Regression Equations for Estimating Flood Flows 

for the 2-, 10-, 25-, 50-, 100-, and 500-Year 

Recurrence Intervals in Connecticut

By Elizabeth A. Ahearn

U.S. Department of the Interior

U.S. Geological Survey

In cooperation with the Connecticut Department of Transportation and the

Connecticut Department of Environmental Protection

Scientific Investigations Report 2004-5160
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2004

For sale by U.S. Geological Survey, Information Services

Box 25286, Denver Federal Center

Denver, CO 80225

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

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endorsement by the U.S. Government.

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.
iii

CONTENTS

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

IIntroduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

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

Previous Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Description of the Study Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Estimating Flood Flows at Stream Sites in Nonurban Basins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Data Used in the Regression Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Analytical Procedures for Regression Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Correlations Between Explanatory Variables. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Regression Analysis Using Ordinary-Least-Squares. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Regression Analysis Using Generalized-Least-Squares . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Diagnostic Evaluation of the Regression Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Evaluation of Possible Hydrologic Subregions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Results of the Regression Analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Accuracy of the Regression Equations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Limitations of the Regression Equations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Simplified Regression Equations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Web Application for Solving the Regression Equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Estimating Flood Flows at Stream Sites in Urban Basins. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Definitions of Equation Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Accuracy and Limitations of the National Urban Regression Equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Comparison of Flood-Flow Estimates From the Urban Equations to Observed Flows in Connecticut . . . 23

Weighted Averages of Flood Flow for Streamflow-Gaging Stations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Evaluation of the Streamflow-Gaging Network for Characterizing Flood Flows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

Spatial Coverage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Record Lengths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Drainage-Basin Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Summary and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Selected References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

APPENDIXES

1. Peak-flow frequency estimates for streams in Connecticut for selected recurrence intervals . . . . . 55

2. Hydrologic characteristics for streamflow-gaging stations used in the regression analysis for

Connecticut . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

FIGURES

1. Map showing locations of the 70 streamflow-gaging stations with upstream drainage basins 

used to develop regression equations for estimating flood flows at stream sites in 

Connecticut . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4

2. Graph showing the lengths of record for 70 streamflow-gaging stations used to develop 

regression equations for estimating flood flows at stream sites in Connecticut . . . . . . . . . . . . . . . . . . . . . .6

3. Map showing residuals values derived from regression equations (using drainage 

area, 24-hour rainfall for the 100-year recurrence interval, and mean basin elevation as 

explanatory variables) for estimating flood flows at stream sites in Connecticut . . . . . . . . . . . . . . . . . . . 12
iv

4. Map showing distribution of 24-hour rainfall for a 100-year recurrence interval with locations 

of the 70 streamflow-gaging stations used to develop regression equations for estimating flood

flows at stream sites in Connecticut . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

5. Map showing land-surface elevations with locations of the 70 streamflow-gaging stations 

used to develop regression equations for estimating flood flows at stream sites in Connecticut . . . 16

6. Graphs showing two-dimensional ranges of explanatory variables for the 100-year regression

equation for estimating flood flows at stream sites in Connecticut . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

7. Schematic diagram showing typical drainage-basin shapes and subdivision into basin thirds. . . . . 21

8. Map showing locations of the 25 core streamflow-gaging stations for flood analysis of 

nonurban basins and ungaged regional basins unaffected by extensive flood-control regulation

in Connecticut . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

9. Graph showing the distribution of the basin characteristics of the 25 core streamflow-gaging 

stations for flood analysis in Connecticut . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

10. Map showing land-surface elevations with locations of the 25 core streamflow-gaging 

stations for flood analysis in Connecticut . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

11. Map showing land-cover data with locations of the 25 core streamflow-gaging stations for 

flood analysis in Connecticut . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

TABLES

1. Descriptions of 70 streamflow-gaging stations used to develop statewide regression equations

for estimating flood flows for the 2-, 10-, 25-, 50-, 100-, and 500-year recurrence intervals at 

stream sites in Connecticut. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

2. Description of hydrologic characteristics used in the development of regression equations to 

estimate flood flows in Connecticut . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

3. Statistical summary of hydrologic characteristics used in the development of regression 

equations to estimate flood flows in Connecticut . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8

4. Regression equations for estimating flood flows for the 2-, 10-, 25-, 50-, 100,- and 500-year 

recurrence intervals at stream sites in nonurban basins in Connecticut and their accuracy . . . . . . . 13

5. Simplified regression equations for estimating flood flows for the 2-, 10-, 25-, 50-, 100,- and 

500-year recurrence intervals at stream sites in nonurban basins in Connecticut and 

their accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

6. National regression equations for estimating flood flows the 2-, 10-, 25-, 50-, 100,- and 500-year 

recurrence intervals in urban basins and their accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

7. Ranges of explanatory variables used in the national regression equations for estimating 

flood-flow statistics at stream sites in urban basins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

8. Flood-frequency computations for urban basins in Connecticut. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

9. Weighted averages of flood flows for selected recurrence intervals based on log-Pearson 

type III analysis of data from streamflow-gaging stations and the regression equations for 

estimating flood flows at stream sites in Connecticut . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

10. Streamflow-gaging stations in the current (2004) network that most represent natural hydrologic 

conditions for regional flood analysis in Connecticut . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
v

Conversion Factors and Datum

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

(NAVD 88).

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

Multiply By To obtain

inch (in.) 2.54 centimeter

foot (ft) 0.3048 meter

mile (mi) 1.609 kilometer

square mile (mi2) 2.590 square kilometer

foot per mile (ft/mi) 0.1894 meter per kilometer

cubic foot (ft3

) 0.02832 cubic meter

cubic foot per second (ft3/s) 0.02832 cubic meter per second

Regression Equations for Estimating Flood Flows for the 

2-, 10-, 25-, 50-, 100-, and 500-Year Recurrence Intervals 

in Connecticut

by Elizabeth A. Ahearn

Abstract

Multiple linear-regression equations were developed to 

estimate the magnitudes of floods in Connecticut for recurrence 

intervals ranging from 2 to 500 years. The equations can be used 

for nonurban, unregulated stream sites in Connecticut with 

drainage areas ranging from about 2 to 715 square miles. Floodfrequency data and hydrologic characteristics from 70 streamflow-gaging stations and the upstream drainage basins were 

used to develop the equations. The hydrologic characteristics—drainage area, mean basin elevation, and 24-hour rainfall—are used in the equations to estimate the magnitude of 

floods. Average standard errors of prediction for the equations 

are 31.8, 32.7, 34.4, 35.9, 37.6 and 45.0 percent for the 2-, 10-, 

25-, 50-, 100-, and 500-year recurrence intervals, respectively. 

Simplified equations using only one hydrologic characteristic—drainage area—also were developed. The regression analysis is based on generalized least-squares regression techniques. 

Observed flows (log-Pearson Type III analysis of the 

annual maximum flows) from five streamflow-gaging stations 

in urban basins in Connecticut were compared to flows estimated from national three-parameter and seven-parameter 

urban regression equations. The comparison shows that the 

three- and seven- parameter equations used in conjunction with 

the new statewide equations generally provide reasonable estimates of flood flows for urban sites in Connecticut, although a 

national urban flood-frequency study indicated that the threeparameter equations significantly underestimated flood flows in 

many regions of the country. Verification of the accuracy of the 

three-parameter or seven-parameter national regression equations using new data from Connecticut stations was beyond the 

scope of this study.

A technique for calculating flood flows at streamflow-gaging stations using a weighted average also is described. Two 

estimates of flood flows—one estimate based on the log-Pearson Type III analyses of the annual maximum flows at the gaging station, and the other estimate from the regression equation—are weighted together based on the years of record at the 

gaging station and the equivalent years of record value determined from the regression. Weighted averages of flood flows 

for the 2-, 10-, 25-, 50-, 100-, and 500- year recurrence intervals 

are tabulated for the 70 streamflow-gaging stations used in the 

regression analysis. Generally, weighted averages give the most 

accurate estimate of flood flows at gaging stations. 

An evaluation of the Connecticut’s streamflow-gaging 

network was performed to determine whether the spatial coverage and range of geographic and hydrologic conditions are adequately represented for transferring flood characteristics from 

gaged to ungaged sites. Fifty-one of 54 stations in the current 

(2004) network support one or more flood needs of federal, 

state, and local agencies. Twenty-five of 54 stations in the current network are considered high-priority stations by the U.S. 

Geological Survey because of their contribution to the longterm understanding of floods, and their application for regionalflood analysis. Enhancements to the network to improve overall 

effectiveness for regionalization can be made by increasing the 

spatial coverage of gaging stations, establishing stations in 

regions of the state that are not well-represented, and adding 

stations in basins with drainage area sizes not represented. 

Additionally, the usefulness of the network for characterizing 

floods can be maintained and improved by continuing operation 

at the current stations because flood flows can be more accurately estimated at stations with continuous, long-term record. 

Introduction

Floods are among the most destructive and costly natural 

disasters in the United States (http://www.fema.gov/library). In 

Connecticut, floods historically have caused millions of dollars 

in damages—in terms of property, disrupted business, and personal trauma. During the 1992 flood that affected the western 

and central coastal areas of Connecticut, three persons lost their 

lives as a result of flooding, property damages were estimated 

at $4.6 million (in 1992 dollars), and 26 homes were destroyed 

(D. Glowacki, Connecticut Department of Environmental Protection, written comm., 2004). Flooding affects every community (169 towns) in Connecticut and continues to be a constant 

threat in the 21st century. The most severe floods in terms of 

magnitude, extent, loss of life, and property damages in Connecticut during the 20th century took place in September 1938, 

August 1955, October 1955, and June 1982. The September 

1938, August 1955, and October 1955 floods were caused by 

major hurricanes and characterized by intense rainfall, significant flooding, and severe damages. The September 1938 flood 

resulted in hundreds of lives lost and damages of about $100 

million (Paulson and others, 1940). The August 1955 flood 

resulted in 87 deaths and property damages of about $100 million in Connecticut (Bogart, 1960). The October 1955 flood 

resulted in 17 deaths and state damages of about $36 million 
2 Regression Equations for Estimating Flood Flows in Connecticut

(Bogart, 1960). The June 1982 flood resulted in 11 deaths and 

damages of about $250 million (L.R. Johnston Associates, 

1983). The most notable floods in Connecticut since 1620 are 

listed chronologically in “Historical Floods in New England” 

(Thomson and others, 1964). Information on memorable floods 

also can be found in Weiss (1991). Consequently, a better 

understanding of the flood magnitude and likelihood of flood 

flows will provide information for the safe design of structures 

along rivers, the effective management of flood-prone areas, 

and development of flood-insurance-rate maps. 

The U.S. Geological Survey (USGS) uses flood-regionalization procedures to relate flood characteristics to watershed 

and climatic characteristics through the use of regression analysis. Flood-regionalization procedures provide a means of predicting or estimating flood flows at stream sites where little or 

no streamflow information is available. Traditionally, Connecticut has been considered to be one hydrologic region and statewide regression equations are used to estimate flood flows at 

stream sites. Flood-regionalization studies are performed periodically because additional data improve the accuracy of the 

regression equations. Various USGS reports provide equations 

for estimating the magnitude and frequency of floods on unregulated streams in Connecticut. Since the last study (Weiss, 

1983), an additional 20+ years of annual maximum flow data 

have been collected, the guidelines for computing station floodfrequency curves (Interagency Advisory Committee on Water 

Data, 1982) were updated, and newer regression procedures—generalized-least-squares—have become available. 

Generalized-least-squares regression accounts for error in flood 

characteristics because of unequal record lengths at gaging stations and the correlation of flood characteristics between sites 

(Stedinger and Tasker, 1985; 1986). These factors warrant revision of techniques previously used.

In an effort to provide updated information for regulatory, 

planning, and design activities in Connecticut, the USGS is conducting a three-part study, in cooperation with the Connecticut 

Department of Transportation (DOT) and the Connecticut 

Department of Environmental Protection (DEP), to improve 

flood-flow statistics for Connecticut. In the first part of the 

study, flood-flow statistics were estimated for 128 streamflowgaging stations in Connecticut for 1.5-, 2-, 10-, 25-, 50-, 100-, 

and 500-year recurrence intervals (Ahearn, 2003, http://

water.usgs.gov/pubs/wri/wri034196). In the second part of the 

study (this report), regional regression equations were developed to estimate flood flows at nonurban, unregulated stream 

sites in Connecticut. The third planned part of the study will 

implement a World Wide Web application, named StreamStats, 

which includes an automated geographic information system 

(GIS) procedure that measures the hydrologic characteristics 

and solves regression equations to estimate flood-flow statistics 

for user-selected sites in Connecticut. The web application also 

will include the flood flows for 128 USGS streamflow-gaging 

stations from the first part of the study. 

Purpose and Scope

This report presents new statewide regression equations 

that can be used to estimate the magnitude and frequency of 

floods for nonurban, unregulated streams in Connecticut. The 

equations can be used to estimate the magnitude of flows for the 

2-, 10-, 25-, 50-, 100-, and 500-recurrence intervals. The equations were developed by incorporating flood-frequency estimates (see appendix 1) from the first phase of the study 

(Ahearn, 2003) with an analysis of 22 drainage-basin and climatic characteristics. Statistical procedures—ordinary-leastsquares regression and generalized least-squares regression—were used in the regional analysis. This report updates 

and supersedes previously published flood-flow frequency 

studies for stream sites in Connecticut.

This report also provides (1) a comparison of observed 

flood flows from streamflow-gaging stations to flood flows 

estimated from the national regression equations for urban 

basins, (2) a technique for calculating flood flows for streamflow-gaging stations using a weighted average, and (3) an evaluation of the streamflow-gaging network for characterizing 

floods in Connecticut. 

Throughout this report, flood flow refers to the flood flow 

of a specified recurrence interval, such as the 2, 10, 25, 50, 100, 

and 500 year; annual maximum flow refers to the annual maximum flow for the water year; and periods of record for streamflow-gaging stations are given in water years. A water year is 

the 12-month period beginning October 1 and ending September 30, is designated by the calendar year in which it ends; for 

example, the 12-month period ending September 30, 2004, is 

called the 2004 water year.

Previous Studies

Several studies by USGS that provide equations for estimating the magnitude and frequency of flood-flows in Connecticut have been published. The earliest regional analysis of flood 

flows was made by Bigwood and Thomas (1955), who presented methods for estimating the mean-annual flood based on 

drainage area and basin slope. Weiss (1975) provided a method 

for estimating flood-flow magnitudes at 2- and 100-year recurrence intervals for urbanized and nonurbanized basins in Connecticut. Regional equations in the 1975 report included drainage area, 24-hour rainfall, main channel stream length, and 

main channel slope as explanatory variables to estimate floodflow magnitudes. Weiss (1983) updated the regional regression 

analysis of flood flows by incorporating additional streamflow 

data and evaluating geologic characteristics. Weiss used multiple regression analysis to define the relation between flood 

flows and some hydrologic characteristics (drainage area, 24-

hour rainfall for selected recurrence intervals, main channel 

length, main channel slope, and the percentage of the basin 

underlain by coarse-grained stratified deposits) at recurrence 

intervals of 2, 10, 25, 50, and 100 years, for unregulated streams 

in Connecticut. Previous studies were limited to basin or hydro-
Estimating Flood Flows at Stream Sites in Nonurban Basins 3

logic characteristics that could be readily measured from topographic maps. 

Description of the Study Area

Connecticut encompasses an area of 5,009 square miles 

(mi2) and can be divided into four physiographic regions: western uplands (northwestern part of the state), eastern uplands 

(northeastern part of the state), central valley (central part of the 

State), and coastal lowlands. The western uplands generally 

have the steepest topography in the state; land-surface elevations range from about 500 to 2,300 ft above NAVD 88 with 

average slopes of about 11 percent. Land-surface elevations in 

the eastern uplands range from about 500 to 1,300 ft above 

NAVD 88 with average slopes of about 8 percent. Topographic 

relief along the coastal lowland and central valley generally is 

low with land-surface elevations ranging from 0 to about 500 ft 

above NAVD 88. Average slopes along the coastal and central 

lowlands are less than 7 percent. 

The climate in Connecticut generally is temperate and 

humid, with four distinct seasons. Prevailing westerly winds 

alternately transport cool, dry, continental-polar, and warm, 

moist, maritime-tropical air masses into the region, resulting in 

frequent weather changes. Major storms are usually (1) coastal 

storms of extratropical origin; (2) frontal storms moving northeasterly along the Appalachian Mountains; or (3) tropical 

cyclones, including hurricanes, that tend to provide the majority 

of high intensity rains during summer and fall (Weiss, 1983). 

Localized thunderstorms tend to provide short-term (hours) 

intensive rainfall and are prevalent during the summer (Miller 

and others, 2003). Precipitation is distributed fairly evenly 

throughout the year and averages about 45 inches (in.) annually 

(based on long-term data from 1920 to 1996). The climate is 

moderated by maritime influences along coastal regions. 

Regional differences in topography, elevation, and proximity to 

the ocean can result in a substantial areal variation in snowfall 

amounts. Mean snowfall ranges from 30 in. along coastal areas, 

to 40 in. inland, to 60 in. in the northwestern part of the state 

(Miller and others, 2003). Average annual temperatures range 

from 51.7 °F in coastal areas (Bridgeport) to 49.9 °F in the central valley (Hartford) (http://met-www.cit.cornell.edu/ccd/

nrmavg.html). 

Land cover in Connecticut is highly mixed, with forests 

dominating the north, and densely populated urban areas featured prominently along the southwestern coastal and central 

valley region. Most of the land cover in Connecticut can be classified as forested, that is, characterized by little urban development or low population density. Based on the National Land 

Cover Data (NLCD) for the early 1990s at a 30-m (meter) resolution grid for Connecticut, 59.1 percent of the land cover is 

categorized as forest (deciduous, evergreen and mixed forest), 

5.0 percent is categorized as urban (commercial, industrial and 

high population density), 11.6 percent is categorized as suburban (low population density), and 24.3 percent is categorized as 

other. 

The surficial geologic materials of Connecticut, described 

by Stone and others (1992; in press), are primarily glacial 

deposits. Unconsolidated glacial deposits of varying thickness 

blanket the bedrock surface across most of the state. Glacial till 

is the most widespread surficial deposit and is generally thin 

(less than 15 ft thick). Till was deposited directly by glacial ice 

and is a nonsorted material ranging in grain size from clay to 

large boulders. Glacial stratified deposits occur primarily in valleys both inland and along the coast of Connecticut; these materials were laid down by glacial meltwater in streams and lakes 

and consist of layers of gravel, sand, silt, and clay. These materials are most widespread in the broad central Connecticut Valley and along the coast. Thick, permeable glacial stratified 

deposits cover much of the area in the broad valleys of the state. 

Till, bedrock, and fine-grained stratified deposits (very fine 

sand, silt and clay) generally have low permeability in comparison to the coarse-grained stratified deposits, which generally 

have high permeability.

Estimating Flood Flows at Stream Sites in 

Nonurban Basins

Regression equations can be used to estimate flood flows 

for nonurban, unregulated stream sites. The equations statistically relate the magnitude and frequency of floods for a group 

of data-collection (streamflow-gaging) stations to the drainagebasin and climatic characteristics of the upstream drainage 

areas. In this report, the combined drainage-basin and climatic 

characteristics are called hydrologic characteristics. The hydrologic characteristics were selected on the basis of their potential 

relation to flood flows, results of previous studies in similar 

hydrologic regions, and on the ability to measure the hydrologic 

characteristics using digital datasets with GIS technology. A 

database for the regional analysis was developed using active 

and discontinued streamflow-gaging stations, and populated 

with flood-flow frequency estimates for selected recurrence 

intervals for the gaging stations and the hydrologic characteristics of the basins upstream from each station. 

Data Used in the Regression Analysis

Flood-flow frequency estimates for the 2-, 10-, 25-, 50-, 

100-, and 500-year recurrence intervals were computed for 128 

streamflow-gaging stations by fitting the Pearson Type III distribution to the logarithms of the annual maximum flows (largest instantaneous streamflow for each water year) using the 

guidelines in Bulletin 17B (Interagency Advisory Committee 

on Water Data, 1982). Of these, flood-flow frequency estimates 

from 70 streamflow-gaging stations (24 active and 46 inactive) 

(fig. 1; table 1, at back of report) were suitable for use in the 

regression analysis. Stations were selected for the regression 

analysis using the following criteria: (1) the station has a 
4 Regression Equations for Estimating Flood Flows in Connecticut

2

3

4

5

1

8

9

7

6

53

47

30

33

12

54

17

70

67

63

61

59

57

56 55

52

45 43

42

40 41

37

31

29

26

25

21

19

15

14

13

10

69

49

36

48

46

38

27

51

39

44

58

65

64

66

68

20

18

28

35

24

62

60

50

34

32

23

22

16

16

11

73 ̊30'

72 ̊

41 ̊

41 ̊30'

42 ̊

0 10 5 MILES

0 10 5 KILOMETERS

EXPLANATION

Basin boundary

Main channel of gaged 

stream

Streamflow-gaging station

and reference number

Connecticut River

Housatonic River

Thames River

Long Island Sound

Figure 1. Locations of the 70 streamflow-gaging stations with upstream drainage basins used to develop regional regression equations 

for estimating flood flows at stream sites in Connecticut. 
Estimating Flood Flows at Stream Sites in Nonurban Basins 5

1 01118300 Pendleton Hill Brook near Clarks Falls

2 01119500 Willimantic River near Coventry

3 01120000 Hop River near Columbia

4 01120500 Safford Brook near Woodstock Valley

5 01121000 Mount Hope River near Warrenville

6 01122000 Natchaug River at Willimantic

7 01122500 Shetucket River near Willimantic

8 01123000 Little River near Hanover

9 01124000 Quinebaug River at Quinebaug

10 01125490 Little River at Harrisville

11 01125500 Quinebaug River at Putnam

12 01125600 Mashamoquet Brook at Abington

13 01126000 Fivemile River at Killingly

14 01126500 Moosup River at Moosup

15 01126600 Blackwell Brook near Brooklyn

16 01127000 Quinebaug River at Jewett City

17 01127500 Yantic River at Yantic

18 01184100 Stony Brook near West Suffield

19 01184300 Gillette Brook at Somers

20 01184490 Broad Brook at Broad Brook

21 01184500 Scantic River at Broad Brook

22 01186500 Still River at Robertsville

23 01187000 West Branch Farmington River at 

Riverton 

24 01187300 Hubbard River near West Hartland

25 01187400 Valley Brook near West Hartland

26 01187800 Nepaug River near Nepaug

27 01188000 Burlington Brook near Burlington

28 01189000 Pequabuck River at Forestville

29 01189200 Stratton Brook near Simsbury

30 01189390 East Branch Salmon Brook at Granby

31 01189500 Salmon Brook near Granby

32 01190000 Farmington River at Rainbow

33 01190600 Wash Brook at Bloomfield

34 01191000 North Branch Park River at Hartford

35 01192500 Hockanum River near East Hartford

36 01192650 Roaring Brook at Hopewell

37 01192700 Mattabasset River at East Berlin

38 01192883 Coginchaug River at Middlefield

39 01193500 Salmon River near East Hampton

40 01193800 Hemlock Valley Brook at Hadlyme

41 01194000 Eightmile River at North Plain

42 01194500 East Branch Eightmile River near 

North Lyme

43 01195000 Menunketesuck River near Clinton

44 01195100 Indian River near Clinton

45 01195200 Neck River near Madison

46 01196500 Quinnipiac River at Wallingford

47 01196580 Muddy River near North Haven

48 01196620 Mill River near Hamden

49 01196700 Wepawaug River at Milford

50 01198500 Blackberry River at Canaan

51 01199050 Salmon Creek at Lime Rock

52 01199200 Guinea Brook at West Woods Road at 

Ellsworth

53 01200000 Tenmile River near Gaylordsville

54 01201190 West Aspetuck River at Sand Road near 

New Milford

55 01201500 Still River near Lanesville

56 01203000 Shepaug River near Roxbury

57 01203510 Pootatuck River at Sandy Hook

58 01204000 Pomperaug River at Southbury

59 01204800 Copper Mill Brook near Monroe

60 01206000 Naugatuck River near Thomaston

61 01206500 Leadmine Brook near Thomaston

62 01208500 Naugatuck River at Beacon Falls

63 01208850 Pequonnock River at Trumbull

64 01208925 Mill River near Fairfield

65 01208950 Sasco Brook near Southport

66 01208990 Saugatuck River near Redding

67 01209500 Saugatuck River near Westport

68 01209700 Norwalk River at South Wilton

69 01211700 East Branch Byram River at Round Hill

70 01212100 East Branch Byram River at Riversville

Reference 

number

U.S. Geological Survey streamflow-gaging station Reference 

number

U.S. Geological Survey streamflow-gaging station

Number Name Number Name
6 Regression Equations for Estimating Flood Flows in Connecticut

minimum of 10 years of continuous-record data (stations with 

less than 10 years of data may not provide a sufficient sampling 

of the variation of the population); (2) no substantial effects of 

flood-control regulation are observed in the basins (basins with 

usable storage of more than 4.5 million cubic feet (103 acre-ft) 

per square mile are considered to have substantial effects of 

flood-detention (Benson, 1962)); (3) no substantial effects of 

urbanization or other man-made influences, such as channel 

improvements, are observed in the basins (basins with more 

than 15 percent of the land cover designated as high-density residential, commercial, or industrial are considered urbanized); 

and (4) the station has its upstream drainage basin within Connecticut or extending no more than 20 mi (miles) outside the 

Connecticut state border. 

Not all stations that meet the criteria listed above were 

selected for use in the study; for example, stations where the 

stage-discharge relation was not reasonably well-defined at 

high (flood) flows were not used. The 70 stations selected for 

regression analysis represent good spatial coverage of streams 

in Connecticut (fig. 1). The lengths of record (number of years 

streamflow data was collected at a station) for the stations range 

from 10 to 75 years with an average of 37 years (fig. 2).

Hydrologic characteristics that were considered to be 

potentially important factors affecting flood flows, and, therefore, useful in developing equations for predicting flood flows, 

were selected by USGS with input from state cooperators (DOT 

and DEP; table 2). Twenty-two hydrologic characteristics were 

quantified for the 70 basins being studied and were divided into 

two groups: drainage-basin characteristics (basin morphology, 

land cover, soils, and geology) and climatic characteristics 

(average rainfall and rainfall frequency and duration). Statistical summaries of the hydrologic characteristics are presented in 

table 3. All the characteristics were determined from digital 

map data. The digital data include, but are not limited to, (1) 

drainage subbasins at 1:24,000 scale (Connecticut Drainage 

Basin Boundaries); (2) elevation from USGS Digital Elevation 

Models (DEMs) at 1:24,000 scale from the USGS National Elevation Dataset (U.S. Geological Survey, 2001); (3) land cover 

at 1:24,000 scale from the USGS National Land Cover Dataset 

(U.S. Geological Survey, 2000); (4) soil characteristics at 

1:250,000 scale from STATSGO soil characteristics (U.S. Geological Survey, 1997; Natural Resources Conservation Service, 

1991); (5) surficial geology at 1:250,000 scale from Surficial 

Materials Map of Connecticut (Stone and others, 1992); (6) centerline hydrography for Connecticut at 1:24,000 scale developed by Connecticut DEP (Danenberg and Bogar, 1994); (7) 

mean annual rainfall characteristics (Daly, 2000; Randall, 

1996); and (8) maximum 24-hour and 6-hour rainfall characteristics (Miller and others, 2003; B.N. Belcher, Northeast 

Regional Climate Center, written commun., 2003). 

Subbasins were aggregated to the streamflow-gaging stations on 1:24,000 USGS topographic maps (Connecticut Drainage Basin Boundaries). Where a gaging station was located 

within a subbasin (not at the subbasin outlet), the basin boundary was digitized from the gaging station to the subbasin boundary. Centerline hydrography data was used to determine the 

total length of streams, in miles, for each basin. Centerline 

hydrography data was developed from digital line graph (DLG) 

datasets (Danenberg and Bogar, 1994). For the majority of the 

quadrangles used to create the centerline hydrography, intermittent and perennial channels were aggregated into one category. 

Average slope of the basins and minimum, maximum, and

10 to 19 20 to 29 30 to 39 40 to 49 50 to 59 60 to 69 70 to 75

0

5

10

15

20

RECORD LENGTH, IN YEARS

NUMBER OF STREAMFLOW-GAGING STATIONS

Figure 2. The lengths of record for 70 streamflow-gaging stations used to develop regression equations for estimating flood flows at 

stream sites in Connecticut. 
Estimating Flood Flows at Stream Sites in Nonurban Basins 7

Table 2. Description of hydrologic characteristics used in the development of regression equations to estimate flood flows 

in Connecticut.

[NAVD 88, North American Vertical Datum of 1988; DEM, digital elevation model]

Hydrologic characteristic Definition

DRAINAGE-BASIN CHARACTERISTICS

Basin Morphology (10 variables)

Drainage area Drainage area that contributes to surface runoff, in square miles

Main channel length Main channel length measured along channel from streamflow-gaging station to the 

headwaters of the channel following the channel with the largest upstream area, in miles

Main channel slope Main channel slope using main channel length and associated upstream and downstream elevation 

points, in feet per mile

Basin perimeter Perimeter following the basin boundary, in miles

Total length of streams Total length of perennial channels in the basin, in miles

Basin shape factor Main channel length squared divided by the drainage area

Maximum basin elevation Maximum basin elevation in the drainage basin derived from the intersection of basin polygon coverages 

and DEMs, in feet above NAVD 88

Mean basin elevation Mean basin elevation in the drainage basin derived from the intersection of basin polygon coverages and 

DEMs, in feet above NAVD 88

Elevation range Maximum basin elevation minus the minimum basin elevation (at the streamflow-gaging station), in feet

Mean basin slope Mean basin slope in the drainage basin derived from the intersection of basin polygon 

coverages and DEMs, in percent

Land Cover (4 variables)

Forest Area of basin classified as forested (deciduous forest, deciduous shrubland, evergreen forest and mixed 

forest were aggregated into forest), in percent

Water Area of basin classified as water, in percent

Wetland Area of basin classified as wetland (woody wetlands and emergent herbaceous were aggregated into 

wetlands), in percent

Storage Area of lakes, ponds, reservoirs and wetlands in a basin, in percent 

Soils (2 variables) 

Hydrologic soil index Hydrologic soil groups A through D

Soil drainage index Soils drainage index 1-8, code identifying the natural drainage condition of the soil and refers to the 

frequency and duration of periods when the soil is free of saturation

Geology (2 variables)

Coarse-grained stratified 

glacial deposits

Area of basin classified as coarse-grained (sand and gravel) stratified glacial deposits and coarse-grained 

stratified deposits over fine-grained (very fine sand, silt, and clay) stratified deposits, in percent

Glacial till Area of basin classified as glacial till, in percent

CLIMATIC CHARACTERISTICS

Rainfall (4 variables)

Mean-annual rainfall, 1951-80 Mean-annual rainfall for 1951-1980, in inches

Mean-annual rainfall, 1961-90 Mean-annual rainfall for 1961-1990, in inches 

24-hour rainfall (inches) Maximum 24-hour rainfall having recurrence interval of 2-, 10-, 25-, 50-, 100-, and 500-year, in inches

6-hour rainfall (inches) Maximum 6-hour rainfall having recurrence interval of 2-, 10-, 25-, 50-, 100-, and 500- year, in inches
8 Regression Equations for Estimating Flood Flows in Connecticut

Table 3. Statistical summary of hydrologic characteristics used in the development of regression equations to estimate flood flows

in Connecticut.

[Hydrologic characteristics in bold represent the explanatory variables used in the final regression equations. NAVD 88, North American Vertical Datum of 1988]

Hydrologic characteristic Maximum 75th 

percentile Median Mean 25th 

percentile Minimum

Number 

of 

observations

DRAINAGE-BASIN CHARACTERISTICS

Drainage area (square miles) 715 78.4 28.8 75.2 12.3 1.69 70

Main channel length (miles) 77.5 21.9 12.7 16.9 8.67 2.29 70

Main channel slope (feet per mile) 141 58.8 42.1 46.7 25.7 9.40 70

Basin perimeter (miles) 228 57.4 33.7 47.2 23.2 6.94 70

Total length of streams (miles) 1,850 179 66.2 183 35.0 5.63 169

Basin shape factor 12.1 7.38 5.48 5.83 4.08 1.58 70

Maximum basin elevation (feet above NAVD 88) 2,450 1,290 997 1,080 773 360 70

Mean basin elevation (feet above NAVD 88) 1,310 736 525 590 371 169 70

Elevation range (feet) 2,080 1,040 723 840 565 218 70

Mean basin slope (percent) 14.6 9.49 8.07 8.52 7.15 4.82 70

Forest (percent) 94.7 76.3 68.2 67.1 58.3 32.4 70

Water (percent) 5.34 2.53 1.54 1.78 0.72 0.10 70

Wetland (percent) 10.2 8.14 6.95 6.71 5.37 0.47 70

Storage (percent) 13.7 10.5 8.40 8.49 7.02 1.71 70

Hydrologic soil index 3.00 2.71 2.60 2.61 2.53 2.16 70

Soil drainage index 3.90 3.54 3.49 3.46 3.36 2.95 70

Coarse-grained stratified glacial 

deposits (percent)

63.1 19.9 10.5 14.6 6.56 0.59 261

Glacial till (percent) 98.8 88.2 84.3 78.7 71.3 31.8 2

61

CLIMATIC CHARACTERISTICS

Mean annual rainfall, 1951-80 (inches) 50.7 48.4 47.8 47.5 46.6 43.1 70

Mean annual rainfall, 1961-90 (inches) 52.9 49.9 48.8 48.7 47.7 43.9 70

2-year, 24-hour rainfall (inches) 3.82 3.62 3.45 3.44 3.25 2.95 70

10-year, 24-hour rainfall (inches) 5.53 5.26 5.17 4.97 4.61 4.15 70

25-year, 24-hour rainfall (inches) 7.00 6.54 6.37 6.14 5.60 4.93 70

50-year, 24-hour rainfall (inches) 8.36 7.70 7.37 7.21 6.49 5.62 70

100-year, 24-hour rainfall (inches) 9.99 9.10 8.60 8.46 7.56 6.41 70

2-year, 6-hour rainfall (inches) 2.56 2.47 2.37 2.17 2.29 2.37 369

10-year, 6-hour rainfall (inches) 3.65 3.56 3.43 3.20 3.32 3.43 3

69

25-year, 6-hour rainfall (inches) 4.55 4.40 4.32 3.86 4.07 4.25 3

69

50-year, 6-hour rainfall (inches) 5.35 5.19 5.09 4.60 4.75 5.00 3

69

100-year, 6-hour rainfall (inches) 6.35 6.13 6.00 5.24 5.65 5.90 3

69

1

Compatible (digital) hydrography maps not available for U.S. Geological Survey station 01200000, Tenmile River near Gaylordsville, Conn.

2Compatible (digital) geology maps not available for nine stations with basins that extend into other states.

3Compatible 6-hour rainfall data not available for U.S. Geological Survey station 01200000, Tenmile River near Gaylordsville, Conn.
Estimating Flood Flows at Stream Sites in Nonurban Basins 9

mean land-surface elevations of the basins were determined by 

use of the 30-m National Elevation Dataset, 1:24,000 scale 

(U.S. Geological Survey, 2001). Some basin characteristics 

were combined to determine additional characteristics for use in 

the regression analysis. These characteristics included (1) basin 

shape, computed by main channel length squared, divided by 

drainage area ((main channel length 2

)/drainage area), and (2) 

range, in feet, computed by subtracting the minimum basin elevation from the maximum basin elevation. 

Land cover was defined using the National Land Cover 

Data (NLCD) for the early 1990s at a 30-m resolution grid (U.S. 

Geological Survey, 2000). Although there are a total of 17 subcategories within the national land-cover dataset, not all subcategories were used in the regression analysis of flood flows. 

Land-cover categories used include forest, water, wetlands, and 

urban. The subcategories water and wetlands were aggregated 

into the subcategory storage. Each of the five land-cover subcategory was tested in the regression analysis. 

Soil hydrologic group classifications used in the analysis 

are contained in the 1:250,000-scale State Soil Geographic 

(STATSGO) data (Natural Resources Conservation Service, 

1991; U.S. Geological Survey, 1997) and are based on generalized soils maps. Detailed county-level digitized soils data are 

not available for all of Connecticut; therefore, the more generalized STATSGO data were used to characterize soils. 

Climatic data consist of (1) mean annual rainfall for the 

period 1951–1980; (2) mean-annual rainfall for the period 

1961–1990; (3) maximum 24-hour rainfall for the 2-, 10-, 25-, 

50-, and 100-year recurrence intervals and; (4) maximum 6-

hour rainfall for the 2-, 10-, 25-, 50-, and 100-year recurrence 

intervals. Mean-annual rainfall data for the period 1951–1980 

was computed as spatially weighted values of rainfall for each 

basin using contour data converted to grided estimates at a 

1,000-ft resolution (Randall, 1996). Mean-annual rainfall data 

for the period 1961–1990 was computed using the Parameterelevation Regressions on Independent Slopes Model (PRISM) 

developed by Oregon State University as part of the Natural 

Resources Conservation Service (NRCS) Spatial Climatic 

Mapping Project (Daly, 2000). A non-proprietary PRISM GIS 

map of mean annual rainfall for the State of Connecticut 

(derived on the basis of rainfall data collected from 1961 to 

1990) was downloaded from the NRCS PRISM Web site (http:/

/www.ftw.nrcs.usda.gov/prism/ prismdata_state.html). The 

NRCS PRISM map was loaded into a GIS application and interpolated to 1,000-ft grids. Basin-wide averages of mean annual 

rainfall (in inches) were computed for each study basin by averaging the values of all 1,000-ft grids within the bounds of the 

drainage basin on an area basis. 

The 24-hour and 6-hour rainfall data are based on records 

from 73 daily-record stations and 71 separate hourly record stations in Connecticut and adjacent parts of New York, Massachusetts, and Rhode Island using the entire period of record 

available for each station. Initial analysis of the 24-hour and 6-

hour rainfall curves from Miller (2003) were scanned, georeferenced, and digitized. The 24-hour rainfall was selected for 

detailed analysis. The 24-hour rainfall values (B.N. Belcher, 

Northeast Regional Climate Center, written commun., 2003) 

were estimated for 0.2-degree evenly spaced points (about 

every 12 mi) across the state by the Northeast Regional Climate 

Center using a Cressman analysis. These data were then recompiled by USGS using GIS to develop 24-hour rainfall values at 

a finer scale of about 1,000 ft. 

Analytical Procedures for Regression Analysis

Multiple-linear regression analysis was used to determine 

which hydrologic characteristics best explain the variations in 

flood flows and to develop equations to predict flood flows, 

such as the 100-year flood discharge, at ungaged stream sites. 

Multiple regression analysis provides a mathematical equation 

of the relation between a response variable (flood-flow frequency estimate for the gaging station) and two or more explanatory variables (drainage-basin and climatic characteristics). 

The regression equation is generally reported in the following 

forms:

, (1)

or

(2)

where

The flood-flow frequency estimates and hydrologic characteristics used in a linear regression typically are log-normally 

distributed, and transformation of the variables to logarithms 

usually is necessary to satisfy the regression assumptions 

(described in section “Diagnostic Evaluation of the Regression 

Models”). Logarithmic (base-10) transformations were made of 

the flood flows and drainage-basin and climatic characteristics 

to linearize the relation between the explanatory variables 

(drainage-basin and climatic characteristics) and response variable (flood flows), to obtain equal variance about the regression 

line, and to help achieve normality. 

Correlations Between Explanatory Variables 

Because many drainage-basin and climatic characteristics 

exhibit some degree of dependence, a correlation matrix of the 

logarithms of the hydrologic characteristics was used initially to 

evaluate the amount of dependence among variables. The coefYT is the response variable (flood flow having 

T-year recurrence interval),

X1 to Xn are explanatory variables (drainage-basin and 

climatic characteristics), and

b0 to bn are regression coefficients estimated through 

generalized-least-squares procedures 

(described in section “Regression 

Analysis Using Generalized-LeastSquares”).

log10YT b0 + b1log10X1 b2log10X2 ... bnlog10Xn = + + ( )

YT 10

b0 X1

b1 ( ) X2b2 ( )... Xnbn = ( )
10 Regression Equations for Estimating Flood Flows in Connecticut

ficient of determination, denoted as R2

, represents the fraction 

of variability in y that can be explained by the variability in x. 

An R2

 value of 1.00 means perfect correlation, a value of -1.00 

means perfect inverse correlation, and a value of 0.00 means no 

correlation or complete independence. Several explanatory 

variables showed relatively high correlation: drainage area and 

total length of streams (R2

=0.99), drainage area and perimeter 

(R2

=0.99), drainage area and main channel length (R2=0.96), 

perimeter and main channel length (R2=0.97), maximum elevation and range (R2

=0.85), 24-hour rainfall and 6-hour rainfall 

(R2

=0.85), and mean annual rainfall 1951–1980 and mean 

annual rainfall 1961–1990 (R2

=0.80). All the variables were 

tested in the regression, and the explanatory variables that had 

high correlation with flood flows and low correlation with other 

explanatory variables were selected for further analysis. Of the 

original 22 variables, 11 variables were selected for further 

analysis: drainage area, main channel slope, basin shape, mean 

basin elevation, average basin slope, forest, storage, soil index, 

drainage index, average annual rainfall 1961-1990, and the 24-

hour rainfall for 2-, 10-, 25-, 50-, and 100-year recurrence intervals. 

Regression Analysis Using Ordinary-Least-Squares

Ordinary-least-squares regression (OLS) was used for 

selecting the explanatory variables that would appear in the 

final regression equations (Helsel and Hirsch, 1992). In OLS 

regression, equal weight is given to all stations in the analysis 

regardless of record length and the possible correlation of the 

flood-flow frequency estimates among stations. Variable selection was done using an “all-possible regression” procedure with 

the commercial statistics and data-management software S-Plus 

(Insightful, Inc., 2000). The all-possible regression procedure 

was used to find the best possible subsets of explanatory variables based on the Mallow’s Cp statistic as an initial criterion 

for discriminating among models. The models with the lowest 

Cp value generated from the all-possible regression procedure 

were then analyzed on the basis of the following statistics: 

• Adjusted-R2

 (adj-R2) measures the proportion of variation that is explained by the explanatory variables in 

the regression model. In contrast to R2

, the adj-R2 is 

adjusted for the number of variables in the model and 

the size of the sample.

• Mean square error (MSE) is a measure of how much 

the calculated value will differ from the true value. It is 

known as the sample model error variance of the estimates for the streamflow-gaging stations included in 

the analysis (Reis and Friesz, 2000). 

• Mallow’s Cp statistic is an estimate of the standardized 

mean square error of prediction. Cp statistic is a compromise between maximizing the explained variance 

by including all relevant variables and minimizing the 

standard error by keeping the number of variables as 

small as possible (Helsel and Hirsch, 1992).

• Predicted Residual Sum of Squares (PRESS) statistic is 

a validation-type estimator of error (Helsel and Hirsch, 

1992). PRESS uses n-1 observations to develop the 

equation, then estimates the value of the one left out. 

The process is repeated for each observation and the 

prediction errors are squared and summed.

Different models from the OLS regression were compared 

based on adj-R2, MSE, Mallow’s Cp, and the PRESS statistics. 

The models with a smaller MSE, Mallow’s Cp, and PRESS statistic and higher adj-R2 were preferred. In addition to the abovelisted statistical parameters used for identifying the best combination of explanatory variables, the explanatory variables were 

selected on the basis of (1) statistical significance at the 95-percent confidence level, (2) correlations among the explanatory 

variables, (3) how the explanatory variables might affect flood 

flows, (4) ease of measuring the variables using GIS, and (5) an 

analysis of the residuals. Explanatory variables that had a 95-

percent probability of effectiveness were classified as significant, and variables that had a 99-percent probability of effectiveness were classified as highly significant. If an explanatory 

variable was significant or highly significant, but had only a 

small effect on the standard error (arbitrarily chosen as less than 

a 3-percent change), it was left out of the model. Potential 

explanatory variables for the final models were further checked 

for redundant (considered highly correlated among themselves) 

and were not both (all) included. 

An all-possible regression was performed on 70 streamflow-gaging stations to determine the best combination of 

explanatory variables to use in the final regression equations. 

The explanatory variables used in the all-possible regression 

include: drainage area, main channel slope, basin shape, mean 

basin elevation, average basin slope, forest, storage, soil index, 

drainage index, average annual rainfall from 1961 to 1990, and 

the 24-hour rainfall for 2-, 10-, 25-, 50- and 100-year recurrence 

interval. Previous studies (Weiss, 1983) in Connecticut have 

shown that multiple-regression methods have successfully 

related flood flows to geology. The geologic characteristics 

were excluded from this analysis because the digital geologic 

maps for the adjacent states of New York, Massachusetts, and 

Rhode Island are not compatible with Connecticut’s digital geologic maps. Because the geologic characteristics in the state 

vary with elevation, it is assumed that the geologic characteristics can be indirectly evaluated with the drainage-basin characteristics—mean basin elevation and soil index. 

Results of the all-possible regression on the 70 gaging stations indicate that drainage area, 24-hour rainfall for selected 

recurrence intervals, and mean basin elevation are the best 

explanatory variables to predict flood flows at ungaged stream 

sites in Connecticut. The explanatory variables in the models 

were found to be statistically significant at the 95-percent confidence level or better, were not correlated with other explanatory variables, and improved the standard errors by more than

3 percent. 
Estimating Flood Flows at Stream Sites in Nonurban Basins 11

Regression Analysis Using Generalized-Least-Squares

Generalized-least-squares (GLS) regression, as described 

by Stedinger and Tasker (1985), was used to compute the final 

regression coefficients in the models (equations) and evaluate 

the models determined from the OLS regression. GLS regression is a more specialized method of regression that accounts 

for time-sampling error (a function of record length) and crosscorrelation between stations, which is not accounted for in OLS 

regression techniques. The USGS computer program GLSNET 

(available at http://water.usgs.gov/software/surface_water) was 

used to compute the regression coefficients. GLSNET also provides model diagnostics that were used to evaluate the adequacy 

of the models. 

Stedinger and Tasker (1985) found that GLS regression 

equations are more accurate and provide a better estimate of the 

accuracy of the equations than OLS regression equations when 

streamflow records at gaging stations are of different and 

widely varying lengths and when concurrent flows at different 

stations are correlated. GLS regression techniques give less 

weight to streamflow-gaging stations that have shorter records 

than other stations. In addition, GLS regression techniques give 

less weight to those stations where concurrent flood flows are 

correlated with other stations (Hodgkins, 1999). Results of the 

GLS equations were compared with the previously published 

equations (Weiss, 1983) and were found to have smaller standard errors associated with the regression.

Diagnostic Evaluation of the Regression Models 

Diagnostic checks were done to test the model adequacy 

and check whether the assumptions underlying the regression 

model are valid. The basic assumptions for a regression model 

include: (1) the equation adequately describes the relation 

between the response and explanatory variables, (2) the mean of 

the residual error is zero, (3) the variance of the residual error is 

constant and independent of the values of Xn, the explanatory 

variables, (4) the residual errors are normally distributed, and 

(5) the residual errors are independent of one another (Helsel 

and Hirsch, 1992). Overall, the model appears to fit the data reasonably well and adequately describes the relation between the 

response and explanatory variables. There was no evidence that 

any of the model assumptions have been violated. The model 

shows no curvature or changing variance. The residuals show a 

slight departure from normality, but it was not considered significant.

Diagnostic checks included looking for outliers and influential observations. The presence of outliers is a subtle form of 

non-normality. Influential observations are data that substantially change the fit of the regression line. Influence of an individual observation on the regressions was measured with 

Cook’s D statistic. Cook’s D statistic is a measure of the change 

in the parameter estimates when an observation is deleted from 

the regression analysis. No influential observations were found 

with Cook’s D statistic that appreciably altered the slope of the 

regression line. Statistical output of the regression program 

GLSNET was used to check possible problems with influential 

observations (high leverage sites) and outliers. The station 

leverage and standard residual values for the 70 observations 

from GLSNET showed no problems with high leverage or outliers.

The relations among the explanatory variables in the final 

models were investigated for problems with collinearity by 

computing variance-inflation factors (VIF) for each explanatory variable. VIF values express the ratio of the actual variance 

of the coefficient of the explanatory variable to its variance if it 

were independent of the explanatory variables (Cavalieri and 

others, 2000). VIF values greater than 5 indicate that a explanatory variable is so highly correlated to other explanatory variables that it is an unreliable explanatory variable and should not 

be included in the equations because the equations may provide 

erroneous results. None of the explanatory variables for this 

study had a VIF value greater than 5. 

Evaluation of Possible Hydrologic Subregions 

Hydrologic regions of homogeneous flood characteristics 

in Connecticut were evaluated primarily through an analysis of 

the areal distribution of the regression residuals for the 10- and 

100-year frequencies. The residuals are defined as the differences between the computed (predicted) values from the regression equations and the flood estimates based on the log Pearson 

Type III analysis of the annual maximum flows at the gaging 

station (observed flood flows). If the residuals show geographical biases, subdividing Connecticut into smaller hydrologic 

regions for analysis possibly can improve the accuracy of the 

regression; however, a balance is needed between isolating 

hydrologically similar regions and meeting minimum samplesize requirements. 

The residuals from the statewide regression model (equation) were plotted at the centroid of their respective drainage 

basins to look for geographical biases in the regression model 

(fig. 3). Regions with large positive residuals mean that 

observed flood flows are greater than computed flood flows 

(regression equations underestimate the flood flow), and 

regions with large negative residuals mean that the observed 

flood flows are lower than computed flood flows (regression 

equations overestimate the flood flow). Regions where the 

regression equations consistently overestimate or underestimate 

the flood flows can indicate the regional relation is inadequate 

and biased; subsequently, separate regression analysis would be 

performed for each subregion. In this study, the residuals from 

the statewide regression model do not show apparent geographical biases or regions with either large positive or negative 

residuals. Therefore, Connecticut was considered one region, 

and only one set of regression equations was developed to estimate flood flows in Connecticut. 
12 Regression Equations for Estimating Flood Flows in Connecticut

(-0.18)

(-0.22)

(-0.02)

0.00

(-0.04)

0.24

0.13

(-0.05)

0.07

0.06

0.06

0.10

0.23

(-0.22)

0.19

(-0.24)

0.31

0.15

0.11

(-0.38)

(-0.01)

0.23

(-0.13)

(-0.08)

(-0.40)

0.05

0.09

0.10

(-0.09)

(-0.18)

0.07

0.01

0.22

0.21

(-0.04)

(-0.17)

0.04

0.26

0.23

(-0.17)

0.02

(-0.26) (-0.21)

0.14

(-0.14)

(-0.16) 0.10

(-0.03)

0.01

0.15

(-0.23)

(-0.01)

0.02

0.20

0.22

(-0.09)

0.10

0.23

0.15

0.02

(-0.05)

(-0.30)

0.03

0.26

(-0.28)

(-0.04)

(-0.26)

(-0.05)

(-0.14)

0.05

73 ̊30'

72 ̊

41 ̊

41 ̊30'

42 ̊

0 10 5 MILES

0 10 5 KILOMETERS

EXPLANATION

Basin boundary with residual

value derived from

regression equations

0.02

Long Island Sound

Figure 3. Residuals values derived from regression equations (using drainage area, 24-hour rainfall for the 100-year recurrence

interval, and mean basin elevation as explanatory variables) for estimating flood flows at stream sites in Connecticut. [Residual values 

are plotted at the centroid of each basin; because some basins are nested within larger basins, some values may appear to overlap 

each other or to be missing.]
Estimating Flood Flows at Stream Sites in Nonurban Basins 13

Results of the Regression Analysis

Regression equations were developed to estimate floodflows for the 2-, 10-, 25-, 50-, 100-, and 500-year recurrence 

intervals (table 4). The explanatory variables in the final regression equations are drainage area, 24-hour rainfall, and mean 

basin elevation (appendix 2). The variables represent the most 

efficient combination for explaining flood flows with the smallest number of variables, as indicated by the average standard 

error of regression (SEr), standard error of prediction (SEp), the 

adjusted R-squared (adj-R2), and the average equivalent years 

of record (AEYR) (table 4). These four measures of accuracy 

are discussed in a later section of the report “Accuracy of the 

Regression Equations.” Although regression equations are traditionally used at ungaged sites, the equations also can be used 

at gaged sites with very short periods of record (less than 10 

years).

The regression equations explain 93 to 89 percent of the 

variation of the magnitude of the floods for recurrence intervals 

of 2- to 100-year flood flows, respectively, and 83 percent of the 

variation of the magnitude of the floods for the 500-year recurrence interval. The explanatory variable—drainage area—was 

found to be the most important variable and explains 85 percent 

of the variation in flood flows at the 100-year recurrence interval. Drainage area is physically related to the magnitude of the 

flow. Generally, larger drainage basins generate larger flood 

flows than smaller drainage basins. The other two explanatory 

variables—24-hour rainfall and mean basin elevation—explain 

an additional 4.0 percent of the variance in flood flows at the 

100-year recurrence interval. 

Rainfall is the primary cause of floods and generally is one 

of the most important predictors of flood flows in regional 

regression equations. The 24-hour rainfall value was found to 

be the second most important hydrologic characteristics (after 

drainage area) affecting flood flows in Connecticut (fig. 4). The 

24-hour rainfall shows a positive correlation to the flood flows 

and is statistically significant at the 99-percent confidence level 

for all recurrence intervals. 

Table 4. Regression equations for estimating flood flows for the 2-, 10-, 25-, 50-, 100,- and 500-year recurrence intervals at stream sites in 

nonurban basins in Connecticut and their accuracy.

[Qx, flood discharge for selected recurrence intervals in cubic feet per second; DA, drainage area in square miles; Px, 24-hour rainfall for x-recurrence interval in 

inches; EL, mean basin elevation in feet; adj-R2, adjusted-R square (or adjusted coefficient of determination) index value; SEr , standard error of regression in 

percent; SEp, standard error of prediction in percent; AEYR, average equivalent years of record]

Flood-flow regression equations for 

given recurrence intervals from 2 to 500 years

Adjusted-R 

square 

(adj-R2

)

Standard error of 

regression

(SEr)

Standard error of 

prediction

(SEp)

Average 

equivalent 

years of 

record 

(AEYR) Average (range),

in percent

Average (range), 

in percent

Q2=0.329 (DA)0.769 (P2)

2.947 (EL)0.262 0.93 30.4 (34.9 to -25.9) 31.8 (36.8 to -26.9) 3.5

Q10=0.510 (DA)0.776 (P10)

2.485 (EL)0.260 .92 30.7 (35.2 to -26.1) 32.7 (37.9 to -27.5) 8.1

Q25=0.947 (DA)0.784 (P25)

2.064 (EL)0.243 .91 31.9 (36.9 to -27.0) 34.4 (40.2 to -28.7) 10.9

Q50=1.37 (DA)0.790 (P50)

1.826 (EL)0.235 .90 33.1 (38.5 to -27.8) 35.9 (42.1 to -29.6) 12.7

Q100=1.86 (DA)0.799 (P100)

1.628 (EL)0.231 .89 34.6 (40.5 to -28.8) 37.6 (44.4 to -30.8) 14.3

1

Q500= 107 (DA)0.790 (EL)0.204

1Q500 equation used only two variables because P500 was not available.

.83 41.9 (50.3 to -33.5) 45.0 (54.7 to -35.4) 14.9
14 Regression Equations for Estimating Flood Flows in Connecticut

73 ̊30'

72 ̊

41 ̊

41 ̊30'

42 ̊

0 10

10.8

5.5

8

6

7

9

10

5 MILES

0 10 5 KILOMETERS

EXPLANATION

Basin boundary

100-YEAR 24-HOUR MAXIMUM

PRECIPITATION, IN INCHES

Streamflow-gaging station

Precipitation data from

Miller and others, 2003

Long Island Sound

Figure 4. Distribution of 24-hour rainfall for a 100-year recurrence interval with locations of the 70 streamflow-gaging stations used to 

develop regression equations for estimating flood flows at stream sites in Connecticut. 
Estimating Flood Flows at Stream Sites in Nonurban Basins 15

Mean basin elevation is an explanatory variable that is not 

in itself a direct cause of variation in flood flows; however, it 

proved to be an important factor affecting flood flows (fig. 5). 

Regional differences in elevation affect the type of precipitation 

that occurs. Average snowfall is about 30 in. along the coast, 40 

in. inland, and 60 in. in the northwestern corner of the state 

(Miller and others, 2003). Snow affects streamflow by temporary storage of precipitation. Also, temperature, vegetation, 

wind, radiation, basin and main channel slope vary with elevation, and their effects are evaluated indirectly, at least in part, by 

mean basin elevation. The explanatory variable—mean basin 

elevation—shows a positive correlation to flow and is statistically significant at the 95-percent confidence level for the 2-, 

10-, 25-, 50-, and 100-year recurrence intervals. 

The 6-hour rainfall and mean annual rainfall were two 

other climatic variables studied. The 24-hour rainfall was a 

slightly better explanatory variable than the 6-hour rainfall. 

Mean-annual rainfall (1961–1990), an explanatory variable that 

describes the general climate, shows a stronger correlation to 

the 2-year flood flows than the 24-hour rainfall. For consistency, the 24-hour rainfall was used in the final equations for all 

recurrence intervals including the 2-year recurrence interval. At 

the 500-year recurrence interval, 24-hour rainfall data are not 

available; therefore, the regression equation to estimate the 500-

year flood flow was developed using only two of the explanatory variables—drainage area and mean basin elevation. Mean 

basin elevation is not statistically significant (p-value = 0.13) at 

the 500-year recurrence interval; however, for consistency in 

the equations, mean basin elevation was included in the final 

500-year equation. 

The regression results indicated that the explanatory variable—total lengths of streams—is as good or slightly better an 

explanatory variable than drainage area for some recurrence 

intervals. Total lengths of streams is a summation of the lengths 

of all the streams, in miles, in a basin (designated as a solid blue 

line on 1:24,000 USGS topographic maps). Total lengths of 

streams affects not only the magnitude of the flood, but the timing of the flood, which is not as precisely accounted for with the 

variable drainage area. Although total lengths of streams 

explained slightly more of the flood-flow variation than drainage area for some recurrence intervals, drainage area was 

selected for use in the final equations, primarily because it is 

easier to measure. 

Accuracy of the Regression Equations 

The most frequently used measures of accuracy of regression equations are the standard error of regression (also commonly called the standard error of estimate) and the standard 

error of prediction. The standard error of regression, reported in 

percent, is a measure of the variation between the regression 

estimates and the observed floods from the streamflow-gaging 

station used in deriving the regression equations; or in other 

words, measures the scatter of the observed values around the 

regression line. The average standard error of regression is, by 

definition, one standard deviation on each side of the regression 

equation; about 67 percent of the data are contained within this 

range. In other words, there is a 67-percent probability that the 

value estimated from the regression equations is within the 

range of the standard error of the regression. The average standard errors of regression computed for the regression equations 

for the 2- to 100-year recurrence interval range from 30.4 to 

34.6 percent (table 4). The standard error of prediction, reported 

in percent, is a measure of the accuracy of the regression equations when predicting values for basins not used in the analysis. 

The standard error of prediction is generally slightly higher than 

the standard error of regression. The standard errors of prediction ranges from 31.8 to 37.6 percent for the 2- to 100-year 

recurrence interval (table 4). The largest standard errors are 

associated with equations where the flood flow records are considerably shorter than the recurrence interval used in the prediction (for example, the 100-year flood). There is a 67-percent 

probability that the true value at a site is within the range of the 

standard error of prediction. 

The adequacy of the regression equations also was measured using the adjusted-R2 and average equivalent years of 

record (AEYR). The adj-R2 can be interpreted as the proportion 

of the variance in the response variable accounted for by the 

number of explanatory variables and takes the size of the sample into effect. The AEYR represents the average number of 

years of gaging-station data needed to achieve results with 

accuracy equal to the regression equations. The AEYR is a 

function of the accuracy of the regression equations, the recurrence interval, and the average variance and skew of the annual 

flood flows at streamflow-gaging stations (Hardison, 1971).

Limitations of the Regression Equations 

The regression equations are statistical models determined 

from digital data using GIS technology. When applying the 

regression equations, the explanatory variables should be computed by the same methods that were used to develop the equations, and the explanatory variables should be derived using the 

same or comparable methods as those documented in this 

report. Using more or less accurate methods of computing the 

explanatory variables (for example, determining the mean basin 

elevation using grid sampling from topographic maps) can 

result in flood-flow frequency estimates of unknown accuracy.

Ranges (maximum and minimum) of explanatory variables used in the final equations are presented in table 3. Extrapolating beyond the range of explanatory variables used in the 

study to values higher or lower is tenuous. In multiple-regression analysis, it is possible to be within the observed range of 

every variable of the sample data and still be considered an 

extrapolation because the variables jointly define the region of 

the observed data (for example, variables need to be within the 

gray area of all plots shown in figure 6). If the explanatory variables (drainage area, 24-hour rainfall, and mean basin elevation) used in the regression equations are outside the ranges 

used to develop these equations (fig. 6), then the accuracy
16 Regression Equations for Estimating Flood Flows in Connecticut

73 ̊30'

72 ̊

41 ̊

41 ̊30'

42 ̊

0 10 5 MILES

0 10 5 KILOMETERS

EXPLANATION

Basin boundary

LAND-SURFACE ELEVATION,

IN FEET ABOVE NAVD 88

0 - 99

100 - 249

250 - 499

500 - 999

1,000 - 1,249

over 1,250

Streamflow-gaging station

Long Island Sound

Elevation data from 

U.S. Geological Survey, 2001

Figure 5. Land-surface elevations with locations of the 70 streamflow-gaging stations used to develop regression equations for estimating flood flows at stream sites in Connecticut. 
Estimating Flood Flows at Stream Sites in Nonurban Basins 17

10 100 1,000

DRAINAGE AREA, IN SQUARE MILES

6

7

8

9

10

11

24-HOUR RAINFALL FOR 100-YEAR

RECURRENCE INTERVAL, IN INCHES

1

10 100 1,000

DRAINAGE AREA, IN SQUARE MILES

0

500

1,000

1,500

MEAN BASIN ELEVATION, IN FEET ABOVE NAVD 88

1

0

500

1,000

1,500

6 89 7 10 11

24-HOUR RAINFALL FOR 100-YEAR RECURRENCE INTERVAL, IN INCHES

MEAN BASIN ELEVATION, IN FEET ABOVE NAVD 88

Figure 6. Two-dimensional ranges of explanatory variables for the regression equations for estimating flood flows at stream sites 

in Connecticut. 
18 Regression Equations for Estimating Flood Flows in Connecticut

of the flood flows from the equations is unknown. 

The regression equations presented in this report were 

derived on the basis of flood-flow data and drainage-basin and 

climatic characteristics of nonurban drainage basins with no 

appreciable flood-control regulation. Applying these equations 

to significantly regulated basins or urbanized basins will yield 

results of unknown error. 

Simplified Regression Equations

Simplified regression equations provide estimates of flood 

flows that are easier to calculate, although less accurate, than 

those computed by the regression equations shown in table 4. 

The simplified regression equations contain only one basin 

characteristic—drainage area. Generally, average standard 

errors for the simplified equations are about 4 percent higher 

than for the full regression equations. For example, the average 

standard error of regression for the 100-year flood is 38.7 percent for the simplified regression equations and 34.6 percent for 

the full regression equation. In addition to the simplified regression equations being easier to apply, the drainage area exponents can be useful in transferring flood flows upstream and 

downstream from a gaged site according to the ratio of the 

ungaged site’s drainage area to ungaged site’s drainage area, 

raised to the exponent power (Wandle, 1983). The simplified 

regression equations for recurrence intervals of 2, 10, 25, 50, 

100, and 500 years and adj-R2

, standard error of regression, 

standard error of prediction, and AEYR are presented in table 5.

Web Application for Solving the Regression Equations

A World Wide Web application, named StreamStats 

(http://water.usgs.gov/osw/programs/streamstats.html), is 

planned as the last phase of this study. The Web application will 

incorporate the new regression equations and provide floodflow frequency estimates for most unregulated streams sites in 

Connecticut. The Web application will include (1) a mapping 

tool that allows users to specify locations on streams where 

flood-flow statistics are needed; (2) a database that includes 

flood-flow frequency statistics, hydrologic characteristics, location, and descriptive information for all USGS streamflow-gaging stations used in the Connecticut flood-flow frequency 

study; and (3) an automated GIS procedure that measures the 

hydrologic characteristics and solves the regression equations 

to estimate flood-flow statistics for user-selected sites.

Table 5. Simplified regression equations for estimating flood flows for the 2-, 10-, 25-, 50-, 100,- and 500-year recurrence intervals at 

stream sites in nonurban basins in Connecticut and their accuracy. 

[Qx, flood flows for selected recurrence intervals in cubic feet per second; DA, drainage area in square miles; adj-R2, adjusted-R square (or adjusted coefficient of 

determination) index value; SEr , standard error of regression in percent; SEp, standard error of prediction in percent; AEYR, average equivalent years of record]

Simplified regression 

equations for selected 

recurrence intervals

Adjusted-R 

square 

(adj-R2

)

Standard error of regression

(SEr)

Standard error of prediction

(SEp)

Average 

equivalent 

years of 

record 

(AEYR) Average (range), in percent Average (range), in percent

Q2= 67.6 (DA)0.755 0.90 34.5 (40.3 to -28.7) 35.5 (41.6 to -29.4) 2.9

Q10= 151 (DA)0.757 .88 35.3 (41.4 to -29.2) 36.8 (43.3 to -30.2) 6.6

Q25= 200 (DA)0.765 .87 36.4 (42.8 to -30.0) 38.2 (45.2 to -31.1) 9.2

Q50= 237 (DA)0.774 .86 37.4 (44.1 to -30.6) 39.3 (46.8 to -31.9) 11.0

Q100= 276 (DA)0.781 .85 38.7 (45.9 to -31.4) 40.7 (48.7 to -32.8) 12.5

Q500= 369 (DA)0.804 .82 42.6 (51.4 to -33.9) 45.2 (54.9 to -35.4) 15.1
Estimating Flood Flows at Stream Sites in Urban Basins 19

Estimating Flood Flows at Stream Sites in 

Urban Basins

Urbanization can sometimes increase runoff because the 

drainage system often is altered by straightening and enlarging 

the channel, installing storm sewers and curb-and gutter systems, or converting land surface from pervious to impervious 

surfaces. These alterations can change the magnitude and timing of the runoff, and ultimately, facilitate faster runoff with an 

increase in floods. Some aspects of urbanization can decrease 

an area’s flood potential by storing runoff in detention ponds 

and releasing it slowly. Also, road embankments, bridges, and 

culverts can act as dams by causing temporary storage of runoff 

behind the structures. Evaluating the effects of urbanization on 

the magnitude and frequency of flood flows involves many factors. 

A national study of flood magnitude and frequency in 

urban areas that evaluated many of the factors listed above was 

conducted by the USGS to develop methods for estimating 

flood-flow characteristics at ungaged stream sites in urban 

basins. Urban basins are defined as those that have at least

15 percent of their drainage area covered with some type of 

commercial, industrial, or residential development. Two sets of 

regression equations (seven-parameter and three-parameter) 

were developed to estimate flood discharges for ungaged 

stream sites for urban basins with recurrence intervals from 2 to 

500 years (Sauer and others, 1983). The national equations are 

described in USGS Water-Supply Paper (WSP) 2207 and are 

based on urban runoff data from 199 basins in 56 cities and 31 

States. The three- and seven-variable regression equations are 

shown in table 6; these regression equations are referred to as 

the "urban" regression equations. The standard error of regression and the standard error of prediction are discussed in a later 

section of the report “Accuracy and Limitations of the National 

Urban Regression Equations.”

Subsequent to the development of the national urban equations, an urban flood-frequency study (Sauer, 1985) indicated 

that the three-parameter equations are biased and significantly 

underestimated flood flows in many regions of the country. It is 

unknown if the three-parameter equations are appropriate for 

Connecticut because the second study did not include stations 

in Connecticut or the Northeast. Sauer’s second study (1985) 

did show that the seven-parameter equations are unbiased in the 

areas tested. 

Table 6. National regression equations for estimating flood flows the 2-, 10-, 25-, 50-, 100,- and 500-year recurrence intervals in 

urban basins and their accuracy.

[From Sauer and others, 1983; standard errors are based on 199 streamflow-gaging stations nationwide. UQx, urban flood-discharge for selected recurrence intervals in cubic feet per second; A, drainage area in square miles; SL, main channel slope in feet per mile; R2, 2-hour rainfall for a 2-year recurrence interval, in 

inches; ST, drainage-basin storage, in percent; BDF, basin development factor; IA, impervious surface area in percent; RQx, flood discharge from regression 

equations for Connecticut in cubic feet per second; SEr , standard error of regression in percent; SEp, standard error of prediction in percent; --, not available]

Flood-flow regression equations for given recurrence interval

(recurrence intervals from 2 to 500 years)

Standard error of 

regression (SEr)

Standard error of 

prediction (SEp)

Average (range), 

in percent

Average (range), 

in percent

Seven-variable equations

UQ2 = 2.35 (A)0.41 (SL)0.17 (R2+3)2.04 (ST+8)-0.65 (13-BDF)-0.32 (IA)0.15 (RQ2)

0.47 38 (46 to -31) 44 (54 to -35)

UQ10 = 2.99 (A)0.32 (SL)0.15 (R2+3)1.75 (ST+8)-0.57 (13-BDF)-0.30 (IA)0.09 (RQ10)

0.58 38 (45 to -31) 45 (55 to -35)

UQ25 = 2.78 (A)0.31 (SL)0.15 (R2+3)1.76 (ST+8)-0.55 (13-BDF)-0.29 (IA)0.07 (RQ25)

0.60 40 (48 to -32) --

UQ50 = 2.67 (A)0.29 (SL)0.15 (R2+3)1.74 (ST+8)-0.53 (13-BDF)-0.28 (IA)0.06 (RQ50)

0.62 42 (50 to -34) --

UQ100 = 2.50 (A)0.29 (SL)0.15 (R2+3)1.76 (ST+8)-0.52 (13-BDF)-0.28 (IA)0.06 (RQ100)

0.63 44 (54 to -35) 53 (66 to -40)

UQ500 = 2.27 (A)0.29 (SL)0.16 (R2+3)1.86 (ST+8)-0.54 (13-BDF)-0.27 (IA)0.05 (RQ500)

0.63 49 (61 to -38) --

Three-variable equations

UQ2 = 13.2 (A)0.21 (13-BDF)-0.43 (RQ2)

0.73 43 (51 to -34) 44 (54 to -35)

UQ10 = 9.51 (A)0.16 (13-BDF)-0.36 (RQ10)

0.79 41 (49 to -33) 43 (51 to -34)

UQ25 = 8.68 (A)0.15 (13-BDF)-0.34 (RQ25)

0.80 43 (51 to -34) --

UQ50 = 8.04 (A)0.15 (13-BDF)-0.32 (RQ50)

0.81 44 (54 to -35) --

UQ100 = 7.70 (A)0.15 (13-BDF)-0.32 (RQ100)

0.82 46 (57 to -36) 49 (61 to -38)

UQ500 = 7.47 (A)0.16 (13-BDF)-0.30 (RQ500)

0.82 52 (65 to -39) --
20 Regression Equations for Estimating Flood Flows in Connecticut

Definitions of Equation Variables 

The definitions listed below are modified from Sauer and 

others (1983):

UQx - The flood flow, in cubic feet per second, for the 

urban drainage basin for recurrence interval x; for example, 

UQ2 = 2-year urban flood flow, UQ5 = 5-year urban flood flow. 

A - The contributing drainage area, in square miles (not 

square kilometers). In urban areas, drainage systems sometimes 

cross topographic divides. Such drainage-area changes should 

be accounted for when computing A. This process may require 

field inspections.

SL - The main channel slope, in feet per mile, measured 

from points that are 10 percent and 85 percent of the main channel length upstream from the study site. The main channel, 

where two channels join, is the one that drains the largest area. 

The main channel length is measured as the distance from the 

study site to the basin divide. For sites where SL is greater than 

70 ft/mi, 70 ft/mi is used in the equations.

R2 - The rainfall, in inches, for the 2-hour 2-year recurrence interval. Determined from National Weather Service 

Technical Publication No. 40 (1961). 

ST - Drainage basin storage, the areal percentage of the 

drainage basin occupied by lakes, reservoirs, and wetlands. Inchannel storage of a temporary nature caused by detention 

ponds, roadway embankments, or other structures is not 

included in the computation of ST. This variable should be 

computed from USGS topographic maps (not U.S. Fish and 

Wildlife Service National Wetland Inventory maps or by other 

methods) for this particular computation. 

IA - The percentage of the drainage basin occupied by 

impervious surfaces, such as houses, buildings, streets, and 

parking lots. This variable should be computed from the best 

available maps or aerial photographs. Field inspections to supplement the maps are useful.

RQx - The flood flow, in cubic feet per second, for an 

equivalent (nonurban) drainage basin for recurrence interval x. 

For Connecticut, the equations in table 4 should be used to calculate RQx. 

BDF - The basin development factor, an index of the prevalence of the drainage factors: (a) storm sewers, (b) channel 

modifications, (c) impervious channel linings, and (d) curband-gutter streets. The range of BDF is from 0 to 12. A value of 

zero for BDF indicates that the above drainage factors are not 

prevalent, but it does not necessarily mean the basin is nonurbanized. A value of 12 indicates full development of the 

drainage factors throughout the basin. 

BDF can be determined from drainage maps and field 

inspections of the drainage basin. After the basin has been 

delineated on a topographic map, the basin is divided into 

upper, middle, and lower thirds on the same map. Each third 

contains approximately one-third of the drainage area and 

drains the upper, middle, or lower reaches of the basin. Because 

travel time is considered when drawing the lines separating the 

basin into thirds, distances along main streams and tributaries 

can be marked to help locate the boundaries of the basin thirds. 

This drawing of the boundaries means that not all thirds of the 

basin have equal travel distances but that within each third, the 

travel distances of two or more streams are about equal. 

Because precise definition of the lines dividing the basin into 

thirds is not considered necessary, the lines can generally be 

drawn on the drainage map by eye, without precise measurements. Schematic diagrams of three typical basin shapes and 

their division into thirds are shown in figure 7. Complex basin 

shapes and drainage patterns are sometimes encountered and 

would require more judgment to subdivide. 

Within each drainage-basin third, four aspects of the drainage system are evaluated; each aspect is assigned a code. The 

guidelines for determining the drainage-system codes are not 

intended to be precise measurements. Measurements should be 

checked in the field to obtain the best estimates. 

1. Channel modifications - If channel modifications such 

as straightening, enlarging, deepening, and clearing are prevalent for the main drainage channels and principal tributaries 

(those that drain directly into the main channel) in a basin third, 

then a code of 1 is assigned. Any or all of these modifications 

would qualify for a code of 1. To be considered prevalent, at 

least 50 percent of the main drainage channels and principal 

tributaries must be modified to some degree over natural conditions. If channel modifications are not prevalent, then a code of 

zero is assigned.

2. Channel linings - If more than 50 percent of the length 

of the main drainage channel and principal tributaries in each 

basin third has been lined with an impervious material, such as 

concrete, then a code of 1 is assigned to this aspect. If less than 

50 percent of these channels are lined, then a code of zero is 

assigned. The presence of channel linings would obviously 

indicate the presence of channel modifications as well. Therefore, this presence is an added factor that indicates a more 

highly developed drainage system.

3. Storm drains (storm sewers) - Storm drains are defined 

as enclosed drainage structures (usually pipes), frequently used 

on the secondary tributaries (those that drain directly into the 

principal tributaries) where the drainage is received directly 

from streets or parking lots. Many of these drains empty into 

open channels; however, in some basins they empty into channels enclosed as box or pipe culverts. When more than 50 percent of the secondary tributaries within a drainage-basin third 

consist of storm drains, then a code of 1 is assigned to this 

aspect; if less than 50 percent of the secondary tributaries consist of storm drains, then a code of zero is assigned. It should be 

noted that if 50 percent or more of the main drainage channels 

and principal tributaries are enclosed, then the aspects of channel modifications and channel linings also would be assigned a 

code of 1.

4. Curb-and-gutter streets - If more than 50 percent of a 

sub-area (third) is urbanized (covered by residential, commercial, and (or) industrial development) and if more than 50 percent of the streets and highways in the drainage-basin third are 

constructed with curbs and gutters, then a code of 1 is assigned 

to this aspect. Otherwise, it is assigned a code of zero. 
Estimating Flood Flows at Stream Sites in Urban Basins 21

Drainage from curb-and-gutter streets frequently empties into 

storm drains. 

BDF is calculated as the sum of the codes assigned to the 

four drainage aspects in each of the basin thirds. The maximum 

value for a fully developed drainage system would be 12; if the 

drainage system were totally undeveloped, then the BDF would 

be zero. Such a condition does not necessarily mean that the 

basin is unaffected by urbanization. A basin could be partially 

urbanized, have some impervious area, and have some modification of secondary tributaries and still have an assigned BDF 

of zero. 

The BDF is a fairly easy index to estimate for an existing 

urban basin. The 50-percent guideline usually will not be difficult to evaluate because many urban areas tend to use the same 

design criteria and therefore have similar drainage aspects. 

Also, the BDF is convenient for projecting the effects of future 

development. Full development and maximum urban effects on 

floods would occur when the BDF is 12. 

Figure 7. Typical drainage-basin shapes and subdivision into basin thirds. (From Sauer and others, 1983, fig. 2.)
22 Regression Equations for Estimating Flood Flows in Connecticut

Accuracy and Limitations of the National Urban 

Regression Equations

As previously stated, the average standard error of regression is, by definition, one standard deviation on each side of the 

regression equation, and so there is a 67-percent probability that 

the true value of a flood flow at a site (a site where a flood flow 

is being estimated) will be within the average standard error of 

regression range. The average standard errors of prediction in 

table 6 were computed by Sauer and others (1983) using a validation method (split sampling).

The urbanization of a drainage basin generally causes 

flood flows to increase for those basins that do not have appreciable in-channel or detention storage. The increase in flood 

flows is usually most dramatic for lower recurrence interval 

flows (which occur frequently) and less pronounced for higher 

recurrence interval flows (Sauer and others, 1983). The location 

of urbanization in a drainage basin may have an effect on flood 

flows that is not accounted for in the urban regression equations. For example, if the lower part of a basin is urbanized and 

the upper part is not, rapid removal of floodwaters from the 

lower part may occur before the upper part can contribute 

appreciable runoff. This pattern of urbanization could potentially decrease flood flows from a drainage basin (Sauer and 

others, 1983). The computed urban flood flow (UQx) should be 

compared to the equivalent nonurban flood flow (RQx) to make 

sure that the urban flood-flow estimate is reasonable.

The ranges of the explanatory variables used in the urban 

regression equations are shown in table 7. Drainage basins of 

the streamflow-gaging stations that were used to develop the 

urban regression equations had at least 15 percent of their drainage area covered with some type of commercial, industrial, or 

residential development. For this reason, the urban equations 

may not be applicable to basins containing less than 15-percent 

developed land. The standard errors of the regression are higher 

for the three-variable urban equations than for the seven-variable urban equations, but the three-variable equations are easier 

to apply. The standard errors of prediction for the two sets of 

urban equations are comparable. If values outside the ranges of 

the explanatory variables are used, then the standard errors may 

be considerably higher than the listed standard errors (table 6). 

As discussed by Sauer and others (1983), the drainage-basin 

storage variable (ST) does not include in-channel storage of a 

temporary nature (resulting from detention ponds, roadway 

embankments, or other structures). This type of storage tends to 

reduce flood flows. Reservoir- and channel-routing techniques 

should be applied to determine the effect that temporary inchannel storage has on flood flows in an urbanized basin.

Table 7. Ranges of explanatory variables used in the national regression equations for estimating flood-flow statistics at 

stream sites in urban basins.

[From Sauer and others, 1983]

Explanatory variable Units Minimum Maximum

Area (A) Square miles 0.2 100

Main channel slope (SL) Feet per mile 3.0 1

70

2-hour rainfall for 2-year 

recurrence interval (R2)

Inches .2 2.8

Storage (ST) Percent 0 11

Basin development factor (BDF) None 0 12

Impervious area (IA) Percent 3 50

1

Maximum value of slope for use in urban equations is 70 feet per mile, although numerous drainage basins used in 

the national study had values as high as 500 feet per mile.
Estimating Flood Flows at Stream Sites in Urban Basins 23

Comparison of Flood-Flow Estimates From the Urban 

Equations To Observed Flows in Connecticut

Development of new regression equations based on 

streamflow-gaging stations in Connecticut to estimate flood 

flows in urban basins in Connecticut was beyond the scope of 

this study; however, a limited analysis (based on five stations) 

of the national urban regression equations was done as part of 

this project. Although it is unknown if the three-variable equations are biased in the northeastern states, both the three- and 

seven-variable regression equations were used to estimate the 

effects of urbanization on the magnitude and frequency of flood 

flows in Connecticut. 

In Connecticut, six drainage basins with USGS streamflow-gaging stations were identified as urban, based on the criterion of having at least 15 percent of the drainage basin with 

commercial, industrial, or high- intensity residential development (Civco, 1998). One station—USGS station 01190200, 

Mill River at Newington—was later omitted because an undetermined amount of flow is diverted outside the basin during 

large rainfall events. Flood-flow frequency estimates in urban 

basins were derived from the three- and seven-parameter urban 

regression equations and compared to the observed flows (frequency curves) from the streamflow-gaging stations (table 8). 

The observed flows are based on log-Pearson Type III procedures described in Bulletin 17B (Interagency Advisory Committee on Water Data, 1982) using station skews. A comparison 

of the 2-, 10-, 25-, 50-, 100-, and 500-year observed flows to the 

estimated flows from the urban regression equations shows the 

three- and seven-variable urban equations used in conjunction 

with the new equations for Connecticut generally provide reasonable estimates of flood flows (table 8). At four of the five 

stations, flood-flow estimates derived from the three- and 

seven-variable equations are within 15- and 25-percent of the 

observed 100-year flows, and 30- and 15-percent of the 

observed 10-year flows, respectively. One station—USGS station 01195490, Quinnipiac River at Southington—was an outlier and the estimated flood flows from the three and seven-variable urban equations and nonurban regression equations did not 

compare well with the observed flood flows. The station has a 

short length of record (1988-2001); hence, the observed flows 

for this station, particularly for higher frequencies (greater than 

10 years), are less reliable. Also, there may be appreciable storage in the headwaters. Temporary storage can cause the 

observed flows to be substantially less than the predicted flows 

from the regression equations. The user should be aware that 

this is a limited analysis of the urban equations and their application in Connecticut because it was based on only five sites. 

No attempt was made to test for biases in the three- or sevenvariable equations using Connecticut data.

Table 8. Flood-frequency computations for urban basins in Connecticut. 

[The seven- and three-parameter national equations are described in Sauer and others, 1983. DA, drainage area in square miles; Px, 24-hour rainfall for selected 

recurrence interval in inches; EL, mean basin elevation in feet above NAVD 88; SL, main channel slope in feet per mile; R2, 2-hour 2-year rainfall from National 

Weather Service maps (1961); ST, drainage-basin storage, in percent; BDF, basin development factor; IA, impervious surface in percent; RQx, flood discharge 

from regression equations for Connecticut, in cubic feet per second; UQx, flood discharge from urban regression equation, in cubic feet per second]

Variable

U.S. Geological Survey streamflow-gaging station (period of record)

01190100 

Piper Brook 

(1955; 

1958-1984)

01190500 

S. Branch 

Park River 

(1936-1972; 

1974-1981)

01191500

Park River 

(1936-1962)

01195490

Quinnipiac 

River 

(1988-2001)

01208873 

Rooster River 

(1978-2001)

Basin characteristics used in 

regression equations

DA 14.4 40.4 73.7 17.8 10.7

P2 3.67 3.61 3.56 3.61 3.61

P10 5.42 5.32 5.25 5.3 5.36

P25 6.78 6.64 6.55 6.61 6.73

P50 8.03 7.85 7.74 7.81 7.99

P100 9.52 9.29 9.16 9.23 9.5

EL 258 230 219 260 249

SL 26.7 21.9 18.6 11.4 37.6

R2 1.6 1.6 1.6 1.6 1.65

ST 5.7 3.9 3.6 3.3 1.3

BDF 4 4 5 1 5

IA 21.9 21.3 18.6 18 27.7
24 Regression Equations for Estimating Flood Flows in Connecticut

Connecticut regression equations

(for nonurban, unregulated basins; see 

table 4)

RQ2 506 1,030 1,560 568 380

RQ10 1,140 2,360 3,590 1,280 874

RQ25 1,540 3,210 4,940 1,720 1,190

RQ50 1,860 3,930 6,090 2,100 1,450

RQ100 2,220 4,730 7,380 2,500 1,730

RQ500 2,730 6,030 9,600 3,240 2,140

Urban seven-parameter 

regression equations

UQ2 738 1,660 2,600 739 857

UQ10 1,510 3,370 5,270 1,530 1,670

UQ25 1,940 4,350 6,880 1,970 2,120

UQ50 2,330 5,220 8,270 2,380 2,530

UQ100 2,780 6,300 10,100 2,830 3,000

UQ500 3,260 7,560 12,200 3,430 3,560

Urban three-parameter 

regression equations

UQ2 846 1,770 2,850 851 679

UQ10 1,720 3,600 5,760 1,760 1,380

UQ25 2,180 4,570 7,360 2,230 1,760

UQ50 2,640 5,650 9,160 2,740 2,140

UQ100 3,150 6,850 11,200 3,270 2,550

UQ500 3,890 8,790 14,700 4,250 3,150

Observed discharges using 

log-Pearson Type III analysis of 

streamflow-gaging station data

Q2 745 1,500 2,150 461 1,140

Q10 1,630 3,660 4,590 785 1,800

Q25 2,160 5,160 6,460 949 2,070

Q50 2,590 6,480 8,220 1,070 2,250

Q100 3,050 7,980 10,400 1,200 2,410

Q500 4,220 12,300 17,200 1,490 2,740

Table 8. Flood-frequency computations for urban basins in Connecticut. —Continued

[The seven- and three-parameter national equations are described in Sauer and others, 1983. DA, drainage area in square miles; Px, 24-hour rainfall for selected 

recurrence interval in inches; EL, mean basin elevation in feet above NAVD 88; SL, main channel slope in feet per mile; R2, 2-hour 2-year rainfall from National 

Weather Service maps (1961); ST, drainage-basin storage, in percent; BDF, basin development factor; IA, impervious surface in percent; RQx, flood discharge 

from regression equations for Connecticut, in cubic feet per second; UQx, flood discharge from urban regression equation, in cubic feet per second]

Variable

U.S. Geological Survey streamflow-gaging station (period of record)

01190100 

Piper Brook 

(1955; 

1958-1984)

01190500 

S. Branch 

Park River 

(1936-1972; 

1974-1981)

01191500

Park River 

(1936-1962)

01195490

Quinnipiac 

River 

(1988-2001)

01208873 

Rooster River 

(1978-2001)
Weighted Averages of Flood Flow for Streamflow-Gaging Stations 25

Weighted Averages of Flood Flow for 

Streamflow-Gaging Stations

For streamflow-gaging stations, a weighted average of two 

independent flood-flow frequency estimates provides the most 

accurate estimate of flood flows for the station. One estimate is 

derived from the log-Pearson Type III analyses of the annual 

flood flows at the gaging station, and the other estimate is from 

the regression equation. The two estimates are weighted based 

on the years of record at the gaging station and the equivalent 

years of record value assigned to the equation (table 4). The 

weighted average is particularly more accurate than a nonweighted average at gaging stations with a short period of 

record. According to guidelines (Interagency Advisory Committee on Water Data, 1982), if two independent estimates (one 

computed from the gaging-station data and one from the regression equation) are weighted inversely proportional to their variances, the variance of the weighted average is less than the variance of either estimate.

Three flood-flow estimates for the 2-, 10-, 25-, 50-, 100-, 

and 500-recurrence intervals are provided for the 70 streamflow-gaging stations used in the regional analysis of flood-flow 

characteristics for Connecticut: (1) a weighted average (W) of 

the two flood-flow frequency estimates (table 9; note that values shown in table 9 are the anti-logarithmic transformations of 

the weighted averages); (2) the flood-flow frequency estimate 

computed from a log-Pearson Type III analysis of the annual 

flood flows from the gaging station (G); and (3) the flood-flow 

frequency estimate computed from the regression equations 

(R). The weighted average flood flow (Qwt) (in base-10 logarithms) is computed from the following equation: 

, (3)

where

Qgage is the gaging-station flood flow (in base-10 

logarithms) for a given recurrence 

interval, calculated by log-Pearson 

Type III analysis,

N is the number of annual flood flows at a 

gaging station (table 9),

Qreg is the regression-equation flood flow (in 

base-10 logarithms) calculated by the 

methods described earlier in the section “Estimating Flood Flows at 

Nonurban, Unregulated Stream 

Sites,” and

AEYR is the average equivalent years of record for 

the appropriate regression equations 

(table 4).

Qwt

( ) Qgage × N + ( ) Qreg × AEYR

( ) N AEYR + = ------------------------------------------------------------------------
Table 9. Weighted flood-frequency estimates for selected recurrence intervals at streamflow-gaging stations in Connecticut and the flood-flow estimregression equations and a log-Pearson Type III analysis of the annual maximum flows at the streamflow-gaging station. [Method (for estimating flood flow): W, weighted averages; G, streamflow-gaging station log-Pearson Type III analysis; R, regression estimate. Weighted flood-frequency estim

than nonweighted estimates at stations with short periods of record. Flood-flow frequency estimates are based on 10 or more years of unregulated flow record. Stations with floowith usable storage of more than 4.5 million cubic feet per square mile. Flood-flow frequency values are rounded to three significant figures. mi2, square miles; N, number of ann

ft3/s, cubic feet per second; nr, near] U.S. Geological Survey streamflow-gaging station Drainage area (mi2

)

Period of record 

used in analysis

Number 

of flood 

flows

(years) 

(N)

Method

Flood-flow frequency estimates for g

(ft3/s)

Number Name 2 

years

10 

years

25

years

5

yea

01118300 Pendleton Hill Brook nr Clarks Falls 4.01 1959-2001 43 W 135 263 342 

G 132 242 303 

R 182 412 552 

01119500 Willimantic River nr Coventry 122 1932-2001 70 W 2,160 5,120 7,360 9,

G 2,160 5,170 7,520 9,

R 2,270 4,740 6,400 7,

01120000 Hop River nr Columbia 74.5 1933-1984 52 W 2,010 4,260 5,720 6,

G 2,040 4,450 6,070 7,

R 1,570 3,230 4,300 5,

01120500 Safford Brook nr Woodstock Valley 4.17 1951-1981 31 W 317 586 747 

G 338 659 860 1,

R 177 374 500 

01121000 Mount Hope River nr Warrenville 29.0 1938, 1941-2001 62 W 1,010 2,180 2,990 3,

G 1,030 2,280 3,180 4,

R 758 1,570 2,110 2,

01122000 Natchaug River at Willimantic 

(flood control after 1951)

170 1931-1951 21 W 3,090 6,660 9,340 11,

G 3,110 6,820 9,770 12,

R 2,960 6,280 8,560 10,

01122500 Shetucket River nr Willimantic

(flood control after 1951)

401 1904-1906, 1920-

1921, 1929-1951

28 W 6,080 12,200 16,800 20,

G 6,120 12,300 16,900 21,

R 5,750 12,000 16,400 20,

01123000 Little River nr Hanover 30.0 1936, 1938, 1952-

2001

52 W 877 1,820 2,420 2,

G 881 1,820 2,410 2,

R 826 1,820 2,450 3,

01124000 Quinebaug River at Quinebaug

(flood control after 1959)

156 1932-1959 28 W 1,950 4,600 6,700 8,

G 1,900 4,510 6,700 8,

R 2,450 4,940 6,690 8,

01125490 Little River at Harrisville 35.7 1936, 1938, 1962-

1976

17 W 707 1,480 2,010 2,

G 682 1,350 1,760 2,

R 842 1,810 2,480 3,
01125500 Quinebaug River at Putnam

(flood control after 1959)

329 1930-1959 30 W 3,850 8,460 11,800 14,

G 3,790 8,300 11,600 14,

R 4,390 9,090 12,500 15,

01125600 Mashamoquet Brook at Abington 11.0 1963-1976 14 W 423 824 1,070 1,

G 434 815 1,020 1,

R 382 839 1,140 1,

01126000 Fivemile River at Killingly 57.8 1936, 1938-1984 48 W 698 1,480 2,030 2,

G 669 1,330 1,750 2,

R 1,260 2,840 3,940 4,

01126500 Moosup River at Moosup 83.5 1933-1984 52 W 1,510 3,020 4,010 4,

G 1,480 2,810 3,610 4,

R 1,950 4,760 6,660 8,

01126600 Blackwell Brook nr Brooklyn 17.0 1962-1976 15 W 515 1,240 1,720 2,

G 515 1,260 1,760 2,

R 515 1,210 1,670 2,

01127000 Quinebaug River at Jewett City

(flood control after 1964)

715 1919-1964 46 W 7,710 16,100 22,400 27,

G 7,640 15,600 21,300 26,

R 8,730 19,500 27,400 34,

01127500 Yantic River at Yantic 89.2 1931-2001 71 W 2,600 5,510 7,420 9,

G 2,640 5,740 7,860 9,

R 1,900 3,830 5,090 6,

01184100 Stony Brook nr West Suffield 10.5 1955, 1960-2001 43 W 397 1,000 1,440 1,

G 405 1,060 1,560 2,

R 313 750 1,050 1,

01184300 Gillette Brook at Somers 3.66 1960-1984 25 W 129 286 387 

G 126 273 367 

R 153 328 437 

01184490 Broad Brook at Broad Brook 15.6 1938, 1962-1976, 

1982-2001

36 W 414 844 1,110 1,

G 417 848 1,100 1,

R 380 824 1,130 1,

Table 9. Weighted flood-frequency estimates for selected recurrence intervals at streamflow-gaging stations in Connecticut and the flood-flow estimregression equations and a log-Pearson Type III analysis of the annual maximum flows at the streamflow-gaging station.—Continued[Method (for estimating flood flow): W, weighted averages; G, streamflow-gaging station log-Pearson Type III analysis; R, regression estimate. Weighted flood-frequency estim

than nonweighted estimates at stations with short periods of record. Flood-flow frequency estimates are based on 10 or more years of unregulated flow record. Stations with floowith usable storage of more than 4.5 million cubic feet per square mile. Flood-flow frequency values are rounded to three significant figures. mi2, square miles; N, number of ann

ft3/s, cubic feet per second; nr, near]

U.S. Geological Survey

streamflow-gaging station Drainage 

area 

(mi2)

Period of record 

used in analysis

Number 

of flood 

flows

(years) 

(N)

Method

Flood-flow frequency estimates for g

(ft3/s)

Number Name 2 

years

10 

years

25

years

5

yea
01184500 Scantic River at Broad Brook 97.7 1929-1984 56 W 1,100 2,400 3,350 4,

G 1,070 2,260 3,080 3,

R 1,670 3,690 5,130 6,

01186500 Still River at Robertsville (flood control after 1961) 85.1 1936, 1938, 1949- 1961 15 W 2,800 6,770 9,530 12,

G 2,880 7,110 10,300 13,

R 2,480 6,190 8,560 10,

01187000 West Branch Farmington River at Riverton 217 1930-1956 27 W 6,340 16,600 24,600 32,

G 6,590 18,100 28,100 38,

R 4,670 12,400 17,800 22,

01187300 Hubbard River nr West Hartland 20.6 11938-1954, 1955, 1957-2001 64 W 1,240 2,730 3,630 4,

G 1,270 2,820 3,750 4,

R 795 2,130 3,020 3,

01187400 Valley Brook nr West Hartland 7.39 11940-1954, 1955-

1972

33 W 350 1,030 1,520 1,

G 350 1,060 1,610 2,

R 347 898 1,260 1,

01187800 Nepaug River nr Nepaug 23.4 11922-1954, 1955, 

1958-1984

61 W 851 2,030 2,810 3,

G 843 1,990 2,750 3,

R 1,000 2,380 3,200 3,

01188000 Burlington Brook at Burlington 4.20 1932-2001 70 W 293 694 934 1,

G 294 702 953 1,

R 276 627 823 

01189000 Pequabuck River at Forestville 45.7 1938, 1942-2001 61 W 1,640 3,760 5,270 6,

G 1,650 3,840 5,470 6,

R 1,400 3,170 4,270 5,

01189200 Stratton Brook nr Simsbury 5.44 1964-1984 21 W 147 381 541 

G 134 316 432 

R 261 616 833 1,

01189390 East Branch Salmon Book at Granby 39.1 1955-1956, 1964-

1976

15 W 811 2,510 3,920 5,

G 764 2,450 4,080 5,

R 1,050 2,620 3,710 4,

Table 9. Weighted flood-frequency estimates for selected recurrence intervals at streamflow-gaging stations in Connecticut and the flood-flow estimregression equations and a log-Pearson Type III analysis of the annual maximum flows at the streamflow-gaging station.—Continued[Method (for estimating flood flow): W, weighted averages; G, streamflow-gaging station log-Pearson Type III analysis; R, regression estimate. Weighted flood-frequency estim

than nonweighted estimates at stations with short periods of record. Flood-flow frequency estimates are based on 10 or more years of unregulated flow record. Stations with floowith usable storage of more than 4.5 million cubic feet per square mile. Flood-flow frequency values are rounded to three significant figures. mi2, square miles; N, number of ann

ft3/s, cubic feet per second; nr, near]

U.S. Geological Survey

streamflow-gaging station Drainage 

area 

(mi2)

Period of record 

used in analysis

Number 

of flood 

flows

(years) 

(N)

Method

Flood-flow frequency estimates for g

(ft3/s)

Number Name 2 

years

10 

years

25

years

5

yea
01189500 Salmon Brook nr Granby2 66.9 1947-1963 17 W 1,880 5,240 7,940 10,

G 1,930 5,780 9,500 13,

R 1,680 4,260 6,010 7,

01190000 Farmington River at Rainbow

(flood control after 1968)

590 1928, 1936-1968 34 W 7,380 19,400 29,400 39,

G 7,120 18,100 27,300 36,

R 10,400 26,200 37,200 46,

01190600 Wash Brook at Bloomfield 5.64 1955, 1959-1971 14 W 197 466 663 

G 198 476 702 

R 193 449 615 

01191000 North Branch Park River at Hartford

(flood control after 1962)

26.5 1936-1962 27 W 1,080 2,230 3,020 3,

G 1,150 2,460 3,430 4,

R 692 1,600 2,200 2,

01192500 Hockanum River nr East Hartford 73.3 1920-1921, 1929-

2001

75 W 1,060 2,260 3,040 3,

G 1,050 2,200 2,940 3,

R 1,320 2,830 3,870 4,

01192650 Roaring Brook at Hopewell 24.2 1962-1976 15 W 533 1,010 1,330 1,

G 507 852 1,060 1,

R 663 1,380 1,830 2,

01192700 Mattabesset River at East Berlin 45.3 1962-1979, 31995-

1998

22 W 1,520 2,720 3,420 4,

G 1,590 2,780 3,400 3,

R 1,140 2,550 3,470 4,

01192883 Coginchaug River at Middlefield 29.7 41962-1980; 1981-

2001

40 W 763 1,630 2,140 2,

G 747 1,550 2,010 2,

R 977 2,060 2,700 3,

01193500 Salmon River nr East Hampton 101 1929-2001 73 W 2,700 6,240 8,860 11,

G 2,730 6,460 9,390 12,

R 2,200 4,550 6,010 7,

01193800 Hemlock Valley Brook at Hadlyme 2.69 1961-1976 16 W 125 256 335 

G 123 245 314 

R 132 280 368 

Table 9. Weighted flood-frequency estimates for selected recurrence intervals at streamflow-gaging stations in Connecticut and the flood-flow estimregression equations and a log-Pearson Type III analysis of the annual maximum flows at the streamflow-gaging station.—Continued[Method (for estimating flood flow): W, weighted averages; G, streamflow-gaging station log-Pearson Type III analysis; R, regression estimate. Weighted flood-frequency estim

than nonweighted estimates at stations with short periods of record. Flood-flow frequency estimates are based on 10 or more years of unregulated flow record. Stations with floowith usable storage of more than 4.5 million cubic feet per square mile. Flood-flow frequency values are rounded to three significant figures. mi2, square miles; N, number of ann

ft3/s, cubic feet per second; nr, near]

U.S. Geological Survey

streamflow-gaging station Drainage 

area 

(mi2)

Period of record 

used in analysis

Number 

of flood 

flows

(years) 

(N)

Method

Flood-flow frequency estimates for g

(ft3/s)

Number Name 2 

years

10 

years

25

years

5

yea
01194000 Eightmile River at North Plain 20.2 1938-1984 46 W 843 1,770 2,350 2,

G 861 1,850 2,500 3,

R 638 1,360 1,810 2,

01194500 East Branch Eightmile River nr North Lyme 22.4 1938-1982 45 W 646 1,360 1,910 2,

G 645 1,350 1,900 2,

R 661 1,450 1,950 2,

01195000 Menunketesuck River nr Clinton 11.3 1938, 1942-1967, 1982 28 W 412 887 1,220 1,

G 410 873 1,210 1,

R 431 935 1,240 1,

01195100 Indian River nr Clinton 5.62 1982-2001 20 W 182 426 601 

G 176 405 578 

R 219 482 646 

01195200 Neck River nr Madison 6.57 1962-1982 21 W 190 429 592 

G 183 401 548 

R 236 513 686 

01196500 Quinnipiac River at Wallingford 110 1931-2001 71 W 2,110 4,210 5,480 6,

G 2,100 4,100 5,260 6,

R 2,400 5,300 7,210 8,

01196580 Muddy River nr North Haven 17.8 1963-1976 14 W 703 1,190 1,500 1,

G 712 1,080 1,280 1,

R 668 1,400 1,830 2,

01196620 Mill River nr Hamden 24.5 1969-1970, 1979-

2001

25 W 926 2,210 3,100 3,

G 946 2,420 3,580 4,

R 793 1,680 2,220 2,

01196700 Wepawaug River at Milford 18.6 1962-1984 23 W 657 1,530 2,150 2,

G 667 1,580 2,280 2,

R 593 1,390 1,900 2,

01198500 Blackberry River at Canaan

(flood control after 1961)

46.0 1949-1961 13 W 1,700 3,760 5,070 6,

G 1,830 4,190 5,740 7,

R 1,310 3,160 4,380 5,

Table 9. Weighted flood-frequency estimates for selected recurrence intervals at streamflow-gaging stations in Connecticut and the flood-flow estimregression equations and a log-Pearson Type III analysis of the annual maximum flows at the streamflow-gaging station.—Continued[Method (for estimating flood flow): W, weighted averages; G, streamflow-gaging station log-Pearson Type III analysis; R, regression estimate. Weighted flood-frequency estim

than nonweighted estimates at stations with short periods of record. Flood-flow frequency estimates are based on 10 or more years of unregulated flow record. Stations with floowith usable storage of more than 4.5 million cubic feet per square mile. Flood-flow frequency values are rounded to three significant figures. mi2, square miles; N, number of ann

ft3/s, cubic feet per second; nr, near]

U.S. Geological Survey

streamflow-gaging station Drainage 

area 

(mi2)

Period of record 

used in analysis

Number 

of flood 

flows

(years) 

(N)

Method

Flood-flow frequency estimates for g

(ft3/s)

Number Name 2 

years

10 

years

25

years

5

yea
01199050 Salmon Creek at Lime Rock 29.4 1949, 1955, 1962-

2001

42 W 595 1,530 2,270 2,

G 588 1,510 2,250 2,

R 684 1,640 2,340 2,

01199200 Guinea Brook at West Woods Rd at 

Ellsworth

3.50 1960-1981 22 W 103 231 318 

G 96 199 262 

R 151 345 472 

01200000 Tenmile River nr Gaylordsville 200 1930-1987, 1992-

2001

68 W 2,980 6,950 9,860 12,

G 2,983 6,945 9,832 12,

R 2,910 6,950 10,000 12,

01201190 West Aspetuck River at Sand Rd nr 

New Milford

23.8 1963-1972 10 W 419 1,070 1,540 1,

G 360 842 1,180 1,

R 648 1,430 1,970 2,

01201500 Still River nr Lanesville 67.6 1932-1966, 51967-1984 53 W 1,260 3,080 4,360 5,

G 1,220 2,940 4,190 5,

R 2,180 4,170 5,260 6,

01203000 Shepaug River nr Roxbury 132 1931-1984 54 W 2,960 7,140 10,300 13,

G 2,960 7,210 10,600 14,

R 3,020 6,670 9,020 11,

01203510 Pootatuck River at Sandy Hook 25.0 1966-1984 19 W 1,250 2,260 2,770 3,

G 1,290 2,390 2,970 3,

R 1,040 1,980 2,460 2,

01204000 Pomperaug River at Southbury 75.3 1933-2001 69 W 2,750 6,100 8,400 10,

G 2,810 6,490 9,190 11,

R 1,790 3,600 4,740 5,

01204800 Copper Mill Brook nr Monroe 2.45 1959-1976 18 W 151 278 347 

G 148 257 312 

R 166 330 414 

01206000 Naugatuck River nr Thomaston 71.9 1931-1959 29 W 3,120 6,600 8,990 11,

G 3,260 7,120 10,000 12,

R 2,200 5,040 6,780 8,

Table 9. Weighted flood-frequency estimates for selected recurrence intervals at streamflow-gaging stations in Connecticut and the flood-flow estimregression equations and a log-Pearson Type III analysis of the annual maximum flows at the streamflow-gaging station.—Continued[Method (for estimating flood flow): W, weighted averages; G, streamflow-gaging station log-Pearson Type III analysis; R, regression estimate. Weighted flood-frequency estim

than nonweighted estimates at stations with short periods of record. Flood-flow frequency estimates are based on 10 or more years of unregulated flow record. Stations with floowith usable storage of more than 4.5 million cubic feet per square mile. Flood-flow frequency values are rounded to three significant figures. mi2, square miles; N, number of ann

ft3/s, cubic feet per second; nr, near]

U.S. Geological Survey

streamflow-gaging station Drainage 

area 

(mi2

)

Period of record 

used in analysis

Number 

of flood 

flows

(years) 

(N)

Method

Flood-flow frequency estimates for g

(ft3/s)

Number Name 2 

years

10 

years

25

years

5

yea
01206500 Leadmine Brook nr Thomaston 24.5 1931-59, 61960-1984 54 W 1,220 3,160 4,590 5,

G 1,240 3,320 5,010 6,

R 989 2,250 2,990 3,

01208500 Naugatuck River at Beacon Falls (flood control after 1959) 260 1920-1959 40 W 8,270 18,800 26,000 32,

G 8,620 20,700 29,900 38,

R 5,170 11,500 15,500 19,

01208850 Pequonnock River at Trumbull 15.5 1955, 1962-1984 24 W 718 1,610 2,150 2,

G 732 1,720 2,380 2,

R 629 1,320 1,710 2,

01208925 Mill River nr Fairfield 28.6 1973-2001 29 W 702 1,620 2,180 2,

G 679 1,530 2,020 2,

R 928 2,010 2,660 3,

01208950 Sasco Brook nr Southport 7.38 1960-2001 42 W 271 708 1,050 1,

G 270 728 1,120 1,

R 279 614 823 

01208990 Saugatuck River nr Redding 20.7 1962-2001 40 W 607 1,290 1,690 2,

G 586 1,220 1,580 1,

R 899 1,730 2,160 2,

01209500 Saugatuck River nr Westport 79.5 1933-1967 35 W 1,600 4,060 5,740 7,

G 1,540 3,950 5,680 7,

R 2,290 4,600 5,940 7,

01209700 Norwalk River at South Wilton 29.9 1956, 1963-2001 40 W 1,020 2,300 3,120 3,

G 1,010 2,320 3,220 4,

R 1,080 2,180 2,800 3,

01211700 East Branch Byram River at 

Round Hill 

1.69 1960-1975 16 W 102 227 304 

G 100 225 306 

R 109 232 301 

01212100 East Branch Byram River at 

Riversville

11.2 1963-1984 22 W 550 1,310 1,750 2,

G 567 1,450 2,020 2,

R 453 985 1,300 1,

Table 9. Weighted flood-frequency estimates for selected recurrence intervals at streamflow-gaging stations in Connecticut and the flood-flow estimregression equations and a log-Pearson Type III analysis of the annual maximum flows at the streamflow-gaging station.—Continued[Method (for estimating flood flow): W, weighted averages; G, streamflow-gaging station log-Pearson Type III analysis; R, regression estimate. Weighted flood-frequency estim

than nonweighted estimates at stations with short periods of record. Flood-flow frequency estimates are based on 10 or more years of unregulated flow record. Stations with floowith usable storage of more than 4.5 million cubic feet per square mile. Flood-flow frequency values are rounded to three significant figures. mi2, square miles; N, number of ann

ft3/s, cubic feet per second; nr, near]

U.S. Geological Survey

streamflow-gaging station Drainage 

area 

(mi2

)

Period of record 

used in analysis

Number 

of flood 

flows

(years) 

(N)

Method

Flood-flow frequency estimates for g

(ft3/s)

Number Name 2 

years

10 

years

25

years

5

yea
1Discharge is a maximum daily average.2Gaging station formerly published under the name West Branch Salmon Brook at Granby, Connecticut. 31995–1998 streamflow data collected at station 01192704, Mattabesset River at Rt. 372 at East Berlin adjusted to site (data transferred using transfer equation 6.12, Drai

Connecticut Dept. of Transportation, January 2000). 41962–1980 streamflow data collected at station 01192890, Coginchaug River at Rockfall adjusted to site (data transferred using transfer equation 6.12, Drainage Manual

Connecticut Dept. of Transportation, January 2000). 51967–1984 streamflow data collected at station 01201510, Still River at Lanesville adjusted to site (data transferred using transfer equation 6.12, Drainage Manual, 

Connecticut Dept. of Transportation, January 2000). 61960–1984 streamflow data collected at station 01206400, Leadmine Brook near Harwinton adjusted to site (data transferred using transfer equation 6.12, Drainage ManConnecticut Dept. of Transportation, January 2000).
34 Regression Equations for Estimating Flood Flows in Connecticut

Evaluation of the Streamflow-Gaging 

Network for Characterizing Flood Flows

The statewide streamflow-gaging network is a multipurpose network funded by federal, state, and local agencies to 

meet numerous water-resources management goals. The 

streamflow-gaging network is a major component of many 

hydrologic investigations, including (1) water-resources design, 

(2) regional flood analysis, flood warning, and flood-plain mapping, (3) water-supply evaluation, (4) stream classification and 

restoration, (5) water-quality management, (6) aquatic-habitat 

improvements, (7) enhanced operations of dams, (8) long-term 

environmental change, (9) verification and calibration of hydrologic models, and (10) watershed analysis. During the last 10 or 

15 years, uncertainties in funding and other factors have led to 

a fragmented network of stations with shorter periods of record 

on which to improve the flood-frequency estimates. Questions 

as to whether an adequate number of gaging stations are in operation across the State, and as to whether the full range of geographic and hydrologic conditions are represented by the current network, have been raised by state and local water-resource 

agencies. To address these concerns, a streamflow-gaging network evaluation was performed as part of this study. Although 

the network serves many purposes, this evaluation focuses on 

whether the number of gaging stations and range of geographic 

and hydrologic conditions represented by the network are adequate to meet federal and state goals related to regional flood 

analysis. Regional flood analysis is used to transfer flood characteristics, such as the 100-year flood discharge, from gaged to 

ungaged sites by relating flood characteristics to basin or climatic characteristics. It is important to emphasize that data from 

discontinued streamflow-gaging stations continue to provide 

information for characterizing floods and are used in regional 

flood analysis. An evaluation that summarizes the variety of 

users and uses of flow data at each individual station currently 

in operation was beyond the scope of this study.

The current (2004) network in Connecticut consists of 54 

streamflow-gaging stations. Overall, the majority of the stations 

(51 of 54) in the network meet one or more federal, state or local 

agency needs for information on floods: flood forecasting (hazard warning), flood-plain mapping and management, operation 

and management of flood-control reservoirs, engineering 

design, evaluation of stream stability and scour, verification and 

calibration of hydrologic models, and flood research. About 20 

stations with real-time telemetry are used by the National 

Weather Service and Connecticut DEP to issue flood warnings 

ensuring that lives and property are protected during floods. 

Another 10 stations are used for the operation and management 

of flood-control reservoirs by the U.S. Army Corps of Engineers. Only three stations were identified as providing minimum information for characterizing floods.

Because the number of ungaged basins will always far 

exceed the number of gaged basins, a core set of USGS streamflow-gaging stations that can be used for transferring flood 

characteristics from gaged to ungaged sites is needed. Core stations are most representative of natural hydrologic conditions, 

have long periods of record, and represent various hydrologic, 

geologic, and physiographic regions of the state. Data from core 

stations often are used as substitute (or surrogate) stations for 

other locations where no streamflow-gaging stations are 

present. Generally, these stations have important value for flood 

applications and meet the needs of many federal and state floodmanagement goals. 

Twenty-five stations in the current network that are unregulated (not affected by appreciable flood control) and have long 

periods of record (greater than 10 years) are considered core stations and can be used for regional flood analysis. The USGS 

considers the 25 core stations high-priority stations because of 

their scientific value for improving our understanding of floods. 

Only 24 of the 25 core stations were used in the regional regression analysis of nonurban basins; one station (01203600 Nonnewaug River at Minortown) was deleted from the analysis 

because the stage-discharge relation is not yet well-defined at 

high flows. This station is a core station because it represents 

natural (flood) flow conditions and can be used for future 

regional flood analysis. Two additional stations (01201487 Still 

River at Brookfield Center and 01203805 Weekeepeemee River 

at Hotchkissville) in the current network will be considered core 

stations in time with continued operation, but as of 2004, the 

periods of record associated with these two stations are less than 

10 years. Three other stations (01195490 Quinnipiac River at 

Southington, 01208873 Rooster River at Fairfield, and 

01209901 Rippowam River near Stamford) in the current network can be used for urban flood analysis and also considered 

to be high priority because of their scientific value to the understanding of floods in urbanized areas. 

Conducting regional flood analysis requires long-term 

records of streamflow from a diverse set of locations. Flood 

characteristics may vary substantially between regions because 

of differences in the climate, elevation, land cover, geology, and 

topography. To improve the transfer information for floods 

from gaged to ungaged sites, the streamflow-gaging network 

should include: (1) stations that represent different physiographic regions and the regional basins, (2) stations that are 

operated for extended periods of time (greater than 10 years), 

and (3) stations that represent various combinations of basin 

sizes, land-cover types, and hydrologic and geologic characteristics. To evaluate whether the number of gaging stations and 

range of geographic and hydrologic conditions are adequately 

represented by the current network, the spatial coverage of the 

network, the length of record of the gaging stations, and the 

drainage-basin characteristics associated with the gaging stations were assessed (table 10). Data layers of topography, geology, land cover, and rainfall were used to evaluate the spatial 

coverage and the drainage-basin characteristics represented by 

these stations. The goal of this network evaluation is to define 

geographical gaps, check for possible overlaps in the current 

network, and develop strategies for improving the network coverage for transferring flood characteristics. 
Evaluation of the Streamflow-Gaging Network for Characterizing Flood Flows 35

Table 10. Streamflow-gaging stations in the current (2004) network that most represent natural hydrologic conditions for regional flood 

analysis in Connecticut.

[Table includes 27 stations for regional analysis of nonurban basins and 3 stations for regional analysis of urban basins. Latitude/longitude given in degrees (°), 

minutes (’) and seconds (”)]

Station 

number Station name

Drainage

area

(square miles)

Period of record 

(water year)

Latitude/

Longitude Town

Pawcatuck River Basin

01118300 Pendleton Hill Brook near Clarks Falls, Conn. 4.01 1959-2004 Lat 41°28’30”, 

long 71°50’03”

North 

Stonington

Thames River Basin

01119500 Willimantic River near Coventry, Conn. 122 1932-2004 Lat 41°45’02”, 

long 72°15’56”

Mansfield

01121000 Mount Hope River near Warrenville, Conn. 29.0 1938, 1941-2004 Lat 41°50’37”, 

long 72°10’08”

Ashford

01123000 Little River near Hanover, Conn. 30.0 1936, 1938, 1952-

2004

Lat 41°40’16”, 

long 72°03’11”

Canterbury

01127500 Yantic River at Yantic, Conn. 89.2 1931-2004 Lat 41°33’32”, 

long 72°07’18”

Norwich

Connecticut River Basin

01184100 Stony Brook near West Suffield, Conn. 10.5 1955, 1960-2004 Lat 41°57’39”, 

long 72°42’38”

Suffield

01184490 Broad Brook at Broad Brook, Conn. 15.6 1938, 1962-1976, 

1982-2004

Lat 41°54’50”, 

long 72°32’58”

East 

Windsor

01187300 Hubbard River near West Hartland, Conn. 20.6 1

1938-1954, 1955, 

1957-2004

Lat 42°02’15”, 

long 72°56’22”

Hartland

01188000 Burlington Brook near Burlington, Conn. 4.20 1932-2004 Lat 41°47’10”, 

long 72°57’54”

Burlington

01189000 Pequabuck River at Forestville, Conn. 45.7 1938, 1942-2004 Lat 41°40’24”, 

long 72°53’59”

Bristol

01192500 Hockanum River near East Hartford, Conn. 73.3 1920-1921, 1929-

2004

Lat 41°46’60”, 

long 72°35’13”

East

Hartford

01192883 Coginchaug River at Middlefield, Conn. 29.7 2

1981-2004 Lat 41°31’12”, 

long 72°42’22”

Middlefield

01193500 Salmon River near East Hampton, Conn. 101 1929-2004 Lat 41°33’08”, 

long 72°26’56”

East

Hampton

01194500 East Branch Eightmile River near North Lyme, 

Conn.

22.4 1938-1982, 2002-

2004

Lat 41°25’39”, 

long 72°20’06”

Lyme

South-Central Coastal Basins

01195100 Indian River near Clinton, Conn. 5.62 1982-2004 Lat 41°18’21”, 

long 72°31’52”

Clinton

01195490 Quinnipiac River at Southington, Conn.3 17.8 1988-2004 Lat 41°36’13”, 

long 72°52’59”

Wallingford

01196500 Quinnipiac River at Wallingford, Conn. 110 1931-2004 Lat 41°22’07”, 

long 72°50’30”

Wallingford

01196620 Mill River near Hamden, Conn. 24.5 1969-1970, 1979-

2004

Lat 41°25’15”, 

long 72°54’12”

Hamden

Housatonic River Basin

01199050 Salmon Creek at Lime Rock, Conn. 29.4 1949, 1955, 1962-

2004

Lat 41°56’32”, 

long 73°23’28”

Salisbury

01200000 Tenmile River near Gaylordsville, Conn. 200 1930-1933, 1935-

1987, 1992-2004

Lat 41°39’32”, 

long 73°31’44”

Wingdale, 

N.Y.

01201487 Still River at State Route 7 at 

Brookfield Center, Conn.

62.3 42002-2004 Lat 41°27’58”, 

long 73°24’13”

Brookfield
36 Regression Equations for Estimating Flood Flows in Connecticut

Housatonic River Basin—Continued

01203600 Nonnewaug River at Minortown, Conn. 17.7 1955, 1963-1976, 

1979, 2001- 2004

Lat 41°34’32”, 

long 73°10’45”

Woodbury

01203805 Weekeepeemee River at Hotchkissville, Conn. 26.8 51979, 2001-2004 Lat 41°33’26”, 

long 73°12’57”

Woodbury

01204000 Pomperaug River at Southbury, Conn. 75.3 1933-2004 Lat 41°28’54”, 

long 73°13’29”

Southbury

Southwest Coastal Basins

01208873 Rooster River at Fairfield, Conn. 3 10.6 1978-2004 Lat 41°10’47”, 

long 73°13’10”

Trumbull

01208925 Mill River near Fairfield, Conn. 28.6 1973-2004 Lat 41°09’55”, 

long 73°16’13”

Fairfield

01208950 Sasco Brook near Southport, Conn. 7.38 1960-2004 Lat 41°09’10”, 

long 73°18’21”

Westport

01208990 Saugatuck River near Redding, Conn. 20.7 1962-2004 Lat 41°17’40”, 

long 73°23’42”

Redding

01209700 Norwalk River at South Wilton, Conn. 29.9 1956, 1963-2004 Lat 41°09’50”, 

long 73°25’11”

Wilton

01209901 Rippowam River at Stamford, Conn.3 34.0 1976, 1978-82, 

2002-2004

Lat 41°03’59”, 

long 73°32’59”

Stamford

1Annual maximum flow for these water years is a maximum daily average.

2

Annual maximum data for 1962–1980 collected at station 01192890, Coginchaug River at Rockfall, drainage area of 34.7 square miles. 3Station data can be used for urban flood-frequency analysis. 4

Less than 10 years of record. Annual maximum data for 1932–1966 collected at station 01201500, Still River near Lanesville, drainage area of 

67.5 square miles; for 1967–1971 collected at station 01201510, Still River at Lanesville, drainage area of 69.8 square miles. 5

Less than 10 years of record.

Table 10. Streamflow-gaging stations in the current (2004) network that most represent natural hydrologic conditions for regional flood 

analysis in Connecticut.—Continued

[Table includes 27 stations for regional analysis of nonurban basins and 3 stations for regional analysis of urban basins. Latitude/longitude given in degrees (°), 

minutes (’) and seconds (”)]

Station 

number Station name

Drainage

area

(square miles)

Period of record 

(water year)

Latitude/

Longitude Town

Spatial Coverage

The spatial coverage of the core stations in the network (in 

2004) contains large gaps (fig. 8). The gaps are in the central 

and southeastern coastal regions (a band about 10 to 15 mi 

inland from Long Island Sound), the northwestern region, and 

the far northeastern regions of the state. In addition, 10 regional 

basins that are unaffected by extensive flood-control regulation 

(and, therefore, potentially valuable for estimating floods) are 

ungaged: Fivemile River Basin (drainage area 76.6 mi2

), 

Moosup River Basin (drainage area 89.3 mi2

). Pachaug River 

Basin (drainage area 63.1 mi2), Scantic River Basin (drainage 

area 114 mi2

), Park River Basin (drainage area 76.2 mi2), Mattabessett River Basin (drainage area 108 mi2), Hollenbeck 

River Basin (drainage 42.3 mi2), Candlewood River Basin 

(drainage 40.4 mi2), Aspetuck River Basin (drainage area 50.6 

mi2

), and Shepaug River Basin (drainage area 156 mi2). One 

subregional basin—Hop River Basin, with a drainage area of 

80.2 mi2—is identified in the spatial coverage analysis because 

of its size. 

Record Lengths

The longer the period of record for a station, the greater the 

accuracy in estimating flood probabilities, especially those in 

the 50- to 100-year recurrence interval. Estimates based on 

shorter periods of record are generally not used because they 

may be more representative of a particular series of unusually 

wet or dry years. Ten years of flood-flow values is generally 

accepted as a minimum requirement for developing a floodprobability estimate (Interagency Advisory Committee on 

Water Data, 1982). The continuous operation of stations has 

fluctuated from year to year. Streamflow-gaging stations are 

often discontinued or have breaks in the systematic record. It 

must be emphasized that the continuity of the record of the gaging station also increases the value of the data from the station. 

Although there are considerably fewer streamflow-gaging stations operating today than in the early 1970s that can be used to 

transfer flood information from gaged to ungaged sites in Connecticut, the streamflow-gaging records are longer and therefore improve some of the uncertainty in estimating the magnitude and frequency of floods.
Evaluation of the Streamflow-Gaging Network for Characterizing Flood Flows 37

41 ̊

41 ̊30'

42 ̊

0 10 5 MILES

0 10 5 KILOMETERS

EXPLANATION

Basin boundaries for 25 core stations

73 ̊

72 ̊

01193500 Streamflow-gaging station 

and station number

Basin boundaries of ungaged regional basins

unaffected by extensive flood-control regulation

Moosup

Pachaug

Scantic

Park

Mattabessett

Hollenbeck

Candlewood

Aspetuck

Shepaug

Hop

Fivemile

Long Island Sound

01119500

01127500 01203600

01196620 01194500

01196500

01192883

01188000

01199050

01193500

01195100

01204000

01208950

01208925

01208990

01209700

01121000

01118300

01123000

01184490

01184100

01189000

01192500

01187300

01200000

Figure 8. Locations of the 25 core streamflow-gaging stations for flood analysis of nonurban basins, and ungaged regional basins unaffected by extensive flood-control regulation in Connecticut. [Hop River Basin is a subregional basin.]
38 Regression Equations for Estimating Flood Flows in Connecticut

As of water year 2004, the number of years of record associated with the 25 core stations ranges from 23 to 78 years with 

an average of 54 years. Eight stations have record lengths 

longer than 70 years, 3 stations have record lengths from 50 to 

70 years; 11 stations have record lengths from 30 to 50 years; 

and 3 stations have record lengths less than 30 years. About 

15 percent of the core stations in the current network have gaps 

in their systematic records. 

Drainage-Basin Characteristics

The streamflow-gaging network is designed to cover a 

wide range of selected basin characteristics and represent various stream types and basins—from steep, mountainous streams 

to low gradient, meandering streams; from small streams to 

large rivers; and from mostly forested to urban environments. 

Regionalization studies in New England have shown that the 

most common drainage basin and climatic characteristics used 

to estimate flows are drainage area, main channel slope, 24-

hour rainfall, mean basin elevation, percentage of basin area 

covered by forest, and percentage of basin area classified as 

coarse-grained stratified glacial deposits.

The core stations in the current (2004) network represent a 

narrow range of stream types and basin characteristics for Connecticut. For example, the range of basin sizes in the current 

network is small (fig. 9). No basins have drainage areas smaller 

than 4.0 mi2

 or larger than 200 mi2, and only three basins have 

drainage areas from 35 to 70 mi2 or larger than 125 mi2

 (fig. 9). 

The distribution of drainage areas is skewed to smaller size 

basins partly because many larger basins in Connecticut have 

streamflows that are regulated by flood-control dams and these 

basins cannot be used to estimate regional flood-flow characteristics. 

Mean basin elevation and main channel slope appear to be 

adequately represented by the current network of core stations, 

except that stations at higher elevations (above a mean of 800 ft 

above NAVD 88) or in mountainous terrain where main channel slopes can be greater than 75 ft/mi are not well represented 

(fig. 9 and 10). Of the 25 core stations, mean basin elevations 

range from 230 to 1,290 ft; only four stations have a mean basin 

elevation above 800 ft and a main channel slope greater than 75 

ft/mi. The distribution of mean basin elevation and main channel slope is slightly skewed to stations with elevations from 200 

to 650 ft and lower gradient channel slopes from 10 to 70 ft/mi. 

Many basins with mean elevations less than 200 ft are tidally 

affected and cannot be used to estimate regional flood-flow 

characteristics.

Urban basins are not well represented by the network of 

core stations. Currently, only three gaging stations 

(01195490—Quinnipiac River at Southington, 

01208873—Rooster River at Fairfield, and 01209910—Rippowam River at Stamford) that cover a combined drainage area 

of 62.5 mi2 represent urban basins in Connecticut. Urban 

basins, as defined for this study, have more than 15 percent of 

their drainage area designated as commercial, industrial, or 

high-intensity residential development. In Connecticut, 255 mi2

of the land cover is classified as commercial, industrial, or highintensity residential development. The total area of the drainage 

basins associated with the 255 mi2

 of urban land cover is considerable greater. By contrast, forested basins are better represented, except for basins with more than 75-percent forest cover 

and less than 40-percent forest cover (figs. 9 and 11).

Coarse-grained stratified glacial deposits in Connecticut 

are concentrated along streams and in the valleys and, therefore, 

do not make up a large percentage of the surficial geologic 

deposits in the state. Although a small percentage of the Connecticut’s surficial geologic materials consists of coarsegrained stratified glacial deposits, the percentage of basin area 

containing more than 15-percent coarse-grained stratified glacial deposits appears to be slightly underrepresented by the core 

stations (fig. 9). About two-thirds of the basins have less than 

15-percent coarse-grained stratified glacial deposits. 

Overall, the network is near or is at its minimum capability 

to adequately support the critical needs of engineers and waterresources managers in federal, state, and local agencies. 

Enhancements to the network to improve overall effectiveness 

for characterizing floods can be made by increasing the density 

of gages, establishing stations in regions that are not well-represented, and maintaining the continuity of the records. It 

should be pointed out that no core station was identified that 

duplicates or only slightly improves information available from 

another core station.

The effectiveness of the active network to estimate flood 

flows could be increased by reactivating streamflow-gaging stations that were previously discontinued and (or) by augmenting 

the network with a crest-stage gaging station network, thereby 

increasing the percentage of drainage area covered by gaging 

stations in different physiographic regions. A crest-stage gaging 

station is one where discrete measurements of flood stage and 

streamflow are collected over a period of time without continuous data being recorded or computed (Buchanan and Somers 

(1968). Generally, the cost of installing, maintaining, and operating a crest-stage gaging station is about 10 to 20 percent the 

cost of a continuous-record gaging station. Reactivated (discontinued) gaging stations and crest-stage gaging stations can be 

used to fill spatial gaps in the network and provide current statistical information on floods.

The network evaluation discussed above does not represent a complete evaluation of the capacity of the streamflowgaging network to meet all federal, state, and local agency needs 

for streamflow information, nor does it reflect an assessment of 

the full range of streamflow-gaging stations uses and needs. No 

attempt was made to evaluate the economic elements of the network. A complete evaluation of the network to meet all needs 

for streamflow information was beyond the scope of this study.
Evaluation of the Streamflow-Gaging Network for Characterizing Flood Flows 39

10 100 1,000

DRAINAGE AREA, IN SQUARE MILES

MEAN BASIN ELEVATION, IN FEET ABOVE NAVD 88

MAIN CHANNEL SLOPE, IN FEET PER MILE

PERCENTAGE OF BASIN AREA CLASSIFIED AS FOREST

PERCENTAGE OF BASIN AREA CONTAINING COARSE-GRAINED STRATIFIED GLACIAL DEPOSITS

(based on 23 streamflow-gaging stations--Tenmile and Hubbard River Basins are excluded)

0 20 10 30 40 50 60 70 80 90 100

0 20 40 60 80 100

0 200 600 800 1,200 1,400

0 30 60 90 120 150

400 1,000

1 5 50 500

10 30 50 70 90

[Each vertical line represents one core streamflow-gaging station. Thicker line represents multiple stations.]

Figure 9. The distribution of the basin characteristics of the 25 core streamflow-gaging stations for flood analysis in Connecticut. 
40 Regression Equations for Estimating Flood Flows in Connecticut

41 ̊

41 ̊30'

42 ̊

0 10 5 MILES

0 10 5 KILOMETERS

EXPLANATION

Basin boundary

LAND-SURFACE ELEVATION, 

IN FEET ABOVE NAVD 88

0 - 99

100 - 249

250 - 499

500 - 999

1,000 - 1,249

over 1,250

73°

72 ̊

Long Island Sound

Streamflow-gaging station

Elevation data from

U.S. Geological Survey, 2001

Figure 10. Land-surface elevations with locations of the 25 core streamflow-gaging stations for flood analysis in Connecticut. 
Evaluation of the Streamflow-Gaging Network for Characterizing Flood Flows 41

Agricultural

Coniferous forest

Forest

Rural residential

Transportation/other

Urban

Water

Wetland

41 ̊

41 ̊30'

42 ̊

0 10 5 MILES

0 10 5 KILOMETERS

EXPLANATION

Basin boundary

LAND-COVER CLASSIFICATION

73 ̊

72 ̊

Streamflow-gaging station

Land-cover data from

U.S. Geological Survey, 2000

Long Island Sound

Figure 11. Land-cover data with locations of the 25 core streamflow-gaging stations for flood analysis in Connecticut. 
42 Regression Equations for Estimating Flood Flows in Connecticut

Acknowledgments

The author thanks Claudia Tamayo of the USGS for her 

extensive help preparing numerous digital data layers and measuring the hydrologic characteristics used in the regression 

analysis. Toby Feaster of the USGS preprocessed and managed 

input data files, provided technical guidance throughout the 

study, as well as performed a thoughtful review of the report. 

Richard Lumia and Glenn Hodgkins of the USGS provided 

essential support and technical reviews of the study. In addition, 

Richard Lumia reviewed the report and made many valuable 

comments. 

Summary and Conclusions

Updated regression equations for calculating the magnitude of flood flows in Connecticut for the 2-, 10-, 25-, 50-, 

100-, and 500-year recurrence intervals were developed by the 

U.S. Geological Survey, in cooperation with the Connecticut 

Department of Transportation and Department of Environmental Protection. Flood-flow frequency data and hydrologic characteristics from 70 streamflow-gaging stations and the upstream 

drainage basins were used to develop the equations using generalized least-squares regression analysis. The final equations 

were chosen on the basis of Mallow Cp statistic, the Predicted 

Residual Sum of Squares statistic, adjusted R-squared, the 

degree of collinearity, and an analysis of the regression residuals. The equations relate flood flows to three hydrologic characteristics—drainage area, 24-hour rainfall, and mean basin elevation—that were determined using digital data. Average 

standard errors of prediction for the regression equations range 

from 31.8 (2-year frequency) to 45.0 percent (500-year frequency). The final equations were compared to previously published equations and generally were found to be more accurate. 

The regression equations are suitable for estimating flood flows 

for nonurban, unregulated stream sites where flood-control 

structures or other man-made factors do not appreciably affect 

the natural flood flows. The equations should be used within the 

range of the explanatory variables used to develop the equations. Furthermore, the characteristics should be determined in 

a manner as consistent as possible with the methods used in this 

study to preserve the statistical validity of the estimating procedures. Simplified equations using only one hydrologic characteristic—drainage area—also were developed. 

The applicability of previously developed, national three 

and seven-variable regression equations designed for urban 

basins used in conjunction with the new statewide equations 

was evaluated. Observed flows were compared to flows estimated from the three- and seven-variable urban regression 

equations at five streamflow-gaging stations in urban basins. 

The comparison shows that both the three- and seven-variable 

urban equations generally provide reasonable estimates of flood 

flows; however, this result is not an indication that the national 

equations provide unbiased estimates of flood flows in Connecticut. Future studies could attempt to verify the accuracy of 

the three-variable urban regression equations and (or) develop 

more simplified urban regression equations than the seven-variable regression equations for the northeastern United States. 

Flood flows were computed for 70 streamflow-gaging stations for the 2-, 10-, 25-, 50-, 100-, and 500-year recurrence 

intervals using a weighted average. The weighted average was 

determined by combining the flood-flow frequency estimate 

from a log-Pearson Type III analysis of the annual maximum 

flood flows for a gaging station with the estimate from the 

regression equations. Generally, a weighted average provides 

the best estimate of flood flows, particularly for stations with 

short length of records. 

It is important that water-resource managers, planners, 

engineers, and emergency management officials have the most 

accurate and current statistical information on floods. The 

streamflow-gaging station network was studied to evaluate the 

capacity of the network to provide better estimates of floods at 

ungaged sites in Connecticut. Increasing the spatial coverage of 

streamflow-gaging stations to include stations in basins that are 

not well represented, as well as continuing operations at the 

core stations, will increase the capacity of the network to provide a better understanding of flood flows in the region. 

Enhancements to the current network could include adding gaging stations with drainage-basin sizes not well represented 

(drainage basins less than 4.0 square miles (mi2

), from 35 to 70 

mi2

, and greater than 125 mi2), and adding stations in regions 

not well represented (mountainous terrain, coastal areas, and 

urban areas). Further enhancements to the streamflow-gaging 

network could include adding crest-stage gaging stations or 

reactivating discontinued gaging stations. 

Selected References

Ahearn, E.A., 2003, Peak-flow frequency estimates for U.S. 

Geological Survey streamflow-gaging stations in Connecticut: U.S. Geological Survey Water-Resources Investigations 

Report 03-4196, 14 p. 

Benson, M.A., 1960, Peak-flow frequency analyses, Manual of 

hydrology, part 3, Flood-flow techniques: U.S. Geological 

Survey Water-Supply Paper 1543-A, p. 51–74. 

Benson, M.A., 1962, Factors influencing the occurrence of 

floods in a humid region of diverse terrain: U.S. Geological 

Survey Water-Supply Paper 1580-B, 64 p. 

Bigwood, B.L., and Thomas, M.P., 1955, A flood-flow formula 

for Connecticut: Geological Survey Circular 365, 15 p. 

Bogart, D.B., 1960, Floods of August-October 1955, New 

England to North Carolina: U.S. Geological Survey WaterSupply Paper 1420, 854 p. 

Buchanan, T.J., and Somers, W.P., 1968, Stage measurement at 

gaging stations: U.S. Geological Survey Techniques of 

Water-Resources Investigations, book 3, chapt. A-7, 28 p.
Selected References 43

Cavalieri, P., Jayawickrama, J., Luca, R., Patetta, M., Scott, K., 

and Walsh, S., 2000, Statistics I—Introduction to ANOVA, 

regression, and logistic regression: Cary, N.C., SAS Institute, 

Inc., 504 p. 

Civco, D.L., 1998, University of Connecticut, Department of 

Natural Resources Management and Engineering, accessed 

on February 18, 2004 at URL http://dep.state.ct.us/gis/dataguides/uconn/layers/support/landus95/.

Cook, R.D., and Weisburg, S., 1994, An introduction to regression graphics: New York, John Wiley and Sons, 253 p.

Daly, Christopher, 2000, PRISM (Parameter-elevation Regressions on Independent Slopes Model), U.S. Department of 

Agriculture and Natural Resources Conservation Service Climate Mapping Project: National Water and Climate Center, 

Oregon State University Partnership. 

Danenberg, Diana, and Bogar, James, 1994, Hydrography subset: Hartford, Conn., Natural Resources Center, Department 

of Environmental Protection, datalayer.

DeGaetano, A.T., 1997, Extreme precipitation analysis for the 

State of Connecticut: Ithaca, N.Y., Northeast Regional Climate Center report, unpaginated.

Feaster, T.D., and Tasker, G.D., 2002, Techniques for estimating the magnitude and frequency of floods in rural basins of 

South Carolina, 1999: U.S. Geological Survey WaterResources Investigations Report 02- 4140, p. 5–11. 

Hardison, C.H., 1971, Prediction error of regression estimates 

of streamflow characteristics at ungaged sites: U.S. Geological Survey Professional Paper 750-C, p. 228–236. 

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

water resources: Studies in Environmental Science 49, New 

York, Elsevier Science Publishers, 529 p.

Hodgkins, G.A., 1999, Estimating the magnitude of peak flows 

for streams in Maine for selected recurrence intervals: U.S. 

Geological Survey Water-Resources Investigations Report 

99-4008, 45 p. 

Insightful, Inc. 2000, S-PLUS 6.1 modern statistics and 

advanced graphics computer software. 

Interagency Advisory Committee on Water Data, 1982, Guidelines for determining flood flow frequency— Bulletin 17B of 

the Hydrology Subcommittee: U.S. Geological Survey, 

Office of Water-Data Collection, 183 p. 

L.R. Johnston Associates, 1983, Realizing the risk—A History 

of the June 1982 floods in Connecticut: Connecticut Department of Environmental Protection Water Planning Report 7, 

142 p. 

Lumia, Richard, 1991, Regionalization of flood discharges for 

rural, unregulated streams in New York, excluding Long 

Island: U.S. Geological Survey Water-Resources Investigations Report 90-4197, 119 p. 

Miller, D.R., Warner, G.S., Ogden, F.L., and DeGaetano, A.T., 

2003, Rainfall in Connecticut: accessed on October 20, 2003 

at URL http://www.ctiwr.uconn.edu/PrecipinCT/precip.pdf.

Natural Resources Conservation Service, National Soil Survey 

Center, 1991, STATe Soil GeOgraphic (STATSGO) database: Miscellaneous Publication 1492, 110 p. (revised July 

1994). 

National Weather Service, 1961, Rainfall frequency atlas of the 

United States for durations from 30 minutes to 24 hours and 

return periods from 1 to 100 years: Technical Paper 40, 

115 p. 

Paulson, C.C., Bigwood, B.L., Harrington, A.W., and others, 

1940, Hurricane floods of September 1938: U.S. Geological 

Survey Water-Supply Paper 867, 562 p. 

Randall, A.D., 1996, Map of mean annual runoff, precipitation 

and evapotranspiration in the glaciated northern United 

States, 1951-80: U.S. Geological Survey Open-File Report 

96-395, plate 2. 

Ries, K.G., and Friesz, P.J., 2000, Methods for estimating lowflow statistics for Massachusetts streams: U.S. Geological 

Survey Water-Resources Investigations Report 00-4135, 

81 p. 

Sauer, V.B., Thomas, W.O. Jr., Stricker, V.A., and Wilson, 

K.V., 1983, Flood characteristics of urban watersheds in the 

United States: U.S. Geological Survey Water-Supply Paper 

2207, 63 p. 

Sauer, V.B., 1985, New studies of urban flood frequency in the 

southeastern United States, in 1985 International Symposium 

on Urban Hydrology, Hydraulic Infrastructures and Water 

Quality Control, University of Kentucky, July 22-25, 1985, 

Proceedings: University of Kentucky, p. 195-201. 

Stedinger, J.R., and Tasker, G.D., 1985, Regional hydrologic 

analysis 1. Ordinary, weighted, and generalized least squares 

compared: Water Resources Research, v. 21, no. 9, p. 1421–

1432.

Stone, J.R., Schafer, J.P., London, E.H., and Thompson, W.B., 

1992, Surficial materials map of Connecticut: U.S. Geological Special Map, 2 sheets, scale 1:125,000. 

Stone, J.R., Schafer, J.P., London, E.H., DiGiacomo-Cohen, 

Mary, Lewis, R.S., and Thompson, W.B., in press, Quaternary geologic map of Connecticut and Long Island Sound, 

with a section on sedimentary facies and morphosequences 

of glacial meltwater deposits by B.D. Stone: U.S. Geological 

Survey Geologic Investigations Map I-2784.

Tasker, G.D., and Stedinger, J.R., 1989, An operational GLS 

model for hydrologic regression: Journal of Hydrology, v. 

111, p. 361–375.

Thomson, M.T., Gannon, W.B., Thomas, M.P., and Hayes, 

G.S., 1964, Historical floods in New England: U.S. Geological Survey Water-Supply Paper 1779-M, 105 p. 

Paulson, R.W., Chase, E.B., Roberts, R.S., and Moody, D.W. 

(compilers), National Water Summary 1988–89—Hydrologic events and floods and droughts: U.S. Geological 

Survey Water- Supply Paper 2375, 591 p.

U.S. Geological Survey, 1997, STATSGO, Soil characteristics 

for the conterminous United States: U.S. Geological Survey 

Open-File Report 656, accessed on January 15, 2003, at URL 

http//water.usgs.gov/GIS/metadata/usgswrd/muid.html.

U.S. Geological Survey, 2000, National Land Cover Dataset: 

U.S. Geological Survey Fact Sheet 108-00, accessed on February 18, 2004 at URL http://erg.usgs.gov/isb/pubs/factsheets/fs10800.html.
44 Regression Equations for Estimating Flood Flows in Connecticut

U.S. Geological Survey, 2001, National Elevation Dataset: U.S. 

Geological Survey, accessed on October 20, 2003 at URL 

http://gisdata.usgs.gov/NED/. 

Wandle, W.S., 1983, Estimating peak discharges of small, rural 

streams in Massachusetts: U.S. Geological Survey WaterSupply Paper 2214, 26 p.

Weiss, L.A., 1975, Floodflow formulas for urbanized and nonurbanized areas of Connecticut, in Watershed Management 

Symposium, Logan, Utah, August 11-13, 1975, Proceedings: 

American Society of Civil Engineers, Irrigation and Drainage Division, p. 658-675. 

Weiss, L.A., 1983, Evaluation and design of a streamflow data 

network for Connecticut: Connecticut Water Resources Bulletin 36, 30 p. 

Weiss, L.A., 1991, Connecticut floods and droughts, with a section on “water management” by C.J. Hughes, in Paulson, 

R.W., Chase, E.B., Roberts, R.S., and Moody, D.W. (compilers), National Water Summary 1988-89—Hydrologic events 

and floods and droughts: U.S. Geological Survey WaterSupply Paper 2375, p. 215-222.
45

Table 1 
Table 1. Descriptions of 70 streamflow-gaging stations used to develop statewide regression equations for estimating flood flows for the 2-, 10-, 25-, 5recurrence intervals at stream sites in Connecticut. [lat, latitude; long, longitude; mi2, square miles; mi, miles; ft, feet; right and left bank are referenced facing downstream; see table 9 or appendix 1 for period of record] U.S. Geological Survey streamflow-gaging station Reference number (see fig. 1) Station number Station name Lat/Long Town Location 1 01118300 Pendleton Hill Brook near Clarks Falls, Conn. Lat 41°28’30”, long 71°50’03” North Stonington New London County, Hydrologic Unit 01090005, on lef

upstream from twin culverts on Grindstone Hill Road, 0.1 State Route 49 in the township of North Stonington, 1.6 west of Clarks Falls, and 3.4 mi northeast of village of Nington. 2 01119500 Willimantic River near Coventry, Conn. Lat 41°45’02”, long 72°15’56” Mansfield Tolland County, Hydrologic Unit 01100002, on left bank upstream from bridge on State Route 31, 1 mi downstream Brook, 2.4 mi southeast of South Coventry, 2.8 mi upstreHop River and 6.3 mi upstream from mouth. 3 01120000 Hop River near Columbia, Conn. Lat 41°43’39”, long 72°18’09” Columbia Tolland County, Hydrologic Unit 01100002, 1,500 ft dow

from Hop River Village near Columbia. 4 01120500 Safford Brook near Woodstock Valley, Conn. Lat 41°55’35”, long 72°03’35” Woodstock Windham County, Hydrologic Unit 01100002, on right bdownstream side of bridge on Hopkins Road, 0.3 mi dowfrom Bradford Brook, 0.3 mi upstream from mouth, and west of West Woodstock.

5 01121000 Mount Hope River near 

Warrenville, Conn.

 Lat 41°50’37”, long 72°10’08”

Ashford Windham County, Hydrologic Unit 01100002, on left ba

downstream from Knowlton Brook, 700 ft upstream from State Route 89, 1.8 mi south of Warrenville, and 3.2 mi sAshford.

6 01122000 Natchaug River at 

Willimantic, Conn.

 Lat 41°43’11”, long 72°11’46”

Windham Windham County, Hydrologic Unit 01100002, on left ba

upstream side of bridge on State Route 66, 1 mi northeas

mantic, 1.6 mi upstream from mouth, and 3.7 mi downstMansfield Hollow Dam.

7 01122500 Shetucket River near 

Willimantic, Conn.

 Lat 41°42’01”, 

long 72°10’57”

Windham Windham County, Hydrologic Unit 01100002, on right b

downstream side of Bingham Bridge on Plains Road, 500 upstream from Penn. Central Co. railroad bridge, 500 ft dfrom Potash Brook, 1.3 mi downstream from confluence mantic and Natchaug Rivers, 1.5 mi southeast of Willim17 mi upstream from mouth.
8 01123000 Little River near 

Hanover, Conn.

 Lat 41°40’16”, long 72°03’11”

Canterbury Windham County, Hydrologic Unit 01100002, on left b

upstream from bridge on Hanover Road, 0.7 mi downstrePeck Brook, 2.3 mi northeast of Hanover, and 6.5 mi upsmouth.

9 01124000 Quinebaug River at 

Quinebaug, Conn.

 Lat 42°01’20”, long 71°57’22”

Thompson Windham County, Hydrologic Unit 01100001, on right b

Quinebaug, 500 ft upstream from bridge on State Route downstream from Massachusetts-Connecticut State line, upstream from French River, and at river mile 46.

10 01125490 Little River at 

Harrisville, Conn.

 Lat 41°55’40”, long 71°55’49”

Putnam Windham County, Hydrologic Unit 01100001, at bridge 

Road, 0.5 mi east of Harrisville.

11 01125500 Quinebaug River at 

Putnam, Conn.

 Lat 41°54’33”, long 71°54’48”

Putnam Windham County, Hydrologic Unit 01100001, on right b

nam, 0.15 mi downstream from Little River, 0.3 mi upstrformer railroad bridge, 2.8 mi downstream from French 3.0 mi downstream from West Thompson Dam, and 36 mfrom mouth.

12 01125600 Mashamoquet Brook at 

Abington, Conn.

 Lat 41°52’27”, long 72°00’34”

Pomfret Windham County, Hydrologic Unit 01100001, at bridge 

Pond Road, 1 mi north of Abington.

13 01126000 Fivemile River at 

Killingly, Conn.

 Lat 41°50’14”, long 71°53’08”

Killingly Windham County, Hydrologic Unit 01100001, at Penn. C

road Co. bridge, 0.6 mi south of Killingly.

14 01126500 Moosup River at 

Moosup, Conn.

 Lat 41°42’38”, long 71°53’10”

Plainfield Windham County, Hydrologic Unit 01100001, at Kaman 

Corp. at Moosup.

15 01126600 Blackwell Brook near 

Brooklyn, Conn.

 Lat 41°45’55”, long 71°57’24”

Brooklyn Windham County, Hydrologic Unit 01100001, on left ba

upstream from bridge on State Highway 169, 1.5 mi souBrooklyn.

16 01127000 Quinebaug River at 

Jewett City, Conn.

 Lat 41°35’52”, long 71°59’05”

Griswold New London County, Hydrologic Unit 01100001, on lef

behind high school on Slater Avenue at Jewett City, 570 

stream from outlet of canal from Wedgewood Mills at mPachaug River, 1,000 ft downstream from railroad bridge mile 6.1.

Table 1. Descriptions of 70 streamflow-gaging stations used to develop statewide regression equations for estimating flood flows for the 2-, 10-, 25-, 5recurrence intervals at stream sites in Connecticut.—Continued[lat, latitude; long, longitude; mi2, square miles; mi, miles; ft, feet; right and left bank are referenced facing downstream; see table 9 or appendix 1 for period of record]

U.S. Geological Survey streamflow-gaging station

Reference 

number 

(see fig. 1)

Station 

number Station name Lat/Long Town Location
17 01127500 Yantic River at Yantic, Conn. Lat 41°33’32”, long 72°07’18” Norwich New London County, Hydrologic Unit 01100003, on lef

Yantic, 700 ft downstream from stone-arch highway briddownstream from Susquetonscut Brook, and 4.8 mi upstrmouth. 18 01184100 Stony Brook near West Suffield, Conn. Lat 41°57’39”, long 72°42’38” Suffield Hartford County, Hydrologic Unit 01080205, at bridge o

Grand Street, 2.1 mi south of West Suffield. 19 01184300 Gillette Brook at Somers, Conn. Lat 41°59’31”, long 72°26’04” Somers Tolland County, Hydrologic Unit 01080205, at twin culvBattle Street, 0.7 mi northeast of Somers. 20 01184490 Broad Brook at Broad Brook, Conn. Lat 41°54’50”, long 72°32’58” East Windsor Hartford County, Hydrologic Unit 01080205, on left ban

upstream from bridge on State Route 191 (Mill Street) at Brook, 0.5 mi upstream from mouth. 21 01184500 Scantic River at Broad Brook, Conn. Lat 41°54’42”, long 72°33’46” East Windsor Hartford County, Hydrologic Unit 01080205, 300 ft upstbridge on State Highway 191, at Broad Brook.

22 01186500 Still River at 

Robertsville, Conn.

 Lat 41°58’01”, long 73°02’02”

Colebrook Litchfield County, Hydrologic Unit 01080207, on left ba

downstream from Sandy Brook, 1 mi southeast of Robernorthwest of Riverton, and 1 mi upstream from mouth.

23 01187000 West Branch Farmington 

River at Riverton, Conn. 

 Lat 41°57’14”, long 73°00’50”

Barkhamsted Litchfield County, Hydrologic Unit 01080207, on right b

downstream from Still River, 0.6 mi south of Riverton.

24 01187300 Hubbard River near 

West Hartland, Conn.

 Lat 42°02’15”, long 72°56’22”

Hartland Hartford County, Hydrologic Unit 01080207, on left ban

chusetts-Connecticut Stateline, 800 ft upstream from bridRoute 20, 0.5 mi upstream from confluence with Valley 2.6 mi northeast of West Hartland.

25 01187400 Valley Brook near 

West Hartland, Conn.

 Lat 42°02’04”, long 72°55’48”

Hartland Hartford County, Hydrologic Unit 01080207, on right ba

upstream from bridge on State Highway 20, 0.25 mi soutchusetts-Connecticut State Line, 0.3 mi upstream from cwith Hubbard River, and 2.25 mi northeast of West Hart

26 01187800 Nepaug River near 

Nepaug, Conn.

 Lat 41°49’15”, long 72°58’13”

New Hartford

Litchfield County, Hydrologic Unit 01080207, beside U.202, 0.2 mi upstream from Nepaug Reservoir.

Table 1. Descriptions of 70 streamflow-gaging stations used to develop statewide regression equations for estimating flood flows for the 2-, 10-, 25-, 5recurrence intervals at stream sites in Connecticut.—Continued[lat, latitude; long, longitude; mi2, square miles; mi, miles; ft, feet; right and left bank are referenced facing downstream; see table 9 or appendix 1 for period of record]

U.S. Geological Survey streamflow-gaging station

Reference 

number 

(see fig. 1)

Station 

number Station name Lat/Long Town Location
27 01188000 Burlington Brook near 

Burlington, Conn.

 Lat 41°47’10”, long 72°57’54”

Burlington Hartford County, Hydrologic Unit 01080207, on left ban

north of Burlington, 3 mi upstream from mouth, 2,000 ft intersection of Covey Road and Hotchkiss Road, and 3 mof Collinsville.

28 01189000 Pequabuck River at 

Forestville, Conn.

 Lat 41°40’24”, long 72°53’59”

Bristol Hartford County, Hydrologic Unit 01080207, on left ban

upstream from bridge on Central Street, 0.2 mi downstreCopper Mine Brook, and 6.5 mi upstream from mouth.

29 01189200 Stratton Brook near 

Simsbury, Conn.

 Lat 41°52’12”, long 72°49’27”

Simsbury Hartford County, Hydrologic Unit 01080207, at bridge o

Village Road, 400 ft upstream from mouth, and 1.0 mi wSimsbury.

30 01189390 East Branch Salmon Brook at 

Granby, Conn.

 Lat 41°57’15”, long 72°46’46”

Granby Hartford County, Hydrologic Unit 01080207, on right ba

stream side of bridge on State Highway 20, 0.5 mi upstre

West Branch Salmon Brook, and 1.8 mi downstream from Lake.

31 01189500 Salmon Brook near 

Granby, Conn.

 Lat 41°56’15”, long 72°46’34”

East Granby Hartford County, Hydrologic Unit 01080207, on left ban

upstream from railroad bridge, 0.5 mi downstream from of East and West Branches Salmon Brook.

32 01190000 Farmington River at 

Rainbow, Conn.

 Lat 41°54’40”, long 72°41’17”

Windsor Hartford County, Hydrologic Unit 01080207, on left ban

bow, 300 ft downstream from Stevens Paper Mill, 0.4 mstream from Farmington River Power Co. dam, 1.3 mi upfrom Poquonock, 6.4 mi downstream from Salmon Brooriver mile 8.2.

33 01190600 Wash Brook at 

Bloomfield, Conn.

 Lat 41°49’33”, long 72°44’21”

Bloomfield Hartford County, Hydrologic Unit 01080205, on right ba

upstream from bridge on Gabb Road, 0.4 mi south of Blo

34 01191000 North Branch Park River at 

Hartford, Conn.

 Lat 41°47’02”, 

long 72°42’32”

Hartford Hartford County, Hydrologic Unit 01080205, on right ba

downstream from stone-arch bridge on Albany Avenue iand 3 mi upstream from confluence with South Branch P

35 01192500 Hockanum River near 

East Hartford, Conn.

 Lat 41°46’60”, long 72°35’13”

EastHartford

Hartford County, Hydrologic Unit 01080205, on left banPreston Street, 0.2 mi upstream from bridge on Walnut Sdownstream from Hop Brook, and 2.8 mi east of East Ha

Table 1. Descriptions of 70 streamflow-gaging stations used to develop statewide regression equations for estimating flood flows for the 2-, 10-, 25-, 5recurrence intervals at stream sites in Connecticut.—Continued[lat, latitude; long, longitude; mi2, square miles; mi, miles; ft, feet; right and left bank are referenced facing downstream; see table 9 or appendix 1 for period of record]

U.S. Geological Survey streamflow-gaging station

Reference 

number 

(see fig. 1)

Station 

number Station name Lat/Long Town Location
36 01192650 Roaring Brook at Hopewell, Conn. Lat 41°39’49”, long 72°34’54” Glastonbury Hartford County, Hydrologic Unit 01080205, at bridge o

Hill Road at Hopewell. 37 01192700 Mattabasset River at East Berlin, Conn. Lat 41°37’08”, long 72°42’46” Berlin Hartford County, Hydrologic Unit 01080205, at bridge o

Street, at East Berlin. 38 01192883 Coginchaug River at Middlefield, Conn. Lat 41°31’12”, long 72°42’22” Middlefield Middlesex County, Hydrologic Unit 01080205, on right upstream from Cider Mill Road, 0.5 mi northeast of Midd0.75 mi upstream from Wadsworth Falls. 39 01193500 Salmon River near East Hampton, Conn. Lat 41°33’08”, long 72°26’56” East Hampton Middlesex County, Hydrologic Unit 01080205, on left ba16 Bridge, 450 ft downstream from New London-Middle

line, 300 ft downstream from Comstock Bridge, 0.7 mi dfrom Dickinson Creek, and 3.5 mi southeast of East Ham

40 01193800 Hemlock Valley Brook at Hadlyme, Conn. Lat 41°25’43”, long 72°25’23” East Haddam Middlesex County, Hydrologic Unit 01080205, on right 

upstream from culvert on Bone Mill Road at Hadlyme, 0upstream from mouth.

41 01194000 Eightmile River at 

North Plain, Conn.

 Lat 41°26’29”, long 72°19’58”

EastHaddam

Middlesex County, Hydrologic Unit 01080205, at bridge Highway 82, at North Plain.

42 01194500 East Branch Eightmile River 

near North Lyme, Conn.

 Lat 41°25’39”, long 72°20’06”

Lyme New London County, Hydrologic Unit 01080205, on lef

State Route 156 bridge, 0.7 mi south of intersection of S82, 0.4 mi upstream from confluence of Eightmile Riverabove mouth.

43 01195000 Menunketesuck River near 

Clinton, Conn.

 Lat 41°18’07”, 

long 72°30’55”

Clinton Middlesex County, Hydrologic Unit 01100004, on right 

Fairy Dell, 100 ft downstream from Cobb's Bridge, 1.7 mClinton, 2.4 mi downstream from Kelseytown Reservoirupstream from mouth.

44 01195100 Indian River near 

Clinton, Conn.

 Lat 41°18’21”, long 72°31’52”

Clinton Middlesex County, Hydrologic Unit 01100004, on right 

stream side of bridge at Hurd Bridge Road, 2.0 mi north 

45 01195200 Neck River near 

Madison, Conn.

 Lat 41°16’58”, long 72°37’08”

Madison New Haven County, Hydrologic Unit 01100004, on left 

upstream from culvert on Fort Path Road, 1.2 mi west of and 3.5 mi upstream from mouth.

Table 1. Descriptions of 70 streamflow-gaging stations used to develop statewide regression equations for estimating flood flows for the 2-, 10-, 25-, 5recurrence intervals at stream sites in Connecticut.—Continued[lat, latitude; long, longitude; mi2, square miles; mi, miles; ft, feet; right and left bank are referenced facing downstream; see table 9 or appendix 1 for period of record]

U.S. Geological Survey streamflow-gaging station

Reference 

number 

(see fig. 1)

Station 

number Station name Lat/Long Town Location
46 01196500 Quinnipiac River at 

Wallingford, Conn.

 Lat 41°26’58”, long 72°50’29”

Wallingford New Haven County, Hydrologic Unit 01100004, on righ

Wilbur Cross Highway, 0.8 mi downstream from bridge iac River in Wallingford, and 2.0 mi upstream from Wha

47 01196580 Muddy River near 

North Haven, Conn.

 Lat 41°22’07”, long 72°50’30”

North Haven New Haven County, Hydrologic Unit 01100004, at bridg

Street, 2 mi southeast of North Haven.

48 01196620 Mill River near 

Hamden, Conn.

 Lat 41°25’14”, long 72°54’10”

Hamden New Haven County, Hydrologic Unit 01100004, 150 ft d

from bridge on Mount Carmel Avenue, 0.4 mi downstreaEaton's Brook, and 2.5 mi north of Hamden.

49 01196700 Wepawaug River at 

Milford, Conn.

 Lat 41°14’10”, long 73°03’28”

Milford New Haven County, Hydrologic Unit 01100004, 50 ft do

from bridge on Walnut Street, at Milford.

50 01198500 Blackberry River at 

Canaan, Conn.

 Lat 42°01’26”, long 73°20’31”

NorthCanaan

Litchfield County, Hydrologic Unit 01100005, on right bbridge on U.S. Highway 44, at Canaan.

51 01199050 Salmon Creek at 

Lime Rock, Conn.

 Lat 41°56’32”, long 73°23’28”

Salisbury Litchfield County, Hydrologic Unit 01100005, on left ba

upstream from bridge on Uptown Salisbury Road, 0.6 mLime Rock, and 3.0 mi upstream from mouth.

52 01199200 Guinea Brook at 

West Woods Road at Ellsworth, Conn.

 Lat 41°49’28”, long 73°25’48”

Sharon Litchfield County, Hydrologic Unit 01100005, on left ba

upstream from culvert on West Woods Road, 0.4 mi souEllsworth, 3 mi west of Cornwall Bridge, and 4.5 mi souSharon.

53 01200000 Tenmile River near 

Gaylordsville, Conn.

 Lat 41°39’32”, long 73°31’44”

Dover, N.Y. Dutchess County, Hydrologic Unit 01100005, 1.2 mi ups

New York-Connecticut stateline, 1.7 mi upstream from m

54 01201190 West Aspetuck River at 

Sand Road near New Milford, Conn.

 Lat 41°36’29”, long 73°25’29”

New Milford

Litchfield County, Hydrologic Unit 01100005, at downsof bridge on Sand Road, off Long Mountain Road, 1,000 State Highway 129, 1 mi northwest of Wellsville, and 2 New Milford.

55 01201500 Still River near 

Lanesville, Conn.

 Lat 41°31’12”, long 73°25’06”

New Milford

Litchfield County, Hydrologic Unit 01100005, on left ba

upstream side of highway bridge, 0.25 mi east of U.S. H1.1 mi south of Lanesville, 3 mi upstream from mouth, asouth of New Milford.

Table 1. Descriptions of 70 streamflow-gaging stations used to develop statewide regression equations for estimating flood flows for the 2-, 10-, 25-, 5recurrence intervals at stream sites in Connecticut.—Continued[lat, latitude; long, longitude; mi2, square miles; mi, miles; ft, feet; right and left bank are referenced facing downstream; see table 9 or appendix 1 for period of record]

U.S. Geological Survey streamflow-gaging station

Reference 

number 

(see fig. 1)

Station 

number Station name Lat/Long Town Location
56 01203000 Shepaug River near Roxbury, Conn. Lat 41°32’59”, long 73°19’47” Roxbury Litchfield County, Hydrologic Unit 01100005, at Weller

1.2 mi southwest of Roxbury. 57 01203510 Pootatuck River at Sandy Hook, Conn. Lat 41°25’12”, long 73°16’56” Newtown Fairfield County, Hydrologic Unit 01100005, at bridge o

Hill Road, at Sandy Hook. 58 01204000 Pomperaug River at Southbury, Conn. Lat 41°28’54”, long 73°13’29” Southbury New Haven County, Hydrologic Unit 01100005, on right 

upstream from bridge on Poverty Road, 800 ft downstreaBullet Hill Brook, 0.6 mi west of Southbury, and 5.8 mi from mouth. 59 01204800 Copper Mill Brook near Monroe, Conn. Lat 41°21’46”, long 73°13’06” Monroe Fairfield County, Hydrologic Unit 01100005, on right baupstream from twin culverts on Hammertown Road, 700 from mouth, 1.2 mi west of State Highway 111, 2.2 mi nMonroe, and 2.2 mi east of Botsford. 60 01206000 Naugatuck River near Thomaston, Conn. Lat 41°42’14”, long 73°03’54” Thomaston Litchfield County, Hydrologic Unit 01100005, on right bdownstream side of Twomile Bridge, 250 ft downstream road bridge, 0.4 mi upstream from Leadmine Brook.

61 01206500 Leadmine Brook near 

Thomaston, Conn.

 Lat 41°42’07”, long 73°03’27”

Thomaston Litchfield County, Hydrologic Unit 01100005, on left ba

downstream from highway bridge, 0.4 mi upstream from 

62 01208500 Naugatuck River at 

Beacon Falls, Conn.

 Lat 41°26’31”, long 73°03’47”

Beacon Falls New Haven County, Hydrologic Unit 01100005, on left 

downstream side of bridge on Bridge Street at Beacon Fa

upstream from Bronson Brook, and at river mile 10.1.

63 01208850 Pequonnock River at 

Trumbull, Conn.

 Lat 41°14’48”, long 73°11’51”

Trumbull Fairfield County, Hydrologic Unit 01100006, at bridge o

Farm Road at Trumbull.

64 01208925 Mill River near 

Fairfield, Conn.

 Lat 41°09’55”, long 73°16’13”

Fairfield Fairfield County, Hydrologic Unit 01100006, on right ba

downstream from bridge on Duck Farm Road, 1.5 mi noFairfield, 14.0 mi downstream from headwater of Mill R

65 01208950 Sasco Brook near 

Southport, Conn.

 Lat 41°09’10”, long 73°18’21”

Westport Fairfield County, Hydrologic Unit 01100006, on left dow

abutment of bridge on Hulls Farm Road, 1.5 mi northwe

Southport.

Table 1. Descriptions of 70 streamflow-gaging stations used to develop statewide regression equations for estimating flood flows for the 2-, 10-, 25-, 5recurrence intervals at stream sites in Connecticut.—Continued[lat, latitude; long, longitude; mi2, square miles; mi, miles; ft, feet; right and left bank are referenced facing downstream; see table 9 or appendix 1 for period of record]

U.S. Geological Survey streamflow-gaging station

Reference 

number 

(see fig. 1)

Station 

number Station name Lat/Long Town Location
66 01208990 Saugatuck River near 

Redding, Conn.

 Lat 41°17’40”, long 73°23’42”

Redding Fairfield County, Hydrologic Unit 01100006, on right do

side of bridge on State Route 53, 100 ft south of intersectRoutes 53 and 107, 0.8 mi upstream from Saugatuck Res1.0 mi southwest of Redding.

67 01209500 Saugatuck River near 

Westport, Conn.

 Lat 41°10’15”, long 73°21’53”

Westport Fairfield County, Hydrologic Unit 01100006, on left ban

Ford Road (Clinton Avenue), 400 ft downstream from W600 ft downstream from Aspetuck River, 2 mi north of W5.5 mi upstream from mouth.

68 01209700 Norwalk River at 

South Wilton, Conn.

 Lat 41°09’50”, long 73°25’11”

Wilton Fairfield County, Hydrologic Unit 01100006, on right ba

upstream side of bridge on Kent Road at South Wilton, 2of Norwalk.

69 01211700 East Branch Byram River at 

Round Hill, Conn.

 Lat 41°05’57”, long 73°40’59”

Greenwich Fairfield County, Hydrologic Unit 01100006, at bridge o

Street, 0.8 mi west of Round Hill.

70 01212100 East Branch Byram River at 

Riversville, Conn.

 Lat 41°03’39”, long 73°40’29”

Greenwich Fairfield County, Hydrologic Unit 01100006, at bridge o

Riversville Road just downstream from Merritt Parkwayupstream from mouth.

Table 1. Descriptions of 70 streamflow-gaging stations used to develop statewide regression equations for estimating flood flows for the 2-, 10-, 25-, 5recurrence intervals at stream sites in Connecticut.—Continued[lat, latitude; long, longitude; mi2, square miles; mi, miles; ft, feet; right and left bank are referenced facing downstream; see table 9 or appendix 1 for period of record]

U.S. Geological Survey streamflow-gaging station

Reference 

number 

(see fig. 1)

Station 

number Station name Lat/Long Town Location
54 Regression Equations for Estimating Flood Flows in Connecticut

Appendix 1 
Appendix 1. Peak-flow frequency estimates for streams in Connecticut for selected recurrence intervals.[Revised from Ahearn, 2003, table 1. Peak-flow frequency estimates based on 10 or more years of unregulated flow record. Period of record includes historical information outs

collection at or near a gaging station. Period of record in italics represents period when flows were affected by flood-control regulation. regulated, flood-control reservoir affects 

drainage area upstream from the gaging station has more than 4.5 million cubic feet of usable storage per mile (Benson, 1962)); mi2, square miles; ft3/s, cubic feet per second; nr

U.S. Geological Survey

streamflow-gaging station Drainage 

area 

(mi2

)

Period of record

 (water years)

Peak-flow frequency estimates for given recurrence int

(ft3/s)

Number Name 1.5

years

2 

years

10 

years

25

years

50 

years

100 

years

PAWCATUCK RIVER BASIN

01118300 Pendleton Hill Brook nr Clarks Falls 4.02 1959-2001 108 132 242 303 351 402 

SOUTHEAST COASTAL BASINS

01118750 Haleys Brook nr Old Mystic 4.37 1962-1984 75 96 234 346 453 585 r01127800 Fourmile River nr E Lyme 4.30 1961-1984 80 96 194 267 334 415 

THAMES RIVER BASIN

01119255 Delphi Brook nr Staffordville 2.59 1964-1976 62 88 260 394 517 664 

01119300 Roaring Brook nr Staffordville 5.61 1960-1984 198 258 559 736 877 1,030 

01119360 Conat Brook at W Willington 2.40 1964-1983 46 59 129 174 212 254 01119450 Eagleville Brook at Storrs 0.36 1953-1969 74 84 123 141 153 165 01119500 Willimantic River nr Coventry 121 1932-2001 1,680 2,160 5,170 7,520 9,730 12,400 2

01119600 Ash Brook nr N Coventry 2.79 1960-1970 134 151 224 261 289 317 01119820 Skungamaug River at N Coventry 24.7 1963-1975 420 556 1,400 2,050 2,640 3,330 01120000 Hop River nr Columbia 74.8 1933-1984 1,610 2,040 4,450 6,070 7,470 9,050 101120500 Safford Brook nr Woodstock Valley 4.15 1951-1981 275 338 659 860 1,030 1,210 01121000 Mount Hope River nr Warrenville 28.6 1938, 1941-2001 816 1,030 2,280 3,180 4,000 4,950 

01121300 Fenton River at E Willington 11.4 1964-1976 272 349 720 933 1,100 1,280 01122000 Natchaug River at Willimantic (regulated) 170 1931-1951, 1952-2001 2,540 3,110 6,820 9,770 12,600 16,100 2

01122500 Shetucket River nr Willimantic (regulated) 404 1904-1906, 1920-1921, 

1929-1951, 1952-2001

6,000 6,120 12,300 16,900 21,100 26,100 4

01122680 Merrick Brook nr Scotland 5.21 1960-1984 211 274 639 894 1,120 1,380 01123000 Little River nr Hanover 30.0 1936, 1938, 1952-2001 698 881 1,820 2,410 2,900 3,430 

01124000 Quinebaug River at Quinebaug (regulated) 155 1932-1959, 1960-2001 1,520 1,900 4,510 6,700 8,880 11,600 2

01124151 Quinebaug River at W. Thompson (regulated) 172 1967-2001, 1920-1940 Entire period of record regulated for flood control 01125300 English Neighborhood Brook at N Woodstock 4.66 1962-1984 130 187 547 805 1,030 1,290 01125490 Little River at Harrisville 35.8 1936, 1938, 1962-1976 548 682 1,350 1,760 2,090 2,440 01125500 Quinebaug River at Putnam (regulated) 328 1930-1959, 1960-2001 rev 3,040 rev 3,790 8,300 11,600 14,500 18,000 201125600 Mashamoquet Brook at Abington 11.1 1963-1976 350 434 815 1,020 1,180 1,340 01125650 Wappoquia Brook nr Pomfret 4.20 1964-1984 167 203 389 508 609 721 01125900 Cady Brook at E Putnam 8.29 1964-1984 231 309 721 979 1,190 1,420 01126000 Fivemile River at Killingly 57.8 1936, 1938-1984 542 669 1,330 1,750 2,100 2,490 01126500 Moosup River at Moosup 83.6 1933-1984 1,210 1,480 2,810 3,610 4,260 4,970 01126600 Blackwell Brook nr Brooklyn 17.0 1962-1976 384 515 1,260 1,760 2,180 2,660 01126700 Kitt Brook nr Canterbury 11.1 1964-1976 176 206 344 421 481 544 

01126950 Pachaug River at Pachaug 53.0 1936, 1938, 1961-1973 502 596 1,010 1,230 1,400 1,570 01127000 Quinebaug River at Jewett City (regulated) 713 1919-1964, 1965-2001 6,270 7,640 15,600 21,300 26,400 32,300 5

01127100 Broad Brook nr Preston City 12.5 1961-1976 292 341 556 672 762 855 01127400 Susquetonscut Brook at Yantic 15.7 1962-1976 378 434 655 761 838 914 
01127500 Yantic River at Yantic 89.3 1931-2001 2,100 2,640 5,740 7,860 9,730 11,800 101127700 Trading Cove Brook nr Thamesville 8.46 1961-1974 286 361 758 1,010 1,220 1,450 01127760 Hunts Brook at Old Norwich Rd. at Quaker Hill 11.5 1964-1976 195 244 512 686 835 1,000 

CONNECTICUT RIVER BASIN 01183990 Jawbuck Brook nr Hazardville 2.16 1967-1976 42 50 89 111 129 147 

01184000 Connecticut River at Thompsonville 9,660 1929-2001 Peak-flow frequency estimates available upon request 01184100 Stony Brook nr W Suffield 10.4 1955, 1960-2001 303 405 1,060 1,560 2,030 2,580 01184260 Namerick Brook nr Warehouse Point 2.70 1964-1984 176 229 505 677 819 972 01184300 Gillette Brook at Somers 3.60 1960-1984 98 126 273 367 446 533 01184490 Broad Brook at Broad Brook 15.5 1938, 1962-1976, 1982-2001 329 417 848 1,100 1,300 1,520 01184500 Scantic River at Broad Brook 98.2 1929-1984 859 1,070 2,260 3,080 3,800 4,640 01186000 W Branch Farmington River at Riverton (regulated) 131 1955-1968, 1969-2001 1,560 2,030 5,660 9,050 12,600 17,300 3

01186100 Mad River at Winsted (regulated) 18.5 1955, 1957-1961, 1962-1969 Less than 10 years of unregulated flow 01186500 Still River at Robertsville (regulated) 85.0 1936, 1938, 1949-1961, 1962-2001 2,200 2,880 7,110 10,300 13,200 16,700 2

01187000 W Branch Farmington River at Riverton 217 1930-1956 5,010 6,590 18,100 28,100 38,200 51,100 901187300 Hubbard River nr W Hartland 19.9 31938-1954, 1955, 1957-

2001

971 1,270 2,820 3,750 4,510 5,320 

01187400 Valley Brook nr W Hartland 7.03 31940-1954, 1955-1972 245 350 1,060 1,610 2,130 2,740 

01187800 Nepaug River nr Nepaug 23.5 31922-1954, 1955, 1958-

1984

637 843 1,990 2,750 3,390 4,110 

01187850 Clear Brook nr Collinsville 0.59 31922-1954, 1955-1973 10 14 35 50 64 81 

01187980 Farmington River at Collinsville (regulated) 360 1928, 1936, 1938, 1955, 

1963-1968, 1969-1977

Less than 10 years of unregulated flow 

01188000 Burlington Brook at Burlington 4.10 1932-2001 217 294 702 953 1,160 1,370 01188090 Farmington River at Unionville (regulated) 378 1978-2001, 1920-1940 Entire period of record regulated for flood control 01188100 Roaring Brook at Unionville 7.60 1962-1984 142 194 518 755 967 1,210 

01189000 Pequabuck River at Forestville 45.8 1938, 1942-2001 1,290 1,650 3,840 5,470 6,960 8,740 1

01189200 Stratton Brook nr Simsbury 5.13 1964-1984 100 134 316 432 528 633 01189390 E Branch Salmon Book at Granby 39.5 1955-1956, 1964-1976 555 764 2,450 4,080 5,810 8,130 101189500 Salmon Brook nr Granby4 67.4 1947-1963 1,440 1,930 5,780 9,500 13,400 18,800 3

01189995 Farmington River at Tariffville (regulated) 577 1913-1939, 1971-2001 Peak-flow frequency estimates not determined at this site 

01190000 Farmington River at Rainbow (regulated) 590 1928, 1936-1968, 1969-1986 5,540 7,120 18,100 27,300 36,500 48,000 801190050 Podunk River at Wapping 4.34 1962-1976 77 93 184 247 303 368 01190070 Connecticut River at Hartford 10,493 1905-2001 Peak-flow frequency estimates available upon request01190100 Piper Brook at Newington Junction5 14.6 1955, 1958-1984 571 745 1,630 2,160 2,590 3,050 

01190200 Mill Brook at Newington 2.65 1955, 1958-1984 159 217 489 636 745 854 

01190300 Trout Brook at W Hartford (regulated) 14.6 1955, 1958-1963, 1964-

1972, 1975, 1980

Less than 10 years of unregulated flow 

Appendix 1. Peak-flow frequency estimates for streams in Connecticut for selected recurrence intervals.—Continued[Revised from Ahearn, 2003, table 1. Peak-flow frequency estimates based on 10 or more years of unregulated flow record. Period of record includes historical information outs

collection at or near a gaging station. Period of record in italics represents period when flows were affected by flood-control regulation. regulated, flood-control reservoir affects 

drainage area upstream from the gaging station has more than 4.5 million cubic feet of usable storage per mile (Benson, 1962)); mi2, square miles; ft3/s, cubic feet per second; nr

U.S. Geological Survey

streamflow-gaging station Drainage 

area 

(mi2

)

Period of record

 (water years)

Peak-flow frequency estimates for given recurrence int

(ft3/s)

Number Name 1.5

years

2 

years

10 

years

25

years

50 

years

100 

years
01190500 S Branch Park River at Hartford5 39.9 1936-1972, 1974-1981 1,130 1,500 3,660 5,160 6,480 7,980 1

01190600 Wash Brook at Bloomfield 5.54 1955, 1959-1971 156 198 476 702 921 1,190 01191000 N Branch Park River at Hartford (regulated) 26.8 1936-1962, 1963-1996 943 1,150 2,460 3,430 4,330 5,400 

01191500 Park River at Hartford5 72.5 1936-1962 1,770 2,150 4,590 6,460 8,220 10,400 1

01191900 Charter Brook nr Crystal Lake 8.51 1965-1984 203 275 647 869 1,050 1,230 01192500 Hockanum River nr E Hartford 73.4 1920-1921, 1929-2001 826 1,050 2,200 2,940 3,550 4,220 01192600 S Branch Salmon Brook at Buckingham 0.94 1961-1976 18 24 66 99 130 167 01192650 Roaring Brook at Hopewell 24.3 1962-1976 439 507 852 1,060 1,240 1,430 01192700 Mattabesset River at E Berlin 46.5 1962-1979, 61995-1998 1,310 1,590 2,780 3,400 3,880 4,360 

01192800 Parmalee Brook nr Durham 2.79 1960-84 174 215 397 494 569 645 

01192883 Coginchaug River at Middlefield 29.8 1962-1980; 71981-2001 581 747 1,550 2,010 2,370 2,750 

01193120 Ponset Brook nr Higganum 5.72 1962-1977, 1982 188 246 598 855 1,090 1,360 01193250 Judd Brook nr Colchester 3.93 1962-1979 116 157 403 578 732 908 01193300 Blackledge River nr Gilead 6.75 1960-1984 161 198 366 459 532 608 01193500 Salmon River nr E Hampton 100 1929-2001 2,140 2,730 6,460 9,390 12,200 15,500 201193800 Hemlock Valley Brook at Hadlyme 2.62 1961-1976 97 123 245 314 368 424 01194000 Eightmile River at N Plain 20.1 1938-1984 679 861 1,850 2,500 3,060 3,680 01194500 E Branch Eightmile River nr N Lyme 22.3 1938-1982 536 645 1,350 1,900 2,430 3,070 

SOUTH CENTRAL COASTAL BASINS

01195000 Menunketesuck River nr Clinton 11.2 1938, 1942-1967, 1982 448 410 873 1,210 1,520 1,890 

01195100 Indian River nr Clinton 5.68 1982-2001 140 176 405 578 738 930 01195200 Neck River nr Madison 6.55 1962-1982 144 183 401 548 675 818 01195490 Quinnipiac River at Southington5 17.4 1988-2001 384 461 785 949 1,070 1,200 

01196500 Quinnipiac River at Wallingford 110 1931-2001 1,690 2,100 4,100 5,260 6,180 7,140 

01196580 Muddy River nr N Haven 18.0 1963-1976 626 712 1,080 1,280 1,440 1,590 01196600 Willow Brook nr Cheshire 9.34 1960-1983 210 278 780 1,230 1,690 2,290 

01196620 Mill River nr Hamden 24.5 1969-1970, 1979-2001 718 946 2,420 3,580 4,670 5,980 101196700 Wepawaug River at Milford 18.4 1962-1984 520 667 1,580 2,280 2,930 3,700 

HOUSATONIC RIVER BASIN

01198500 Blackberry River at Canaan (regulated) 45.9 1949-1961, 1962-1981 1,400 1,830 4,190 5,740 7,070 8,550 101198860 Deming Brook nr Huntsville 1.08 1971-1984 62 91 309 495 676 900 01199000 Housatonic River at Falls Village 634 1913-2001 5,250 6,260 11,600 15,100 18,100 21,400 301199050 Salmon Creek at Lime Rock 29.4 1949, 1955, 1962-2001 450 588 1,510 2,250 2,970 3,850 01199150 Furnace Brook at Cornwall Bridge 13.3 1945, 1949, 1955, 1962-

1976

229 312 921 1,460 1,990 2,680 

01199200 Guinea Brook at W Woods Rd at Ellsworth 3.50 1960-1981 76 96 199 262 314 369 01200500 Housatonic River at Gaylordsville 996 1901-1914, 1924, 1928-2001 8,660 10,600 21,200 28,100 33,900 40,500 5

Appendix 1. Peak-flow frequency estimates for streams in Connecticut for selected recurrence intervals.—Continued[Revised from Ahearn, 2003, table 1. Peak-flow frequency estimates based on 10 or more years of unregulated flow record. Period of record includes historical information outs

collection at or near a gaging station. Period of record in italics represents period when flows were affected by flood-control regulation. regulated, flood-control reservoir affects 

drainage area upstream from the gaging station has more than 4.5 million cubic feet of usable storage per mile (Benson, 1962)); mi2, square miles; ft3/s, cubic feet per second; nr

U.S. Geological Survey

streamflow-gaging station Drainage 

area 

(mi2)

Period of record

 (water years)

Peak-flow frequency estimates for given recurrence int

(ft3/s)

Number Name 1.5

years

2 

years

10 

years

25

years

50 

years

100 

years
01201190 W Aspetuck River at Sand Rd nr New Milford 23.8 1963-1972 276 360 842 1,180 1,480 1,820 01201500 Still River nr Lanesville 67.5 1932-1966, 91967-1984 938 1,220 2,940 4,190 5,320 6,630 1

01201890 Pond Brook nr Hawleyville 11.9 1963-1976 316 425 1,100 1,600 2,050 2,580 

01201930 Marshepaug River nr Milton 9.24 101968-1981 182 224 392 472 530 588 

01202500 Shepaug River at Woodville 38.0 1936-1988 956 1,280 3,120 4,320 5,340 6,460 

01202700 Butternut Brook nr Litchfield 2.42 1960-1984 176 229 506 677 817 968 01203000 Shepaug River nr Roxbury 132 1931-1984 2,320 2,960 7,210 10,600 14,000 rev 18,000 301203100 Jacks Brook nr Roxbury Falls 7.90 1961-1984 410 542 1,230 1,650 2,000 2,370 01203510 Pootatuck River at Sandy Hook 24.8 1966-84 1,040 1,290 2,390 2,970 3,410 3,860 01203600 Nonnewaug River at Minortown 17.7 1955, 1963-1979, 2001 1,070 1,460 3,740 5,320 6,680 8,220 101203700 Wood Creek nr Bethlehem 3.39 1962-1984 154 197 458 649 824 1,030 01204000 Pomperaug River at Southbury 75.1 1933-2001 2,210 2,810 6,490 9,190 11,600 14,600 201204800 Copper Mill Brook nr Monroe 2.45 1959-1976 122 148 257 312 352 391 01205500 Housatonic River at Stevenson 1,544 1924-1925, 1928-2001 15,400 20,000 45,200 61,700 75,700 91,200 101205600 W Branch Naugatuck River at Torrington (regulated) 33.8 1955, 1957-1961, 1962-1996 Less than 10 years of unregulated flow 01205700 E Branch Naugatuck River at Torrington (regulated) 13.6 1955, 1957-1963, 1964-1996 Less than 10 years of unregulated flow 01206000 Naugatuck River nr Thomaston 71.0 1931-1959 2,630 3,260 7,120 10,000 12,600 15,800 201206500 Leadmine Brook nr Thomaston 24.3 1931-59, 111960-1984 938 1,240 3,320 5,010 6,640 8,640 1

01206900 Naugatuck River at Thomaston (regulated) 99.8 1955, 1960-2001 Less than 10 years of unregulated flow 01208013 Branch Brook nr Thomaston (regulated) 20.8 1971-2001, 1920-1940 Entire period of record regulated for flood control 01208100 Hancock Brook nr Terryville 1.18 1960-1981 95 118 219 274 315 357 

01208400 Hop Brook nr Middlebury 9.43 1955, 1962-1975 333 436 1,050 1,480 1,870 2,310 01208420 Hop Brook nr Naugatuck (regulated) 16.3 1955, 1970-2001 Less than 10 years of unregulated flow 01208500 Naugatuck River at Beacon Falls (regulated) 260 1920-1959, 1960-2001 6,700 8,620 20,700 29,900 38,400 48,500 801208700 Little River at Oxford 4.54 1960-1984 186 243 600 870 1,120 1,420 

SOUTHWEST COASTAL BASINS

01208850 Pequonnock River at Trumbull 15.6 1955, 1962-1984 555 732 1,720 2,380 2,950 3,580 01208873 Rooster River at Fairfield5 10.6 1978-2001 959 1,140 1,800 2,070 2,250 2,410 

01208900 Patterson Brook nr Easton 1.21 1960-1984 61 72 116 139 156 173 01208925 Mill River nr Fairfield 28.6 1973-2001 507 679 1,530 2,020 2,390 2,780 01208950 Sasco Brook nr Southport 7.38 1960-2001 205 270 728 1,120 1,500 1,970 01208990 Saugatuck River nr Redding 21.0 1962-2001 456 586 1,220 1,580 1,860 2,160 01209500 Saugatuck River nr Westport 79.8 1933-1967 1,140 1,540 3,950 5,680 7,220 8,990 101209600 Comstock Brook at N Wilton 3.50 1960-1975 116 137 251 325 388 459 

01209700 Norwalk River at S Wilton 30.0 1956, 1963-2001 778 1,010 2,320 3,220 4,000 4,880 

01209770 Fivemile River nr Norwalk 8.96 1956, 1962-1984 431 536 1,130 1,540 1,890 2,300 01211700 E Branch Byram River at Round Hill 1.69 1960-1975 78 100 225 306 376 452 01212100 E Branch Byram River at Riversville 11.1 1963-1984 409 567 1,450 2,020 2,490 3,000 

Appendix 1. Peak-flow frequency estimates for streams in Connecticut for selected recurrence intervals.—Continued[Revised from Ahearn, 2003, table 1. Peak-flow frequency estimates based on 10 or more years of unregulated flow record. Period of record includes historical information outs

collection at or near a gaging station. Period of record in italics represents period when flows were affected by flood-control regulation. regulated, flood-control reservoir affects 

drainage area upstream from the gaging station has more than 4.5 million cubic feet of usable storage per mile (Benson, 1962)); mi2, square miles; ft3/s, cubic feet per second; nr

U.S. Geological Survey

streamflow-gaging station Drainage 

area 

(mi2)

Period of record

 (water years)

Peak-flow frequency estimates for given recurrence int

(ft3/s)

Number Name 1.5

years

2 

years

10 

years

25

years

50 

years

100 

years
1Peak flow augments by release of storage from dam failure. 2Peak flow affected by dam failure.3Peak flow for these water years is a maximum daily average.4Gaging station formerly published under the name West Branch Salmon Brook at Granby, Connecticut. 5Discharge is affected by urbanization or channelization. 61995–1998 streamflow data collected at station 01192704, Mattabessett River at Rt. 372 at East Berlin, adjusted to site (data transferred using transfer equation 6.12, Dra

Connecticut Department of Transportation, January 2000). 71962–1980 streamflow data collected at station 01192890, Coginchaug River at Rockfall adjusted to site (data transferred using transfer equation 6.12, Drainage Manual, 

Connecticut Dept. of Transportation, January 2000). 8Peak flow affected by dam failure.

91967–1984 streamflow data collected at station 01201510, Still River at Lanesville adjusted to site (data transferred using transfer equation 6.12, Drainage Manual, 

Connecticut Dept. of Transportation, January 2000). 10Peak flow frequency estimates based on water years 1972-81. 111960–1984 streamflow data collected at station 01206400, Leadmine Brook near Harwinton adjusted to site (data transferred using transfer equation 6.12, Drainage Man

Connecticut Dept. of Transportation, January 2000).
60 Regression Equations for Estimating Flood Flows in Connecticut

Appendix 2 
Appendix 2 61

Appendix 2. Hydrologic characteristics for streamflow-gaging stations used in the regression analysis for Connecticut. 

[Hydrologic characteristics determined using digital data sets (Connecticut Drainage Basin Boundaries, USGS Digital Elevation Models, and 24-hour rainfall 

from the Northeast Regional Climate Center) and GIS technology. Elevations referenced to North American Vertical Datum of 1988. mi2

, square miles; ft, foot; 

in., inches; DA, drainage area in square miles; EL, mean basin elevation in feet; Px, 24-hour rainfall for x-recurrence interval in inches]

U.S. Geological Survey

streamflow-gaging station Hydrologic characteristics

Number Name

Drainage 

area

(mi2

)

Mean basin 

elevation 

(ft)

2-year,

24-hour 

rainfall 

(inches)

10-year, 

24-hour 

rainfall 

(inches)

25-year, 

24-hour 

rainfall 

(inches) 

50-year, 

24-hour 

rainfall 

(inches)

100-year, 

24-hour 

rainfall 

(inches)

DA EL P2 P10 P25 P50 P100

01118300 Pendleton Hill Brook near Clarks Falls 4.01 348 3.53 5.20 6.48 7.66 9.06

01119500 Willimantic River near Coventry 122 701 3.20 4.44 5.34 6.15 7.08

01120000 Hop River near Columbia 74.5 607 3.25 4.51 5.41 6.23 7.16

01120500 Safford Brook near Woodstock Valley 4.17 748 3.23 4.56 5.56 6.46 7.50

01121000 Mount Hope River near Warrenville 29.0 653 3.23 4.50 5.43 6.27 7.22

01122000 Natchaug River at Willimantic 170 612 3.25 4.55 5.51 6.37 7.36

01122500 Shetucket River near Willimantic 401 621 3.25 4.51 5.44 6.28 7.24

01123000 Little River near Hanover 30.0 508 3.37 4.84 5.94 6.95 8.12

01124000 Quinebaug River at Quinebaug 156 760 3.06 4.15 4.93 5.62 6.41

01125490 Little River at Harrisville 35.7 546 3.22 4.54 5.54 6.43 7.46

01125500 Quinebaug River at Putnam 329 679 3.10 4.25 5.10 5.85 6.70

01125600 Mashamoquet Brook at Abington 11.0 686 3.28 4.70 5.79 6.77 7.91

01126000 Fivemile River at Killingly 57.8 554 3.25 4.68 5.77 6.76 7.92

01126500 Moosup River at Moosup 83.5 512 3.45 5.18 6.53 7.78 9.26

01126600 Blackwell Brook near Brooklyn 17.0 477 3.35 4.94 6.17 7.28 8.61

01127000 Quinebaug River at Jewett City 715 543 3.26 4.64 5.69 6.63 7.73

01127500 Yantic River at Yantic 89.2 409 3.43 4.76 5.74 6.62 7.63

01184100 Stony Brook near West Suffield 10.5 256 3.39 5.06 6.36 7.55 8.97

01184300 Gillette Brook at Somers 3.66 678 3.21 4.55 5.54 6.44 7.49

01184490 Broad Brook at Broad Brook 15.6 300 3.22 4.57 5.57 6.48 7.54

01184500 Scantic River at Broad Brook 97.7 389 3.22 4.58 5.60 6.53 7.61

01186500 Still River at Robertsville 85.1 1210 3.45 5.23 6.62 7.93 9.49

01187000 West Branch Farmington River at 

Riverton

217 1310 3.33 5.12 6.55 7.88 9.48

01187300 Hubbard River near West Hartland 20.6 1290 3.38 5.27 6.80 8.24 9.98

01187400 Valley Brook near West Hartland 7.39 1100 3.38 5.21 6.68 8.06 9.72

01187800 Nepaug River near Nepaug 23.4 844 3.67 5.53 7.00 8.36 9.99

01188000 Burlington Brook near Burlington 4.20 920 3.68 5.48 6.89 8.18 9.72

01189000 Pequabuck River at Forestville 45.7 635 3.54 5.19 6.46 7.63 9.01

01189200 Stratton Brook near Simsbury 5.44 405 3.63 5.47 6.92 8.26 9.86

01189390 East Branch Salmon Brook at Granby 39.1 506 3.41 5.17 6.57 7.87 9.43

01189500 Salmon Brook Near Granby 66.9 580 3.44 5.24 6.66 7.99 9.58

01190000 Farmington River at Rainbow 590 880 3.48 5.28 6.71 8.04 9.63

01190600 Wash Brook at Bloomfield 5.64 181 3.49 5.18 6.48 7.68 9.10

01191000 North Branch Park River at Hartford 26.5 227 3.52 5.20 6.49 7.68 9.09

01192500 Hockanum River near East Hartford 73.3 447 3.17 4.44 5.36 6.19 7.15

01192650 Roaring Brook at Hopewell 24.2 540 3.29 4.60 5.55 6.41 7.40

01192700 Mattabesset River at East Berlin 45.3 216 3.64 5.34 6.65 7.85 9.27

01192883 Coginchaug River at Middlefield 29.7 348 3.70 5.32 6.54 7.66 8.96

01193500 Salmon River near East Hampton 101 490 3.44 4.82 5.82 6.72 7.76
62 Regression Equations for Estimating Flood Flows in Connecticut

01193800 Hemlock Valley Brook at Hadlyme 2.69 362 3.50 5.02 6.17 7.20 8.42

01194000 Eightmile River at North Plain 20.2 407 3.49 4.99 6.12 7.15 8.35

01194500 East Branch Eightmile River Near 

North Lyme

22.4 366 3.47 5.01 6.17 7.24 8.49

01195000 Menunketesuck River near Clinton 11.3 355 3.60 5.22 6.45 7.57 8.89

01195100 Indian River near Clinton 5.62 236 3.56 5.19 6.44 7.57 8.92

01195200 Neck River near Madison 6.57 169 3.61 5.25 6.50 7.65 9.00

01196500 Quinnipiac River at Wallingford 110 303 3.61 5.24 6.50 7.65 9.00

01196580 Muddy River near North Haven 17.8 276 3.79 5.47 6.75 7.91 9.28

01196620 Mill River near Hamden 24.5 302 3.67 5.28 6.50 7.61 8.91

01196700 Wepawaug River at Milford 18.6 263 3.62 5.41 6.81 8.10 9.63

01198500 Blackberry River at Canaan 46.0 1220 3.26 4.83 6.03 7.14 8.45

01199050 Salmon Creek at Lime Rock 29.4 1170 2.95 4.28 5.30 6.22 7.30

01199200 Guinea Brook at West Woods Rd 

at Ellsworth

3.50 1290 3.05 4.40 5.42 6.34 7.43

01200000 Tenmile River near Gaylordsville 200 819 3.02 4.37 5.40 6.33 7.42

01201190 West Aspetuck River at Sand Rd 

near New Milford

23.8 787 3.17 4.52 5.54 6.45 7.52

01201500 Still River near Lanesville 67.6 538 3.77 5.22 6.28 7.22 8.32

01203000 Shepaug River near Roxbury 132 1020 3.34 4.78 5.86 6.85 8.00

01203510 Pootatuck River at Sandy Hook 25.0 510 3.82 5.30 6.37 7.34 8.44

01204000 Pomperaug River at Southbury 75.3 653 3.37 4.66 5.60 6.44 7.41

01204800 Copper Mill Brook near Monroe 2.45 490 3.77 5.35 6.53 7.60 8.85

01206000 Naugatuck River near Thomaston 71.9 1030 3.51 5.16 6.42 7.59 8.97

01206500 Leadmine Brook near Thomaston 24.5 875 3.60 5.31 6.63 7.85 9.29

01208500 Naugatuck River at Beacon Falls 260 783 3.44 4.95 6.08 7.13 8.34

01208850 Pequonnock River at Trumbull 15.5 413 3.72 5.35 6.58 7.70 9.02

01208925 Mill River near Fairfield 28.6 350 3.67 5.32 6.58 7.73 9.10

01208950 Sasco Brook near Southport 7.38 230 3.61 5.27 6.55 7.72 9.11

01208990 Saugatuck River near Redding 20.7 575 3.78 5.26 6.35 7.33 8.47

01209500 Saugatuck River near Westport 79.5 456 3.73 5.25 6.38 7.40 8.58

01209700 Norwalk River at South Wilton 29.9 478 3.71 5.25 6.39 7.43 8.63

01211700 East Branch Byram River at Round Hill 1.69 469 3.62 5.24 6.47 7.60 8.92

01212100 East Branch Byram River at Riversville 11.2 398 3.63 5.28 6.53 7.68 9.03

Appendix 2. Hydrologic characteristics for streamflow-gaging stations used in the regression analysis for Connecticut. —Continued

[Hydrologic characteristics determined using digital data sets (Connecticut Drainage Basin Boundaries, USGS Digital Elevation Models, and 24-hour rainfall 

from the Northeast Regional Climate Center) and GIS technology. Elevations referenced to North American Vertical Datum of 1988. mi2

, square miles; ft, foot; 

in., inches; DA, drainage area in square miles; EL, mean basin elevation in feet; Px, 24-hour rainfall for x-recurrence interval in inches]

U.S. Geological Survey

streamflow-gaging station Hydrologic characteristics

Number Name

Drainage 

area

(mi2

)

Mean basin 

elevation 

(ft)

2-year,

24-hour 

rainfall 

(inches)

10-year, 

24-hour 

rainfall 

(inches)

25-year, 

24-hour 

rainfall 

(inches) 

50-year, 

24-hour 

rainfall 

(inches)

100-year, 

24-hour 

rainfall 

(inches)

DA EL P2 P10 P25 P50 P100