

1961–1990 for these sites ranged from 31.96 inches (in.) in 

Monroe County at the Greater Rochester International Airport 

to 50.56 in. at Claryville in Sullivan County and 50.36 in. at 

Big Moose SE in Herkimer County. Total annual precipitation 

at these sites ranged from about 22 to 69 in. for the 30-year 

period from 1961 through 1990 (Northeast Regional Climate 

Center, 2008). 

Land Use

The USGS Geographic Information Retrieval and 

Analysis System (GIRAS) was used as a reference for 

identifying land use and land cover characteristics in 1979 

and 1984 (U.S. Geological Survey, 1979a, 1979b, 1984a, 

1984b) for the contributing drainage area of the eight USGS 

streamgages analyzed in this study. GIRAS is a digital 

database of land use and land cover compiled from USGS 

1:250,000 scale maps created in the 1970s and 1980s. These 

data were compared to land use and land cover data from 

the National Land Cover Database (NLCD) 2001, which are 

derived from Landsat satellite imagery representing land cover 

of the study basins in 2001 (U.S. Geological Survey, 2003). 

The NLCD 2001 was assumed to represent current landcover conditions. 

Although comparisons between the GIRAS and NLCD 

datasets are difficult and uncertain, the decision was made to 

rely on GIRAS because of the inability to identify historical 

data of higher quality. The GIRAS and NLCD datasets use 

different scales and classification methods to interpret landuse and land-cover data. The differences in methodology 

resulted in overlapping classes that may have lead to 

misclassifying land cover in the GIRAS dataset, thereby 

creating inconsistencies between the datasets. The Landsat 

satellite imagery used for the NLCD is able to identify landuse boundaries at a much smaller scale and, therefore, allow 

conceptually clearer classifications. The historical GIRAS 

dataset is based on 22 classes of land use including an “Other” 

class whereas the NLCD 2001 dataset is based on 16 classes. 

Some GIRAS classes were eliminated or consolidated because 

they were considered unnecessary or redundant in the NLCD 

2001 classification scheme, and other classes were expanded 

into several more specific NLCD classes. Although the NLCD 
6 Changes in Low-Flow Frequency 1976–2006 at Selected Streamgages in New York, Excluding Long Island

Table 1. Northeast Regional Climate Center station name, number, location, elevation, and 30-year average annual precipitation 

(1961–1990) for climate records analyzed in this study. 

[Station names are listed in table exactly as they appear on the Northeast Regional Climate Center data page. SE, southeast; Univ, University; NE, northeast; 

Gtr, Greater; Intl, International; AP, Airport; SW, southwest; Co, County]

Northeast Regional 

Climate Center 

station name

Station 

number

Latitude 

(degrees)

Longitude 

(degrees)

Elevation 

(feet)

30-year 

average 

precipitation 

(1961–90), 

in inches

Big Moose 3 SE 300668 43.80 74.87 1,760 50.36

Ithaca Cornell Univ 304174 42.45 76.45 960 35.40

Liberty 1 NE 304731 41.80 74.73 1,549 48.29

Rochester Gtr Intl AP 307167 43.12 77.68 533 31.96

Rock Hill 3 SW 307205 41.58 74.62 1,270 46.28

Suffern 308322 41.11 74.16 270 49.32

Millbrook 305334 41.85 73.62 820 40.65

Poughkeepsie Dutchess Co AP 306820 41.63 73.88 166 40.72

Claryville 301521 41.92 74.57 1,653 50.56

2001 has fewer classes the dataset better defines the different 

impacts that various land uses have on physical characteristics 

of the land than does GIRAS. Analysis of both datasets 

resulted in a decision to develop seven consolidation classes 

of land use to allow for the comparison of the two datasets 

even though several land-cover and land-use classes were 

consolidated into a single class. The consolidation scheme is 

outlined in table 2.

The seven consolidation classes and the estimated 

percentage of change in total area for each basin are listed 

in table 3. An increase in developed area and a decrease in 

agricultural and forest areas are indicated for most of the 

basins. The indicated increase in developed area was less 

than 8 percent for most sites, except for Allen Creek near 

Rochester, which indicated a nearly 13-percent increase. A 

minor decrease in forested area was identified at the three 

sites in the Catskill Mountain region (Rondout Creek near 

Lowes Corners, Beaver Kill at Cooks Falls, and Neversink 

River at Godeffroy), but relatively large decreases of 

31.4 percent in the Independence River basin and 39.1 percent 

in the Fall Creek basin of northern and western New York 

were identified. A 21-percent increase in wetland areas in 

the Independence River basin is paired with a 31-percent 

decrease in forested area in the basin, but this may be caused 

by differences in the GIRAS and NLCD classification 

schemes and not an actual change of the land cover. An 

alternative theory suggests that the 21-percent increase in 

wetland areas was a reclassification of some forested area so 

that the resultant difference of about 10 percent might be a 

better estimate of the actual decrease in forested areas. The 

increase in agricultural land in the Fall Creek basin may be 

fairly reliable, primarily because of recently increased dairy 

production (Johnson and others, 2007). An apparent increase 

in the percentage of wetlands and a decrease in forested area 

in many basins do not seem consistent with the apparent 

increased development and decreased agriculture in those 

areas. As was suggested for similar data in the Independence 

River basin, the GIRAS dataset may have misclassified some 

wetlands as forest.

Frequency Analysis

Streamflow at a streamgage is affected by many natural 

factors including climate, geomorphology of the channel, 

geology, and groundwater levels. Human activities such as 

building reservoirs and dams, and making withdrawals from 

surface-water and groundwater systems for municipal and 

industrial use also affect streamflow. 

Low-flow statistics are usually computed by using 

continuous record from streamgages having a period of 

record of 10 years or greater. Daily mean discharge values 

are used to compute the lowest mean discharge for a series of 

consecutive days for each year. The USGS generally computes 

annual discharge statistics on a water-year basis (October 1 to 

September 30), but low-flow discharge statistics are computed 

by using an annual period called a climatic year (April 1 to 

March 31) because the annual low-flow period is usually 

during the late summer and fall. A climatic year includes the 
Frequency Analysis 7

Table 2. Consolidation classes for U.S. Geological Survey (USGS) Geographic Information Retrieval and Analysis System (GIRAS) and 

National Land Cover Database 2001 (NLCD 2001) classes used in this study.

GIRAS class Consolidation class NLCD 2001 class

 0 - Other Developed 21 ‐ Developed, open space

11 - Residential 22 ‐ Developed, low intensity

12 - Commercial and services 23 ‐ Developed, medium intensity

13 - Industrial 24 ‐ Developed, high intensity

14 ‐ Transportation, communications and utilities

15 - Industrial and commercial complexes

16 - Mixed urban or built-up land

17 - Other urban or built-up land

21 - Cropland and pasture Agricultural 81 - Pasture/hay

22 - Orchard, grove, vineyard, nursery and ornamental horticultural areas 82 - Cultivated crops

23 ‐ Confined feeding operations

24 - Other agricultural land

32 - Shrub and brush rangeland Scrub/ shrub 52 - Scrub/ shrub

71 ‐ Grassland/herbaceous

41 ‐ Deciduous forest land Forest 41 ‐ Deciduous forest

42 - Evergreen forest land 42 - Evergreen forest

43 - Mixed forest land 43 - Mixed forest

52 ‐ Lakes Open water 11 - Open water

53 - Reservoirs

61 - Forested wetland Wetlands 90 ‐ Woody wetlands

62 - Nonforested wetland 95 - Emergent herbaceous wetland

75 - Strip mines Barren land/ mines 31 ‐ Barren land

76 ‐ Transitional areas

entire period April 1 to March 31, but by using a water year 

for analysis would separate the sustained low-flow period 

between 2 years. 

An often used low-flow series is the minimum average 

7-consecutive-day flow. Many State and local agencies 

use a frequency statistic computed by using an annual 

7-consecutive-day series to assist them in making regulatory 

decisions. The minimum average 7-consecutive-day low flow 

is computed for each climatic year of data available. A log 

Pearson Type III frequency distribution is then computed to 

determine the low-flow statistics at selected streamgages. 

Frequency analysis of the annual 7-day low flows provides 

a means for estimating the probability of occurrence of a 

given discharge. Low-flow frequency commonly is referred 

to in terms of recurrence interval or the probability of being 

exceeded (one is the reciprocal of the other). The minimum 

average 7-day 10-year low-flow discharge (7Q10), for 

example, has a probability of 0.1 (10 percent chance) of not 

being equaled or exceeded in any given year. 

The minimum average 7-day, 2-year (7Q2) and 7-day 

10-year (7Q10) low-flow statistics were computed for all eight 

streamgages in this report by using the full period of record 

available for each streamgage, data for the period from 1976 

through 2006 (31-year period), and data from 1997 through 

2006 (10-year period). These values were then compared with 

the values determined in Bulletin 74 (Eissler, 1979). These 

values are listed in table 4.
8 Changes in Low-Flow Frequency 1976–2006 at Selected Streamgages in New York, Excluding Long Island Table 3. Indicated percent change in land use or land cover from 1970 throuogh 2003 as a percent of drainage basins for eight streamgages in New York (N.Y.). Streamgage number Streamgage name and location Percentage of change in land use or land cover (by consolidation class) Developed Agricultural Scrub/shrub Forest Open water Wetlands Barren land/ mines 01365000 Rondout Creek near Lowes Corners, N.Y. 0.5 -1.6 0.1 0.6 0.0 0.4 0.0 01372500 Wappinger Creek near Wappingers Falls, N.Y. 5.8 -3.0 5.4 -13.2 0.3 5.1 0.0 01376800 Hackensack River at West Nyack, N.Y. 7.6 -0.9 0.4 -13.7 4.0 9.5 0.0 01420500 Beaver Kill at Cooks Falls, N.Y. 1.4 -1.6 0.9 -5.9 0.8 4.5 0.0 01437500 Neversink River at Godeffroy, N.Y. 3.1 -3.8 0.5 0.8 1.5 2.4 0.0 04232050 Allen Creek near Rochester, N.Y. 12.8 -25.8 2.3 6.5 0.3 4.0 0.0 04234000 Fall Creek near Ithaca, N.Y. 4.5 19.6 7.6 -39.1 0.3 7.1 0.0 04256000 Independence River at Donnattsburg, N.Y. 0 0 8.8 -31.4 1.8 20.8 0.0
Frequency Analysis 

9

Table 4. Estimated 7-day, 2-year and 10-year low-flow frequency discharges for eight streamgages in New York (N.Y.) computed from log-Pearson type III analysis and compared with estimated low-flow frequency discharges published in Bulletin 74.[USGS, U.S. Geological Survey; mi2, square miles; ft, feet; ft3/s, cubic feet per second; 7Q2, 7-day, 2-year low flow frequency discharge; 7Q10, 7-day, 10-year low flow frequency discharge; --, no data available; U, unregulated; R, regulated; NWIS, National Water Information System. Example NWIS URL: http://nwis.waterdata.usgs.gov/ny/nwis/nwisman/?site_no=01365000&agency_cd=USGS. Streamgage 

locations shown on figure 1]

USGS 

NWIS 

streamgage 

number

Streamgage 

name

Periodofrecord 

(years)

Drainage 

area 

(mi2) 

reported 

in 1979

 Drainage 

area 

(mi2) 

2006

Regulated 

or 

unregulated

Recurrence 

interval, 

Bulletin 74, 

1979 

(ft3/s) 

Recurrence 

interval, 

data from 

1976–2006

(ft3/s) 

Percent 

difference 

1976–2006

Recurrence 

interval, 

period-of-record 

(ft3/s)

Percent 

difference 

period-of-record

7Q2 7Q10 7Q2 7Q10 7Q2 7Q10 7Q2 7Q10 7Q2 7Q10

01365000 Rondout Creek 

near Lowes Corners, N.Y. 

1938–2006 38.5 38.3 U 9.4 5.8 12.7 6.2 35.1 6.9 10.8 6.0 14.8 2.9

01372500 Wappinger 

Creek near Wappingers Falls, N.Y. 

1930–2006 181 181 U 15.0 6.1 18.1 6.3 20.7 3.3 16.1 6.2 7.2 1.6

01376800 Hackensack 

River at West 

Nyack, N.Y.*

1960–2006 29.4 30.7 R 9.9 4.3 11.4 7.1 15.2 65.1 11.3 6.0 14.0 38.4

01420500 Beaver Kill at 

Cooks Falls, N.Y. 

1914–2006 241 241 U 60.0 35.0 68.3 39.0 13.8 11.4 62.9 36.4 4.8 4.0

01437500 Neversink River 

at Godeffroy, NY1*

1938–1953 302 307 U 76.0 45.0 -- -- -- -- -- -- -- --

1954–2006 302 307 R 40.0 28.0 94.3 61.6 136 120 85.4 53.8 114 92.1

04232050 Allen Creek near 

Rochester, N.Y.*

1961–2006 30.1 30.1 R 6.2 2.8 5.4 3.3 -12.9 17.9 5.7 3.1 -8.8 10.9

04234000 Fall Creek near 

Ithaca, N.Y. 

1926–2006 126 126 U 17.0 9.4 19.9 9.4 17.1 0.0 17.9 9.4 5.5 -0.1

04256000 Independence 

River at Donnattsburg, N.Y.

1944–2006 91.7 88.7 U 27.0 21.0 36.4 24.9 34.8 18.6 30.9 21.9 14.4 4.5

1Period of unregulated streamflow record (1938–53) not used in this analysis.*Sites in green have regulated streamflows.
10 Changes in Low-Flow Frequency 1976–2006 at Selected Streamgages in New York, Excluding Long Island

Data 1976–2006 (31-year period)

The 7Q10 discharges computed by using the 31 

years (1976 through 2006) of data collected from the eight 

streamgages collected since the Bulletin 74 statistics (Eissler, 

1979) indicated changes ranging from no decrease to about 

19 percent increase at streamgages on unregulated streams 

when compared with discharges published in Bulletin 74 

(Eissler, 1979). The 7Q2 discharges computed by using data 

from 1976 through 2006 indicated that all sites increased from 

14 to 35 percent at streamgages on unregulated streams when 

compared with the discharges in Bulletin 74 (Eissler, 1979). 

The 7Q10 for Wappingers Creek near Wappingers Falls 

and Rondout Creek near Lowes Corners, indicate increases 

of about 3 to 7 percent, whereas the computed 7Q10 for 

Fall Creek near Ithaca, indicated zero change. The 7Q10 

for Beaver Kill at Cooks Falls and Independence River at 

Donnattsburg, indicated increases of 11.4 and 18.6 percent, 

respectively. The 7Q2 for all five of these streamgages show 

increases, with Rondout Creek and Independence River 

both showing the largest increases (35.1 and 34.8 percent, 

respectively) when compared with values computed for 

Bulletin 74 (Eissler, 1979). 

Among the streamgages located in an urbanized area, 

the 7Q10 for those at Hackensack River at West Nyack and 

at Allen Creek near Rochester show an increase of 65.1 

and 17.9 percent, respectively. Both streams are regulated 

upstream from the streamgages. Hackensack River diversions 

are used for municipal supply for the town of Nyack, and 

Allen Creek diversions are made during the operating season 

from the Erie Canal through sets of siphons upstream from 

the station and used for augmenting low flows and golf-course 

irrigation. The Hackensack River at West Nyack and the 

Allen Creek near Rochester streamgages are both located in 

urbanized areas; therefore, changes in the low-flow frequency 

may reflect changes in patterns of climate, urbanization, 

and regulation. 

The data from Neversink River at Godeffroy indicate the 

largest increase of all streamgages, a result that is obvious in 

all computed statistics since the Neversink Reservoir began 

regulating streamflow in 1954. The 7Q10 statistics for the 

Neversink River at Godeffroy streamgage indicate the largest 

increase (120 percent) when the statistics were computed 

by using data from 1976 through 2006 and when this value 

was compared with the statistics computed in Bulletin 74 

(Eissler, 1979). This large increase in low-flow frequency may 

be partially attributed to changes in the release operations 

from Neversink Reservoir. The 7Q10 at Neversink River at 

Godeffroy computed by using data from 1976 through 2006 

shows a 37-percent increase when compared with the 7Q10 

computed in Bulletin 74 (Eissler, 1979) for the period before 

regulation, 1938–1953.

Data for the Entire Period of Record

Comparing statistics computed by using data for the 

entire period of record for unregulated sites indicated changes 

ranging from no decrease to a 4.5 percent increase in the 

computed 7Q10 when compared with data from Bulletin 

74 (Eissler, 1979). The 7Q10 for Independence River at 

Donnattsburg and Beaver Kill at Cooks Falls indicated 

increases of 4.5 and 4.0 percent, respectively. The 7Q10 for 

Fall Creek near Ithaca indicated virtually no change when 

compared with data from Bulletin 74 (Eissler, 1979).

The 7Q10 for Hackensack River at West Nyack indicated 

a 38.4-percent increase, whereas 7Q10 for Allen Creek near 

Rochester streamgage indicated a 10.9 percent increase. The 

7Q10 for Neversink River at Godeffroy, regulated by releases 

from Neversink Reservoir, indicated the largest increase of 

92.1 percent for the regulated period from 1954 through 2006.

The computed 7Q2 for the unregulated sites indicated 

increases ranging from 4.8 to 14.8 percent when compared 

with data from Bulletin 74 (Eissler, 1979). The 7Q2 

for Rondout Creek near Lowes Corners and 7Q2 for 

Independence River at Donnattsburg indicated the largest 

increases of 14.8 and 14.4 percent, respectively. The 7Q2 

for Wappingers Creek near Wappingers Falls streamgage 

indicated a 7.2 percent increase whereas the 7Q2 for Fall 

Creek near Ithaca indicated an increase of 5.5 percent.

The 7Q2 for Hackensack River at West Nyack and the 

Allen Creek near Rochester streamgages, located in urbanized 

areas with regulated streamflows, indicated an increase of 

14.1 percent and a decrease of -8.8 percent, respectively, when 

compared with data from Bulletin 74 (Eissler, 1979). 

Trends

Minimum Annual 7-Day Low Flow

The 7-day low-flow discharges computed for each of 

the streamgages that used the full period of record were 

analyzed by using Kendall’s tau test (Helsel and Hirsch, 1992) 

to identify trends. The Kendall’s tau test is a nonparametric 

test that can be used to identify trends in streamflow and 

precipitation data over time. A positive value for Kendall’s 

tau indicates a positive trend and a negative tau indicates a 

negative trend. The p-value is the probability of obtaining a 

value of the tested data (7-day low flow) with equal magnitude 

to the observed extremes, assuming the null hypothesis is true. 

The trend was considered to be significant if the probability 

value (p-value) was less than or equal to 0.05. This value 

represents a 95-percent confidence level. 
Trends 11

Data from six of the streamgages tested showed a 

positive trend, one showed a negative trend, and one showed 

no trend when evaluating the entire period of record at each 

streamgage. Data from two of the streamgages, Independence 

River at Donnattsburg and Neversink River at Godeffroy, 

indicated a significant positive trend and data from Rondout 

Creek near Lowes Corners indicated a nearly significant 

positive trend. The annual 7-day low flows show an obvious 

pattern of increasing 7-day low flows beginning in the 

1970s for six of the eight streamgages studied (fig. 2). The 

pattern is most obvious at the unregulated rural streamgages. 

Independence River, Rondout Creek, Wappinger Creek, and 

Fall Creek clearly illustrate this pattern. The annual 7-day 

low flows for the Wappinger Creek near Wappingers Falls 

streamgage were generally less than 30 cubic feet per second 

(ft3

/s) during the 1930s through the late 1960s, but since about 

1970, the pattern generally seems to be increasing. Prior to 

1970, only 2 years had annual 7-day low flows greater than 

50 ft3

/s, but since about 1970, six of the annual 7-day low 

flows have been greater than 50 ft3

/s. A similar pattern is 

visible at the Fall Creek near Ithaca streamgage. Prior to about 

1975, only one annual 7-day low flow was greater than about 

30 ft3

/s, but since 1975, seven annual 7-day low flows have 

been greater than 30 ft3

/s. The minimum annual 7-day low 

flow for the period of record and the Kendall’s tau trends for 

each streamgage are shown in figure 2.

Precipitation

Changes in precipitation patterns can cause changes in 

the minimum annual 7-day low flow. Precipitation data from 

eight National Weather Service stations located near the eight 

USGS streamgages were analyzed and Kendall’s tau test 

was used to test for trends in the annual precipitation data 

for these stations. All eight stations showed a positive trend 

in annual precipitation. Three of the eight precipitation sites 

showed significant positive trends, and another two stations 

showed nearly significant positive trends. Annual precipitation 

from 1961 to 2006 for all eight stations is shown in figure 3, 

along with each Kendall tau trend. The World Meteorological 

Organization (WMO) defines normals as the average of 

climatological data computed for consecutive 30-year periods 

starting from January 1, 1901. A 30-year average (normal) of 

annual precipitation was computed by using data from 1961 

through 1990 for each of these stations. Figure 4 shows the 

departure from this average during 1987–2006 (20 years). 

Seven of the precipitation stations showed a net positive 

departure from the 30-year average, whereas one station 

showed a decreasing departure from the 30-year average. The 

largest positive change at many of the stations is most evident 

during the past 10 years (1997–2006) with the exceptions of 

Suffern and Big Moose 3 SE. The Liberty 1 NE station shows 

an increasing trend in annual precipitation, which included 

some of the largest annual precipitation totals occurring in 

the last 10 years when data from 1960 through 2006 were 

analyzed. This trend may be reflected in the 35.1-percent 

increase in the 7Q2 at the Rondout Creek near Lowes Corners 

streamgage when the 7Q2 is computed by using data since 

1976 and compared with the 7Q2 value determined in Bulletin 

74 (Eissler, 1979). The 7Q10 for Rondout Creek near Lowes 

Corners also shows a net increase of 6.9 percent when this 

statistic is computed using data since 1976 and compared 

with the 7Q10 determined for this streamgage in Bulletin 74 

(Eissler, 1979).

A 2007 study (Burns and others, 2007) has determined 

that mean precipitation in the Catskill Mountain region has 

increased significantly by 136 millimeters per 50 years and 

that the annual mean air temperature has also increased 

0.6°C per 50 years. Burns and others (2007) also reported 

that peak snowmelt is occurring earlier each year than in the 

past. It is possible that a smaller volume of water stored in 

snowpack later in the spring combined with changes in climate 

could affect low-flow frequency and magnitude in New York 

streams. Further analysis of climatic data throughout New 

York State would allow for greater insight into these types 

of changes.

Groundwater

The USGS operates a network of groundwater 

observation wells to meet several objectives such as 

monitoring the effects of climate change on long-term 

groundwater storage and the seasonal variability in recharge. 

Typically, the effects of climate change on groundwater levels 

would be best monitored by observation wells screened in 

major unconfined sand and gravel aquifers that are minimally 

affected by pumping or artificial recharge. The water level 

of these large unconfined aquifers is often referred to as “the 

water table” and changes in the elevation of the water table 

reflect seasonal and long-term changes in groundwater storage. 

Low-flow conditions throughout the State are influenced 

by many factors including the presence of large unconfined 

aquifers in the drainage basin of the gaged stream. Where 

a gaged stream crosses and is hydraulically connected to 

the aquifer, fluctuations in groundwater levels can help to 

explain changes in annual low flows. Unfortunately, many 

groundwater wells in the New York State network were 

discontinued in 1995 and not reactivated until 2000 to 2002. 

Data from three groundwater observation wells were included 

in this study as a simple means of identifying possible trends 

in groundwater levels in various regions of New York State. 

Two of the observation wells included in this study, CY-7 and 

OE-151, are in sand and gravel (water-table) aquifers. The 

third well, DU-321, is in a confined bedrock aquifer (fig. 5, 

table 5).

The wells in the sand and gravel aquifers were the nearest 

wells with 30 years of data to the selected USGS streamgages. 

The 30-year record at these wells shows a fairly constant level 

of groundwater storage, although a slight positive trend for 

CY-7 and a slight negative trend for OE-151 were identified. 
12 Changes in Low-Flow Frequency 1976–2006 at Selected Streamgages in New York, Excluding Long Island

Suro_fig 02

0

80604020

200180160140120100

1900 1920 1940 1960 1980 2000 2020

Date

1420500 Beaver Kill at Cooks Falls, N.Y.

tau = 0.00

50

353025201510

1930 1940 1950 1960 1970 1980 1990 2000 2010

7-day low-flow discharge, in cubic feet per second 

01365000 Rondout Creek near Lowes Corners, N.Y.

tau = 0.118

0

908070605040302010

100

1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

Date

01372500 Wappinger Creek near Wappingers Falls, N.Y.

tau = 0.053

50

25201510

1950 1960 1970 1980 1990 2000 2010

01376800 Hackensack River at West Nyack, N.Y.

tau = 0.025

7-day low-flow discharge, in cubic feet per second 

p-value = 0.153 p-value = 0.811

p-value = 0.495 p-value = 1.00

Kendall tau trend

Figure 2.

Minumum annual 7-day low flow discharges and Kendall tau trend for eight streamgages in New York State [stations shown in figure 1].
Trends 13 Suro_fig 02 con’t

0

50

300250200150100

1930 1940 1950 1960 1970 1980 1990 2000 2010

01437500 Neversink River at Godeffroy, N.Y.

tau = 0.169

0

70605040302010

1940 1950 1960 1970 1980 1990 2000 2010

04256000 Independence River at Donnattsburg, N.Y.

tau = 0.274

0

605040302010

1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

Date

04234000 Fall Creek near Ithaca, N.Y.

tau = 0.044

86420

141210

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010

04232050 Allen Creek near Rochester, N.Y. p-value = 0.043

7-day low-flow discharge, in cubic feet per second p-value = 0.562

Date

7-day low-flow discharge, in cubic feet per second 

p-value = 0.272tau = -0.113

p-value = 0.002

Kendall tau trend

Figure 2. Minumum annual 7-day low flow discharges and Kendall tau trend for eight streamgages in New York state [stations shown in figure 1].
14 Changes in Low-Flow Frequency 1976–2006 at Selected Streamgages in New York, Excluding Long Island

Suro_fig 03

0

8070605040302010

1950 1960 1970 1980 1990 2000 2010

301521 Claryville

tau = 0.339

0

605040302010

1950 1960 1970 1980 1990 2000 2010

305334 Millbrook 306820 Poughkeepsie Dutchess Co Ap

tau = 0.185

0

70605040302010

1950 1960 1970 1980 1990 2000 2010

304174 Liberty 1 NE

tau = 0.192

0

70605040302010

1950 1960 1970 1980 1990 2000 2010

307205 Rock Hill 3 SW

tau = 0.272

p-value = 0.000

p-value = 0.072 p-value = 0.008

p-value = 0.061

Annual precipitation, in inches 

Date Date

Annual precipitation, in inches 

Kendall tau trend

ork ons in New Y nnual Precipitation from 1961 to 2006 and Kendall tau trend at selected Northeast Regional Climate Center precipitation stati A Figure 3.

[stations shown in figure 1].
Trends 15 Suro_fig 03 con’t

0

8070605040302010

1950 1960 1970 1980 1990 2000 2010

Annual precipitation, in inches 

308322 Suffern

tau = 0.045

50

504540353025201510

1950 1960 1970 1980 1990 2000 2010

307167 Rochester Gtr Intl Ap

tau = 0.289

0

605040302010

1950 1960 1970 1980 1990 2000 2010

Date

304174 Ithaca Cornell Univ

tau = 0.223

0

8070605040302010

1950 1960 1970 1980 1990 2000 2010

Date

300668 Big Moose 3 SE

tau = 0.056

p-value = 0.663

p-value = 0.294

p-value = 0.005

p-value = 0.589

Annual precipitation, in inches 

Kendall tau trend

ork Annual precipitation from 1961 to 2006 and Kendall tau trend at selected Northeast Regional Climate Center precipitation stations in New Y Figure 3.

[stations shown in figure 1].
16 Changes in Low-Flow Frequency 1976–2006 at Selected Streamgages in New York, Excluding Long Island

Suro_fig 04

50

3025201510

1987 1989 1991 1993 1995 1997 1999 2001 2003 2005

Date

301521 Claryville

50

201510

1987 1989 1991 1993 1995 1997 1999 2001 2003 2005

Departure from average, in inches

50

201510

304731 Liberty 1 NE

1987 1989 1991 1993 1995 1997 1999 2001 2003 2005

50

201510

1987 1989 1991 1993 1995 1997 1999 2001 2003 2005

307205 Rock Hill 3 SW 305334 Millbrook - 306820 Poughkeepsie Dutchess Co Ap

30-year average = 48.29 inches

-5

-10-15

Date

-5

-10-15

-5

-10-15

-5

-10-15

Departure from mean, in inches

30-year average = 50.56 inches

30-year average = 40.72 inches 30-year average = 46.28 inches

ork State, from the 30-year average (1961–90), eparture of total annual precipitation at selected Northeast Regional Climate Center stations in New Y D

[stations shown in figure 1].

Figure 4.in inches 
Trends 17Suro_fig 04 con’t

50

201510

307205 Suffern

1987 1989 1991 1993 1995 1997 1999 2001 2003 2005

50

1510

307167 Rochester Gtr Intl Ap

1987 1989 1991 1993 1995 1997 1999 2001 2003 2005

50

1510

1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006

Date

304174 Ithaca Cornell Univ

50

10

1987 1989 1991 1993 1995 1997 1999 2001 2003 2005

Date

300668 Big Moose 3 SE

-5

-10-15-20

Departure from average, in inches

30-year average = 31.96 inches

-5

-10

-5

-10

-5

-10

Departure from mean, in inches 30-year average = 50.36 inches

30-year average = 49.32 inches30-year average = 35.40 inches

ork State, from the 30-year average (1961–90), Departure of total annual precipitation at selected Northeast Regional Climate Center stations in New Y Figure 4.

in inches [stations shown in figure 1].
18 Changes in Low-Flow Frequency 1976–2006 at Selected Streamgages in New York, Excluding Long Island

Suro_fig 05

0

5

10

15

20

25

30 10/2/76

10/2/7810/1/8010/1/829/30/849/30/869/29/889/29/909/28/929/28/949/27/969/27/989/26/009/26/029/25/049/25/06

Water level below land surface, in feet

Date

65

66

67

68

69

70

71

72

73

0

5

10

15

20

25

30

35

A CY-7 Observation well

C DU-321 Observation well

B OE-151 Observation well

Linear trend line

Figure 5. Thirty-year average (1976–2006) hydrographs of weekly water levels at selected 

U.S. Geological Survey (USGS) groundwater observation wells in New York State. (Well 

locations are shown in figure 1).
Trends 19

Table 5. Selected data for three observation wells in the U.S. Geological Survey observation-well network in New York State.[USGS, U.S. Geological Survey; NWIS, National Water Information System; County abbreviations: CY, Cayuga County; DU, Dutchess County; OE, Oneida County. Online data at (1) http://nwis.waterdata.usgs.gov/ny/nwis/nwisman/?site_no=424158076251901&agency_cd=USGS, and (2) http://groundwaterwatch.usgs.gov/AWLSites.asp?S=424158076251901]

County 

well 

number

USGS NWIS 

well number

Period of record 

(years)

Aquifer 

material

Aquifer 

type

Well 

type

Well 

depth 

(feet)

Physiographic region Climatic zone

CY–7 424158076251901 1965–95, 1997–2010 Gravel Water table Drilled 28 Appalachian Upland Hudson ValleyDU–321 414737073563301 1948–50, 1953–97, 

2004–2010

shale confined drilled 126 Appalachian Upland Central Lakes

OE–151 433112075091501 1926–45, 1948–2010 sand water table dug 31 Adirondack Upland Northern Plateau
20 Changes in Low-Flow Frequency 1976–2006 at Selected Streamgages in New York, Excluding Long Island

These trends are minor and do not necessarily reflect an 

impact on low-flow trends at the nearby USGS streamgage. 

This lack of relation is evident when a comparison is made 

between the Independence River at Donnattsburg streamgage 

and well OE-151. The 7Q10 and 7Q2 flows for the streamgage 

at Independence River at Donnattsburg have increased about 

19 and 35 percent, respectively, when computed by using 

data from 1976 through 2006 and compared to statistics 

computed by using data from Bulletin 74 (Eissler, 1979). 

Conversely, the long-term groundwater storage at OE-151 

has remained relatively constant and actually shows a minor 

decreasing trend. 

The data from the bedrock well DU-321 shows a fairly 

constant decrease in groundwater storage during the period 

1976–2006. The 7Q10 and 7Q2 flows from the nearby 

Wappinger Creek streamgage near Wappingers Falls shows 

an increase of about 3 and 21 percent, respectively, during 

the period 1976–2006 when compared with data published 

in Bulletin 74 (Eissler, 1979). The 7Q10 for the Wappinger 

Creek streamgage shows about a 2 percent increase and 

about a 7 percent increase in the 7Q2 when data for the entire 

period of record are used to compute updated statistics. 

Changes in storage of a confined bedrock aquifer are not 

usually a good indicator of changes in surface-water baseflows 

because bedrock aquifers in New York State are hydraulically 

connected to streams in only a few areas of the State. 

These groundwater data indicate a need to install or 

reactivate groundwater wells to monitor aquifer storage in 

aquifers that contribute to the base flows at long-term USGS 

streamgages to assess the effects of groundwater level changes 

on low flows. Simply selecting locations for groundwater 

wells on the basis of political boundaries and aquifer type may 

not provide sufficient data to analyze the effects of changes in 

groundwater storage on surface-water low flows.

Summary

Low-flow data are used by many Federal, State, and 

local permitting and regulatory agencies and by watersupply planners to set policy on surface-water withdraws 

and to manage reservoirs. Low-flow data also are vital for 

determining the total maximum daily loads (TMDLs) and 

effluent limits of point and nonpoint sources of contamination 

on surface water during periods of decreased streamflow. This 

study analyzed streamflow data at eight selected streamgages 

in New York State and data on land use, precipitation, 

and groundwater near these selected sites to investigate 

the hypothesis that low-flow conditions and the natural 

characteristics that strongly influence low flows have changed 

throughout the State from 1976 through 2006, the period 

subsequent to that analyzed in Bulletin 74. 

Although the study is limited and cannot be used to 

determine statewide changes in the low-flow conditions of 

New York streams, the study does identify an important 

pattern when data from 1976 through 2006 are used to 

compute low-flow frequencies and are compared with the 

low-flow frequencies computed by using data prior to 1976, 

which were published in Bulletin 74.

The general pattern is that the 7-day 10 year low-flow 

frequencies showed an increase of about 3 to 19 percent 

at rural streamgages with unregulated streamflows when 

computed by using data from 1976 through 2006. The 

7-day, 2-year low-flow frequency increased from about 

14 to 35 percent at the rural streamgages with unregulated 

streamflows when the same time period were analyzed. 

 Streamgages affected by urbanization and regulation for 

water supply showed the most significant change in the 7-day 

2- and 10-year low-flow frequencies. The 7-day 10-year lowflow frequency increased about 65 percent at the Hackensack 

River at West Nyack when the data since 1976 were used 

and about 38 percent when the entire period of record 

since regulation (46 years) was used. The 7Q10 frequency 

magnitude for Neversink River at Godeffroy streamgage 

showed a 120-percent increase when computed by using data 

from 1976 through 2006 and compared with data published in 

Bulletin 74. The 7Q10 for Neversink streamgage also showed 

an increase of about 92 percent when data for the entire 

period of record since regulation began (1954–2006) were 

used to compute the low-flow frequency and compared with 

Bulletin 74. An analysis comparing current (2010) low-flow 

conditions to preregulation conditions would be necessary to 

evaluate the effect of regulation on unregulated streamflow 

and stream ecology at regulated sites. 

Seven of the eight precipitation stations near the 

selected streamgages showed a net positive departure from 

the 30-year (1976–2006) average precipitation data, and one 

showed a decreased departure from the 30-year average. The 

positive change was most evident during the most recent 

10-year (1997–2006) period at many stations. The data for 

groundwater wells located in sand and gravel aquifers showed 

minor changes in the groundwater storage from 1976 through 

2006 and a lack of relation between the nearby streamgage and 

the groundwater well. 

Groundwater wells used for monitoring water availability 

in New York State, whether newly installed or reactivated, 

would need to monitor groundwater storage in aquifers that 

contribute to the base flows at long-term U.S. Geological 

Survey streamgages in order to assess the effects of changes 

in groundwater level on low flows. Selecting locations for 

groundwater wells simply on the basis of political boundaries 

and type of aquifer may not provide sufficient data to analyze 

the effects of changes in groundwater storage on surfacewater flows. 

This study has identified a major deficiency in the 

number of unregulated long-term U.S. Geological Survey 

streamgages that have remained active since Bulletin 74 was 

published in 1979 and a complete lack of a network of partialrecord streamgages to determine the low-flow conditions in 

New York State. Trends in temperature, precipitation, and 

streamflow have been identified in other studies done in New 
Selected References 21

York and the surrounding area. Recent low-flow studies in 

adjacent states have identified several statistically significant 

characteristics of basins (such as area of stratified drift, 

average basin slope, and (or) maximum basin elevation) 

in addition to drainage area, for improving the accuracy of 

regression equations to estimate low flows.

This study also has identified the need to use the 

additional data collected from 1976 through 2006 to compute 

new low-flow statistics to update those published in two 

previous reports from 1975 and 1979. These low-flow 

determinations could be used in conjunction with updated 

basin characteristics to develop statewide or regional 

equations for estimating low flows at ungaged streams in New 

York State.

Selected References

Barnes, C.R., 1986, Method for estimating low-flow statistics 

for ungaged streams in the Lower Hudson River basin, 

New York: U.S. Geological Survey Water-Resources 

Investigations Report 85-4070, 22 p.

Burns, D.A., Klaus, J., and McHale, M.R., 2007, Recent 

climate trends and implications for water resources in the 

Catskill Mountain region: Journal of Hydrology, v. 336, 

p. 155–170.

Eissler, B.B., 1979, Low-flow frequency analysis of streams in 

New York: New York State Department of Environmental 

Conservation Bulletin 74, 176 p.

Flynn, R.H., 2003, Development of regression equations to 

estimate flow durations and low flow frequency statistics in 

New Hampshire streams: U.S. Geological Survey WaterResources Investigations Report 02–4298, 30 p.

Helsel, D.R., and Hirsch, R.M., 1992, Statistical methods in 

water resources: New York, Elsevier, 522 p.

Johnson, M.K., Woodbury, P.B., Pell, A.N., and Lehman, J., 

2007, Land-use change and stream water fluxes—Decadal 

dynamics in watershed nitrate exports: Ecosystems, v. 10, 

p. 1182–1196.

Ku, H.F.H., Randall, A.D., and MacNish, R.D., 1975, 

Streamflow in the New York part of the Susquehanna River 

basin: New York State Department of Environmental 

Conservation Bulletin 71, 130 p.

Lumia, R., 1998, Flood of January 19-20, 1996 in New 

York State: U.S. Geological Survey Water-Resources 

Investigations Report 97–4252, 61 p.

New York City Department of Environmental Protection, 

2007, History of drought and water consumption: City of 

New York, N.Y., accessed December 3, 2009, at http://www.

nyc.gov/html/dep/html/drinking_water/droughthist.shtml.

Northeast Regional Climate Center, 2008, Climate Information 

for Management and Operational Decisions (CLIMOD): 

Ithaca, N.Y., at http://climod.nrcc.cornell.edu/.

Randall, A.D., 2010, Low flow of streams in the Susquehanna 

River basin of New York: U.S. Geological Survey 

Scientific Investigations Report 2010–5063, 57 p.

Reynolds, R.J., 2000, Evaluation of the Federal-State 

cooperative observation well network in upstate New 

York, 1995–97: U.S. Geological Survey Open-File Report 

99–468, 36 p.

Ries, K.G., III, 1994, Development and application of 

generalized-least-squares regression models to estimate 

low-flow duration in Massachusetts: U.S. Geological 

Survey Water-Resources Investigations Report 94–4155, 

25 p.

Riggs, H.C., 1968, Frequency curves: U.S. Geological Survey 

Techniques of Water-Resources Investigations, book 4, 

chap. B1, 18 p.

Riggs, H.C., 1972, Low-flow investigations: U.S. Geological 

Survey Techniques of Water-Resources Investigations, book 

4, chap. A2, 15 p.

U.S. Geological Survey, 1979a, Land use and land cover, 

1970–76, Hartford, Connecticut; New York; New Jersey; 

Massachusetts: U.S. Geological Survey Land-Use Map 

Series, Map L–79, scale 1:250,000.

U.S. Geological Survey, 1979b, Land use and land cover, 

1974, Scranton, Pennsylvania; New York; New Jersey: U.S. 

Geological Survey Land-Use Map Series, Map L–35, scale 

1:250,000.

U.S. Geological Survey, 1980, Water resources data for 

New York: U.S. Geological Survey Water-Data Report 

NY–79–1, 538 p.

U.S. Geological Survey, 1984a, Land use series, Binghamton 

[quadrangle], New York: U.S. Geological Survey OpenFile Report 84–532, 5 sheets.

U.S. Geological Survey, 1984b, Land use series, Rochester 

[quadrangle], New York, U.S.; Ontario, Canada: U.S. 

Geological Survey Open-File Report 84–30, 1 sheet.

U.S. Geological Survey, 1984c, Land use series, Utica 

[quadrangle], New York: U.S. Geological Survey OpenFile Report 84–533, 5 sheets.

U.S. Geological Survey, 1985, Land use series, Elmira 

[quadrangle], New York; Pennsylvania: U.S. Geological 

Survey Open-File Report 85–318, 5 sheets.

U.S. Geological Survey, 2003, Digital map file of the 2001 

National Land Cover Database for New York state, edition 

1.0, 30-meter Landsat thematic mapper data: Earth 

Resource Observation Systems (EROS) Data Center, at 

http://www.mrlc.gov/nlcd.php
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For additional information write to:

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or visit our Web site at:

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Suro and Gazoorian—Changes in Low-Flow Frequency from 1976–2006 at Selected Streamgages in N.Y., Excluding Long Island—Scientific Investigations Report 2011–5112