Prepared in cooperation with the New York City Department of Environmental Protection

Estimates of Natural Streamflow at Two 

Streamgages on the Esopus Creek, New York, 

Water Years 1932 to 2012

Scientific Investigations Report 2015–5050

U.S. Department of the Interior

U.S. Geological Survey
Cover. The West Basin of Ashokan Reservoir at sunset. Photograph by Elizabeth Nystrom, 2013.
Estimates of Natural Streamflow at Two 

Streamgages on the Esopus Creek, New 

York, Water Years 1932 to 2012

By Douglas A. Burns and Christopher L. Gazoorian

Prepared in cooperation with the

New York City Department of Environmental Protection

Scientific Investigations Report 2015–5050

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2015

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Suggested citation:

Burns, D.A., and Gazoorian, C.L., 2015, Estimates of natural streamflow at two streamgages on the Esopus Creek, 

New York, water years 1932–2012: U.S. Geological Survey Scientific Investigations Report 2015–5050, 20 p., 

http://dx.doi.org/10.3133/sir20155050.

ISSN 2328-0328 (online)
iii

Contents

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

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

Purpose and Scope .......................................................................................................................................3

Study Site ........................................................................................................................................................3

Methods...........................................................................................................................................................3

Study Approach.....................................................................................................................................5

Sources of Discharge Data.................................................................................................................6

Analyses of Discharge Data ...............................................................................................................7

Estimated Natural Discharge Results ........................................................................................................7

Regression Models for Estimating Natural Discharge at the Coldbrook Streamgage .............8

Comparison of Estimated Natural Discharge and Gaged Discharge at the Coldbrook 

Streamgage............................................................................................................................10

Comparison of Estimated Natural Discharge and Historical Discharge at the

Mount Marion Streamgage .................................................................................................12

Comparison of Natural Annual Runoff Estimates for the Coldbrook and Mount Marion 

Streamgages with Annual Runoff at Nearby Catskill Streamgages.............................14

Effects of Discharge Through the Shandaken Tunnel on the Interaction of Esopus Creek 

With Alluvial Groundwater ..................................................................................................14

Summary........................................................................................................................................................17

References Cited..........................................................................................................................................18

Figures

1. Maps showing Esopus Creek and West of Hudson/Catskill and Delaware System

watersheds, New York.................................................................................................................4

2. Graphs showing daily-discharge values predicted by two multiple regression

models applied to estimate natural daily discharge at the Esopus Creek at

Coldbrook streamgage (01362500) from October 1, 1931, to September 30, 2012 ..............9

3. Graph showing flow-duration curves based on gaged daily discharge and natural

daily-discharge estimates for the Coldbrook streamgage (01362500) from October 1,

1931, to September 30, 2012 ......................................................................................................10

4. Graph showing flow-duration curves based on gaged daily discharge and natural

daily-discharge estimates for the Mount Marion streamgage (01364500) from

October 1, 1970, to September 30, 2012...................................................................................12

5. Graph showing flow-duration curves based on natural daily-discharge estimates

for the Mount Marion streamgage (01364500) from October 1, 1931, to September

30, 2012, and from October 1, 1970, to September 30, 2012..................................................13

6. Graphs showing discharge measures from streamgages on the Esopus Creek and

the Shandaken Tunnel, New York ............................................................................................16
iv

Tables

1. Basin characteristics of the Esopus Creek watershed at the Coldbrook, New York,

streamgage (01362500), at the Mount Marion, N.Y., streamgage (01364500), and at

the point of discharge to the Hudson River..............................................................................5

2. Basin characteristics of seven streamgages that were used either as predictive

variables in the estimation of natural discharge in the Esopus Creek at the

Coldbrook, New York, streamgage or in calculations of annual runoff for

comparison with gaged and natural runoff in the Esopus Creek at Coldbrook and

Mount Marion, N.Y., streamgages .............................................................................................7

3. Description of flow metrics calculated by the Indicators of Hydrologic Alteration

software for each discharge record analyzed in this study..................................................8

4. Range of gaged daily discharge and natural daily-discharge estimates as well as

mean relative difference between these values as a function of flow exceedance

range for the Coldbrook, New York, streamgage (01362500) from October 1, 1931, to

September 30, 2012.....................................................................................................................11

5. Gaged daily discharge and estimated natural daily discharge as well as the

difference and relative differences between these values as a function of mean

annual flow metrics for consecutive-day low-flow and high-flow days for the

Coldbrook, New York, streamgage (01362500) from October 1, 1931, to September

30, 2012 .........................................................................................................................................11

6. Range of gaged daily discharge and natural daily-discharge estimates as well as

mean relative difference of these values as a function of flow exceedance values

for the Mount Marion, New York, streamgage (01364500) from October 1, 1970, to

September 30, 2012.....................................................................................................................12

7. Gaged daily discharge and estimated natural daily discharge as well as the

difference and relative difference of these values as a function of mean annual

flow metrics for consecutive-day low-flow and high-flow days for the Mount

Marion streamgage (01364500) from October 1, 1970, to September 30, 2012 .................13

8. Mean annual runoff based on gaged and estimated natural daily discharge at the

Coldbrook, New York (01362500), and Mount Marion, N.Y. (01364500), streamgage

sites along with mean annual runoff for three other gaged sites in the Catskill

Mountain region from October 1, 1931, to September 30, 2012, and seven other

gaged sites from October 1, 1970, to September 30, 2012. Annual runoff values are

expressed in inches ...................................................................................................................15
v

Conversion Factors

Inch/Pound to International System of Units

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2

) 259.0 hectare (ha)

square mile (mi2

) 2.590 square kilometer (km2) 

Flow rate

cubic foot per second (ft3

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

million gallons per day (Mgal/d) 0.04381 cubic meter per second (m3

/s)

Datum

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

(NAVD 88).

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

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

Abbreviations

IHA Indicators of Hydrologic Alteration software

NYSET New York Streamflow Estimation Tool

USGS U.S. Geological Survey

WY water year

Estimates of Natural Streamflow at Two Streamgages on 

the Esopus Creek, New York, Water Years 1932 to 2012

By Douglas A. Burns and Christopher L. Gazoorian

Abstract

Streamflow in the Esopus Creek watershed is altered 

by two major watershed management activities carried out 

by the New York City Department of Environmental Protection as part of its responsibility to maintain a water supply 

for New York City: (1) diversion of water from the Schoharie 

Creek watershed to the Esopus Creek through the Shandaken 

Tunnel, and (2) impoundment of the Esopus Creek by a dam 

that forms the Ashokan Reservoir and subsequent release 

through the Catskill Aqueduct. Stakeholders in the Catskill 

region are interested and concerned about the extent to which 

these watershed management activities have altered streamflow, especially low and high flows, in the Esopus Creek. To 

address these concerns, natural (in the absence of diversion 

and impoundment) daily discharge from October 1, 1931, to 

September 30, 2012, was estimated for the U.S. Geological 

Survey streamgages at Coldbrook (station number 01362500), 

downstream of the Shandaken Tunnel discharge, and at Mount 

Marion (01364500), downstream of the Ashokan Reservoir.

A multiple linear regression approach, using nearby 

discharge records from unimpounded streams as predictive 

variables, was applied to estimate natural discharge at the 

Coldbrook streamgage. Estimated values of natural daily 

discharge at the Coldbrook streamgage were lower than values 

of gaged daily discharge throughout the flow range at this site. 

At moderate- and low-flow conditions, gaged daily-discharge 

values were about two to three times greater than natural 

daily-discharge estimates, whereas the difference between the 

two records was less than 5 percent for the highest 1 percent of 

daily-discharge values. These results indicate that Shandaken 

Tunnel discharge has a minor effect on flooding in the Esopus 

Creek Basin. However, a difference of 5 percent is within the 

uncertainty of the regression-based natural discharge estimates 

for Coldbrook; thus, it cannot be stated with certainty that the 

Tunnel has on average any effect on flow for the highest 1 

percent of daily discharge values.

Natural discharge at the Mount Marion streamgage was 

estimated by summing the natural discharge estimated for the 

Coldbrook streamgage and the discharge estimated for the 

intervening basin area through application of the New York 

Streamflow Estimation Tool, recently developed for estimating unaltered streamflow at ungaged locations in the State. 

Estimates of natural daily discharge at the Mount Marion 

streamgage were about three times greater than gaged daily 

discharge throughout the moderate- to low-flow range from 

October 1, 1970, to September 30, 2012, the period of record 

for full water years at this streamgage. The relative difference between the two discharge time series declined as flow 

increased beyond the moderate range, but gaged daily discharge was still 25 to 43 percent less than estimated natural 

daily discharge for the high-flow metrics calculated in this 

analysis, and the mean relative difference was 43 percent for 

the annual 1-day maximum discharge. Overall, these estimates 

of natural discharge reflect the absence of effects of the Shandaken Tunnel and Ashokan Reservoir on flows in the Esopus 

Creek over broad time frames. However, caution is warranted 

if one is attempting to apply the natural estimates at short time 

scales because the regression prediction intervals indicate that 

uncertainty at a daily time step ranges from about 40 to 80 

percent.

Introduction

The natural flow regimes of Earth’s streams and rivers 

are greatly altered by human activities such as withdrawals for 

water supply and irrigation, discharges from point sources, and 

impoundment (Vörösmarty and Sahagian, 2000; Eng and others, 2013). As examples of the extent of this alteration, about 

20 percent of global continental runoff is stored behind registered dams (Vörösmarty and Sahagian, 2000), and more than 

half of global runoff accessible to humans is withdrawn or 

maintained for instream uses (Postel and others, 1996). Human 

exploitation of rivers has wide ranging implications, including 

changes in the following: the flow and thermal regimes of rivers, aquatic and riparian ecological communities, the geomorphology of channel networks and flood plains, water quality, 

and land-use patterns (Rosenberg and others, 2000; Magilligan 

and Nislow, 2005). Temporal patterns in population growth 

and per capita water use indicate that human alteration of the 

global water cycle and river flow regimes is increasing and 

suggest that further increases are likely in the future (Wada 

and others, 2013). Although many of the effects of human 

water use have deleterious environmental consequences, 

the benefits of water use are numerous and include food 
2 Estimates of Natural Streamflow at Two Streamgages on the Esopus Creek, New York, Water Years 1932 to 2012

production, electricity generation, recreation, flood reduction, 

and improved health outcomes (Gleick, 1996). This complex mix of benefits and adverse effects often creates water 

management conflicts among interest groups with divergent 

priorities. Successful resolution of water-use conflicts presents 

a challenge and forms the basis of good water management 

(Wolf and others, 2005).

The Catskill Mountain region of southeastern New 

York is valued for its water resources, and its water has long 

been used for human activities such as recreation, industry, 

and water supply (Francis, 1988; Swaney and others, 2006). 

Streamflow and water quality in the region have been altered 

by human activities that date to at least the 19th century, when 

leather tanneries diverted flow and severely polluted waterways (Swaney and others, 2006). New York City began to play 

a large role in the use and management of water resources in 

the Catskills in the early 20th century. The city’s water-supply 

needs were forecast to exceed the available Croton water supply, and the newly established Bureau of Water Supply then 

began to acquire land in the Catskills to build dams, reservoirs, and aqueducts for what would become the City’s West 

of Hudson water supply (Iwan, 1987).

The first reservoir completed, the Ashokan, began 

delivering water to the city through the Catskill Aqueduct 

in 1915 (Galusha, 1999). Development of the West of Hudson water supply continued through the mid-1960s until the 

sixth reservoir, the Cannonsville, was completed and began 

to deliver water downstate through the Delaware Aqueduct. 

The extensive use of water from the Catskills by New York 

City has often pitted the interests of the city in protecting its 

water supply against those of governmental agencies at the 

municipal, county, and state levels, as well as those of regional 

citizen and interest groups (Soll, 2013). These conflicts have 

played out against a backdrop of regulatory enforcement that 

has driven watershed and water-supply management requirements that New York City must meet under laws such as the 

Safe Drinking Water Act (42 U.S.C. §300f et seq.) and Clean 

Water Act (33 U.S.C. §1251 et seq.; McClure, 2007; Smith 

and Porter, 2010).

The six reservoirs in the Catskills that form the New 

York West of Hudson water-supply system, along with smaller 

reservoirs and impoundments, have altered the natural flow 

regime in the region. Although the City’s reservoirs were not 

designed for flood control, impoundment has diminished and 

attenuated flood peaks and decreased base flow (Zembrzuski 

and Evans, 1989; Suro and Firda, 2006; Milone and MacBroom, Inc., 2009). The New York City Department of Environmental Protection manages the City’s water-supply system in 

accordance with several regulations that dictate releases from 

the reservoirs. This mix of mandatory and voluntary regulations is carried out under the guidance of the New York State 

Department of Environmental Conservation (2014a), the U.S. 

Environmental Protection Agency (2014), and the Delaware 

River Basin Commission (http://www.state.nj.us/drbc/). Some 

of these regulations are designed to minimize downstream 

flood effects by increasing releases in advance of periods 

when flood risk is high and when refilling of reservoirs in the 

near future is likely.

The Esopus Creek is an example of a watershed in which 

diversions for New York City’s water supply have altered the 

natural flow regime. Water is transferred from the Schoharie 

Reservoir to the Esopus Creek through the 18-mile (mi) Shandaken Tunnel. This diversion increases the flow of the Esopus 

Creek, and businesses that offer float trips on inner tubes and 

other watercraft are dependent on this enhanced discharge 

during summer low-flow periods. Regulations govern releases 

from the tunnel to serve these recreational needs (New York 

State Department of Environmental Conservation, 2014b). 

Because the Shandaken Tunnel provides up to 600 million gallons per day (Mgal/d; about 930 cubic feet per second [ft3

/s]) 

to the Esopus Creek, the issue of whether the tunnel increases 

the magnitude of floods in the Creek has long been controversial. Reports as far back as the mid-20th century blame 

Shandaken Tunnel discharges for increased flood damage (Van 

Burkalow, 1959).

Downstream of the U.S. Geological Survey (USGS) 

streamgage at Coldbrook (station number 01362500), the 

Esopus Creek is impounded by a dam that forms the Ashokan Reservoir. About 350 to 400 Mgal/d of water is diverted 

downstate from this reservoir through the Catskill Aqueduct 

to serve New York City’s water-supply needs. A spillway 

discharges back into the Esopus Creek (upstream of the USGS 

streamgage at Mount Marion [01364500]) when the Reservoir 

is full or when gated releases are occurring to serve objectives 

such as minimizing downstream floods. Lowered base flow 

and decreased high flow in the Esopus Creek downstream of 

the dam have been noted previously (Suro and Firda, 2007; 

Milone and MacBroom, Inc., 2009). In addition to the Ashokan Reservoir, there are three other impoundments in the Esopus Creek Basin, the largest of which is Cooper Lake, a water 

supply for the city of Kingston (http://www.kingston-ny.gov/

content/76/78/1005/default.aspx). However, this lake is quite 

small, with a storage volume equivalent to about 1 percent of 

the Ashokan Reservoir volume.

The Esopus Creek is also an example of a Catskill watershed about which the water management interests of New 

York City have often conflicted with the interests of various 

stakeholders, such as governmental organizations and citizen 

groups (Postel and Thompson, 2005; Kane and Erickson, 

2007). A variety of issues have arisen about the management 

of flow and water quality in the Esopus Creek that include 

protecting fish habitat, supporting water recreation businesses, minimizing flooding and flood damage, and managing 

excessive turbidity. Turbidity in the Esopus Creek Basin has 

been a prominent issue, and a successful lawsuit in which 

the Shandaken Tunnel was determined to be a point source 

of sediment pollution under the Clean Water Act (McClure, 

2007) has resulted in considerable effort by the New York City 

Department of Environmental Protection to minimize tunnel 

diversions during periods of high turbidity.

The focus of the current study by the U.S. Geological 

Survey, in cooperation with the New York City Department 
Methods 3

of Environmental Protection, is to determine the natural 

flow conditions in the Esopus Creek at two key locations, 

represented by USGS streamgages at (1) Coldbrook, N.Y. 

(01362500), and (2) Mount Marion, N.Y. (01364500). The 

Coldbrook streamgage is 10.5 mi downstream of the Shandaken Tunnel and therefore receives additional discharge 

on days when flow is exiting the tunnel. The Mount Marion 

streamgage is 31.6 mi downstream of the Ashokan Reservoir 

and therefore receives less discharge than would naturally be 

received from upstream sources.

Purpose and Scope

This report describes the development and application of 

a method that provides estimates of natural discharge in the 

Esopus Creek at two streamgages (Coldbrook and Mount Marion) for water years (WYs) 1932 through 2012 (a water year 

begins on October 1 of the preceding calendar year and ends 

on September 30). Flow at these streamgages is affected by 

the water management activities of the New York City Department of Environmental Protection. The purpose of this report 

is to provide broad multiyear comparisons of the natural dailydischarge estimates with the gaged daily-discharge record 

across a range of flow conditions at these sites to describe the 

extent to which the Shandaken Tunnel and Ashokan Reservoir 

affect streamflow in the Esopus Creek Basin. The report evaluates the validity of the estimates of natural daily discharge by 

(1) comparing estimated natural discharge at the Coldbrook 

streamgage with gaged discharge for days when water management effects were negligible and (2) comparing natural 

runoff at these two streamgages with natural runoff from six 

streamgages in the region that are not affected by diversion or 

impoundment.

“Natural” in this instance indicates Esopus Creek flow 

that is affected neither by discharge from the Shandaken 

Tunnel nor by impoundment in the Ashokan Reservoir and 

subsequent diversion of flow to New York City through the 

Catskill Aqueduct. The term “gaged discharge” at these two 

streamgages refers to values that are influenced by discharge 

from the Shandaken Tunnel and by impoundment and consequent diversion downstate from the Ashokan Reservoir. Other 

human development, including roads, residences, and towns in 

the Esopus Creek Basin, likely has some effect on streamflow. 

Furthermore, the effect of human development on streamflow 

in the basin likely increased over the study period. This report 

assumes that the effect of human development in the Esopus 

Creek Basin, which is minor compared with the heavy development of urbanized watersheds, is likely affecting streamflow 

much less than that of New York City’s water-supply management activities. Specifically, the report is predicated on 

the assumption that the Shandaken Tunnel and the Ashokan 

Reservoir and Catskill Aqueduct are the two major human 

influences on streamflow in the Esopus Creek Basin. There 

are other small impoundments in the Esopus Creek Basin, but 

the scale of these structures is much smaller than that of the 

Ashokan Reservoir. The results of this study should provide an 

improved understanding of the effects of flow management on 

low flows and floods in this basin.

Study Site

The Esopus Creek, a tributary of the Hudson River with 

headwaters in the Catskill Mountains, drains a watershed area 

of 424 square miles (mi2

) at the point of discharge to the river 

(fig. 1). The Esopus Creek at the Mount Marion streamgage, 

downstream of the Ashokan Reservoir, drains a watershed area 

of 419 mi2

, and the Esopus Creek at Coldbrook streamgage, 

upstream of the Reservoir but downstream of the Shandaken 

Tunnel discharge, drains a smaller watershed area of 192 

mi2

 (fig. 1). A watershed area of 256 mi2 is regulated by the 

Ashokan Reservoir, and when the influence of the reservoir is 

combined with that of three additional impoundments in the 

Esopus watershed, the effective drainage area of the Mount 

Marion streamgage is 112 mi2

, about 27 percent of the true 

drainage area. However, a spillway and gates release excess 

water from the reservoir that re-enters the Esopus Creek 

downstream of the dam but upstream of the Mount Marion 

streamgage. These releases occur primarily during high flow, 

but water can be released for other purposes. During WYs 

1971 through 2012, the daily mean discharge in the spillway 

channel was 158.3 ft3

/s, but there was no flow release on 80.1 

percent of the days (J. Porter, New York City Department of 

Environmental Protection, written commun., 2014). For the 

19.9 percent of days when discharge was noted, the effective 

drainage area of the reservoir is uncertain.

The headwaters of the Esopus Creek watershed originate 

on Slide Mountain, which has the highest elevation in the 

Catskills. These headwaters receive mean annual precipitation of 63.6 inches (in.; Northeast Regional Climate Center, 

2014), a value among the highest in New York and all of the 

northeastern United States. The Esopus Creek watershed 

drains steeply from the westernmost headwaters to the USGS 

streamgage at Allaben, N.Y. (01362200), a basin of 63.7 mi2

. 

At the Coldbrook streamgage, downstream of the Shandaken 

Tunnel, the watershed remains steep, with a mean slope of 

31.0 percent (table 1), and is mainly forested (97.8 percent of 

basin), with only minor influence from urban land use (0.61 

percent of basin). Downstream of the reservoir at the Mount 

Marion streamgage, the basin is, on average, less steep than 

at the Coldbrook streamgage, with less forest cover and more 

urban land, but mean annual precipitation remains quite high 

for New York at 48.1 in. (table 1).

Methods

The approach described in the project proposal was 

changed after an initial exploration of the data showed that 
4 Estimates of Natural Streamflow at Two Streamgages on the Esopus Creek, New York, Water Years 1932 to 2012

01350000

01413500

01423000

01420500

01435000

01365000

Base from U.S. Geological Survey,

The National Map National Elevation Dataset,

http://ned.usgs.gov. Shaded relief was generated

at 315 degrees NW azimuth at an altitude of 45 degrees

EXPLANATION

USGS streamgage

Esopus Creek watershed boundary

74°30' 74°20' 74°10' 74°

42°10'

42°5'

42°

41°55'

41°50'

Ashokan

Reservoir

01362200

01362500

01364500

Slide 

Mountain

Esopus 

Creek

Ashokan 

Dam

Hudson

River

Shandaken

Tunnel

discharge

Cooper

Lake

0 2.5 5 MILES

0 2.5 5 KILOMETERS

01362200 Esopus Creek at Allaben, N.Y.

01362500 Esopus Creek at Coldbrook, N.Y.

01364500 Esopus Creek at Mount Marion, N.Y.

0 5 10 MILES

0 5 10 KILOMETERS

West of Hudson/Catskill system

 watershed boundary

01423000

01350000 Schoharie Creek at Prattsville, N.Y.

01365000 Rondout Creek near Lowes Corners, N.Y.

01420500 Beaver Kill at Cooks Falls, N.Y.

01423000 West Branch Delaware River at Walton, N.Y.

01435000 Neversink River near Claryville, N.Y.

01413500 East Branch Delaware River at Margaretville, N.Y.

75°20' 75° 74°40' 74°20' 74°

42°30'

42°20'

42°10'

42°

41°50'

NEW YORK

A

B

Figure 1. Esopus Creek and West of Hudson/Catskill and Delaware System watersheds, New York. A, watershed boundaries and six 

U.S. Geological Survey (USGS) streamgages located outside of the Esopus Creek and used in this study. B, Esopus Creek watershed 

with USGS streamgages at Allaben, Coldbrook, and Mount Marion; Slide Mountain, the outflow of the Shandaken Tunnel, the Ashokan 

Reservoir with dam, Cooper Lake, and the Hudson River are also shown. N.Y., New York.
Methods 5

Table 1. Basin characteristics of the Esopus Creek watershed at the Coldbrook, New York, streamgage (01362500), at the Mount 

Marion, N.Y., streamgage (01364500), and at the point of discharge to the Hudson River. 

[Data are from U.S. Geological Survey (2014a). N.Y., New York; Mt., Mount; mean annual values are for the period 1951–1980]

Basin characteristics Streamgage

Esopus Creek at Coldbrook, N.Y. Esopus Creek at Mt. Marion, N.Y. Esopus Creek at Hudson River

Drainage area, in square miles 192 419 424

Mean basin slope, in percent 31.0 22.3 22.2

Basin storage1

, in percent 0.26 3.42 3.45

Forested area, in percent 97.8 88.7 88.3

Urban area, in percent 0.61 2.85 3.03

Mean annual runoff, in inches 31.6 28.6 28.4

Mean annual precipitation, in inches 50.9 48.1 47.9

1

Percentage of total area in lakes, ponds, and wetlands.

the assumptions of the proposed methodology were not met. 

This originally proposed approach was to subtract the Shandaken Tunnel discharge from that of the Coldbrook streamgage 

to estimate natural discharge at Coldbrook. This proposed 

approach failed to provide discharge estimates that could 

confidently be assumed to represent natural flow conditions. 

The results of this originally proposed approach are briefly 

described in a later section of this report. The alternative 

approach that was applied to obtain natural streamflow estimates is described in this section.

Study Approach

Natural discharge at the Coldbrook streamgage was 

estimated through a multiple linear regression approach for 

days when the Shandaken Tunnel had little effect on discharge 

in the Esopus Creek, which were assumed to be days when 

Shandaken Tunnel discharge did not exceed 1 percent of the 

discharge at the Coldbrook streamgage. A subset of Coldbrook 

daily-discharge data was selected for days when this condition 

was met: 6,874 daily-discharge values from October 1, 1931, 

to September 30, 2012. Natural discharge on these 6,874 days 

was calculated by subtracting the gaged daily discharge of the 

Shandaken Tunnel from the gaged daily discharge at Coldbrook. Natural discharge for the other 22,712 days during the 

81-year period was estimated by applying a multiple regression approach as described in the following paragraphs.

Two multiple regression models were fit to Coldbrook 

discharge for the 6,874 days assumed to represent natural 

discharge. These regressions were then applied to estimate natural discharge on the 22,712 days on which Shandaken Tunnel 

discharge exceeded 1 percent of discharge at the streamgage. 

Daily-discharge values at nearby streamgages that are not 

greatly affected by either diversions or impoundment were 

selected as potential independent variables in the regression 

models. For the period from October 1, 1931, to February 3, 1937, discharge data were available for two nearby 

streamgages, Schoharie Creek at Prattsville, N.Y. (01350000), 

and Beaver Kill at Cooks Falls, N.Y. (01420500). For the 

period from February 4, 1937, to September 30, 2012, discharge data for two additional gages were available for potential inclusion in the second multiple regression model: East 

Branch Delaware River at Margaretville, N.Y. (01413500), 

and Rondout Creek near Lowes Corners, N.Y. (01365000). 

Discharge values for the day before and the day after each day 

for which natural discharge was estimated were also explored 

as potential predictive variables in the regression. A few other 

streamgages in the region were explored for possible inclusion 

in models as well, but these sites showed weak relations to 

the daily discharge at the Coldbrook streamgage and were not 

considered further.

Multiple linear regression models were developed 

through use of a best subsets approach that explored all possible models for predicting natural flow at the Coldbrook 

streamgage with the combination of variables available for 

each of the two time periods. The final “best” models were 

chosen by examining fit metrics, including the standard 

error of estimate and the coefficient of determination (R2

). 

Parsimony was achieved by adding an additional independent variable to a model only if the adjusted R2

 (Theil, 

1961) increased by at least 0.02. Independent variables were 

included in regression models only if the significance level 

(p value) was less than 0.05 and the variance inflation factor (a measure of multicollinearity) was less than 10. Models 

with comparable fit were then explored graphically for bias. 

Independent variables and the dependent variable were log 

(base 10) transformed, new regression models were calculated, 

and these models were compared with models developed 

with untransformed variables for improvement of fit and bias. 

Values derived from a regression model in which the dependent variable was log transformed were bias corrected by 
6 Estimates of Natural Streamflow at Two Streamgages on the Esopus Creek, New York, Water Years 1932 to 2012

using a smearing estimator (Duan, 1983) before analysis and 

presentation.

The New York Streamflow Estimation Tool (NYSET) 

was used to estimate natural daily discharge for the watershed 

area between the Mount Marion and Coldbrook streamgages. 

This estimation was done by subtracting the daily discharge 

estimated by NYSET for the Coldbrook streamgage from the 

daily discharge estimated by NYSET for the Mount Marion 

streamgage. This difference was then added to the regressionestimated daily-discharge values that were derived for the 

Coldbrook streamgage (as described previously) to represent 

natural flow at the Mount Marion streamgage. This approach 

was considered superior to simply using the NYSET-estimated 

discharge at the Mount Marion streamgage because the multiple regression approach provided a better fit than NYSET to 

natural flow days at the Coldbrook streamgage.

NYSET was recently developed to estimate unaltered 

daily discharge at ungaged sites on streams and rivers in New 

York for the period from October 1, 1960, to September 30, 

2010 (Gazoorian, 2015). NYSET can also be used to estimate 

natural discharge at gaged streams and rivers where discharge 

is affected by human water management activities. NYSET 

uses data from existing streamgages in New York to predict 

natural flow at an ungaged location or, as in this case, at a 

streamgage affected by water management (impoundment 

and withdrawal). Briefly, a reference gage is identified for 

the ungaged site through a geostatistical approach termed 

“map correlation” (Archfield and Vogel, 2010), and the flowduration value from the reference gage is assigned to each 

day for which an estimated flow is sought for the ungaged 

site. “Flow duration” refers to the number of days on which 

the daily flow is exceeded within a period of record (Foster, 

1934); the period of WYs 1961 through 2010 was used in 

the development of NYSET. For example, a day whose flow 

was exceeded on 40 percent of the days during the period 

of record would have a flow-duration exceedance value of 

40 percent; this is equivalent to a 60-percentile value. The 

daily discharge was then estimated for 17 points along this 

hypothetical flow-duration curve on the basis of regression 

equations derived from 90 reference gages in New York, for 

which various basin characteristics, such as measures of slope, 

basin area, elevation, and precipitation, serve as dependent 

variables; the full suite of variables used in these regressions is 

described by Gazoorian (2015). In a final step, the full dailyflow record was filled between these 17 values through log-log 

interpolation. This approach, termed the QPPQ method, was 

specifically developed to estimate natural flow conditions in a 

river, and has been applied in Massachusetts and other states 

surrounding New York. Additional details of the method are 

described by Archfield and Vogel (2010) and Archfield and 

others (2010).

In this study, the Schoharie Creek at Prattsville 

streamgage was used as the reference gage to determine 

flow-duration values for the Esopus Creek because discharge 

at this site was the most strongly correlated with the Coldbrook streamgage over the entire study period for days when 

Shandaken Tunnel discharge was minimal. An additional 

modification to the NYSET approach here was to extend the 

estimates back to October 1, 1931, and forward to September 

30, 2012. This modification was consistent with the originally 

proposed study approach of providing an estimated natural 

flow record at the Mount Marion streamgage that extends from 

WY 1932 to WY 2012.

Sources of Discharge Data

Stream discharge data were obtained from the USGS 

New York Water Science Center Web site (U.S. Geological 

Survey, 2014b) for the entire periods of record at the Esopus 

Creek at Coldbrook (WYs 1932 through 2012) and the Esopus 

Creek at Mount Marion (WYs 1971 through 2012). Stream 

discharge data were obtained for the Schoharie Creek at Prattsville and the Beaver Kill at Cooks Falls streamgages for the 

period of WYs 1932 through 2012 and from the East Branch 

Delaware River at Margaretville and the Rondout Creek near 

Lowes Corners streamgages for the period from February 4, 

1937, to September 30, 2012 (table 2). Additional data were 

obtained from the West Branch Delaware River at Walton 

(01423000), Neversink River near Claryville (01435000), and 

Esopus Creek at Allaben streamgages for WYs 1971 through 

2012 to provide a comparison of gaged and natural annual 

runoff (discharge per unit basin area, reported in inches per 

year) at the Coldbrook and Mount Marion streamgages with 

the runoff of other regional streams (table 2). Finally, a dataset 

was assembled for the Shandaken Tunnel discharge to the Esopus Creek (diversion from Schoharie Reservoir, 01362230). 

Data before December 18, 1996 were obtained from records 

compiled by the New York City Department of Environmental 

Protection (J. Porter, New York City Department of Environmental Protection, written commun., 2014), and data for subsequent dates were obtained from USGS streamgage records.

Stream discharge was calculated from a stage-discharge 

rating curve according to methods described by Rantz and others (1982a,b). The USGS provides a general assessment of the 

accuracy of discharge data from a streamgage largely on the 

basis of the judgment of the hydrographer who processes the 

flow record each year. The accuracy categories are “excellent” 

(95 percent of daily values within 5 percent of actual discharge), “good” (95 percent of daily values within 10 percent 

of actual discharge), “fair” (95 percent of daily values within 

15 percent of actual discharge) and “poor” (lower than “fair” 

quality). The flow records at the Esopus Creek streamgages at 

Coldbrook and Mount Marion are generally rated as “good,” 

as are those of the Beaver Kill at Cooks Falls, East Branch 

Delaware River at Margaretville, West Branch Delaware River 

at Walton, and Neversink River near Claryville streamgages. 

In contrast, the streamgages at Schoharie Creek at Prattsville, 

Rondout Creek near Lowes Corners, and Esopus Creek at 

Allaben are generally rated as “fair.”
Estimated Natural Discharge Results 7

Table 2. Basin characteristics of seven streamgages that were used either as predictive variables in the estimation of natural 

discharge in the Esopus Creek at the Coldbrook, New York, streamgage or in calculations of annual runoff for comparison with gaged 

and natural runoff in the Esopus Creek at Coldbrook and Mount Marion, N.Y., streamgages. 

[Data from U.S. Geological Survey (2014a). Ck., Creek; N.Y., New York; E., East; R., River; W., West; USGS, U.S. Geological Survey; ID, identification 

number]

Basin 

characteristics Streamgage

Schoharie 

Ck. at

Prattsville, 

N.Y.

Beaver Kill 

at Cooks 

Falls, N.Y.

E. Branch 

Delaware R. at 

Margaretville, 

N.Y.

Rondout Ck. 

near Lowes 

Corners, 

N.Y.

W. Branch 

Delaware R. 

at Walton, 

N.Y.

Neversink 

R. near 

Claryville, 

N.Y.

Esopus Ck. at 

Allaben, N.Y.

USGS streamgage ID

Drainage area, in square 

miles

Mean basin slope, 

in percent

Basin storage1

, 

in percent

Forested area,

in percent

Urban area, in percent

Mean annual runoff, in 

inches, 1951–1980

Mean annual 

precipitation, in 

inches, 1951–1980

01350000

237

20.7

0.54

91.8

0.55

27.5

46.8

01420500

241

17.8

1.3

94.7

0.50

30.5

49.1

01413500

163

23.1

0.17

90.3

0.45

27.6

46.3

01365000

38.3

27.9

0.08

99.0

0.02

34.0

51.7

01423000

332

18.1

0.30

75.4

0.56

24.6

44.0

01435000

66.6

22.7

0.09

99.3

0.08

37.5

55.4

01362200

63.7

31.5

0.03

98.7

0.21

33.5

52.4

1

Percentage of total area in lakes, ponds, and wetlands.

Analyses of Discharge Data

The estimated records of natural daily discharge at the 

Esopus Creek at Coldbrook and Mount Marion streamgages 

were compared to the daily gage records affected by tunnel 

discharge and impoundment, respectively. First, the cumulative percentage of days throughout the study period on 

which the flow was exceeded was calculated for each record 

as a basis for comparing natural and gaged flow-duration 

or exceedance values. Second, the Indicators of Hydrologic 

Alteration (IHA) software was applied to calculate several 

high-flow and low-flow metrics (table 3) as a basis for further 

comparisons between the natural and gaged discharge records 

(Richter and others, 1996). Comparisons of estimated natural and human-influenced daily discharge were made for the 

Coldbrook streamgage for WYs 1932 through 2012, whereas 

comparisons for the Mount Marion streamgage were for WYs 

1971 through 2012 to encompass the maximum number of 

full water years available (the Mount Marion record began 

March 1, 1970). Flow metrics were also calculated by using 

IHA for the estimated natural daily discharge at the Mount 

Marion streamgage for WYs from 1932 through 2012 to 

facilitate comparisons with the estimated natural discharge 

record at the Coldbrook streamgage. Finally, annual runoff 

was calculated for seven streamgage sites in the Catskills that 

were not affected by impoundment during WYs 1932 through 

2012 and WYs 1971 through 2012 for comparing to the gaged 

and natural annual runoff at the Coldbrook and Mount Marion 

streamgages.

Estimated Natural Discharge Results

Results for two multiple linear regression equations 

developed to estimate natural discharge at the Coldbrook 

streamgage are described along with the ability of these 

regressions to predict natural flow in the Esopus Creek. 

The resulting natural discharge estimates for the Coldbrook 

streamgage are then compared with gaged discharge throughout the range of flow at this site, including several low-flow 

and high-flow metrics. A similar comparison is then made 

between natural discharge estimates and gaged discharge for 

the Esopus Creek at Mount Marion streamgage. Finally, the 

natural estimates of discharge and gaged discharge at the two 

Esopus Creek sites are compared with discharge at several 

nearby Catskill region streamgages through calculation of 

mean annual runoff..
8 Estimates of Natural Streamflow at Two Streamgages on the Esopus Creek, New York, Water Years 1932 to 2012

Table 3. Description of flow metrics calculated by the Indicators of Hydrologic 

Alteration software for each discharge record analyzed in this study.

[Mean values were calculated for each period of record analyzed]

Flow metric Description

1-day minimum flow Annual 1-day minimum daily discharge

3-day minimum flow Annual consecutive 3-day minimum daily discharge

7-day minimum flow Annual consecutive 7-day minimum daily discharge

30-day minimum flow Annual consecutive 30-day minimum daily discharge

90-day minimum flow Annual consecutive 90-day minimum daily discharge

1-day maximum flow Annual 1-day maximum daily discharge

3-day maximum flow Annual consecutive 3-day maximum daily discharge

7-day maximum flow Annual consecutive 7-day maximum daily discharge

30-day maximum flow Annual consecutive 30-day maximum daily discharge

90-day maximum flow Annual consecutive 90-day maximum daily discharge

Regression Models for Estimating Natural 

Discharge at the Coldbrook Streamgage

Two different multiple regression models were developed 

to predict natural discharge at the Coldbrook streamgage. 

From October 1, 1931, through February 3, 1937, the beginning of the period for which estimations were made, only 

two nearby discharge records were available as independent 

variables in a regression: Schoharie Creek at Prattsville and 

Beaver Kill at Cooks Falls. A regression model with two independent variables was selected as having the best fit and least 

predictive bias while meeting the conditions of acceptability 

as described in the methods section. Log transformation of 

the variables improved the fit and bias, and therefore, a model 

with log-transformed variables was selected.

The equation for this regression model is

log Coldbrook = 0.0568 + (0.418 × log Cooks) + (0.553 × log 

Prat), (1)

where

log Coldbrook is the log base 10 of the predicted natural 

discharge at the Coldbrook streamgage 

(equation was fit to Coldbrook discharge 

minus Shandaken Tunnel discharge),

log Cooks is the log base 10 of discharge at the Beaver 

Kill at Cooks Falls streamgage, and

log Prat is the log base 10 of discharge at the 

Schoharie Creek at Prattsville streamgage.

All discharge values are in units of cubic feet per second. 

This regression model was based on 6,874 days during WYs 

1932 through 2012 on which discharge at the Coldbrook 

streamgage minus the Shandaken Tunnel discharge was less 

than 1 percent of the Coldbrook streamgage discharge. The 

adjusted R2

 value of this model is 0.931, and both independent variables are highly significant (p<0.001). The predicted 

discharge values of this regression are shown relative to the 

Coldbrook streamgage minus Shandaken Tunnel discharge 

values in figure 2A. Because this regression was developed 

with log-transformed data, the uncertainty as reflected by 

the 95-percent prediction intervals is not symmetrical about 

the regression line and varies in untransformed discharge 

units throughout the full range of flow at this site. However, 

the prediction intervals are approximately constant relative 

to the predicted discharge, and vary by about +79 percent 

and –44 percent throughout the flow range at the Coldbrook 

streamgage. These results highlight the limitations of using 

the regression model developed to predict natural discharge on 

any individual day. Although the multiple regression model to 

estimate natural flow at the Coldbrook streamgage is highly 

significant and can account for about 93 percent of the variation in the full set of days when the tunnel did not impact 

streamflow, the model cannot predict discharge with high 

accuracy on any particular day, and these results should be 

applied on a daily basis with caution.

The second multiple regression model to represent natural flow at the Coldbrook streamgage was developed for the 

period from February 4, 1937, through September 30, 2012, 

when in addition to the Schoharie Creek at Prattsville and the 

Beaver Kill at Cooks Falls streamgages, two additional nearby 

flow records were available for the Rondout Creek near Lowes 

Corners and the East Branch Delaware River at Margaretville 

(not used in final predictive regressions). A multiple regression 

model with three independent variables was selected as having 

the best fit and least predictive bias while meeting the conditions of acceptability (as described in the methods section). 
Estimated Natural Discharge Results 9

A. Discharge predictions from regression, 1932–2012

Log discharge, Coldbrook streamgage minus Shandaken Tunnel, in cubic feet per second

0

1

2

3

4

5

B. Discharge predictions from regression, 1937–2012

0 1 2 3 4 5

0

1

2

3

4

5

Log discharge, predicted from regression, in cubic feet per second

Figure 2. Daily-discharge values predicted by two multiple regression models applied to estimate natural daily discharge at the 

Esopus Creek at Coldbrook streamgage (01362500) from October 1, 1931, to September 30, 2012. Discharge values predicted by the 

regressions are shown relative to the difference of Coldbrook streamgage discharge and Shandaken Tunnel discharge for days when 

Tunnel discharge was less than 1 percent of Coldbrook streamgage discharge. The solid black line represents the regression relation, 

and the two red lines indicate the upper and lower limits for the 95-percent prediction intervals of the regression relation. A, Gaged and 

regression-derived values for the period from October 1, 1931, to September 30, 2012, shown for 6,874 days when the discharge at the 

Coldbrook streamgage was assumed to represent natural flow conditions. This regression was applied to estimate natural discharge 

at the Coldbrook streamgage for days from October 1, 1931, to February 3, 1937, when gaged discharge did not represent natural flow 

conditions. B, Gaged and regression-derived values from February 4, 1937, to September 30, 2012, shown for 6,298 days when the 

discharge at the Coldbrook streamgage was assumed to represent natural flow conditions. This regression was applied to estimate 

natural discharge at the Coldbrook streamgage for days from February 4, 1937, to September 30, 2012, when gaged discharge did not 

represent natural flow conditions.
10 Estimates of Natural Streamflow at Two Streamgages on the Esopus Creek, New York, Water Years 1932 to 2012

Improved fit and bias were achieved by log-transforming the 

dependent and independent variables.

The equation for this regression model is

log Coldbrook = 0.263 + (0.430 × log Prat) + (0.525 × log 

Rondout) + (0.0744 × log Cooks – 1), (2)

where

log Coldbrook is the log base 10 of the predicted natural 

discharge at the Coldbrook streamgage 

(equation was fit to Coldbrook discharge 

minus Shandaken Tunnel discharge),

log Prat is the log base 10 of the discharge at the 

Schoharie Creek at Prattsville streamgage,

log Rondout is the log base 10 of the discharge at the 

Rondout Creek near Lowes Corners 

streamgage, and

log Cooks-1 is log base 10 of the discharge at the Beaver 

Kill at Cooks Falls streamgage on the day 

preceding the day for which predictions 

were made.

All discharge values are in units of cubic feet per second. 

This regression model was based on 6,298 days during the 

period on which the discharge at the Coldbrook streamgage 

minus the Shandaken Tunnel discharge was less than 1 percent 

of the Coldbrook streamgage discharge. The adjusted R2

 value 

of this model was 0.946, and all three independent variables 

were highly significant (p<0.001). The predicted discharge 

values of this regression are shown relative to the difference 

of the Coldbrook streamgage discharge and the Shandaken 

Tunnel discharge in figure 2B. Because this regression was 

developed with log-transformed data, the uncertainty in predictions is not symmetrical about the regression line and varies 

in untransformed discharge units throughout the full range of 

flow at this site. However, the prediction intervals are approximately constant relative to the predicted flow, and vary by 

about +66 percent and –40 percent throughout the flow range 

at the Coldbrook streamgage.

Comparison of Estimated Natural Discharge and 

Gaged Discharge at the Coldbrook Streamgage

Daily-discharge values at the Coldbrook streamgage were 

used directly to represent natural daily discharge in the Esopus 

Creek on days when the Shandaken Tunnel discharge was 

less than 1 percent of the discharge at the streamgage, which 

occurred on 6,874 days, or 23.2 percent of the days throughout the full study period. To estimate natural discharge for the 

remaining 76.8 percent of the daily record, the two regression 

models discussed in the previous section were applied to predict daily-discharge values: the first regression for the period 

from October 1, 1931, to February 3, 1937, and the second 

regression for the period from February 4, 1937, to September 30, 2012. The complete record of estimated natural daily 

discharge was then compared with the Coldbrook streamgage 

daily-discharge record through flow-duration curves and several high-flow and low-flow statistics computed by using IHA.

The estimated natural daily discharge at the Coldbrook 

streamgage was less than the gaged daily discharge throughout 

the flow range, and the mean difference peaked in the 10- to 

20-percent flow-exceedance range (fig. 3; table 4). The mean 

difference declined gradually through the 80- to 90-percent 

flow-exceedance range before dropping off sharply thereafter. 

The mean difference also declined fairly sharply and by nearly 

half from the 10- to 20-percent flow-exceedance range to the 

0- to 1-percent flow-exceedance range. The mean relative difference between the Coldbrook gaged discharge and estimated 

natural discharge shows a different pattern of change than 

that of the mean difference, with the mean relative difference 

peaking in the 80- to 90-percent flow-exceedance range and 

sharply declining to the 99- to 100-percent flow-exceedance 

range. The mean relative difference also decreased gradually 

and successively from the 80- to 90-percent flow-exceedance 

range to the 10- to 20-percent flow-exceedance range and 

then decreased sharply to the 0- to 1-percent flow-exceedance 

range. The gaged discharge exceeded the estimated natural 

discharge for Coldbrook streamgage by a mean of 4.3 percent 

for the highest 1 percent of flows. The low-flow metrics calculated by IHA indicate that the Coldbrook gaged discharge was 

66.8 ft3

/s greater than the estimated natural discharge for the 

1-day annual minimum flow, and this value increased successively and by more than threefold to 227.9 ft3

/s for the 90-day 

annual minimum flow (table 5). Gaged discharge at the Coldbrook streamgage was consistently about three times greater 

than natural flow estimates throughout the low-flow range 

shown in table 5. The relative differences between these two 

records for the high-flow metrics are much smaller than those 

calculated for the low-flow metrics, indicating that the relative 

influence of the Shandaken Tunnel on Esopus Creek discharge 

Coldbrook, gaged 

Coldbrook, estimated natural

EXPLANATION

Percentage of days on which discharge was equalled or exceeded

0 20 40 60 80 100

Daily discharge, in cubic feet per second

1

10

100

1,000

10,000

100,000

Figure 3. Flow-duration curves based on gaged daily 

discharge and natural daily-discharge estimates for the 

Coldbrook streamgage (01362500) from October 1, 1931, to 

September 30, 2012.
Estimated Natural Discharge Results 11

Table 4. Range of gaged daily discharge and natural daily-discharge estimates as well as mean relative difference between 

these values as a function of flow exceedance range for the Coldbrook, New York, streamgage (01362500) from October 1, 1931, 

to September 30, 2012.

[ft3/s, cubic feet per second]

Flow-exceedance range,

in percent

Gaged daily discharge 

across exceedance 

range,

in ft3/s

Natural daily-discharge 

estimates across exceedance range,

in ft3/s

Mean Discharge Difference (ft3

/s)

Mean relative difference,

as percentage of natural 

discharge

99–100 32–9.3 19.4–11.6 +8.8 +53.4

90–100 185–9.3 49.1–11.6 +71.6 +193.7

80–90 280–185 88.9–49.1 +166.3 +246.9

70–80 360–280 138–88.9 +208.6 +187.6

60–70 444–360 196–138 +234.4 +142.3

50–60 538–444 269–196 +257.3 +111.9

40–50 665–538 354–269 +288.6 +93.6

30–40 830–665 468–354 +336.6 +82.7

20–30 1,070–830 653–468 +385.9 +70.2

10–20 1,440–1,070 1,040–653 +413.6 +51.6

0–10 29,030–1,440 29,015–1,040 +339.5 +22.1

0–1 29,030–3,901 29,015–3,750 +228.7 +4.3

Table 5. Gaged daily discharge and estimated natural daily discharge as well as the difference and relative differences between 

these values as a function of mean annual flow metrics for consecutive-day low-flow and high-flow days for the Coldbrook, New York, 

streamgage (01362500) from October 1, 1931, to September 30, 2012.

[ft3/s, cubic feet per second]

Annual flow metric Gaged daily discharge,

in ft3/s

Estimated natural daily 

discharge,

in ft3/s

Discharge difference,

in ft3/s

Relative difference,

as percentage of natural 

discharge

1-day minimum flow 100.8 34.0 66.8 196.5

3-day minimum flow 107.6 35.5 72.1 203.1

7-day minimum flow 122.5 39.4 83.1 210.9

30-day minimum flow 196.8 59.4 137.4 231.3

90-day minimum flow 362.4 134.5 227.9 169.4

1-day maximum flow 8,898 8,602 296 3.4

3-day maximum flow 5,541 5,253 288 5.5

7-day maximum flow 3,518 3,235 283 8.7

30-day maximum flow 1,919 1,561 358 11.1

90-day maximum flow 1,291 954.1 336.9 35.3
12 Estimates of Natural Streamflow at Two Streamgages on the Esopus Creek, New York, Water Years 1932 to 2012

is substantially less at high flow. Although the differences in 

discharge between these two records are greater at high flow, 

the relative differences in discharge are much greater at low 

flow.

Comparison of Estimated Natural Discharge 

and Historical Discharge at the Mount Marion 

Streamgage

The estimated natural daily discharge at the Mount 

Marion streamgage, developed by summing the estimated 

natural daily discharge at the Coldbrook streamgage and the 

NYSET-estimated natural daily discharge for the intervening 

area between Coldbrook and Mount Marion streamgages, was 

compared with the gaged daily discharge at the Mount Marion 

streamgage for the period from October 1, 1970, to September 30, 2012. The gaged discharge at the Mount Marion 

streamgage was less than the estimated discharge throughout 

the full range of flow conditions during WYs 1971 through 

2012 (fig. 4). The mean difference in discharge between these 

two flow records increased with increasing flow and decreasing flow exceedance value (table 6). The mean difference in 

discharge ranged from 21.5 ft3

/s in the 99- to 100-percent 

flow-exceedance range and increased successively to a value 

of 3,221 ft3

/s in the 0- to 1-percent flow-exceedance range.

The gaged discharge at Mount Marion streamgage 

was about two-thirds less than that of the estimated natural 

Percentage of days on which discharge was equalled or exceeded

0 20 40 60 80 100

Daily discharge, in cubic feet per second

1

10

100

1,000

10,000

100,000

Mt Marion, gaged 

Mt Marion, estimated natural

EXPLANATION

Figure 4. Flow-duration curves based on gaged daily discharge 

and natural daily-discharge estimates for the Mount Marion 

streamgage (01364500) from October 1, 1970, to September 30, 

2012.

discharge throughout the 40- to 100-percent flow-exceedance 

range, and this difference narrowed successively to 27.7 

percent less in the flow-exceedance range of 0 to 1 percent. 

These high-flow results are consistent with previous observations that the Ashokan Reservoir and other reservoirs in 

the Catskills operated by the New York City Department of 

Table 6. Range of gaged daily discharge and natural daily-discharge estimates as well as mean relative difference of these 

values as a function of flow exceedance values for the Mount Marion, New York, streamgage (01364500) from October 1, 1970, to 

September 30, 2012.

[ft3/s, cubic feet per second]

Flow exceedance range, 

in percent

Gaged daily discharge 

across exceedance 

range, 

in ft3

/s

Natural daily discharge 

estimates across exceedance range, in ft3

/s

Mean Discharge Difference

 (ft3/sec)

Mean relative difference, 

as percentage of natural 

discharge

99–100

90–100

80–90

70–80

60–70

50–60

40–50

30–40

20–30

10–20

0–10

0–1

13–5.6

34–5.6

60–34

95–60

138–95

191–138

270–191

418–270

696–418

1,290–696

24,200–1,290

24,200–4,979

30–17

75–17

129–75

212–129

316–212

437–316

588–437

808–588

1,149–808

1,821–1,149

46,237–1,821

46,237–6,173

-21.5

-44.6

-93.3

-153.4

-243.3

-345.2

-445.6

-546.6

-640.7

-752.4

-1,310

-3,221

-66.5

-66.0

-66.7

-66.5

-68.0

-68.0

-66.2

-62.0

-54.4

-44.6

-33.2

-27.7
Estimated Natural Discharge Results 13

Table 7. Gaged daily discharge and estimated natural daily discharge as well as the difference and relative difference of these 

values as a function of mean annual flow metrics for consecutive-day low-flow and high-flow days for the Mount Marion streamgage 

(01364500) from October 1, 1970, to September 30, 2012.

[ft3/s, cubic feet per second]

Estimated natural daily Relative difference, Gaged daily discharge, Discharge difference, Annual flow metric discharge, as percentage of natu- in ft3

/s in ft3/s in ft3

/s ral discharge

1-day minimum flow 22.4 67.8 45.4 -67.0

3-day minimum flow 23.3 71.0 47.7 -67.2

7-day minimum flow 25.9 80.4 54.5 -67.8

30-day minimum flow 41.8 122.2 80.4 -65.8

90-day minimum flow 114.8 277.4 162.6 -58.6

1-day maximum flow 8,307 14,540 6,233 -42.9

3-day maximum flow 6,393 8,938 2,545 -28.5

7-day maximum flow 4,193 5,624 1,431 -25.4

30-day maximum flow 1,974 2,916 942 -32.3

90-day maximum flow 1,166 1,880 714 -38.0

Environmental Protection, though not designed as flood-control reservoirs, decrease downstream flood peaks substantially 

(Zembrzuski and Evans, 1989; Suro and Firda, 2007; Milone 

and MacBroom, Inc., 2009). The analysis by Milone and MacBrook, Inc. (2009) indicated that instantaneous flood peaks are 

reduced by about 60 percent as a result of the emplacement 

of the Ashokan Reservoir. In contrast, the current analysis 

indicates that the 1-day annual maximum daily discharge 

was diminished by a lower mean value of about 43 percent 

during WYs 1971 through 2012. The higher diminishment 

value from the analysis in Milone and MacBrook, Inc.(2009) 

may reflect the report’s basis on only 5 instantaneous peaks, 

whereas the analysis in this report reflects 1-day annual values 

based on 43 peak-flow days. Streamgage remarks from the 

National Water Information System (U.S. Geological Survey, 

2014c) as well as the analysis of Milone and MacBroom, Inc. 

(2009) describe the natural discharge at the Mount Marion 

streamgage as representative of an effective watershed area of 

112 mi2

 during nonflood conditions, whereas the analysis in 

this report indicates a value of about one-third of the drainage 

area, or 140 mi2

.

The annual low-flow metrics showed a similar pattern 

to those of the flow-exceedance values (table 7). The Mount 

Marion streamgage discharge was about two-thirds less than 

Percentage of days on which discharge was equalled or exceeded

0 20 40 60 80 100

Daily discharge, in cubice feet per second

10

100

1,000

10,000

100,000

Water years 1932–2012 

Water years 1971–2012

EXPLANATION

Mt Marion, estimated natural discharge

Figure 5. Flow-duration curves based on natural daily-discharge 

estimates for the Mount Marion streamgage (01364500) from 

October 1, 1931, to September 30, 2012, and from October 1, 1970, 

to September 30, 2012.
14 Estimates of Natural Streamflow at Two Streamgages on the Esopus Creek, New York, Water Years 1932 to 2012

the estimated natural discharge for the 1-day, 3-day, 7-day, 

and 30-day annual minimum flows and decreased slightly to 

58.6 percent less for the 90-day annual minimum flow. At high 

flow, the relative differences in flow were less than those at 

low flow; relative differences at high flow ranged from 25.4 to 

42.9 percent less than the estimated natural discharge for the 

five metrics for annual maximum flow, and the greatest relative difference was for the annual 1-day maximum flow.

Natural discharge was also estimated for the period of 

WYs 1932 through 2012 for the Mount Marion streamgage. 

The estimated natural discharge values for this period were 

lower than the exceedance values for the previously computed 

period of WYs 1971 through 2012 (fig. 5). The differences 

between these estimates, however, were smaller than the differences previously described for comparisons of the effects 

of Shandaken Tunnel discharge on Coldbrook streamgage 

discharge and the effects of impoundment of the Ashokan 

Reservoir on the Mount Marion streamgage discharge. For 

example, the estimated natural discharge at the Mount Marion 

streamgage for WYs 1971 through 2012 was 17.0 percent 

greater than the estimated natural discharge for WYs 1932 

through 2012 at 90-percent flow exceedance, 11.1 percent 

greater at 50-percent flow exceedance, and 6.3 percent greater 

at 10-percent flow exceedance.

Comparison of Natural Annual Runoff 

Estimates for the Coldbrook and Mount Marion 

Streamgages with Annual Runoff at Nearby 

Catskill Streamgages

Mean annual runoff values from several streamgages 

operated by the USGS in the Catskill Mountain region 

were compared with values computed from the gaged and 

natural daily discharge at the Coldbrook and Mount Marion 

streamgages. Only streamgages upstream of the New York 

City water-supply reservoirs were chosen for this analysis to 

minimize the effects of impoundment and therefore provide 

a stronger basis for comparison. The comparisons were made 

for two periods—from October 1, 1931, to September 30, 

2012, and from October 1, 1970, to September 30, 2012—to 

include as many complete water years as possible within the 

full periods of record for the Coldbrook and Mount Marion 

streamgages, respectively. The data indicate a wide range of 

natural variation in annual runoff in streams of this region. 

Across both time periods examined, mean annual runoff at 

the regional streamgages ranged from 26.3 in at the West 

Branch Delaware River at Walton streamgage to 42.3 in at the 

Neversink River near Claryville streamgage (table 8). Mean 

annual runoff values at the two streamgages affected by flow 

alteration that resulted from New York City’s water-supply 

activities were outside this range of natural variation. Mean 

annual runoff was 53.5 in at the Esopus Creek at Coldbrook 

streamgage during WYs 1932 through 2012 (and 53.0 in during WYs 1971 through 2012), whereas mean annual runoff 

at the Esopus Creek at Mount Marion streamgage was 17.2 

in during WYs 1971 through 2012. In contrast, the estimated 

annual natural runoff during the same two periods was 34.4 

in. and 37.1 in., respectively, at the Coldbrook streamgage 

and 29.6 and 32.0 in, respectively, at the Mount Marion 

streamgage. These results show that estimation of natural 

flow in the Esopus Creek shifted annual runoff values from 

being the highest (Coldbrook) and lowest (Mount Marion) in 

the region to being well within the range of natural variation 

observed for Catskill region streams. These natural runoff 

estimates are greater than those provided in table 1, which are 

based on the analysis of Randall (1996) for the period of 1951 

through 1980.

The difference between mean annual runoff and the mean 

annual estimated natural runoff for the Coldbrook streamgage 

provides an estimate of the extent to which Shandaken Tunnel 

discharge increases natural flow in the Esopus Creek. During 

WYs 1932 through 2012, the difference was 19.1 in., compared with a mean annual runoff value of 20.7 in. based on the 

gaged discharge of the Shandaken Tunnel for the same period 

(table 8). During WYs 1971 through 2012, the calculated 

difference was 15.9 in., whereas the value based on gaged 

Shandaken Tunnel discharge was 18.2 in.

Effects of Discharge Through the Shandaken 

Tunnel on the Interaction of Esopus Creek With 

Alluvial Groundwater

The originally proposed study approach was to estimate 

natural discharge at the Coldbrook streamgage by subtracting 

the daily Shandaken Tunnel discharge from the daily discharge 

at the streamgage. This approach did not provide accurate estimates of natural daily discharge at Coldbrook. This proposed 

calculation would be appropriate if the Esopus Creek behaved 

like a pipe, transferring all of the discharge from the Shandaken Tunnel in one day to the streamgage at Coldbrook, but 

the data show that the creek did not consistently behave like a 

pipe along the approximately 10.5-mi reach between the two 

measurement sites (fig. 6). Additionally, a lag is likely between 

the time when water is discharged from the Shandaken Tunnel and when this discharge is detected at the Coldbrook 

streamgage, 10.5 mi downstream. This lag is expected to vary 

according to stream velocity and resulting traveltimes, but a 

value as high as half a day to 2 days is likely based on typical 

stream velocities measured at the Coldbrook streamgage.

Figure 6 provides examples of two periods in which the 

daily discharge of the Shandaken Tunnel exceeded that at the 

Coldbrook streamgage, resulting in negative values of natural 

daily discharge when calculated by the subtraction approach, 

an unrealistic situation. Negative discharge values resulted for 

12 days during July 1966 (fig. 6A) and 3 days during August 

1965 (fig. 6B). Negative daily discharge during these two 

periods reached values of –114.5 ft3

/s on July 18, 1966, and 

–85.5 ft3

/s on August 14, 1965. The magnitude of these calculated negative discharge values suggest that they did not likely 

result solely from uncertainty in the two discharge measures. 
Estimated Natural Discharge Results 15

Table 8. Mean annual runoff based on gaged and estimated natural daily discharge at the Coldbrook, New York (01362500), and 

Mount Marion, N.Y. (01364500), streamgage sites along with mean annual runoff for three other gaged sites in the Catskill Mountain 

region from October 1, 1931, to September 30, 2012, and seven other gaged sites from October 1, 1970, to September 30, 2012. Annual 

runoff values are expressed in inches.

[--, no data]

Streamgage site or flow record Mean annual runoff, in inches

Esopus Creek at Coldbrook gaged discharge

Esopus Creek at Coldbrook natural discharge

Esopus Creek at Mount Marion gaged discharge

Esopus Creek at Mount Marion natural discharge

Esopus Creek at Allaben (01362200) gaged discharge

Schoharie Creek at Prattsville (01350000) gaged discharge1

Beaver Kill at Cooks Falls (01420500) gaged discharge 

Rondout Creek near Lowes Corners (01365000) gaged discharge

East Branch Delaware River at Margaretville (01413500) gaged discharge

West Branch Delaware River at Walton (01423000) gaged discharge

Neversink River near Claryville (01435000) gaged discharge

Shandaken Tunnel2

October 1, 1931–

September 30, 2012

53.5

34.4

--

29.6

--

27.8

32.4

--

--

--

--

20.7

October 1, 1970–

September 30, 2012

53.0

37.1

17.2

32.0

34.3

30.6

34.8

39.1

28.3

26.3

42.3

18.2

1

Discharge was affected by withdrawals for snowmaking during winter.

2

Runoff was calculated by dividing tunnel discharge by the drainage area of the Esopus Creek at Coldbrook streamgage.

Shandaken Tunnel discharge exceeded that at the Coldbrook 

streamgage on 1,503 days during water years (WYs) 1932 

through 2012, about 5.1 percent of the total days.

On many days, this difference calculation did not yield 

negative values but still provided unrealistically low estimates 

of natural discharge for the Esopus Creek at Coldbrook. For 

example, between August 13, 1965, and August 15, 1965, the 

estimated difference was less than the discharge measured 

at the upstream Esopus Creek at Allaben, N.Y., streamgage 

(01362200), a site that drains a watershed only one-third 

the size of the watershed at Coldbrook (fig. 6B). An additional consideration is that the drainage area of the Esopus 

Creek where the Shadaken Tunnel enters is 67.2 mi2

, much 

smaller than the drainage area of 192 mi2

 at the Coldbrook 

streamgage, and many tributaries enter the Creek along the 

10.5-mi reach between the two sites. If the contributions of all 

inflows to Esopus Creek are considered, then many additional 

days are likely for which subtraction of these two flow records 

would yield unrealistically low values.

The calculated difference between the Shandaken Tunnel 

discharge and that of the Coldbrook streamgage also seemed 

to be unrealistically high for many days. For example, during 

August 14–18, 1965, this calculated difference increased during a period when flows on the Esopus Creek (as shown by the 

Allaben streamgage) were generally receding (fig. 6B).

The evidence of unrealistically low and high daily 

discharge values when calculated by the originally proposed 

subtraction approach suggest that the Esopus Creek channel does not behave like a pipe and that, instead, streamflow 

likely reflects dynamic interaction between stream water and 

the streambanks and flood plain, which in the Esopus valley generally consist of permeable sand and gravel deposits 

(Rich, 1935). Dynamic interaction of streamflow with adjacent 

alluvial groundwater has been well known in hydrology for 

decades (Cooper and Rorabaugh, 1963; Pinder and others, 1969). This interaction is heightened during floods and 

is further amplified when stream channels receive artificial 

floods such as those derived by release from an irrigation 

canal or discharged from a tunnel, aqueduct, or pipe (Hancock, 

2002) as occurs in the Esopus Creek. Gaged annual runoff of 

the Shandaken Tunnel exceeded the difference between the 

annual runoff at the Coldbrook streamgage and the estimated 

natural mean annual runoff at Coldbrook, which is consistent 

with small net annualized losses caused by induced infiltration along the reach. But these differences were small on an 

annual basis, averaging 8.4 percent of the difference between 

Coldbrook streamgage flow and natural estimates during 

WYs 1932 through 2012 and 14.5 percent of the difference 

during WYs 1971 through 2012. Factors such as increased 

evaporative losses from artificially elevated levels of shallow 
16 Estimates of Natural Streamflow at Two Streamgages on the Esopus Creek, New York, Water Years 1932 to 2012

A. July 1966

7/1/66 7/5/66 7/9/66 7/13/66 7/17/66 7/21/66 7/25/66 7/29/66 

-200

0

200

400

600

800

Coldbrook, gaged

Shandaken Tunnel, gaged

Coldbrook streamgage minus Shandaken Tunnel

B. August 1965

Date

8/1/65 8/5/65 8/9/65 8/13/65 8/17/65 8/21/65 8/25/65 8/29/65 

-200

-100

0

100

200

300

400

500

600

Coldbrook streamgage minus Shandaken Tunnel

Shandaken Tunnel, gaged

Allaben, gaged

Discharge, in cubic feet per second

EXPLANATION

EXPLANATION

Figure 6. Discharge measures from streamgages on the Esopus Creek and the Shandaken Tunnel, New York. 

A. Coldbrook streamgage (01362500), Shandaken Tunnel, and Coldbrook streamgage minus Shandaken Tunnel 

discharge during July 1966, and B. Coldbrook streamgage minus Shandaken Tunnel, Shandaken Tunnel, and Allaben 

streamgage (01362200) discharge during August 1965. Red horizontal line on each plot indicates discharge of zero.
Summary 17

groundwater may have played a role in the apparent long-term 

loss of tunnel discharge.

These imbalances in long-term runoff, when considered 

along with the day-by-day analysis presented in this section, 

might be interpreted to indicate that induced infiltration resulting from Shandaken Tunnel discharge causes long-term losses 

of stream water to groundwater. This interpretation, however, 

may be unwarranted because measured discharge values have 

uncertainty of about 10 percent, and the uncertainty of natural 

flow estimates is even greater. Consideration of uncertainty 

indicates that the combined runoff values cannot be viewed 

conclusively as exceeding those of natural runoff. For some 

days, comparisons of the Shandaken Tunnel and Coldbrook 

discharge records also show exfiltration of excess flow from 

adjacent alluvial groundwater into the Esopus Creek (the 

opposite effect of induced infiltration), which is consistent 

with the idea that induced infiltration results in only temporary 

losses of streamflow. Regardless of what causes Shandaken 

Tunnel discharge to exceed that measured at the Coldbrook 

streamgage on many days throughout the record, the existence 

of this phenomenon resulted in a change from the originally 

proposed study approach (subtracting the tunnel flow from 

Coldbrook discharge to yield a natural discharge record) to 

the use of a regression-based statistical approach with nearby 

streams as predictive variables.

Summary

This report summarizes the results of a study by the U.S. 

Geological Survey (USGS) in cooperation with the New York 

City Department of Environmental Protection to estimate natural discharge in the Esopus Creek at two USGS streamgages 

on the Esopus Creek at Coldbrook (10362500) and Mount 

Marion (01364500), New York. The Coldbrook streamgage is 

affected by out-of-basin discharge from the Shandaken Tunnel, 

and the Mount Marion streamgage is affected by impoundment in the Ashokan Reservoir and subsequent release through 

the Catskill Aqueduct. Natural daily discharge values for water 

years (WYs) 1932 to 2012 were estimated at the Coldbrook 

streamgage by multiple linear regression using nearby daily 

discharge values from streamgages that are not greatly affected 

by human activities. These natural discharge estimates for the 

Coldbrook streamgage were then added to those estimated by 

the New York Streamflow Estimation Tool for the basin area 

from Coldbrook downstream to the Mount Marion to estimate 

natural daily discharge at the Mount Marion streamgage for 

WYs 1932 through 2012. These estimated natural daily discharge values were compared with those of the measured daily 

discharge at the two streamgages for consecutive-day low- and 

high-flow metrics and for the full range of flow exceedance 

values. Annual runoff values derived from these natural estimates and gaged discharge values were compared with each 

other and with those derived from nearby streamgages with 

little impact from human activities.

Gaged discharge in the Esopus Creek at two USGS 

streamgage sites produced mean annual runoff values that fell 

outside the current range observed in unimpounded streams in 

the Catskill region of New York. However, natural discharge 

estimates produced runoff values well within this range that 

are consistent with natural runoff patterns in this region. 

Natural discharge estimates at the Coldbrook streamgage 

(01362500) were lower than gaged discharge values, which 

is consistent with the expected effects of inflow from the 

Shandaken Tunnel to the Esopus Creek. Gaged daily-discharge 

values varied from two to more than three times greater than 

natural discharge estimates in the low- to moderate-flow range 

(50- to 99-percent flow exceedance), but the relative difference between gaged and natural discharge declined sharply at 

the lowest 1 percent of daily-discharge values.

One plausible explanation for this lower relative difference is that tunnel inflow is most likely to induce infiltration 

into the adjacent alluvial aquifer at the lowest flow conditions. The presence of streamflow losses that were likely due 

to infiltration induced by Shandaken Tunnel discharge was 

identified in an analysis according to which daily tunnel discharge exceeded daily discharge at the Coldbrook streamgage 

on about 5 percent of the days during WYs 1932 through 

2012. The infiltration losses are probably not of long duration, and exfiltration back to the stream is likely following a 

rapid increase in tunnel discharge. Long-term runoff patterns 

suggest the possibility of slight permanent losses of tunnel 

discharge, which could also be caused in part by increased 

evaporation from shallow groundwater. However, the sum 

of tunnel discharge and Coldbrook discharge exceeded the 

estimated natural discharge at the Coldbrook streamgage by 

only 8.4 percent during WYs 1932 through 2012 and by 14.5 

percent during WYs 1971 through 2012 (values that are close 

to the 10-percent uncertainty of gaged discharge values at 

the Coldbrook streamgage). Additionally, with consideration 

of the uncertainty of these natural flow estimates, which is 

substantially greater than 10 percent of Coldbrook discharge, 

permanent losses over long periods of time caused by induced 

infiltration cannot be confirmed on the basis of this analysis.

Flow-exceedance values and annual consecutive-day 

high-flow metrics indicate that the relative difference between 

gaged daily discharge and estimated natural daily discharge 

at the Coldbrook streamgage narrows at the highest flows. 

For example, 1-day, 3-day, and 7-day maximum annual gaged 

discharge differed by less than 10 percent from natural discharge, within the 10-percent uncertainty range of gaged daily 

discharge values at this site. In contrast to previous reports of 

greatly increased flooding induced by Shandaken Tunnel discharges, these results indicate that the tunnel has only a minor 

effect on streamflow in the Esopus Creek during floods.

Impoundment of the Esopus Creek in the Ashokan Reservoir sharply diminishes discharge at the downstream Mount 

Marion streamgage (01364500) relative to natural flow estimates for this site. Estimated natural discharge is about three 

times greater than gaged discharge throughout the moderateto low-flow range (50- to 90-percent flow exceedance). These 
18 Estimates of Natural Streamflow at Two Streamgages on the Esopus Creek, New York, Water Years 1932 to 2012

natural discharge values are lower than would be expected 

considering that the true watershed area (419 square miles 

[mi2]) is about 3.7 times greater than the effective watershed area (112 mi2

) when water is not being diverted from 

the reservoir. This diversion of flow into the Esopus Creek, 

which is reflected in the gaged discharge at the Mount Marion 

streamgage, may explain in part why the gaged discharge is 

greater than would be expected from the effective watershed 

area. The results described in this report indicate an effective 

watershed area of about 140 mi2 at low to moderate flow conditions based on the Mount Marion streamgage record.

These relative differences between the Mount Marion 

gaged discharge and estimated natural discharge decrease at 

high flow, but gaged daily discharge is still 25 to 43 percent 

less than estimated natural daily discharge for all calculated 

annual high-flow metrics, and this mean relative difference is 

27.7 percent for the 1 percent of highest flows from October 1, 

1970, to September 30, 2012. These high-flow results are consistent with previous observations that the Ashokan Reservoir 

and other Catskill reservoirs operated by the New York City 

Department of Environmental Protection, though not designed 

as flood-control reservoirs, decrease downstream flood peaks. 

The analysis in this report indicates that the highest daily discharge values are diminished by a mean of 43 percent during 

WYs 1971 through 2012, whereas a previous analysis indicated a 60 percent diminishment of flood peaks. The previous 

analysis was based on 5 instantaneous flood peaks, whereas 

the current analysis is based on 43 daily peaks, which may 

account for this difference.

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For additional information write to:

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