U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2004-5134

Hydrogeology and Water Quality of the Pepacton 

Reservoir Watershed in Southeastern New York

Part 2. Hydrogeology, Stream Base Flow, and Ground-Water Recharge 

In cooperation with

NEW YORK STATE DEPARTMENT OF ENVIRONMENTAL CONSERVATION
Cover: Ground water discharging from a contact spring exposed in a roadcut of Catskill 

Mountain Series rocks (Upper Walton Formation), Ulster County, N.Y. Springs are used 

extensively throughout the Catskill Mountain region for domestic and municipal water 

supplies, and provide a major contribution to the base flow of streams in the region. 

(Photo by Jason Zatorsky, U.S. Geological Survey, January 2003.)
Hydrogeology and Water Quality of 

the Pepacton Reservoir Watershed in 

Southeastern New York

Part 2. Hydrogeology, Stream Base Flow, and 

Ground-water Recharge

By Richard J. Reynolds

In cooperation with the New York State Department of Environmental Conservation

Scientific Investigations Report 2004-5134

U.S. Department of the Interior

U.s. Geological Survey
ii

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

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

reproduce any copyrighted materials contained within this report.
iii

Contents

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

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

Objectives and Approach ................................................................................................................... 3

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

Previous Studies .................................................................................................................................. 3

Hydrogeology ................................................................................................................................................ 4

Deglaciation of the East Branch Delaware River Basin ............................................................... 4

Surficial Geology .................................................................................................................................. 5

Surficial Geologic Units ...................................................................................................................... 5

Till .......................................................................................................................................... 5

Colluvium ...................................................................................................................................... 7

Ice-contact Stratified Drift ........................................................................................................ 7

Kames ................................................................................................................................. 7

Kame Terraces ................................................................................................................. 10

Kame Deltas ...................................................................................................................... 10

Kame Moraines ................................................................................................................ 10

Outwash, Inwash, and Alluvium ............................................................................................. 10

Outwash ............................................................................................................................ 10

Alluvium ............................................................................................................................. 12

Inwash............................................................................................................................... 12

Bedrock Geology ............................................................................................................................... 12

Stratigraphy ............................................................................................................................... 12

Stress Relief and Fracturing ................................................................................................... 14

Well Yields .................................................................................................................................. 15

Saline Ground Water ................................................................................................................ 15

Ground-Water Flow System .................................................................................................... 16

Springs ........................................................................................................................................ 16

Stratified-Drift Aquifers ................................................................................................................... 18

Upper East Branch Delaware subbasin ................................................................................ 19

Tremper Kill subbasin ............................................................................................................... 19

Platte Kill Subbasin ................................................................................................................... 19

Bush Kill Subbasin .................................................................................................................... 20

Dry Brook Subbasin .................................................................................................................. 20

East Branch Delaware River Valley ....................................................................................... 20

Base Flow and Ground-Water Recharge ................................................................................................ 21

Base-flow and Recharge Analysis ................................................................................................. 21

Program RECESS ...................................................................................................................... 22

Program PART ........................................................................................................................... 23

Program RORA .......................................................................................................................... 23

Precipitation, Recharge, and Base Flow, by Subbasin ................................................................ 24

Precipitation .............................................................................................................................. 24

Recharge .................................................................................................................................... 24
iv

Base Flow ................................................................................................................................... 24

Base Flow in Relation to Geology ................................................................................................... 28

Bedrock as a Source of Base Flow ....................................................................................... 28

Thick Till Deposits as a Source of Base Flow ...................................................................... 28

Summary ...................................................................................................................................................... 29

Selected References .................................................................................................................................. 29

FIGURES

 1. Map showing location of East Branch Delaware River Basin and its major subbasins 

in Southeastern New York .................................................................................................................... 2

 2. Map showing distribution of surficial geologic units and locations of selected wells 

and geologic sections within the East Branch Delaware River Basin ......................................... 8

 3. Geologic section A-A’ at the site of the present Downsville dam ............................................... 11

 4. Geologic section B-B’ along the Shavertown bridge at Shavertown, N.Y. ............................... 13

 5. Map showing distribution of major bedrock units within the East Branch Delaware 

River Basin ............................................................................................................................................ 14

 6. Generalized geologic section showing features of stress-relief fracturing .............................. 15

 7. Diagram showing conceptual ground-water flow within bedrock and stratified drift 

aquifers within the East Branch Delaware River Basin ................................................................ 17

 8. Plot of measured yield as a function of altitude for springs in Delaware County, 

New York ............................................................................................................................................... 18

 9. Map showing locations of stream gaging stations and subbasins within the East 

Branch Delware River Basin ............................................................................................................. 22

10. Diagram showing use of the recession-curve displacement method to estimate 

ground-water recharge in response to a recharge event ............................................................. 25

 11. Map showing distribution of mean annual precipitation, runoff, and zones of 

evapotranspiration within the East Branch Delaware River Basin ............................................ 27

TABLES

1. Ranking of subbasins with respect to potential for glacial meltwater inflow across 

saddles on drainage divides in the East Branch Delaware River Basin ......................................... 6

2. Characteristics of recession analysis and master recession curves for subbasins in the 

East Branch Delaware River Basin in Southeastern New York ..................................................... 21

3. Mean components of hydrologic budgets for subbasins in the East Branch Delaware 

River Basin in Southeastern New York, 1998-2001 ........................................................................... 26
v

CONVERSION FACTORS AND DATUMS

Length

inch (in.) 2.54 centimeter

foot (ft) 0.3048 meter

mile (mi) 1.609 kilometer

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

Area

square mile (mi2) 2.590 square kilometer

Flow rate

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

cubic foot per second (ft3/s) 28.32 liter per second

cubic foot per second per square 0.01093 cubic meter per second per 

 mile [(ft3/s)/mi2] square kilometer [(m3/s)/km2]

gallon per minute (gal/min) 0.06309 liter per second

gallon per day per square mile 0.001461 cubic meter per day per

 [(gal/d)/mi2] square kilometer

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

million gallons per day (Mgal/d) 3,785 cubic meters per day

million gallons per day per square 1,461 cubic meter per day per

 mile [(Mgal/d)/mi2] square kilometer

gallons per day per square mile 0.001462 cubic meters per day per

 [(gal/d)/mi2] square kilometer

gallon per day per foot [(gal/d)/ft ] 0.0001437 liter per second per meter

Hydraulic Units

transmissivity, feet squared per day 0.0929 meter squared per day

 (ft2/d)

hydraulic conductivity*, feet per day 0.3048 meter per day

 (ft/d)

specific capacity, gallons per

minute per foot (gal/min)/ft 0.2070 liter per second per meter

pound per cubic foot (lb/ft3 ) 0.01602 gram per cubic centimeter

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 

(NGVD 29); horizontal coordinate information is referenced to the North American Datum of 1927 

(NAD 27).

* Hydraulic conductivity: The standard unit for hydraulic conductivity is cubic foot per day per 

square foot of aquifer cross-sectional area (ft3/d/ft2. In this report, the mathematically reduced 

form, feet per day (ft/d) is used for convenience.
vi
Abstract

 The hydrogeology of the 372-square-mile Pepacton 

Reservoir watershed (herein called the East Branch Delaware 

River Basin) in the southwestern Catskill Mountain region 

of Southeastern New York is described and depicted in a 

detailed surficial geologic map and two geologic sections. An 

analysis of stream discharge records and estimates of mean 

annual ground-water recharge and stream base flow for eight 

subbasins in the basin are included.

Analysis of surficial geologic data indicates that the most 

widespread geologic unit within the basin is till, which occurs 

as masses of ablation till in major stream valleys and as thick 

deposits of lodgment till that fill upland basins. Till covers 

about 91.5 percent of the Pepacton Reservoir watershed, 

whereas stratified drift (alluvium, outwash, and ice-contact 

deposits) accounts for 6.3 percent. The Pepacton Reservoir 

occupies about 2.3 percent of the basin area. Large outwash 

and ice-contact deposits occupy the valleys of the upper East 

Branch Delaware River, the Tremper Kill, the Platte Kill, the 

Bush Kill, and Dry Brook. These deposits form stratified-drift 

aquifers that range in thickness from 90 feet in parts of the 

upper East Branch Delaware River Valley to less than 30 feet 

in the Dry Brook valley, and average about 50 feet in the main 

East Branch Delaware River Valley near Margaretville. 

An analysis of daily mean stream discharge for the 

six eastern subbasins for 1998-2001, and for two western 

subbasins for 1945-52, was performed using three computer 

programs to obtain estimates of mean annual base flow and 

mean annual ground-water recharge for the eight subbasins. 

Mean annual base flow ranged from 15.3 inches per year for 

the Tremper Kill subbasin to 22.3 inches per year for the Mill 

Brook subbasin; the latter reflects the highest mean annual 

precipitation of all the subbasins studied. Estimated mean 

annual ground-water recharge ranged from 24.3 inches per 

year for Mill Brook to 15.8 inches per year for the Tremper 

Kill. The base flow index, which is the mean annual base 

flow expressed as a percentage of mean annual streamflow, 

ranged from 69.1 percent for Coles Clove Kill to 75.6 percent 

for the upper East Branch Delaware River; most subbasin 

indices were greater than 70 percent. These high base flow 

indices indicate that because stratified drift covers only a 

small percentage of subbasin areas (generally 5 to 7 percent), 

most of the base flow is derived from the fractured sandstone 

bedrock that underlies the basin.

Introduction

The Pepacton Reservoir watershed, which is composed 

of the upper reaches of the East Branch Delaware River Basin, 

encompasses a drainage area of approximately 372 mi2 in the 

Catskill Mountain region of Southeastern New York. The 

location of the East Branch Delaware River Basin and its eight 

subbasins are shown in figure 1.

Most productive aquifers in Upstate New York consist 

of stratified-drift (unconsolidated) deposits of sand and gravel 

that occupy major river and stream valleys. In the Catskill 

Mountain region of New York, these aquifers occupy narrow, 

steep-walled valleys and are typically underlain or overlain by 

large thicknesses of till. Many small municipalities, individual 

residences, and non-municipal community water systems 

withdraw ground water from these aquifers for domestic and 

public supply. Stratified-drift aquifers occupy eight major 

subbasins within the East Branch Delaware River Valley 

and supply ground water to the villages of Margaretville 

and Fleishmanns (fig. 1), and to the Roxbury and Arkville 

Water Districts, which have a combined population of 

2,200. In addition, ground water in most of these aquifers 

is hydraulically connected to streams or rivers within these 

valleys and is discharged to streams during periods of little or 

no precipitation to sustain stream base flow.

Knowing the location, extent, and potential for groundwater development of these stratified-drift aquifers is an 

important first step toward managing the ground-water 

resources of this region. In 2000, the U.S. Geological Survey 

Hydrogeology and Water Quality of the Pepacton 

Reservoir Watershed in Southeastern New York

Part 2. Hydrogeology, Stream Base Flow, and Ground-water 

Recharge

By Richard J. Reynolds
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Figure 1. Location of the East Branch Delaware 

River Basin and its major subbasins in 

Southeastern New York.

2 Hydrogeology and Water Quality of the Pepacton Reservoir Watershed in Southeastern New York Part 2
(USGS), in cooperation with the New York State Department 

of Environmental Conservation, began a multi-phase study 

of the hydrology of the Pepacton Reservoir watershed that 

entailed delineating the extent of stratified drift aquifers within 

the basin, conducting a survey of pesticides and herbicides 

in ground water and surface water, and conducting a general 

water-quality assessment of ground water and surface water in 

the basin. The results of these studies are being published as 

four separate reports: part 1 (Phillips and Heisig, 2004) gives 

concentrations of pesticides and their degredates in stream 

base flow from 2000-2001; part 3 (Heisig and Phillips, 2004) 

describes the responses of stream base-flow chemistry to 

hydrogeologic factors and non-point sources of contamination; 

and part 4 (Heisig, 2004) describes the quantity and quality of 

ground water and tributary contributions to stream base flow 

in selected main-valley reaches. This report, part 2 (Reynolds, 

2004), describes the hydrogeology and extent of stratified-drift 

aquifers within the basin and gives estimates of mean annual 

base flow and recharge in each subbasin.

Objectives and Approach

The primary objective of this phase of the study was to 

map the distribution of the stratified-drift aquifer systems in 

the East Branch Delaware River valley above the Pepacton 

Reservoir — within Delaware, Ulster, and Greene Counties 

— and produce a digital 1:48,000-scale map of the surficial 

geology of the basin. The mapped area consists of all or 

part of thirteen 7.5- minute quadrangles and covers an area 

of 372 mi2. The map was derived from four sources: (1) 

reconnaissance-level mapping of surficial geology at 1:24,000 

scale, on file with the New York State Geological Survey; (2) 

maps and illustrations from unpublished doctoral dissertations 

by Kirkland (1973) and Ozvath (1985); (3) unpublished 

reconnaissance-level surficial geologic maps at 1:62,500 

scale, prepared during a previous USGS study of the groundwater resources of Delaware County (Soren, 1958, 1963); and 

(4) soils maps of Delaware County produced by the Natural 

Resources Conservation Service of the U.S. Department of 

Agriculture. Surficial geology was compiled at 1:24,000 

scale; the mapping included upland areas that are adjacent 

and tributary to the main valley-fill aquifers. A secondary 

objective was to provide estimates of average annual base 

flow and recharge to each of the subbasins within the East 

Branch Delaware Basin; this was accomplished through an 

analysis of daily mean stream discharge using computer 

programs developed by Rutledge (1993). 

Purpose and Scope

This report contains two major sections. The first (1) 

outlines the deglacial chronology that led to the deposition of 

major morphostratigraphic units, (2) describes the distribution 

and character of surficial geologic units within the East 

Branch Delaware River Basin and its subbasins, (3) describes 

the general stratigraphy and hydrogeology of the bedrock 

that underlies the basin and, (4) discusses the approximate 

thickness and character of stratified-drift aquifers within 

the six largest subbasins. The second part (1) describes 

the computation of mean annual base flow and groundwater recharge through automated hydrograph separation 

procedures for the period 1998-2001, (2) presents mean annual 

precipitation, recharge, and base flow estimates by subbasin for 

the period 1998-2001, and (3) discusses the relation of stream 

base flow in the subbasins to bedrock and surficial geology.

Previous Studies

Several investigators have studied various hydrologic and 

geologic aspects of the East Branch Delaware River Basin. 

Rich (1935) studied the surficial geology of the western and 

central Catskills, including the eastern half of the East Branch 

Delaware River Basin, and reported evidence of two distinct 

drift sheets, which he termed “early” and “late” Wisconsinan. 

Later investigations of the glacial geology within this basin 

included doctoral dissertations by Kirkland (1973) and 

Ozvath (1985). Kirkland’s study encompassed most of the 

East Branch Delaware River Basin, whereas Ozvath’s study 

included only the westernmost part of the basin.

 Investigations by the USGS in the 1950’s and early 

1960’s, in cooperation with the New York State Department 

of Conservation, produced reports on the ground-water 

resources of Delaware (Soren, 1963) and Greene (Berdan, 

1954) Counties, which contain most of the East Branch 

Delaware River Basin. Geotechnical and engineering studies 

conducted by the New York City Board of Water Supply from 

the 1930’s through the 1940’s, in conjunction with the planned 

construction of the Pepacton Reservoir, provided various 

unpublished memoranda reports describing the results of test 

borings at planned dam sites (for example, Bird and Berkey, 

1944) and two published summary reports (Fluhr, 1953; Fluhr 

and Terenzio, 1984). Perlmutter and Salvas (Perlmutter, N.M., 

and Salvas, E.H., U.S. Geological Survey, written commun., 

1957) described various aspects of ground water in the New 

York part of the Delaware River basin; their findings were 

later incorporated into a larger USGS study of the entire 

Delaware River Basin conducted in the early 1960’s by Parker 

and others (1964). As part of that same basinwide USGS 

study, Hely and Olmsted (1963) investigated the relation 

between low-flow statistics and watershed characteristics 

in the entire Delaware River Basin. A comparison study of 

Catskill and Susquehanna River Basin streams by Coates 

(1971) investigated the relation of bedrock type and joint 

spacing to stream base flow, and provided streamflow and 

low-flow statistics for all gaged streams in the Delaware River 

Basin in New York. Reynolds (2000) delineated the surficial 

geology of the Beaver Kill basin to the south, and suggested a 

relation between the permeability of Catskill Mountain Series 

sandstones and the high stream base flows within that basin.

Introduction 3
Hydrogeology

The thickness, type, and areal extent of surficial geologic 

units within the East Branch Delaware River Basin are 

the direct result of the mode of deglaciation that occurred 

there, and are among the principal factors that determine the 

ground-water availability within the basin and the amount and 

distribution of ground water and surface runoff to the eight 

subbasins within the Pepacton Reservoir watershed.

Deglaciation of the East Branch Delaware 

River Basin

The style of deglaciation that occurred in the East 

Branch Delaware River Basin resulted in the emplacement 

of numerous deposits of ice-contact stratified drift and large 

masses of ablation till, particularly in the upland tributary 

valleys. The mode of deglaciation in the East Branch 

Delaware differed from that in the West Branch, primarily 

because of the steepness of the valley walls combined with the 

sinuosity of the East Branch Delaware River (Kirkland, 1979). 

Ice retreat in the valley of the West Branch was characterized 

by a series of ice-tongue detachments that resulted in six 

zones of stagnant ice (Ozvath, 1985). These stagnant-ice 

zones ranged in length from 10 to 15 mi each, and together 

display three or four fluvial morphosequences (Koteff, 1979), 

in which a complete morphosequence consists of outwash 

grading upstream to pitted outwash, and then to kame moraine 

or kame terrace. Deglaciation in the more sinuous East 

Branch, in contrast, occurred through the ablation of stagnant 

ice masses, which in turn resulted in a succession of proglacial 

lakes in the lower reaches of the East Branch Delaware 

Valley, downstream of the present study area. Kirkland (1979) 

identified three glacial-lake stages that he named after the 

locations of the ice dams that produced them: the Harvard 

Lake Stage, at 1,099 ft above NGVD 29; the Shinhopple Lake 

Stage, at 1,161 ft; and the Shavertown Lake Stage, at 1,299 ft. 

These proglacial lake stages are evidenced by numerous kame 

deltas along lower East Branch valley walls, and by well logs 

that show deposits of lacustrine silt and clay in the lower East 

Branch Valley, below the Downsville dam.

Deglaciation in the East Branch Delaware River Basin 

began with the mass stagnation of ice within the adjacent 

Beaver Kill and Willowemoc Creek valleys to the south. 

The ice sheet then thinned over the southern escarpment (the 

divide between the East Branch Delaware and the Beaver Kill 

Basins) and ablated to the extent that large masses of stagnant 

ice in the lower East Branch Valley, southeast of Shinhopple, 

began to melt. During this time, the Harvard Lake Stage 

(1,099 ft) was formed by an ice dam at Harvard and extended 

an unknown distance up the East Branch Delaware Valley. 

This lake, and the two successive lake stages — Shinhopple 

at 1,161 ft, and Shavertown at 1,299 ft — received meltwater 

from the active ice margin to the north, through cols and 

notches in the divide between the East and West Branches of 

the Delaware Valley (Ozvath and Coates, 1986). As the active 

ice retreated upvalley past Shavertown, both the Harvard 

and Shinhopple Lake stages drained, as evidenced by kameterrace deposits in several southward-flowing tributaries 

grading to outwash in the lower East Branch Delaware River 

Valley (Ozvath and Coates, 1986). Both of these lake stages 

produced kame deltas at several locations in the main East 

Branch Delaware valley southwest of the Downsville dam, 

and small, discontinuous deposits of lacustrine silt and sand 

in the main valley, as seen in test borings for the Downsville 

dam (fig. 3, section A-A’). The third and highest lake level 

(1,299 ft above NGVD 29) was formed by an ice dam near 

the now-extinct hamlet of Shavertown, which was demolished 

during the construction of the Pepacton Reservoir. The hamlet 

was located at a sharp southward bend in the East Branch 

Delaware River near the mouth of the Tremper Kill. Evidence 

of this lake is seen as a kame delta along the southern valley 

wall just northeast of Arena (fig. 2), that grades to the 1,299-

ft lake elevation. The extent of the Shavertown lake stage is 

unknown, however, the present stream grade indicates that 

it probably extended upstream to near Margaretville. These 

three glacial lakes were relatively small in size and short-lived 

when compared to regional glacial lakes such as Glacial Lake 

Albany because: (1) relatively little fine-grained lacustrine 

material appears in the logs of test borings or wells within the 

valley, (2) drainage in the main East Branch Delaware River 

Valley and its tributaries was away from the retreating ice; 

thus, the only dams that could have impounded lake water 

would have to have consisted of either morainic material or 

detached ice blocks, and (3) if detached ice blocks were the 

temporary dams, they would not have lasted long (probably 

tens of years) in an ice-ablation climate.

Ozvath and Coates (1986) note that the steep valley 

walls and sinuosity of the East Branch Delaware River would 

have caused much larger bodies of stagnant ice to persist here 

than in the West Branch, and the long time needed for these 

large bodies of stagnant ice to ablate, allowed a variety of 

ice-contact depositional features to develop in the East Branch 

valley. This ice-stagnation mode of glacial retreat is the reason 

why all principal morphostratigraphic units that occupy the 

southward draining tributaries to the East Branch Delaware 

River are ice-contact deposits (kames, kame terraces, kame 

moraines), deposited by glacial meltwater against stagnant ice. 

These detached and ablating ice masses in these tributaries 

were also the source of large amounts of ablation till that 

were deposited in the valley bottoms, through the melt-out of 

englacial and supraglacial debris. Ozvath (1985) notes that, 

in many parts of these tributary valleys, exposures of ablation 

till grade almost imperceptibly into the stratified drift of kame 

moraines or kame terraces.

When all of the ablating ice masses had melted in the 

East Branch Delaware River Valley, and the ice front was 

well north of the East Branch Delaware River Basin, glacial 

meltwater from glacial Lake Grand Gorge (Rich, 1935), 

which occupied the Schoharie Valley to the east, began to 

cascade through the bedrock notch at Grand Gorge (fig. 2), 

4 Hydrogeology and Water Quality of the Pepacton Reservoir Watershed in Southeastern New York Part 2
at an elevation of 1,600 ft above NGVD 29, and flow into 

the head of the upper East Branch Delaware River Valley. 

Glacial Lake Grand Gorge was not an open-water lake at that 

time but, rather, was partly filled with stagnant glacial ice 

(Cadwell, 1986). Meltwater flowed around and across this ice 

toward the Grand Gorge outlet until another, lower, drainage 

channel —the Franklinton Channel at 1,200 ft. — became ice 

free. The opening of the Franklinton Channel allowed glacial 

Lake Grand Gorge to drain northward through this outlet to 

Catskill Creek and then eastward to the Hudson Valley. The 

large volumes of meltwater flowing through the Grand Gorge 

outlet and into the East Branch Delaware River Valley caused 

erosion and resedimentation of the ice-contact and till deposits 

within the valley, and also transported large amounts of 

sediment from outside the basin divide. As a result, the upper 

East Branch Delaware now contains the eroded remnants of 

kame terraces and a well-developed valley train of outwash 

overlain by post glacial alluvium throughout its upper reach.

Because the Upper East Branch Delaware was linked 

to, and received drainage from a saddle (Grand Gorge) in 

the northern basin divide it is the primary reason that this 

subbasin contains more outwash and ice-contact sand and 

gravel than any other subbasin in the East Branch Delaware 

River Basin. Randall (2001) points out that the valleys in 

rugged terrain in the glaciated Northeast that were connected 

by the lowest elevation cols or saddles apparently became 

trunk drainage systems during deglaciation, and therefore 

conveyed much larger volumes of meltwater and sediment 

than did other parallel or transverse valleys. Moreover, coarse 

sand-and-gravel outwash in these trunk drainage valleys is 

likely to be thicker, better sorted, and more continuous than 

in the intervening or adjacent valleys in which less meltwater 

was available (Randall, 2001). The relations between streams 

in the East Branch Delaware River Basin, saddles in the 

drainage divides, and remarks on glacial meltwater flows are 

summarized in Table 1.

 Many of the subbasins had no inlet for meltwater 

drainage from outside the basin divide; nevertheless these 

subbasins contain extensive deposits of outwash and 

alluvium (fig. 2). For example, the subbasins of Mill Brook, 

Dry Brook, Platte Kill, Bush Kill, and Terry Clove contain 

detached segments of outwash on the valley floor, separated 

by deposits of ice-contact sand and gravel or till, even though 

these subbasins could not have received exterior meltwater 

drainage (table 1). In these valleys, glacial ice probably 

became detached, then stagnated and melted in place at nearly 

the same time as ice in adjacent valleys, and internal drainage 

developed within the decaying ice to form a system of tunnels 

and (or) crevasses. The stagnant ice deposited much of its 

sediment load as ablation till or ice-contact deposits, and 

mass movements over the next several hundred years of postglacial climate carried sediment to late-deglacial streams that 

deposited alluvium along the floors of these valleys. These 

deposits, although mapped as outwash are, therefore, largely 

of late-deglacial to postglacial in origin.

Surficial Geology

The primary objective of this study was to produce a 

generalized surficial geologic map of the East Branch Delaware 

River Basin above the Downsville dam that can be used to 

identify the location and extent of stratified-drift (sand and 

gravel) aquifers within the drainage area of the Pepacton 

Reservoir. The surficial geologic map (fig. 2) includes all or 

part of thirteen 7.5-minute quadrangles and represents an area of 

372 mi2. The map is based on the interpretation of surficial units 

as depicted in four sources: (1) unpublished reconnaissancelevel surficial geologic maps by D. H. Cadwell, at 1:24,000 

scale, on file with the New York State Geological Survey; (2) 

plates and illustrations in doctoral dissertations by Kirkland 

(1973) and Ozvath (1985); (3) unpublished reconnaissancelevel surficial geologic maps at 1:62,500 scale, prepared as part 

of a previous USGS study by Soren (1963), and (4) digital soils 

maps of Delaware County produced by the Natural Resources 

Conservation Service (NRCS) in Walton, N.Y. 

During map compilation, morphostratigraphic units 

delineated either by Cadwell (1998), Kirkland (1973), or 

Ozvath (1985) in valley sections were compared with each 

other and with field notes and observations made by Soren 

(1958). Generally, the contacts of morphostratigraphic units 

delineated by Cadwell, Kirkland, or Ozvath were retained or 

were modified slightly. Areas of thick lodgment till (greater 

than 5 ft) in upland valleys were delineated by overlaying 

the NRCS digital soils data onto 1:24,000 topographic maps, 

then comparing the distribution of soils units derived from till 

parent material to the topography, using breaks in slope as a 

guide for delineating geologic contacts. 

Surficial Geologic Units

The four major types of surficial (unconsolidated) 

sediments within the East Branch Delaware River Basin 

are till, ice-contact stratified drift, outwash, and postglacial 

alluvium. Till is, by far, the most widespread surficial unit 

within the basin and is mapped (fig. 2) as three distinct 

units: thin till (ground moraine), thick till, and colluvium. 

Ice-contact stratified drift occurs as small, isolated kames, 

kame terraces, kame moraines, and kame deltas emplaced 

along the sides of the East Branch Delaware River Valley 

and some of its larger tributaries. Outwash and postglacial 

alluvium form a narrow deposit of coarse gravel and sand that 

floors the East Branch Delaware River valley and some of its 

major tributaries. Bedrock is commonly exposed along steep 

hillsides and ridgetops. The origin and distribution of these 

major units are discussed below.

Till

Till is the predominant surficial geologic unit within the 

East Branch Delaware River Basin and covers approximately 

91.5 percent of the area within the basin. Till occurs within the 

Hydrogeology 5
basin as: (1) thin till (ground moraine), (2) thick till of varying 

origins, and (3) colluvium, which is till that has moved 

downslope and has been redeposited. The predominance of till 

in the East Branch Delaware Basin is the direct result of the 

mode of deglaciation, the steep topography, and the limited 

volume of external meltwater that entered the basin during 

deglaciation. Deglaciation in the East Branch Delaware River 

Basin progressed as “ice-stagnation retreat”, which resulted 

in the deposition of large amounts of ablation till in the valley 

bottoms (Kirkland, 1973; Ozvath, 1985). Ozvath (1985) 

refers to the ablation till as “valley diamicton,” and ascribes 

its deposition to the resedimentation of supraglacial and 

englacial debris during ice melting. Large masses of ablation 

till in the western Catskills are commonly associated, and 

may interfinger with, ice-contact stratified-drift, especially 

in the smaller tributary valleys. This interfingering is a result 

of the multiple depositional processes (resedimentation, 

meltwater runoff, and slope colluviation) that occurred 

during ice ablation, and which resulted in wide variability 

in till composition (Ozvath, 1985). The ablation till in the 

western Catskills has been described as a massive, matrixsupported deposit of sand, silt, and gravel with clasts ranging 

up to boulder size, and is much less compact than the upland 

lodgment till (Ozvath, 1985). 

Thick deposits of ablation till in the East Branch 

Delaware River Basin, and in much of the western Catskills, 

form a variety of recognizable landforms, such as drumlinlike 

hills, morainal loops, ridges, knobs, and lateral embankments 

(large deposits of ablation till containing some stratified 

beds that were emplaced along the lower valley walls). Rich 

(1935) was the first to recognize such features as ablation 

till and ascribed their occurrence to deposition by active ice 

margins. Later investigators, notably Kirkland (1973) and 

Ozvath (1985), maintained that these features resulted from 

Table 1. Ranking of subbasins in the East Branch Delaware River Basin in Southeastern New York with respect to potential for glacial 

meltwater inflow across saddles on drainage divides. [Locations shown on fig. 2; ft, feet] 

Stream subbasin Drainage saddle and elevation Remarks

Upper East Branch Delaware River Grand Gorge, 1600 ft above NGVD 29. Lowest saddle anywhere on the DelawareMohawk divide. Likely to have carried 

substantial meltwater flow throughout 

deglaciation.

Tremper Kill Lake Delaware-Bigger Hollow, 1,830 ft. The lowest saddle on the divide between 

the East and West Branch Delaware Basins, 

and adjoins the large valley of the Little 

Delaware River. Prolonged meltwater flow 

across the divide during deglaciation is 

more likely here than in any other tributary 

subbasin.

Bush Kill/Emory Brook Highmount, 1,890 ft. Saddle opens to the southeast, to head of 

stream draining to Big Indian on Esopus 

Creek. Not clear if there was any flow or 

which direction meltwater would have 

flowed.

Bush Kill/Vly Brook Mountaintop ridge, no saddle No remarks.

Terry Clove Kill Saddle at 1,910 ft. Lack of major valley to north precludes any 

meltwater flow across divide.

Coles Clove Kill Saddle at 2,190 ft at head of Falls

Clove Kill tributary.

Lack of major valley to north precludes any 

meltwater flow across divide.

Platte Kill Winter Creek saddle at 2,450 ft.

Bryant Creek saddle at 2,210 ft.

All other tributaries head in mountaintop 

ridges, no saddles.

Lack of major valley to north of Winter 

Creek saddle precludes any meltwater flow 

across divide. Bryant Creek saddle breaches 

divide with adjacent Tremper Kill, not 

towards any retreating ice.

Dry Brook Mountaintop ridge, no saddle North-draining tributary. No possibility of 

meltwater flow into this subbasin.

Mill Brook Mountaintop ridge, no saddle. North-draining tributary. No possibility of 

meltwater flow into this subbasin.

6 Hydrogeology and Water Quality of the Pepacton Reservoir Watershed in Southeastern New York Part 2
the disintegration of stagnant ice within the stream valleys, 

especially valleys that are oriented perpendicular to the 

southwestward direction of continental ice flow, or that are 

extremely sinuous in their course. Ozvath (1985) first used 

the term “lateral embankments” and suggested that they 

represented supraglacial and englacial debris that collected in 

troughs between the ablating ice and the valley wall. Large 

lateral embankments of ablation till are present in many of the 

valleys tributary to the East Branch Delaware River, as well 

as many other streams in the western Catskills, and typically 

fill the original bedrock valley to a level that has caused the 

present-day stream course to shift considerably from the 

original bedrock thalweg (Reynolds, 2000; Randall, 2001). 

Some of these lateral embankments may consist partly or 

entirely of resedimented till. Rich (1935) first described and 

mapped “thick drift” deposits of “non-morainic” or ablation 

till in the Catskills and noted that the till in the lee of spurs 

on valley walls can be so thick as to block the mouths of 

tributaries and shift them downvalley. Coates (1966) was 

the first to use the term “till shadows” for thick deposits 

of lodgment till that were deposited on the southern, or 

lee, side of bedrock hills with respect to ice flow and later 

(1971) showed how these till shadows on the south sides 

of hills in the Appalachian Plateau had shifted the thalwegs 

of small upland valleys. Large masses of till, whether as 

lateral embankments or till shadows, can be hydrologically 

important, in that they can overlie and confine saturated sandand-gravel deposits to form locally confined aquifers. Randall 

(2001) notes that thick till shadows that locally protrude into 

valley bottoms in the Catskills are actually products of upland 

depositional processes and are not related to the deglaciation 

of the valleys, but rather to the glaciation of the adjacent 

uplands. The sand and gravel aquifers that are confined by 

these masses of ablation till are presumably of small areal 

extent, and may or may not be hydraulically connected to the 

surficial outwash aquifer that underlies most of the valleys in 

the Catskills.

Some of the thickest deposits of till in the East Branch 

Delaware Basin are deposits of lodgment till that fill upland 

bedrock valleys and hollows in the headwaters of upland 

tributaries. These deposits are the result of ice movement 

over the divides that separate the East Branch Delaware River 

Basin from the Schoharie Creek and West Branch Delaware 

River Basins, and are the product of three glacial processes: 

(1) the dropout of subglacial debris as glacial ice moved over 

lee side depressions, (2) the meltout of debris from active ice, 

and (3) the meltout of debris from stagnant basal ice (Ozvath, 

1985). Lodgment till in upland valleys and on south facing 

slopes can exceed 60 ft in thickness (Soren, 1963); elsewhere, 

it is deposited as a veneer of ground moraine that generally 

does not exceed 6 ft in thickness. Lodgment till and ground 

moraine in the East Branch Delaware Basin range in matrix 

composition from sandy silt to silty loam, with minor amounts 

of clay and are poorly sorted, with embedded clasts as large as 

gravel size (Ozvath, 1985).

Colluvium

Colluvium is one of the major surficial geologic units in 

the East Branch Delaware River Basin and covers about 1.2 

percent of the basin (fig. 2). It is prevalent in the steep-sloped, 

southward draining Platte Kill and Batavia Kill, as well as 

along the southern wall of the East Branch Delaware River 

southwest of Margaretville. Ozvath (1985) lists three types of 

colluvium common to the western Catskills: (1) remobilized 

till, (2) talus, and (3) landslide. Remobilized till forms gently 

sloping, hummocky deposits at the base of steep hillsides 

and is derived from lodgment till that has moved gradually 

downslope through gravity during periglacial conditions. 

Talus consists of angular, boulder-sized slabs of local bedrock 

that became dislodged from steep valley walls or cliffs and 

accumulated at the base of the slope. Landslides are abrupt 

downslope movements of large masses of earth material that 

form convex mounds of colluvium at the base of steep slopes, 

and leave a corresponding concave depression in the hillside, 

directly upslope, where the material was removed. Large 

glacial or postglacial deposits of colluvium can partly block a 

valley in much the same way that a lateral embankment or till 

shadow can, and alter the course of the modern-day stream. 

Colluvium that is composed of remobilized till can cover and 

obscure older sand-and-gravel deposits on the valley floor and 

locally confine ground water in the underlying aquifer.

Ice-contact stratified drift

Geomorphic features within the East Branch Delaware 

River Basin that consist of ice-contact stratified drift include 

kames, kame terraces, kame deltas, and kame moraines. These 

units (fig. 2) are closely associated with, and commonly 

interfinger with, adjacent or nearby deposits of ablation till. 

Most of these deposits are along the valleys of the main East 

Branch Delaware River and its southward-draining tributaries.

Kames

Kames are small, rounded hills that occupy the valley 

floor or lower hillslopes of a valley and range to 60 ft in height 

and 800 ft in width. Kames consist of englacial or supraglacial 

debris that accumulated during ice melting and typically have 

a wide range in grain size, sorting, and roundness. The large 

variability in grain size and sorting reflects the rapid changes 

in the rates of meltwater flow and sediment release in the 

glacial environment (Ozvath, 1985). 

Kames can be indicative of areas that are favorable for 

water-supply development where they are largely saturated 

and hydraulically connected to ice-contact stratified drift at 

depth or to adjacent or overlying saturated outwash. Surface 

exposures of kames, even if mostly unsaturated, can be 

recharge areas for saturated ice-contact deposits at depth. 

Hydrogeology 7
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Figure 2. Distribution of surficial geologic units and locations of selected wells and geologic sections within the East 

Branch Delaware River basin.

8 Hydrogeology and Water Quality of the Pepacton Reservoir Watershed in Southeastern New York Part 2
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Hydrogeology 9
Kame Terraces

Kame terraces are large deposits of fluvially deposited 

ice-contact sediments that once occupied the area between 

the bedrock valley wall and the ablating ice within the valley. 

Kame terraces within the East Branch Delaware River Basin 

can have surface elevations as high as 100 ft above the valley 

floor and can be as much as 1,000 ft wide and up to a halfmile long. Many kame terraces have been partly eroded by 

postdepositional slumping or floods, especially in the upper 

East Branch Delaware Valley where meltwater drainage from 

glacial lake Grand Gorge flowed. Kame-terrace sediments 

within the East Branch Delaware River Basin range from silt 

to cobble-sized gravel and are crossbedded locally. Ozvath 

(1985) identified four common sediment facies within 

kame terraces in the western Catskills, all of which indicate 

deposition in a braided glacial-stream environment. 

Kame Deltas

Kame deltas are terrace-like deposits of stratified sand, 

gravel, silt, and clay that were deposited by meltwater into 

a proglacial lake. Ozvath (1985) recognized two forms of 

kame deltas in the western Catskills—those that flank a valley 

wall (terraces), and those that extend across the valley. Kame 

deltas emplaced as flanking terraces were deposited in lakes 

that developed between the valley wall and the ablating ice 

tongue, whereas cross-valley kame deltas were deposited in 

proglacial lakes that developed between the toe of the ice 

tongue and older downvalley deposits. Kame deltas typically 

have flat-topped terrace surfaces that developed at a former 

lake level and consist of three units of sedimentation—topset 

beds, foreset beds, and bottomset beds. Topset beds are the 

result of the deposition of coarse sediment at or above lake 

level by meltwater streams. Foreset beds are the result of 

meltwater-stream deposition of sediment at the leading edge of 

a delta advancing into a proglacial lake; these deposits overlap 

bottomset beds, which are the result of the deposition of finegrained sediment in deeper parts of the proglacial lake. Foreset 

and topset beds of kame deltas can contain considerable 

amounts of coarse sand and gravel, but their elevations are 

typically above the modern stream grade and, therefore, 

are largely unsaturated and generally do not represent areas 

of large ground-water development. They may, however, 

represent potential ground-water recharge areas for saturated 

ice-contact deposits at depth. Kame deltas are found only in 

the lower reaches of the East Branch Delaware River Valley, 

generally downstream of Margaretville, because these were 

the only reaches that contained glacial lakes.

Kame Moraines

Kame moraines are ice-contact landforms that are closely 

associated with a temporary pause in the retreat of the ice 

front within a valley; they also are known as “outwash heads.” 

Kame moraines generally consist of extremely coarse-grained 

(to boulder size), well-sorted material that indicates deposition 

by fast-moving meltwater streams close to the ice. These 

deposits generally show deformation features from ice shove 

and can contain inclusions of till. Kame moraines indicate the 

location of a former ice-margin position within the valley, 

and their downvalley ends generally grade into outwash 

deposits. Kame moraines may have potential for ground-water 

development where they are overlapped by and buried beneath 

later outwash and alluvium and are mostly saturated. Kame 

moraines in the East Branch Delaware River Basin have been 

mapped near the mouths of two southward-draining tributaries 

— Coles Clove and Tremper Kill — and on the south wall of 

the East Branch Delaware River near the confluence with Mill 

Brook (fig. 2).

Outwash, Inwash, and Alluvium

Outwash

Outwash consists of well-sorted, coarse-grained 

sediments deposited by meltwater streams issuing from 

the ice front. The relatively steep stream gradients within 

the East Branch Delaware River Basin, coupled with variable meltwater flows and an abundance of coarse-grained 

sediments, resulted in high-energy, braided streams that 

deposited tens of feet of coarse sand and gravel outwash. This 

outwash accumulated on the valley floor as the ice receded 

and formed what is known as a “valley train.” Floods and 

meltwater drainage commonly reworked and redistributed the 

outwash and adjacent ice-contact deposits during late glacial 

and postglacial time to form a veneer of alluvium over the 

outwash. Sheets of outwash and alluvium in the lower East 

Branch Delaware River Valley (fig. 2) are generally less than 

2,000 ft wide but can be as much as a half-mile wide at major 

valley confluences; where saturated, they form the principal 

stratified-drift aquifers in the basin. Outwash, or alluvium 

underlain by outwash, floors the main valleys tributary to the 

East Branch Delaware River except in their upper reaches, but 

these deposits rarely exceed 1,000 ft in width.

 The thickness and horizontal continuity of outwash 

within the East Branch Delaware River Basin is highly 

variable as a result of the locally varying amounts of 

meltwater and sediment that were available within the basin. 

The retreat of stagnant ice, as mentioned earlier, resulted in the 

deposition of large amounts of ablation till in tributary valleys 

during early stages of deglaciation; thus, most of the valley-fill 

sediments within the upland tributary valleys are ablation till 

or a combination of ablation till, ice-contact stratified drift, 

and alluvial sediments. Where outwash is present in tributary 

valleys, it typically forms only a thin veneer over these older 

deposits. Outwash is as much as 100 ft thick locally within 

the main East Branch Delaware River Valley, however. Test 

borings that were made in the 1930’s for the New York City 

Board of Water Supply, in conjunction with the eventual 

construction of the Downsville dam that impounds the 

Pepacton Reservoir, revealed wide variability in outwash and 

10 Hydrogeology and Water Quality of the Pepacton Reservoir Watershed in Southeastern New York Part 2
Figure 3. Geologic section A-A’ along the Downsville dam, near Downsville, N.Y. Trace of section shown on fig.2. (Modified from Bird and Berkey, 194

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till thicknesses. Geologic section A-A’, at the Downsville 

dam on the East Branch Delaware River (fig. 3), indicates 

that the bulk of the valley fill here consists of till, outwash, 

possibly some ice-contact sand and gravel, colluvium, but 

little lacustrine material. Slightly further upstream, near 

Shavertown, a series of test borings drilled for the construction 

of a bridge (Section B-B’, fig. 4) over the Pepacton Reservoir 

reveal about 80 ft of till on the south side of the valley; this 

till thins to about 20 ft in the valley center and is overlain by 

10-15 ft of alluvium. The saturated thickness of the alluvium 

at this locality is only about 10 ft. No lacustrine sediments are 

indicated in well logs at this site.

Alluvium

Alluvium consists primarily of fluvially deposited 

sediment ranging in size from silt to fine to coarse sand and 

fine gravel, generally less than 6 ft in thickness, that occurs 

as overbank deposits on flood plains. Alluvium is derived 

from reworked and fluvially sorted outwash, ice-contact, 

and till deposits, and is generally confined to modern flood 

plains within a valley or deposited as alluvial fans where 

small tributaries enter major valleys. Alluvium is generally 

permeable and typically overlies outwash on valley floors. 

Inwash

Inwash deposits are stratified accumulations of sand and 

gravel deposited in main valleys by tributaries of nonglacial 

origin. Inwash commonly takes the form of an alluvial fan and 

may have been deposited atop outwash, or against stagnant 

ice, where it coalesced with a developing kame terrace. 

Inwash deposits are typically found in upland headwaters, 

where outwash deposits would not be expected. Deposits 

of inwash are found in the upper reaches of the Batavia Kill 

tributary to the East Branch Delaware River. The permeability 

of inwash is generally high, like that of outwash, but is highly 

variable. 

Bedrock Geology

The bedrock that underlies the East Branch Delaware 

River Basin not only provides ground water to bedrock wells 

but is a major contributor of ground-water flow (base flow) to 

the East Branch Delaware River and its tributaries.

Stratigraphy

The entire Catskill region, including the East Branch 

Delaware River Basin, is underlain by the Catskill Mountain 

Series (of Mather, 1840; Chadwick, 1936) of Upper Devonian 

age, which is as much as 6,000 ft thick and consists of 

marine and nonmarine red and gray sandstones, shales, and 

conglomerates. Beds within the formation are nearly flat 

lying and dip gently to the southeast and northwest as a 

result of broad folds whose axes trend northeastward (Soren, 

1963). The Catskill Mountains were formed by the dissection 

of a bedrock plateau, by streams, and by glacial erosion. 

The highest hilltops and ridges, which form the principal 

drainage divides, are underlain by various beds of siliceous 

conglomerates that are highly resistant to erosion, whereas 

the lower hilltops and ridges are capped by various sandstone 

units that also are highly resistant. The valleys are developed 

along the strike of less competent siltstones and shales, gentle 

anticlines, and fracture zones (Soren, 1961, 1963). 

Fisher and others (1970) and Rickard (1975) divided the 

Catskill Mountain Series (or Catskill Facies, which pertains to 

the nonmarine red and gray sandstones and shales) in the East 

Branch Delaware River Basin into five major formations— the 

Honesdale, Slide Mountain, and Upper Walton Formations 

of the West Falls Group, the Lower Walton Formation of the 

Sonyea Group, and the Oneonta Formation of the Genesee 

Group (Rickard, 1975, pl. 3). The stratigraphically higher 

(and younger) Honesdale and Slide Mountain Formations 

occupy the ridgetops of the southern escarpment, which forms 

the drainage divide between the East Branch Delaware River 

Basin and the Beaver Kill basin to the south. The Upper 

Walton Formation generally occupies the uplands within the 

East Branch Delaware River Basin and its ridges form the 

drainage divides between the seven main tributaries to the 

East Branch Delaware River. The Lower Walton Formation 

underlies the lower slopes and valleys of the main tributaries, 

whereas the Oneonta Formation underlies the upper East 

Branch Delaware River from Grand Gorge south to Stratton 

Falls (Fisher and others, 1970). The areal distribution of these 

major bedrock formations within the East Branch Delaware 

River basin is illustrated in figure 5.

Three intersecting fracture planes, one of which is 

parallel to the bedding and two of which are vertical, appear 

in most of the beds of the Catskill Mountain Series. The 

two vertical fracture sets are roughly perpendicular—one 

striking northeast, from N15° to N35°E, and the other striking 

northwest, from N25° to N80°W (Perlmutter, N.M., and 

Salvas, E.H., U.S. Geological Survey, written commun., 

1957). These roughly orthogonal vertical fracture sets 

facilitated the quarrying of bedrock from hillsides by glacial 

ice to produce horizontal surfaces and ledges bounded by 

nearly vertical cliffs, commonly tens of feet high (Parker and 

others, 1964; Soren, 1963). The fracture sets also serve to 

increase the permeability of the rock and are the reason for 

the relatively high yield of bedrock wells in this region. The 

beds of the Catskill Mountain Series have been characterized 

as poor to moderately good aquifers whose well yields can 

vary widely within hundreds of feet (Parker and others, 1964). 

In general, the sandstone beds are much more permeable than 

the shale beds, although some massive sandstone units are 

so completely cemented and lacking in fractures that they 

transmit little water (Parker and others, 1964). 

12 Hydrogeology and Water Quality of the Pepacton Reservoir Watershed in Southeastern New York Part 2
Figure 4. Geologic section B-B’ along the Shavertown Bridge, at Shavertown, NY. Trace of section shown on fig. 2. (From Fluhr, 1946)

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Stress Relief and Fracturing

Stress relief of sedimentary rocks in valleys has been 

shown to be the principal factor in enhancing the secondary 

permeability of clastic rocks (Wyrick and Borchers, 1981). 

Stress relief refers to the fracturing, arching, and faulting 

of sedimentary rocks beneath a bedrock valley floor and 

along the valley walls in response to the removal of rock by 

erosional processes. The removal of rock to produce a bedrock 

valley creates tensional forces along the valley walls and 

compressional (upward) forces on the bedrock that directly 

underlies the valley floor. These forces typically produce 

vertical tensile fractures in bedrock on the valley walls and 

vertical fracturing, low-angle faulting, and arching of beds 

Figure 5 Distribution of major bedrock units within the East Branch Delaware River basin in southeastern New York.

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14 Hydrogeology and Water Quality of the Pepacton Reservoir Watershed in Southeastern New York Part 2
beneath the valley floor. The tensile fractures on the valley 

walls can result in blocky or steep cliffs with near-vertical 

faces, especially in structurally competent rocks such as 

massive sandstones, whereas the vertical and horizontal 

compressional forces on the rock underlying the valley floor 

typically produce arching of beds and associated vertical 

fracturing. Soren (1963) reports anticlinal crests in the East 

and West branches of the Delaware River near the Pepacton 

Reservoir, and Heisig (1999) depicts stratigraphy (based on 

natural gamma radiation logs) in the Batavia Kill valley near 

Windham, N.Y. (in the adjacent Schoharie Creek Basin to the 

east), that appears to show arching of beds (or an anticlinal 

structure) beneath the thalweg of the bedrock valley there. 

Therefore, a similar anticlinal bed structure and its associated 

vertical fracturing can be reasonably assumed to be present 

beneath the main East Branch Delaware River Valley and 

many of its larger tributaries. Valley-bottom and valley-side 

fracturing because of stress relief improves the secondary 

permeability of rock to the extent that yields from bedrock 

wells in these areas are typically higher than those of wells 

drilled into the upper hillsides or hilltops. The general features 

of stress relief fracturing are illustrated in figure 6.

Well Yields

Records of 199 wells drilled into the Catskill Mountain 

Series within Delaware County (Soren, 1963) indicate well 

depths ranging from 18 to 687 ft, with a median depth of 

158 ft; most well depths range from 100 to 300 ft. Yields 

range from 0 to 450 gal/min, with a median yield of about 

15 gal/min (Soren, 1963). Specific capacities range from 

less than 0.2 to 4 gal/min per foot of drawdown (Perlmutter, 

N.M., and Salvas, E.H., U.S. Geological Survey, written 

commun., 1957). The maximum known yield from a bedrock 

well in the East Branch Delaware River basin (450 gal/min) 

is from well D-56, an 8-in diameter well that is 404 ft deep 

and is completed in the Oneonta Formation, which underlies 

the northern part of the East Branch Delaware River valley. 

Similarly, the maximum known yield from a bedrock well 

within Delaware County is from well D-15, a 12-in. industrial 

well completed in sandstone and shale near Sidney, N.Y. 

(within the Susquehanna River Basin) and yielding 550 gal/

min. Wells that are completed in the Catskill Mountain Series 

and are situated on valley floors commonly exhibit artesian 

flow because potentiometric heads in the underlying bedrock 

are typically above land surface in these locations. Data from 

exploratory borings made in the 1930’s and 1940’s along 

the routes of the nearby East and West Delaware Aqueducts 

indicate artesian flow at several borings in valley floor and 

lower hillside settings because the potentiometric surface was 

more than 10 ft above land surface.

Saline Ground Water

Ground water from the Catskill Mountain Series 

generally is of excellent quality and is typically used for 

domestic, industrial, and municipal supplies without treatment 

(Parker and others, 1964). Hardness ranges from very soft 

to moderately hard, and dissolved solids concentrations are 

generally low. Iron concentrations are typically low (0.01 to 

0.17 mg/L), but can be elevated locally (Parker and others, 

1964). 

Saline ground water, however, has been reported at depth 

within the East Branch Delaware River Basin and within the 

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Figure 6. Generalized geologic section showing features of stress-relief fracturing (from Wyrick and Borchers, 1981, fig.3.2-2).

Hydrogeology 15
Schoharie Creek Basin to the east. Soren (1963) reported two 

bedrock wells, with depths of 140 and 420 ft, which produced 

water with elevated chloride concentrations. Water from 

well D-59 at Grand Gorge (140 ft deep, fig. 2) had a chloride 

concentration of 170 mg/L when it was sampled in 1946, 

and a sample from D-102 at Walton (420 ft deep) initially 

had a chloride concentration of 800 mg/L in 1946, but had 

a reduced chloride concentration of 126 mg/L when it was 

resampled in 1957. Perlmutter and Salvas (Perlmutter, N.M. 

and Salvas, E.H., U.S. Geological Survey, written commun., 

1957) reported a 462-ft-deep bedrock well in central Sullivan 

County that produced natural gas (methane) and saline water 

from a gray shale unit, and a 208-ft-deep well at Deposit, 

N.Y. (D-114), in the West Branch Delaware River Basin, that 

produced water with a chloride concentration of 310 mg/L. All 

of these wells probably intersect marine shales of the Catskill 

Series and tap naturally occurring brine (connate water). Salty 

ground water has also been reported in deep wells completed 

in Catskill Series rocks within the adjacent Schoharie Creek 

Basin to the east. Heisig (1999) reported three bedrock 

wells within the Batavia Kill valley near Windham, N.Y. 

that produce very saline ground water from depths ranging 

from 420 to 489 ft. Several bedrock wells within the Batavia 

Kill valley were geophysically logged in that study and 

water samples were obtained at selected intervals within the 

boreholes with a down-hole point sampler. Dissolved chloride 

concentrations in ground water entering the boreholes from 

fractures ranged from 370 to 13,000 mg/L (Heisig, 1999, 

appendix B). Not all deep bedrock wells that intercept saline 

water at depth produce impotable water, however, because 

considerable mixing generally occurs within the borehole. 

Moreover, the location of the well with respect to the valley 

will affect the flow within the borehole. Wells that are located 

high up on the valley walls or on hilltops are generally in 

areas of ground-water recharge, where the vertical gradient 

and, thus, the flow within the well bore, are downward. 

Conversely, wells on the valley floor are in areas of groundwater discharge, where the vertical gradient and, thus, the flow 

within the well bore, are upward. Therefore most bedrock 

wells located on valley floors, and that intercept saline ground 

water at depth, usually yield water that becomes impotable 

through the upward movement of saline water within the well 

bore.

Ground-Water Flow System

The sequence of permeable sandstone units alternating 

with less permeable shale units within the Catskill Mountain 

Series (of Mather, 1840) forms a series of stacked aquifers 

separated by confining units of varying, but generally lesser 

thickness, than that of the aquifer units. Weathering, glacial 

loading and unloading, and stress relief through the erosion 

of bedrock in the valleys have made exposed rock, and rock 

nearest to the mountainsides, typically more jointed and 

fractured than the unexposed rock deep within each mountain 

or ridge. This enhanced jointing increases the secondary 

permeability in these areas, and allows ground water in the 

highly jointed sandstone to flow readily to points of discharge 

(springs) along the hillside, just above confining shale beds, 

as illustrated in figure 7. A small, unsaturated zone develops 

in the upper part of each sandstone unit near the hillside, 

although the entire rock section within the core of the 

mountain is saturated. Thus, the mountainsides commonly 

exhibit a stepwise pattern of steep sandstone faces with springs 

at their bases, separated by gentler slopes of shale. Horizontal 

joints and permeable bedding-plane fractures typically form 

at the contact between lithologically dissimilar rock units; 

therefore, ground water within a saturated sandstone unit 

typically discharges at the contact with the underlying shale 

unit as a contact spring. These springs can be ephemeral 

features that flow only during the wet spring season (March 

and April), but may flow continuously if the saturated zone 

above the springs is thick enough. These sandstone units also 

are saturated where they extend into the core of the mountain, 

but here they are less fractured or jointed, and ground water 

within these units moves mainly as slow, diffuse flow through 

small intergranular spaces and small, discontinuous fractures 

and, therefore, probably contribute little to contact springs.

Ground water within saturated sandstone that is overlain 

by thick deposits of ablation till (lateral embankments), is 

generally confined, with ground water levels typically above 

land surface in bedrock wells on or near the valley floor. 

For example, records of two test holes drilled into sandstone 

and basal till along the proposed route of the East Branch 

Delaware aqueduct crossing of Beaver Kill at Lewbeach, in 

adjacent Sullivan County, indicate that artesian flow from a 3-

in-diameter casing ranged to as much as 18 gal/min, and heads 

ranged from 12 to 13 ft above the valley floor (Reynolds, 

2000). The large head difference between the water table in 

the valley and the potentiometric surface of the confined, 

fractured sandstone at depth may explain the relatively high 

base flows of streams in this basin.

Springs

Springs are a major source of domestic ground-water 

supplies for much of the population of the East Branch 

Delaware River Basin, and much of the Catskill Mountain 

region. Well and spring inventories were conducted by 

the USGS during previous ground-water investigations of 

Delaware County (Soren, 1963) and of the Delaware River 

Basin in New York (Perlmutter, N.M., and Salvas, E.H., U.S. 

Geological Survey, written commun., 1957). Perlmutter and 

Salvas inventoried about 300 springs that discharge from the 

Catskill Mountain Series rocks in the “Appalachian Plateau 

Unit,” which includes the West and East Branch Delaware 

Basins and most of adjoining Sullivan County. Nearly all of 

these springs were situated on hillsides, and originated from 

bedding-plane openings or other fractures at rates as high as 

106 gal/min, although most discharged only 2 to 3 gal/min 

(Perlmutter, N.M., and Salvas, E.H., U.S. Geological Survey, 

written commun., 1957). Similarly, Soren (1963) published 

16 Hydrogeology and Water Quality of the Pepacton Reservoir Watershed in Southeastern New York Part 2
Figure 7. Conceptual diagram of 

York (from Reynolds, 2000, fig.8)

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records of 184 representative springs in Delaware County, 

nearly all of which discharged from Catskill Mountain Series 

sandstones. Most of these springs were used as domestic 

supplies for homes and farms, although some were used for 

public water supplies for several villages. Yields of 178 of 

these springs were measured by the USGS from October 

1948 through February 1949; the highest yield (100 gal/min) 

was from a group of three springs that supply the village of 

Downsville. The average yield for these 178 springs, which 

includes 11 public water supplies, was 8.5 gal/min, with a 

median yield of 3 gal/min. Most domestic springs in Delaware 

County have yields ranging from 3 to 5 gal/min (Soren, 1963). 

Records of these 178 springs show that they range in altitude 

from 1,000 ft to 2,360 ft above NGVD 29, with a mean 

altitude of 1,676 ft. A plot of yield as a function of altitude 

for these 178 springs (fig. 8) indicates that most of the yields 

are less than 10 gal/min, and that most of the springs are at 

altitudes between 1,400 and 2,000 ft above NGVD 29.

Many small villages within the East Branch Delaware 

River Basin still rely on springs as their primary source 

of water, but often use wells as an auxiliary supply. Soren 

(1963) reported that, in 1960, more than half of the small 

municipalities (17 out of 31) within Delaware County relied 

on springs to supply all or part of their drinking water, and 

water-use data for 2000 (New York State Department of 

Health, written commun., 2000) indicated that there are 15 

small municipalities in Delaware County that obtain all or part 

of their water needs from springs. The prolific use of springs 

for domestic and municipal water supplies in the Catskill 

Mountain region indicates a lesser dependence on drilled 

wells for drinking water than in other parts of New York. The 

spatial distribution of well records in the USGS National Water 

Information System (NWIS) database for the East Branch 

Delaware River basin appears to support this conclusion, in 

that the majority of the drilled wells are on the valley floor, 

with relatively few wells drilled into bedrock in the uplands. 

The abundance of springs issuing from Catskill Mountain 

Series sandstones (Soren, 1963) reflects the relatively high 

secondary permeability of these rocks because of the presence 

of both vertical and bedding-plane fracture sets. The abundance 

of springs also has a pronounced effect on the base flow of 

tributary streams within the East Branch Delaware Basin (as 

discussed later), as many of the small tributaries originate as 

spring discharge from hillsides (Perlmutter, N.M., and Salvas, 

E.H., U.S. Geological Survey, written commun., 1957).

Stratified-Drift Aquifers 

The locations of stratified-drift aquifers in the East 

Branch Delaware River Basin are directly related to the 

distribution of outwash and ice-contact deposits within each 

subbasin. In general, the sediments in the tributary valleys 

become less stratified and sorted as the valley size decreases. 

Most of the small tributary valleys contain little stratified drift, 

and whatever is present consists of poorly sorted ice-contact 

material (Ozvath and Coates, 1986). The valleys of the upper 

East Branch Delaware, Tremper Kill, Platte Kill, Bush Kill, 

and Dry Brook (fig. 2), however, contain large amounts of 

ice-contact sand and gravel and outwash that together form 

productive sand and gravel aquifers that are generally under 

water-table (unconfined) conditions. The distribution and 

characteristics of aquifers in each of these valleys is discussed 

in the following sections.

Figure 8 Yield of 178 springs in Delaware County, N.Y., as a function of altitude (Data from Soren, 1963).

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18 Hydrogeology and Water Quality of the Pepacton Reservoir Watershed in Southeastern New York Part 2
Upper East Branch Delaware subbasin

The upper East Branch Delaware subbasin encompasses 

about 81 mi2, and contains large deposits of outwash and 

ice-contact sand and gravel. This subbasin represents the East 

Branch Delaware River Basin upstream from Margaretville, 

including its main tributary, the Batavia Kill. Till comprises 

the largest surficial unit in this subbasin and covers 89.4 

percent (72.4 mi2) of the subbasin. The headwater valleys in 

this subbasin contain several large areas of thick ablation till 

that occupy 10.7 percent (8.68 mi2) of the subbasin. Most of 

the East Branch Delaware River valley above Halcottsville 

is flanked with large kame-terrace deposits, which were 

deposited against and atop stagnant glacial ice by large flows 

of meltwater. Ice-contact deposits cover about 4.4 percent 

(3.54 mi2) of the subbasin area. The main stratified-drift 

aquifer in this valley consists of outwash and alluvium which 

occupies the valley floor and represent 5.5 percent (4.47 mi2) 

of the subbasin area. All of the stratified drift units combined 

account for 10 percent (8.16 mi2) of the subbasin area. Several 

communities obtain municipal ground-water supplies from the 

outwash aquifer, including the Roxbury Water District (pop. 

750), the hamlet of Hallcottsville (pop. 43), and the Grand 

Gorge Water District (pop. 380) (New York State Department 

of Health, written commun., 2000). Only a limited amount 

of well data in this valley is available to define the thickness 

and hydraulic properties of the aquifer because of the sparse 

population, but several well records in the USGS database 

provide some information on the range in thickness of the 

outwash aquifer. Well records appear to indicate that the 

outwash aquifer ranges up to almost 90 ft in thickness locally, 

and averages about 50 ft thick. Well D-58, a domestic well 

about 2 mi north of Hallcottsville is 86 ft deep, is completed 

open-ended in sand and gravel, and yields about 35 gal/min. 

Well D-438, at Kelly Corners near the confluence of the upper 

East Branch Delaware and the Batavia Kill, is 54 ft deep, is 

also finished open-ended, and produces 12 gal/min. Drillers 

logs for both wells record the material penetrated as “sand 

and gravel.” Further north, in the village of Roxbury, the 

Town of Roxbury uses two supply wells that are completed 

in outwash sand and gravel and have been pumped at 350 

and 207 gal/min, respectively, during tests. A newly installed 

supply well for the Town of Roxbury (D-817) is 70 ft deep 

and is completed with 15 ft of stainless-steel well screen. 

This well has yielded 350 gal/min with a specific capacity 

of 26.6 (gal/min)/ft of drawdown in a 72-hour aquifer test. A 

second well is 36 ft deep and is equipped with a 10-ft screen, 

and has yielded 207 gal/min with a specific capacity of 51.9 

(gal/min)/ft of drawdown (GeoLogic NY, unpublished report, 

2001). A nearby well drilled to bedrock in Roxbury indicates 

that the sand and gravel aquifer here is from 80 to 85 ft thick 

and is underlain by till. Drillers’ logs indicate that the sand 

and gravel aquifer here contains many cobbles and boulders, 

and is very silty, as is common in areas where glacial ice 

has melted in place. Well records from the adjacent Batavia 

Kill valley indicate that the sand and gravel ranges from 60 

to 80 ft thick in the northern part of the basin (wells D-449, 

D-451) to about 20 ft thick near its confluence with the upper 

East Branch Delaware River Valley (D-441). Yields of two 

domestic wells completed open ended in the sand and gravel 

aquifer in the adjacent Batavia Kill valley range from 12 to 30 

gal/min. Drillers’ logs for the few well records in this valley 

seem to indicate that the upper reach of the valley, northward 

of the Roxbury Town line, contains mostly sand and gravel, 

and that the narrower, southernmost reach contains ablation 

till (“hardpan”) and sand and gravel.

Tremper Kill subbasin

The Tremper Kill subbasin encompasses approximately 

33 mi2 and also contains appreciable deposits of ice-contact 

sand and gravel and outwash. Till is the most extensive 

mapping unit in this subbasin; all till units, including 

colluvium, total 92.4 percent of the basin (30.5 mi2), but 

upland valleys filled with thick deposits of lodgment till 

account for about 27.2 percent (9 mi2) of the subbasin area. 

Stratified-drift units, including alluvium, outwash, and 

ice-contact deposits, account for 7.2 percent (2.36 mi2) of 

the subbasin area. Drillhole data from the Tremper Kill 

subbasin are sparse, and represent only a few well records 

near the village of Andes; however, these data indicate that 

the thickness of valley fill near Andes is around 80 ft. Well 

D-87, at Andes, was drilled though 78 ft of till into bedrock. 

Wells D-168 and D-76, also at Andes, were completed in 

the outwash aquifer here for a dairy-processing facility; 

they are 32 and 21 ft deep and produce 55 and 130 gal/min., 

respectively; the latter with only 4 ft of drawdown, and a 

specific capacity of 32.5 (gal/min)/ft of drawdown. Some well 

logs illustrate the thickness of lodgment till in some of the 

upland basins. Well D-423, just north of the divide between 

the Tremper Kill subbasin and the West Branch Delaware 

River Basin, penetrated 184 ft of till overlying bedrock.

Platte Kill Subbasin

The Platte Kill subbasin comprises approximately 34.9 

mi2, and contains appreciable ice-contact deposits, as well 

as outwash. Stratified drift (alluvium, outwash, and icecontact deposits) accounts for about 7 percent (2.45 mi2

) of 

the subbasin area, and till (including areas of thin till and 

exposed rock) accounts for 93 percent (32.45 mi2). Thick till 

deposits in the headwater valleys of the Platte Kill are one of 

the largest single mapping units and cover about 8.2 percent 

(2.85 mi2) of the subbasin. Exposed bedrock, another large 

mapping unit, comprises 7.5 percent (2.61 mi2) of the subbasin 

area, and is seen primarily on the steep, eastern valley wall of 

the Platte Kill. Well data are sparse, as in the other subbasins, 

but records from a few wells can provide some information on 

the thickness of the valley fill along this tributary. Data from 

several wells at New Kingston, where three upland tributary 

valleys meet, indicate that the thickness of the stratified-drift 

Hydrogeology 19
aquifer there ranges from 50 to 60 ft. Well D-272, drilled for a 

dairy-processing facility at New Kingston, is 4 in. in diameter, 

40 ft deep, equipped with 4 ft of screen, and yields 120 gal/

min. Drillers’ logs of two nearby bedrock wells (D-509 and D344), which intercept bedrock at 50 ft and 61 ft, respectively, 

report only sand and gravel overlying bedrock. The only 

indication of the aquifer thickness further south is from well 

D-268, a 24-ft deep driven well about 2 mi north of Dunraven. 

Most of the smaller tributary valleys contain largely till, but 

some contain narrow stratified-drift sand-and-gravel aquifers. 

An example is seen in the western end of the Bryant’s Brook 

tributary valley, also mapped as Palmer Hollow, where well 

D-396 is completed open-ended in 65 ft of gravel with a 

reported yield of 30 gal/min.

Bush Kill subbasin

The Bush Kill subbasin encompasses about 46.7 mi2, 

and is characterized by large upland bodies of thick till, and a 

well-developed valley train of outwash overlain by alluvium. 

Stratified-drift deposits occupy about 5 percent (2.3 mi2) 

of the subbasin, and till covers the other 95 percent (44.38 

mi2). Upland valleys filled with thick deposits of lodgment 

till represent 22 percent (10.45 mi2) of the subbasin, whereas 

alluvium and outwash account for about 3 percent (1.5 mi2). 

Scattered well data from throughout the Bush Kill subbasin 

and its tributaries provide some information on the relative 

thickness of the valley fill aquifer. At the mouth of the Bush 

Kill subbasin, near Arkville, the valley merges with the Dry 

Brook valley just upstream from the East Branch Delaware 

River Valley, where the valley width reaches a half-mile in 

places. Well logs from the supply wells of the Arkville Water 

District indicate that the outwash aquifer here is at least 60 ft 

thick in the valley center. The log of well D-436, at Arkville, 

indicates an upper unit of alluvium and outwash that is 20 ft 

thick, underlain by 30 ft of lacustrine fine sand (“quicksand”) 

that overlies 10 ft of gravel. The presence of lacustrine 

sediment here indicates that a local, temporary proglacial lake 

formed here and lasted long enough for 30 ft of fine-grained 

sediment to be deposited. The depth to bedrock decreases 

further up the narrow Bush Kill valley and the outwash aquifer 

becomes thinner. The log of well D-437 at Covesville, just 

west of Fleischmanns, indicates bedrock at 28 ft, overlain 

by sand and gravel. Further east, at Fleischmanns, the valley 

narrows considerably and the aquifer thins. Well D-67, a 

150-gal/min supply well for the Village of Fleischmanns, is 

located just east of the village, and intercepts bedrock at 16 ft. 

Further upstream, into the Vly Creek valley, the log of well 

D-491 indicates that the depth to bedrock is about 25 ft. The 

Vly Creek valley widens further upvalley near Halcott Center 

in Greene County, however, and well records (G-830, G-15, 

and G-16) indicate depths to bedrock ranging from 60 to 90 

ft in the valley center. Drillers’ logs here typically report the 

material penetrated as “boulders, hardpan, and gravel”; an 

indication of the increasing amount of lodgment till present 

in these headwater valleys. Records from three wells about a 

mile further up the Vly Creek valley report depths to bedrock 

ranging from 55 to 95 ft (wells G-19, G-21) and indicate from 

40 to 60 ft of till and colluvium (hardpan) overlying icecontact sand and gravel (wells G-18, G-21), which represent 

locally confined aquifers. Most of the other smaller tributaries 

in the upper reaches of the subbasin contain thick deposits of 

lodgment till. For example, well D-281, in the Little Red Kill 

valley, 2 mi north of the village of Fleischmanns, penetrates 

110 ft of till overlying bedrock.

Dry Brook subbasin

The Dry Brook subbasin, or that part of the larger Dry 

Brook basin that is gaged along with the Bush Kill subbasin, 

comprises about 35.4 mi2. This subbasin contains prominent 

kame terraces near the confluence of Dry Brook with the 

Bush Kill, and the valley is floored along most of its length 

by alluvium and outwash. Stratified drift occupies about 5 

percent (1.74 mi2) of the subbasin area, whereas till covers the 

remaining 95 percent (33.74 mi2). Upland hollows filled with 

thick lodgment till account for 4.5 percent (1.6 mi2) of the 

subbasin area, and exposed or thinly covered bedrock accounts 

for 6.2 percent (2.2 mi2). Well data from the Dry Creek valley 

are sparse and give little indication of the thickness of the 

outwash aquifer, except for well D-75, which was drilled on 

the valley flat near the mouth of the Dry Brook valley and 

intercepts bedrock at 50 ft. Field notes and mapping by Soren 

(Soren, Julian, U.S. Geological Survey, unpublished field 

maps, 1955) indicate that the entire length of the Dry Brook 

valley is floored with sand and gravel and that, in some places, 

alluvial or outwash terraces overlie bedrock on both banks of 

the stream. The stream, in other areas, notably just upstream 

from the Ulster County boundary near the confluence with 

Rider Hollow, runs directly on bedrock. This suggests that the 

outwash aquifer throughout most of the length of Dry Brook is 

extremely thin and probably does not exceed 30 ft in thickness 

anywhere.

East Branch Delaware River Valley

Well data from several wells in the main East Branch 

Delaware River valley near Margaretville show that the 

average thickness of the valley fill aquifer here is around 50 ft. 

Well D-79, a 6-in.-diameter supply well for a dairy-processing 

facility, is in the valley center across from the “nose” of 

Pakatakan Mountain. This well is 45 ft deep, is screened in 

sand and gravel, and yields 240 gal/min. Nearby, well D-298, 

a supply well for the village of Margaretville, is 35 ft deep 

and is also screened in sand and gravel. The greatest reported 

thickness of stratified drift is at well D-83, located on a kame 

terrace on the west side of Pakatakan Mountain. This well 

penetrated 140 ft of stratified drift before reaching bedrock, 

and its location atop a kame terrace that is 60 ft above the 

valley floor indicates that the depth to bedrock beneath the 

valley floor here is about 80 ft.

20 Hydrogeology and Water Quality of the Pepacton Reservoir Watershed in Southeastern New York Part 2
Base Flow and Ground-Water 

Recharge

The East Branch Delaware River Basin contains eight 

subbasins in which streamflow is gaged, or has been gaged 

in the past, by the USGS. Some of these gaging stations have 

periods of record as long as 65 years, whereas others are 

relatively new (less than 5 years old), and still others have 

been discontinued. The construction of two new streamgaging stations in 1996-97 — Bush Kill near Arkville, N.Y. 

and Dry Brook at Arkville, N.Y. — permits a comparison of 

streamflow statistics from six currently gaged subbasins and 

of one ungaged subbasin (by subtraction of flow of at one 

station from that at another). Four of these stations have long 

periods of record (greater than 65 years), but a comparison of 

streamflow statistics among them requires a common period 

of record (1998-2001), as dictated by the short period of 

record (less than 5 years) of the two newest stations. Records 

from two discontinued gages at the western end of the basin 

— Terry Clove Kill near Pepacton, N.Y. and Coles Clove Kill 

near Pepacton, N.Y. — allow a comparison of flow at these 

two stations for the period 1945-52.

Base-flow and Recharge Analysis

 For this analysis, daily mean stream discharge values 

for the six eastern gaged subbasins were retrieved from the 

USGS database for the 4-year period beginning January 1998 

and ending December 2001. Daily mean values of streamflow 

for a seventh subbasin, herein called the upper East Branch 

Delaware River, were computed as the daily mean flow for the 

East Branch Delaware River at Margaretville, N.Y. minus the 

daily mean flow for Dry Brook at Arkville, N.Y. In addition, 

daily mean discharge for the two discontinued western stations 

— Terry Clove Kill and Coles Clove Kill — were retrieved 

for the period January 1945 through December 1952. A 

data file for each of these nine stations was constructed for 

subsequent use in analytical programs developed by Rutledge 

(1993, 1998, 2000). Stations used in this study are listed in 

table 2; station locations are shown in figure 9.

The data files of daily mean flow for these nine stations 

were applied to three computer programs developed by 

Rutledge (1993, 1998, 2000) to: (1) partition the streamflow 

hydrograph for each station into baseflow and runoff 

components, and (2) estimate the amount of ground-water 

recharge that occurred in each subbasin for the period of 

record analyzed. These three computer programs, called 

RECESS, PART, and RORA, are described below.

����� �� Recession-analysis and master recession-curve data for eight subbasins (nine gaging stations) in the East Branch

Delaware River Basin in southeastern New York 

[mi2, square miles; MRC, master recession curve; min log Q; minimum value of logarithm of stream discharge; 

max log Q, maximum value of logarthm of stream discharge. Locations are shown in fig. 1. n/a, no record ] 

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 Station name

Station

Number

Drainage

area

(mi

2)

Period of

record

Period

analyzed

(month/year)

Number of

recession

segments

used

 Median

recession index

(days per

log cycle)

Range of MRC

Min.

log Q

Max

log Q

Bush Kill near Arkville 01413398 46. 10/97 - present 1/1998 - 12/2001 8 56.3 1.509 2.048

Dry Brook at Arkville 01413408 82. 12/96 - present 1/1998 - 12/2001 15 65.6 1.197 2.232

East Branch Delaware 

 River at Margaretville 

01413500 163.0 02/37 - present 1/1998 - 12/2001 11 55.1 1.507 2.526

Upper East Branch 

 Delaware Ruver

* 80.8 n/a 1/1998 - 12/2001 12 57.8 1.010 2.177

Platte Kill at Dunraven 01414000 34. 10/41 - 09/62 1/1998 - 12/2001 6 42.8 .578 1.720

12/96 - present

Mill Brook at Dunraven 01414500 25. 02/37 - present 1/1998 - 12/2001 14 61.9 .827 1.937

Tremper Kill near Andes 01415000 33. 02/37 - present 1/1998 - 12/2001 8 59.1 .928 1.771

Terry Clove Kill near

 Pepacton

01415500 13.6 1937 - 62 1/1945 - 12/1952 10 40.8 .506 1.698

Coles Clove Kill near 

 Pepacton

01416500 28.0 1945-53 1/1945 - 12/1952 15 31.3 .509 1.818

Base Flow and Ground-Water Recharge 21
Program RECESS

The RECESS program works interactively with the 

user to: (1) select segments of ground-water recession from 

a station hydrograph of daily mean values, and (2) develop a 

master recession curve (MRC), which describes the recession 

of streamflow under base-flow conditions. In this context, 

base flow conditions exist when all streamflow is derived from 

ground-water discharge to the stream, recharge from previous 

precipitation has ceased, and the profile of ground-water head 

distribution from the divide to the stream is nearly stable. In 

use, the program reads a user-constructed file of daily mean 

discharge values and identifies all streamflow-recession 

segments that exceed some specified length of time during a 

specified period of interest. The user then selects segments 

of these recessions that appear as straight lines on a semilog 

plot; these segments represent ground-water discharge, or base 

flow. The program determines, for each selected segment, 

the best linear equation that best relates time, in days, to the 

logarithm of streamflow, then extracts from this equation a 

recession index (K) for the segment expressed in days per 

log cycle. The user repeats this procedure interactively for all 

recessions of a specified minimum length (usually 10 days) 

and for a specified season (usually the nongrowing season) 

for the period of record to be analyzed. Next, the previously 

calculated values of K are used in the program as data points 

to define the best linear equation for K as a function of the 

logarithm of flow. Coefficients of this equation are then used 

to construct the master recession curve (MRC), which is a 

second-order polynomial expression for time as a function 

Figure 9 Locations of stream gaging stations in the East Branch Delaware River Basin in Southeastern New York.

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22 Hydrogeology and Water Quality of the Pepacton Reservoir Watershed in Southeastern New York Part 2
of the logarithm of flow (Rutledge, 1998). The MRC is 

subsequently used in the program RORA, described later, to 

quantify ground-water recharge for the basin being studied for 

a given period.

Program PART

The program PART estimates the base-flow component 

of a streamflow hydrograph through a partitioning 

(hydrograph-separation) procedure. The program uses the 

daily mean discharge values dataset that has been previously 

constructed by the user, and applies a linear interpolation 

method to estimate the base-flow component under each 

hydrograph peak (the base-flow contribution during periods of 

runoff). This method differs from other published automated 

partitioning methods in that it is based on antecedent 

streamflow, whereas other methods are based on antecedent 

precipitation (Rutledge, 1998). During execution, the program 

creates a one-dimensional array of daily streamflow values, 

then searches the array for days that fit an antecedent recession 

requirement. On each of these days, ground-water discharge 

is designated as being equal to streamflow as long as it is not 

followed by a daily decline of more than 0.1 log cycle. The 

program then searches the array again, estimating by linear 

interpolation the ground-water discharge on days when surface 

runoff has occurred, then correcting any estimated groundwater discharge values that have exceeded the streamflow 

value for that day (Rutledge, 1998). Finally, the program 

writes the results to several output files, and calculates the 

mean base flow for the period of record being analyzed—in 

inches per year and as a percentage of the mean streamflow 

for the period (the base-flow index).

Program RORA

The program RORA (Rutledge, 1993, 1998, 2000) 

is a theoretically based procedure using the recessioncurve-displacement method to estimate total ground-water 

recharge to a basin from daily mean values of streamflow. 

The recession-curve-displacement method (Meyboom, 

1961) is based on the upward shift that occurs in the 

streamflow-recession curve in response to a recharge event. 

A recharge event will increase the total potential groundwater discharge from an aquifer to a stream, represented as 

V, which represents the total volume of water that will drain 

from the aquifer if allowed to do so for an infinite period 

without further recharge. Meyboom (1961) expressed V in 

the following equation, which is based on a linear relation 

between the logarithm of ground water discharge and time as:

 V = Q x K (1)

 2.3026

where: 

V = total potential ground-water discharge (L3),

Q = ground-water discharge at an initial time (L3/T), and

K = recession index, which is the time required for 

ground-water discharge to decline 1 log cycle after 

critical time (Tc).

The critical time (Tc) is the time necessary for the 

ground-water head distribution in a given aquifer 

to stabilize after a recharge event, after which time 

the logarithm of streamflow as a function of time 

is linear. Rorabaugh (1964) approximated critical 

time through the following equation as:

Tc = 0.2 (a2) S (2)

 Tr

where:

Tc = critical time (T)

 a = average distance from the stream to the groundwater divide,

 S = storage coefficient, and

Tr = transmissivity (L2/T).

 Rorabaugh (1964) showed that the total potential 

ground-water discharge to a stream at critical time (eq. 2) 

after a streamflow peak is approximately equal to half of the 

total volume of water that recharged the ground-water system 

during the peak. Therefore, total recharge is calculated in 

program RORA by the following equation (Rutledge, 1993) 

as:

R = 2(Q2 - Q1)K (3)

 2.3026

where:

R = total volume of recharge resulting from the precipitation event (L3

)

 Q1 = ground-water discharge at critical time, as extrapolated from the pre-event streamflow recession 

(L3/T), and

 Q2 = ground-water discharge at critical time, as extrapolated from the post-event streamflow recession 

(L3/T).

Rutledge (1993) derives an equation for critical time 

expressed as a function of recession index (K) by combining 

two equations to obtain:

Tc = 0.2144 (K) (4)

Base Flow and Ground-Water Recharge 23
The recession-curve-displacement method consists of the 

following steps, as illustrated in figure 10:

1. Compute the recession index (K) from the hydrograph 

during periods of no recharge

2. Compute the critical time (Tc) using equation 4.

3. Calculate the hypothetical ground-water discharge to the 

stream at Tc if recharge had not occurred by extrapolating 

the pre-event recession curve.

4. Calculate the ground-water discharge to the stream, 

including recent recharge, by extrapolating the post-event 

recession curve to Tc.

5. Solve equation 3 to obtain the recharge volume (R).

In practice, RORA first locates days in the streamflow 

record that fit an antecedent recession requirement, then 

identifies periods of ground-water recession, which can consist 

of one or more days that each fit the requirement. The program 

then defines a peak as the largest value of streamflow between 

two consecutive periods of ground-water recession and labels 

that peak as a recharge event. The program then estimates the 

ground-water recharge associated with each peak by using 

the recession-curve-displacement method (fig. 10). Finally, 

the program sums all of the calculated recharge volumes for 

the period of record being analyzed and calculates a mean 

recharge for the basin for the period of record, expressed in 

inches per year.

Precipitation, Recharge, and Base Flow, by 

Subbasin

Daily mean values of streamflow for the five gaged 

subbasins and one ungaged subbasin in the East Branch 

Delaware River Basin were compiled for 1998-2001, and daily 

mean discharge values for the two western gaged subbasins 

were compiled for 1945-52, as described earlier. These data 

were analyzed by two different methods, programs PART 

and RORA, to obtain estimates of mean annual base flow 

(equivalent to mean annual recharge) for each subbasin for 

the periods analyzed. Both estimates (table 3) are based on 

measured streamflow, and, therefore, should be considered net 

or effective ground-water recharge values, which exclude any 

recharge that may have been lost through evapotranspiration 

before reaching the streams.

Precipitation

 Data from a precipitation station at Arkville were 

obtained for each year from 1998 to 2001 from the Northeast 

Regional Climate Center database at Cornell University 

(http://climod.nrcc.cornell.edu). Mean annual precipitation 

for the Arkville station for this period was 42.25 in. This 

value compares favorably with the areal distribution of mean 

annual precipitation over the East Branch Delaware River 

Basin for the longer period 1951-80 as depicted in Randall 

(1996); therefore, the contours of mean annual precipitation 

in Randall were used as a guide to estimate the mean annual 

precipitation for each of the eight subbasins for 1998-2001. 

Mean precipitation for each subbasin was estimated as the 

sum of mean streamflow from gaging station records for 

1998-2000, and mean evapotranspiration (ET), using data 

from Randall (1996). The resulting precipitation values, along 

with mean values for components of hydrologic budgets for 

the eight subbasins, are given in table 3. Long-term mean 

annual precipitation in the East Branch Delaware River 

Basin reflects the topography, and ranges from 40 in/yr in the 

valley at Margaretville and Dunraven to 55 in/yr in the higher 

altitudes of the Mill Brook subbasin (Randall, 1996). The 

distribution of mean annual precipitation, runoff, and zones 

of evapotranspiration within the East Branch Delaware River 

Basin is shown in figure 11.

Recharge

The estimates of mean annual recharge for the six eastern 

subbasins obtained through the RORA analysis for 1998-

2001, and for the two western subbasins for 1945-52, range 

from 15.8 to 24.3 in/yr (table 3). The highest recharge rate 

is in the Mill Brook subbasin (station 01414500), which is 

also the subbasin with the greatest mean annual precipitation 

for 1998-2000 (50 in/yr). Several other subbasins have mean 

annual recharge rates in excess of 20 in/yr; these include 

Coles Clove Kill (23.07 in/yr), Terry Clove Kill (22.43 in/yr), 

and Dry Brook (20.10 in/yr). Recharge rates are determined 

primarily by the amount of precipitation, but are also affected 

by the hydraulic properties of the soil zone, the position 

of the water table with respect to land surface, the surface 

slope, and antecedent-soil moisture content. Programs RORA 

and PART, which use different mathematical procedures to 

estimate ground-water discharge as a surrogate for recharge, 

are essentially estimating the same parameter through two 

different forms of hydrograph separation. The results of the 

separate RORA and PART analyses conducted on the eight 

subbasins (nine gaging stations) are both included in table 3, 

however, for comparison. 

Base Flow

Mean annual base-flow values for the two periods of 

record studied were calculated through streamflow partitioning 

(program PART) and are given in table 3. Mean annual baseflow values for the six eastern subbasins for 1998-2001 ranged 

from 15.31 in/yr for the Tremper Kill near Andes, N.Y. to 

22.28 in/yr for Mill Brook at Dunraven, N.Y. Values of mean 

annual base flow for the two western subbasins for 1945-52 

were 18.94 in/yr for Coles Clove Kill and 19.41 in /yr for 

Terry Clove Kill. A more useful statistic for use in comparing 

mean annual base flows of these eight subbasins is the “base24 Hydrogeology and Water Quality of the Pepacton Reservoir Watershed in Southeastern New York Part 2
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Figure 10. Procedure for use of the recession 

curve displacement method to estimate groundwater recharge in response to a recharge event 

(modified from Rutledge, 1993, fig. 6)

Base Flow and Ground-Water Recharge 25
����� �. Mean hydrologic-budget values for the eight subbasins (nine gaging stations) in the East Branch Delaware River Basin in

southeastern New York, 1998-2001.

[mi2, square miles; in/yr, inches per year; ft3/s, cubic feet per second; (ft3/s)/mi2, cubic feet per second per square mile;(in/mi2)/yr, inches per square mile per year. Locations are shown in fig. 9.]

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2 ���� ��� �������� ���� ������� ������3 ���� ���������� ����� ���� ���� ���� ������ ���� ������ �������� ���� ����������������� ������ ��� ���� �������� ���������� ���� ������ ����������� ��� ��������

# Mean daily discharge valuesfor thissubbsasin were calculated by subtracting the flowofstation 01413408 (DryBrook) fromstation 01413500 (East Branch Delaware).

Station name

Station

number

Drainage

area

(mi2)

Mean 1

precipitation

Mean

evapotranspiration

(in/yr)

Mean 3

streamflow

Mean

groundwater

recharge

rate(in/yr)

Mean

base flow

(in/yr)

Mean

direct

runoff

3

(in/yr)

Base flow

index

(percent)

Mean

base flow

(ft3/s)/mi2

ft3/s

Bush Kill �������� 46.7 47 18.5 24.98 85.90 19.73 18.48 6.50 74.0 1.36 Dry Brook �������� 82.2 48 18.5 27.35 165.52 20.10 18.99 8.36 69.4 1.40 

East Branch Delaware �������� 163.0 46 18.5 25.17 301.99 19.53 17.88 7.29 71.0 1.32 

Upper East BranchDelaware

� 80.8 45 18.5 22.94 136.47 17.87 17.38 5.56 75.7 1.28 

Platte Kill �������� 34.9 43 18.5 23.64 60.74 18.62 16.92 6.72 71.6 1.25 Mill Brook �������� 25.2 50 18.0 29.69 55.08 24.33 22.28 7.41 75.0 1.64 

Tremper Kill �������� 33.2 43 19.0 20.77 50.76 15.80 15.31 5.46 73.7 1.13 

Terry Clove Kill* �������� 13.6 44 19.0 27.66 27.69 22.43 19.41 8.25 70.2 1.43 Coles Clove Kill* �������� 28.0 43 19.0 27.41 56.51 23.07 18.94 8.48 69.1 1.40 

2

(in/yr) in/yr

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flow index,” which is the ratio of the mean annual base flow to 

the mean annual streamflow for each gaging station, expressed 

as percent. The base-flow indices for the eight subbasins (nine 

gaging stations) ranged from 69.1 percent for Coles Clove Kill 

to 75.7 percent for the Upper East Branch Delaware River. 

Mill Brook at Dunraven, N.Y. had the second-highest baseflow index (75 percent), which indicates that this basin has 

one of the highest base flows of the East Branch Delaware 

Basin, as well as the highest recharge rate (24.33 in/yr) and 

the highest mean annual streamflow (29.69 in/yr) of the basins 

analyzed. 

A high base-flow index for a stream generally indicates 

that the basin contains a large amount of ground water in 

storage, generally in saturated stratified drift within the 

valley or in storage in fractured bedrock aquifers. Another 

statistic that indicates large ground-water storage within a 

given basin is the duration of base-flow recession. Coates 

(1971) developed base-flow recession curves for 25 basins 

in the Delaware and Susquehanna River Basins and, from 

these curves, calculated two indices that are a measure of 

the base-flow recession for each stream: (1) the number of 

days required for flow to decline from its mean annual value 

to 10 percent of its mean annual flow, and (2) the number 

of days required for flow to decline from 1 (ft3/s)/mi2 to 0.1 

(ft3/s)/mi2. Both these indices of base-flow recession show that 

Mill Brook and East Branch Delaware River at Margaretville 

Figure 11. Distribution of mean annual precipitation, runoff, and zones of evapotranspiration within the East Branch Delaware 

River basin (modified from Randall, 1996).

Base Flow and Ground-Water Recharge 27
have among the longest base-flow recessions in the Catskills 

— 36.1 and 34.7 days, respectively, to decline from 1 

(ft3/s)/mi2 to 0.1 (ft3/s)/mi2 (Coates, 1971, table 1; Reynolds, 

2000, table 1). Only two other subbasins, the Beaver Kill at 

Craigie Claire and the Beaver Kill at Cooks Falls, display 

longer base-flow recessions; 37.6 and 36.7 days, respectively. 

Six of the eight subbasins (nine gaging stations) analyzed for 

this study were included in Coates’s 1971 study; these are 

Terry Clove Kill, Coles Clove Kill, Mill Brook, Tremper Kill, 

Platte Kill, and East Branch Delaware River.

The results of the recession analysis (RECESS) 

performed on the eight East Branch Delaware subbasins 

(table 2) indicate similar results. The highest winter median 

recession indices, in days per log cycle of flow, were 61.9 

days for Mill Brook at Dunraven (the station with the longest 

recession in the East Branch Delaware River Basin in Coates’ 

1971 study), and 65.6 days for Dry Brook (a tributary to the 

East Branch Delaware River at Margaretville), not analyzed 

by Coates.

Base Flow in Relation to Geology

Stream base flows in the glaciated Northeastern United 

States that are well sustained, and (or) that are relatively 

large in terms of volume per square mile of basin area, 

have generally been ascribed to the discharge of ground 

water stored in stratified-drift aquifers that occupy the 

valley bottoms, or from partly saturated ice-contact deposits 

emplaced against valley walls (Wandle and Randall, 1994; 

Randall and Johnson, 1988; Coates, 1971). Coates (1971) 

delineated the percentage of land area occupied by stratified 

drift (valley fill) within each of 13 Catskill subbasins and 

found that stratified drift comprises only 2.9 percent of 

the Mill Brook subbasin and 5 percent of the East Branch 

Delaware River subbasin, which includes the upper East 

Branch Delaware, Bush Kill, and Dry Brook subbasins 

(Coates, 1971; Reynolds, 2000). The other Catskill subbasins 

studied by Coates had higher percentages of stratified drift, but 

yet had lower, and less sustained, base flows (Coates, 1971, 

table 1; Reynolds, 2000, table 1); therefore most of the base 

flow in the East Branch Delaware River Basin must be coming 

from storage in aquifers other than that composed of stratified 

drift.

Bedrock as a Source of Base Flow

One possible source of stream base flow is the bedrock 

that underlies these basins, particularly the Mill Brook, 

Bush Kill, Dry Brook, and Upper East Branch Delaware 

subbasins. Bedrock in the Catskill Mountains consists of 

sandstone, shale, and siltstone, of which sandstone is the most 

permeable because it contains extensive joints. Coates (1971) 

developed two indices that describe the amount of sandstone 

present and the degree of jointing within the sandstone for 

each of the 13 Catskill basins that he studied (Coates, 1971, 

table 1; Reynolds, 2000, table 1). The first, his “sandstone 

index,” is the average percentage of sandstone present within 

measured stratigraphic sections within each subbasin; the 

second, his “massiveness index,” is a measure of sandstone 

competency and fabric and is used to quantify the percentage 

of rock units as seen in outcrops that exceed 1 ft in thickness 

between bedding planes. Coates observed that the joints 

in the sandstones of the Catskill Mountains are larger and 

less numerous, but more continuous than those in the shale 

bedrock of the Susquehanna River basin to the west, and 

concluded that most of the sandstone within the 13 Catskill 

subbasins that he studied is massive and, therefore, would 

tend to have large, continuous joints that serve to transmit 

ground water. The highest sandstone indices of the 13 Catskill 

subbasins studied by Coates were in the Beaver Kill and 

Willowemoc Creek subbasins (98 percent for the Beaver Kill 

and 91 percent for Willowemoc Creek); these two subbasins 

also had the highest massiveness indices (96 percent for the 

Beaver Kill and 88 percent for Willowemoc Creek) (Reynolds, 

2000, table 1).

Of the East Branch Delaware River subbasins, Mill 

Brook has the highest sandstone index (91 percent), and the 

highest massiveness index (88 percent); the Tremper Kill 

had the next highest sandstone and massiveness indices of 

88 and 79 percent, respectively. The remaining four East 

Branch Delaware River subbasins that were analyzed by 

Coates (1971) had sandstone indices of 78 to 79 percent, and 

massiveness indices of 70 to 73 percent (Reynolds, 2000). The 

high percentage of sandstone in all eight of the East Branch 

Delaware River subbasins is consistent with the high baseflow indices of 69 to 75 percent of total flow (table 3), despite 

the small amounts of stratified drift within these subbasins 

— not more than 6 percent, and less than 5 percent in most 

of the subbasins (Reynolds, 2000, table 1). Coates (1971) has 

shown through statistical analysis that the sandstone index 

and the massiveness index show close correlation with the 

90-percent flow duration (Q90); the discharge that is exceeded 

90 percent of the time, and a commonly used measure of base 

flow. Each index was correlated with (1) Q90 divided by mean 

annual discharge, and (2) Q90 divided by drainage area; the 

correlation coefficients (R2) were 0.76 for the sandstone index 

and 0.79 for the massiveness index, relative to either base flow 

term (Coates, 1971, table 6; Reynolds, 2000). This positive 

relation supports the theory that the sustained high base flows 

in the East Branch Delaware River subbasins, especially Mill 

Brook, may be wholly or partly derived from ground-water 

discharge from the jointed-sandstone aquifer. Reynolds (2000) 

reached this same conclusion concerning the adjacent Beaver 

Kill and Willowemoc Creek subbasins to the south.

Thick Till Deposits as a Source of Base Flow

The thick till that covers many upland areas in the East 

Branch Delaware River subbasins may seem to be a potential 

source of sustained stream base flow. Recent studies in New 

York and New England have shown, however, that the ground 

28 Hydrogeology and Water Quality of the Pepacton Reservoir Watershed in Southeastern New York Part 2
water in thick till deposits does not contribute appreciably 

to the base flow of streams in glaciated basins. Randall and 

Johnson (1988) present four low-flow equations, each one 

developed from a separate low-flow study in the glaciated 

Northeast, to estimate the average minimum 7-day low flow, 

known as the 7Q10. All four equations include independent 

variables that account for the area covered by stratified drift, 

the area covered by till, the mean runoff or the mean altitude, 

and the area occupied by wetlands (Randall and Johnson, 

1988, table 1). In each of these studies, the regression 

coefficients for the area covered by stratified drift was from 

9 to 25 times larger than that for the area covered by till. 

Regression coefficients for stratified drift ranged from 0.46 to 

2.16, whereas those for till ranged from 0.05 to 0.10. These 

small coefficients for till indicate that the till within a basin 

makes only a minor ground-water contribution to streamflow.

A similar study by Wandle and Randall (1994) on the 

effects of surficial geology, lakes, swamps, and annual water 

availability on the low flows of streams in central New 

England reached similar conclusions regarding the relative 

effects of till and stratified drift on base flow. The regression 

equations developed in that study to estimate 7Q10 for highand low-relief areas of central New England indicated that 

ground-water discharge from coarse stratified drift was from 

4 to 8 times greater than the ground-water discharge from 

till. Tills of New England generally have a much lower clay 

content and, thus, greater permeability, than those of New 

York, and this would imply that the relative amount of ground 

water discharged to streams from tills in New York would 

be much less than in New England. The results of these two 

studies (1) indicate that the large upland subbasins of ablation 

till within the East Branch Delaware River Basin probably 

contribute little ground-water discharge to the base flow of 

streams, and (2) provide additional evidence that the high base 

flows of the eight subbasins are derived from ground-water 

discharge from the underlying fractured and jointed sandstone.

Summary

The East Branch Delaware River Basin (Pepacton 

Reservoir watershed) drains about 372 mi2 of the southwestern 

Catskill Mountains. The most widespread geologic unit within 

the basin is till, which occurs as masses of ablation till in 

major stream valleys and as thick deposits of lodgment till 

that partly fill upland basins. Till covers approximately 91.5 

percent of the basin, and stratified drift, which consists of 

alluvium, outwash, and ice-contact deposits, accounts for 6.25 

percent. The Pepacton Reservoir occupies about 2.25 percent 

of the basin area. Large outwash and ice-contact deposits 

occupy the valleys of the upper East Branch Delaware River, 

the Tremper Kill, the Platte Kill, the Bush Kill, and Dry 

Brook. These deposits form stratified-drift aquifers that range 

in thickness from 90 ft in parts of the upper East Branch 

Delaware River valley to less than 30 ft in the Dry Brook 

valley, and average about 50 ft for the main East Branch 

Delaware River valley near Margaretville. 

Stream discharge records from the six eastern subbasins 

for 1998-2001, and the two western subbasins for 1945-52, 

were analyzed using computer programs to obtain estimates 

of mean annual base flow and mean annual ground-water 

recharge for each subbasin. Mean annual base flow ranged 

from 15.3 in/yr for the Tremper Kill subbasin to 22.3 in/yr for 

the Mill Brook subbasin; the latter reflects the highest mean 

annual precipitation of all subbasins studied. Estimated mean 

annual ground-water recharge ranged from 24.3 in/yr for Mill 

Brook subbasin to 15.8 in/yr for the Tremper Kill subbasin. 

The base-flow index, which is the mean annual base flow 

expressed as a percentage of mean annual streamflow, ranged 

from 69.1 percent for Coles Clove Kill to 75.7 percent for the 

upper East Branch Delaware River; most subbasin indices 

were greater than 70 percent.

Stratified drift, which is the principal source of base 

flow in most drainage basins in the northeastern United 

States, underlies only 5 to 7 percent of most gaged subbasins 

in the East Branch Delaware Basin; yet, the base-flow 

indices were high relative to gaged streams elsewhere with 

similar percentages of stratified drift. Therefore, it appears 

that the sandstone bedrock, which is recharged by abundant 

precipitation and discharges through numerous springs, is 

the probable source of most base flow in the East Branch 

Delaware River Basin.

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Selected References 29
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Selected References 31