Hydrogeology of the Beaver Kill Basin in

Sullivan, Delaware, and Ulster Counties,

New York

U.S. GEOLOGICAL SURVEY

WATER RESOURCES INVESTIGATIONS REPORT 00-4034

Prepared in cooperation with the

TOWN OF ROCKLAND

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 00-4034

Hydrogeology of the Beaver Kill Basin 

in Sullivan, Delaware, and Ulster

Counties, New York

By Richard J. Reynolds

Troy, New York

2000

Prepared in Coooperation with the 

Town of Rockland
U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

The use of firm, trade, and brand names in this report is for identification purposes only and does

not constitute endorsement by the U.S. Geological Survey

For additional information write to: Copies of this report can be purchased

U.S. Geological Survey

Information Services

Box 25286

Federal Center

Denver, CO 80225

from:

ii

U.S. Geological Survey

425 Jordan Road

Troy, NY 12180-8349
Contents III

CONTENTS

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

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

Objectives and approach ............................................................................................................................................. 1

Purpose and scope ...................................................................................................................................................... 3

Previous studies .......................................................................................................................................................... 3

Geology ................................................................................................................................................................................ 3

Surficial geologic units ................................................................................................................................................ 3

Till ..................................................................................................................................................................... 6

Ground moraine ................................................................................................................................................ 10

Colluvium ......................................................................................................................................................... 10

Ice-contact stratified drift .................................................................................................................................. 10

Outwash and Alluvium ..................................................................................................................................... 11

Bedrock ....................................................................................................................................................................... 11

Stratigraphy ....................................................................................................................................................... 14

Ground water .................................................................................................................................................... 14

Ground-water flow system ................................................................................................................................ 14

Hydrology ............................................................................................................................................................................. 16

Precipitation, runoff, and evapotranspiration .............................................................................................................. 16

Sources of stream baseflow ........................................................................................................................................ 18

Subbasin runoff ........................................................................................................................................................... 20

Summary ............................................................................................................................................................................... 22

Selected references ............................................................................................................................................................... 22

FIGURES

1. Map showing location and major geographic features of the Beaver Kill basin, in southeastern New York . . . . . . . . . 2

2. Map showing areal distribution of major surficial geologic units and location of geologic sections within the 

Beaver Kill basin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3. Geologic section A-A' across Willowemoc Creek, at Roundtop, Sullivan County. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

4. Geologic section B-B' across the Beaver Kill valley at Cragie Claire, Sullivan County . . . . . . . . . . . . . . . . . . . . . . . . 8

5. Geologic section C-C' across the Beaver Kill valley at Jersey Brook, Sullivan County . . . . . . . . . . . . . . . . . . . . . . . . 9

6. Geologic section D-D' along the East Branch Delaware aqueduct crossing of the Beaver Kill valley at Lewbeach,

Sullivan County . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

7. Geologic section E-E' along the East Branch Delaware aqueduct crossing of Willowemoc Creek, 

Sullivan County . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

8. Diagram showing conceptual ground-water flow within the Beaver Kill basin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

9. Map showing lines of equal of precipitation and runoff, and zones of evapotranspiration within the Beaver 

Kill basin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

10. Graph showing relations between summer runoff from natural storage and average annual precipitation for 

23 Catskill Mountain streams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

TABLES

1. Comparison of hydrogeologic characteristics of 13 Delaware River drainage basins within the Catskill Mountains, 

New York . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
IV Contents

CONVERSION FACTORS AND VERTICAL DATUM

Sea level: In this report, "sea level" refers to the National Geodetic Vertical Datum of 1929 (NGVD

of 1929)—a geodetic datum derived from a general adjustment of the first-order level nets of both

the United States and Canada, formerly called Sea Level Datum of 1929.

Multiply By To obtain

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 

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

gallon per day per square mile [(gal/d)/mi2

] 0.001461 cubic meter per day per 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 mile

 [(Mgal/d)/mi2] 1,461 cubic meter per day per square kilometer 

gallons per day per square mile [(gal/d)/mi2] 0.001462 cubic meters per day per square kilometer 

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

Hydraulic Units

transmissivity, feet squared per day (ft2

/d) 0.0929 meter squared per day 

hydraulic conductivity, feet per day (ft/d) 0.3048 meter per day 

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
Introduction 1

Hydrogeology of the Beaver Kill Basin, in Sullivan, 

Delaware, and Ulster Counties, New York

by Richard J. Reynolds

ABSTRACT

The hydrogeology of the 299-square-mile 

Beaver Kill basin in the southwestern Catskill 

Mountains of southeastern New York is depicted 

in a surficial geologic map and five geologic 

sections, and is summarized through an analysis 

of low-flow statistics for the Beaver Kill and its 

major tributary, Willowemoc Creek. Surficial 

geologic data indicate that the most widespread 

geologic units within the basin are ablation and 

lodgment till. Large masses of ablation till as 

much as 450 feet thick were deposited as lateral 

embankments within the narrow Beaver Kill and 

Willowemoc Creek valleys and have displaced the 

modern stream courses by as much as 1,000 feet 

from the preglacial bedrock-valley axis.

Low-flow statistics for the Beaver Kill and 

Willowemoc Creeks indicate that the base flows 

(discharges that are exceeded 90 percent of the 

time) of these two streams--0.36 and 0.39 cubic 

feet per square mile, respectively--are the highest 

of 13 Catskill Mountain streams studied. High 

base flows elsewhere in the glaciated northeastern 

United States are generally associated with large 

stratified-drift aquifers, however, stratified drift in 

these two basins accounts for only about 5 percent 

and 4.4 percent of their respective surface areas, 

respectively. The high base flows in these two 

basins appear to correlate with an equally high 

percentage of massive sandstone members of the 

Catskill Formation, which underlies the entire 

region. Ground-water seepage from these 

sandstone members may be responsible for the 

high base flows of these two streams.

INTRODUCTION

The Beaver Kill basin, which encompasses 

the drainage areas of the Beaver Kill, 

Willowemoc Creek, and several smaller 

tributaries, drains approximately 299 mi2 of the 

rugged southwestern Catskill Mountains in 

Delaware, Sullivan, and Ulster Counties, N.Y. 

(fig. 1). The Beaver Kill, and its tributary, 

Willowemoc Creek, both support a large trout 

fishery that attracts many anglers each year. 

Recent concern among residents, natural-resource 

managers, and anglers over the health of this trout 

fishery prompted the Town of Rockland to enter 

into a cooperative agreement with the U.S. 

Geological Survey (USGS) in 1997 to 

characterize the hydrogeologic system within the 

Beaver Kill basin. The location and major 

geographic features of the Beaver Kill basin, are 

shown in figure 1.

Objectives and Approach

The primary objectives of this study were to (1) 

assemble all available surficial geologic maps for the 

thirteen 7.5-minute quadrangles that encompasses the 

Beaver Kill basin, (2) reconcile differences among 

mapped units delineated by various investigators, and 

(3) produce a 1:48,000-scale digital map of the 

surficial geology of the basin. All available surficial 

geologic mapping of the Beaver Kill basin was found 

to be generally on a reconnaissance level and was 

limited to those sections of the basin that were within 

the study areas of Kirkland (1973), Ozvath (1985), and 

Gubitosa (1980). Copies of some 1:24,000-scale 

quadrangles showing surficial geologic mapping by 

these three authors were obtained from the New York 
2 Hydrogeology of the Beaver Kill Basin, in Sullivan County, Delaware, and Ulster Counties, New York

01420500

01418500

01429500

75°07'30"

41°

52'

30"

42°

75° 74°52'30" 30' 45' 37'30"

Base from New York State Department of Transportation

1:250,000 series, 1983

HARDENBURGH

DENNING

NEVERSINK

HANCOCK

ANDES

ROCKLAND

COLCHESTER

CALLICOON

LIBERTY

FREMONT

Delaware County

S

Ulster County

ullivan County

LIVINGSTON

MANOR

CRAIGIE

CLAIRE

ROSCOE

NEW YORK

0 1 2 34 5 MILES

0 1 2 34 5 KILOMETERS

Streams

Beaver Kill Basin drainage boundary

Subbasin drainage boundary

County boundaries

Town or municipal boundaries

Stream gaging station and USGS station number

EXPLANATION

01420500

Herkimer

Montgomery

Schenectady

Albany

Rensselaer

Otsego

Schoharie

Delaware

Greene

Ulster

Sullivan

Orange PutmanDutchess Columbia

MA

CT

PA

S chohari e Creek

Hudson River

West Delaware Aqueduct

East Delaware Aqueduct

WILL OWEMOC

CREEK

BEAVER

KILL

COOKS

FALLS

Mohaw k River

Beaver Kill

Basin

Susquehanna

River Basin

Delaware

River Basin

Delaware River

Schoharie

Creek

Basin

Figure 1. Location and major geographic 

features of the Beaverkill basin in 

southeastern New York.
3

State Geological Survey. Major surficial geologic units 

in parts of the basin that were not mapped by these 

investigators, were delineated from 1:62,500-scale 

reconnaissance field maps produced by Soren as part 

of two USGS ground-water investigations in Sullivan 

and Delaware Counties (Soren, 1961, 1963).

Purpose and Scope

This report (1) describes the major surficial 

geologic units that occupy the basin, (2) briefly 

outlines the bedrock geology, (3) compares the 

hydrogeologic characteristics of the Beaver Kill basin 

with those of 12 other Delaware river drainage basins, 

and (4) relates mean annual base flow of the Beaver 

Kill and Willowemoc Creek to specific hydrogeologic 

characteristics of the Beaver Kill basin. The report 

also presents three previously published geologic 

sections by Fluhr (1948a,b; 1949) and two new ones to 

illustrate the stratigraphic relationships of major 

mapped units within the basin.

Previous Studies

Several previously published works discuss 

various hydrologic and geologic aspects of the Beaver 

Kill basin. The surficial geology and deglacial 

sequences of the western and central Catskill 

Mountains were first studied in depth by Rich (1935), 

who studied the western and central Catskills 

including the eastern upland part of the Beaver Kill 

and Willowemoc Creek subbasins, and concluded that 

there was 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), and a Master’s thesis by Gubitosa 

(1980). Of these three studies, only Kirkland’s 

encompassed the entire Beaver Kill basin; Ozvath’s 

study included only the northwesternmost part of the 

basin, and Gubitosa’s study included only the 

southwesternmost part. 

Two water-resource investigations by the USGS 

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

Department of Conservation, produced reports on the 

ground-water resources of Sullivan and Delaware 

Counties (Soren, 1961, 1963), which contain part of 

the Beaver Kill basin. Geotechnical and engineering 

studies conducted by the New York City Board of 

Water Supply in the mid-to-late 1940’s in conjunction 

with the planned construction of the Delaware 

Aqueduct System provided several unpublished 

memoranda reports describing the results of test 

borings at planned dam sites and aqueduct tunnel 

crossings (Fluhr, 1948a, b, 1949; Reed and Fluhr, 

1948), and two published summary reports (Fluhr, 

1953; Fluhr and Terenzio, 1984). 

A USGS study of the entire Delaware Basin 

conducted in the early 1960’s by Hely and Olmsted 

(1963) and Parker and others (1964), and a 

comparison study of Catskill and Susquehanna River 

basin streams by Coates (1971) provided streamflow 

and low-flow data and statistics for both the Beaver 

Kill and Willowemoc Creek.

GEOLOGY

The thickness, type and areal extent of surficial 

geologic units and bedrock units within the Beaver 

Kill basin are the main factors that determine (1) 

ground-water availability in the basin, and (2) the 

amount and distribution of ground water and surface 

runoff to the Beaver Kill and Willowemoc Creeks.

Surficial Geologic Units

The major surficial geologic units within the 

Beaver Kill basin are till, ice-contact stratified drift, 

outwash, recent alluvium, colluvium, and areas of 

exposed rock (fig. 2). Till is the most widespread unit 

within the basin by far and is mapped as six distinct 

forms: thin till over rock, till, thick till (greater than 5 

ft), ground moraine, till moraine, and colluvium. Icecontact stratified drift occurs as small, isolated kames, 

kame terraces, and kame deltas emplaced along the 

hillsides within the major valleys. Outwash and recent 

alluvium form a thin, narrow deposit of coarse gravel 

and sand that floors most of the Beaver Kill and 

Willowemoc Creek valleys. Colluvium, an unsorted 

mixture of redeposited till and postglacial sediments, 

is present along the base of many of the steep valley 

walls along the Beaver Kill and Willowemoc Creek. 

Bedrock is commonly exposed along steep hillsides 

and ridgetops. The origin and distribution of these 

major units are discussed below.

Geology
4 Hydrogeology of the Beaver Kill Basin, in Sullivan County, Delaware, and Ulster Counties, New York

75 07 30

41°

52'

30"

42°

75 74 52 30

Base from New York State Department of Transportation

1:250,000 series, 1983

Alluvium

Alluvial fan

Colluvium

Peat and muck

Ground moraine

Kames

Outwash

Till

Thick till

Till moraine

Bedrock

Location of geologic section. Geologic

 sections shown in figures 3-7.

Rivers and Streams

Beaver Kill Basin drainage boundary

EXPLANATION

HOLOCENE

DEVONIAN

PLEISTOCENE

B

B'

A A'

NEW YORK

Beaver

Kill

COOKS

FALLS

ROSCOE

Willowemoc

County boundaries

Figure 2. Distribution of major surficial geologic units within the Beaver Kill basin, and locations of geologic sections 

depicted in figures 3 through 7.
5

45' 37'30" 30'

Geology modified by R.J. Reynolds, 1998 from Soren (1961,

1963); Kirkland (1973);Gubitosa (1980), and Ozrath (1985) 

A

A'

C

D

C'

D'

E

E'

East Branch

Delaware Aqueduct

West Branch

Delaware Aqueduct

0 1 2 34 5 MILES

0 1 2 34 5 KILOMETERS

Beaver Kill

c

Creek

LIVINGSTON

MANOR

CRAIGIE

CLAIRE

Delaware County

Ulster County

Sullivan County

Geology
6 Hydrogeology of the Beaver Kill Basin, in Sullivan County, Delaware, and Ulster Counties, New York

Till 

Till is the predominant surficial geologic unit 

within the Beaver Kill basin and covers approximately 

92 percent of the area within the basin. It occurs as (1) 

thin till over bedrock, (2) ablation and lodgment till in 

varying thicknesses, (3) areas of thick ground 

moraine, and (4) till moraine, deposited as distinctive 

landforms. The wide areal extent of till in the Beaver 

Kill basin is a direct result of the mode of deglaciation 

(thinning and melting of glacial ice) that occurred 

there. Continental glacial ice moving southward over 

the high northern drainage divide of the Beaver Kill 

basin became detached from the main ice sheet 

covering the Catskills and ceased to flow. This 

detached glacial ice that occupied the main Beaver 

Kill and Willowemoc valleys then stagnated and 

melted during what is known as “ice-stagnation 

retreat.” This topographically controlled, massive ice 

stagnation resulted in the deposition of large amounts 

of ablation till in the valley bottoms and of lodgment 

till in high upland tributaries. Ozvath (1985) refers to 

the ablation till as “valley diamicton” and ascribes its 

deposition to the resedimentation of supraglacial and 

englacial debris during the melting of underlying ice. 

Large masses of ablation till in the western Catskills 

are commonly associated with, and may interfinger 

with, ice-contact stratified drift (kame) deposits. This 

interfingering resulted a wide variability in till 

composition, and was produced by the multiple 

depositional processes that occurred during ice 

ablation, such as resedimentation, meltwater runoff, 

and slope colluviation (Ozvath, 1985). The ablation till 

in this basin has been described as a massive, matrixsupported deposit of clasts of sand, silt, and gravel 

ranging from granule to boulder size. The matrix 

ranges from loamy silt to silty sand and is reported to 

be less compacted than adjacent upland ground 

moraine (Ozvath, 1985). 

Thick deposits of ablation till in the western 

Catskills form a variety of recognizable landforms 

such as drumlins, morainal loops, ridges, knobs, and 

lateral embankments. Rich (1935) was the first to 

recognize such features as ablation till and to ascribe 

their occurrence as indicative of deposition by active 

ice margins. Later workers, notably Kirkland (1973) 

and Ozvath (1985), maintain that these features 

resulted from the disintegration of stagnant ice within 

the stream valleys, especially in valleys that are 

oriented perpendicular to the southwestward direction 

of continental ice flow. These large ablation till 

landforms have appreciably altered the courses of the 

Beaver Kill and Willowemoc Creek, as well as many 

other streams in the western Catskills. Test borings 

conducted in the 1940’s by the New York City Board 

of Water Supply as part of geotechnical investigations 

for the East Branch and West Branch Delaware 

aqueducts and for several proposed dam sites reveal 

that large thicknesses of ablation till emplaced against 

the valley walls (lateral embankments) have displaced 

the postglacial valley axis as much as 1,000 ft from the 

preglacial bedrock valley axis. Geologic section A-A' 

(fig. 3) across shows approximately 750 ft of 

southwestward axis displacement to in the 

Willowemoc Creek valley at Roundtop (Fluhr, 1948b).

Logs of test borings at this site indicate that nearly all 

of the valley-fill sediment is till with interbedded 

lenses of sand and gravel or sand and silt. A maximum 

till thickness of 360 ft was penetrated by test boring 2 

(fig. 3).

Geologic section B-B (fig. 4) indicates the 

stratigraphy and valley geometry across the Beaver 

Kill valley at Craigie Claire to be similar to that in the 

Willowemoc Creek valley at Roundtop, except that the 

ablation till here contains more lenses of sand and 

gravel, and of silt and clay (fig. 4). The thickest section 

of till penetrated at this site is between test borings 3 

and 4, where approximately 350 ft of till and 

interbedded lacustrine sand and silt was penetrated 

(Fluhr, 1948a). The lenses of lacustrine silt and sand 

are the result of small, short-lived glacial lakes that 

occupied the depressions between the valley wall, 

previously deposited till, and the disintegrating ice 

mass. The lenses of sand and gravel are the result of 

fluvial resedimentation and sorting of till by glacial 

meltwater. Fluhr called these lenses “modified glacial 

drift” and, although some individual beds are 

permeable, none are areally extensive. As at the 

Roundtop site, the lateral embankment of ablation till 

here has moved the course of the postglacial Beaver 

Kill about 1,000 ft north of the bedrock valley axis. 

A third example of a thick lateral embankment 

of ablation till is depicted in geologic section C-C at 

a formerly proposed damsite across the Beaver Kill 

valley at Jersey Brook (fig. 5). This section shows a 

480-ft-thick embankment of ablation till emplaced

along the north wall of the Beaver Kill valley at this 

location and which completely fills the original bedrock valley. This lateral embankment has shifted the 

course of the Beaver Kill about 1,000 ft to the south, 

such that the stream now flows across a bedrock ledge. 

′

′
7

EXPLANATION

ALTITUDE, IN FEET ABOVE SEA LEVEL

WILLOWEMOC CREEK

ROAD

RT. 17

T.B.5

T.B.1

T.B.2

T.B.6

T.B.12

1200

1300

1400

1500

1600

1700

1800

1900

SW

A

NE

A'

VERTICAL EXAGGERATION 10X Geology from T.W. Fluhr,

November 1948

TILL - thin till over rock

TILL, THICK - lateral embankment

of ablation till

SAND, GRAVEL - lenses of sand and gravel

within ablation till

LACUSTRINE SAND AND SILT - small bodies

of sand and silt within ablation till

ROCK - Catskill Formation rocks

DECAYED ROCK

T.B.2

TEST BORING - number is designation given

in original memorandum report.

(Fluhr, 1948b)

GEOLOGIC CONTACT - dashed

where inferred

0

0

500

250

1000 FEET

METERS

Figure 3. Geologic section A-A' across the Willowemoc Creek valley at Roundtop, N.Y. Trace of section is shown in 

figure 2 (Modified from Fluhr, 1948b).

Geology
8 Hydrogeology of the Beaver Kill Basin, in Sullivan County, Delaware, and Ulster Counties, New York

B B'

SOUTH NORTH

EXPLANATION

ALLUVIUM-recent sand and silt deposits

TILL-ablation till emplaced as lateral

embankment or as thin till over

rock

LACUSTRINE SAND AND SILT--small

bodies of sand and silt within 

ablation till

SAND AND GRAVEL--lenses of sand

and gravel within ablation till

ROCK--Catskill Formation rocks

TEST BORING--number is designation

given in original memorandum

report (Fluhr, 1948a)

GEOLOGIC CONTACT--dashed where

inferred

1700

1600

1500

1400

1300

1200

1100

ALTITUDE, IN FEET ABOVE SEA LEVEL

VERTICAL EXAGGERATION X10 Geology from 

T.W. Fluhr, 1948

0

0

500 1000

250

FEET

METERS

T.B.8

T.B.4

T.B.3

T.B.11

BEAVERKILL CREEKROAD

T.B.4

Figure 4. Geologic section B-B across the Beaver Kill valley at Craigie Claire, Sullivan County, N.Y. Trace of 

section shown in figure 2 (Modified from Fluhr, 1948a).
9

EXPLANATION

TILL - thin till over rock

TILL, THICK - lateral embankment

of ablation till

SAND, GRAVEL - lenses of sand and gravel

within ablation till

LACUSTRINE SAND AND SILT - small bodies

of sand and silt within ablation till

ROCK - Catskill Formation rocks

T.B.2

TEST BORING - number is designation given

in original memorandum report.

(Fluhr, 1949)

GEOLOGIC CONTACT - dashed

where inferred

2000

1900

1800

1700

1600

1500

1400

1300

ALTITUDE, IN FEET ABOVE SEA LEVEL

T.B.14

T.B.21

T.B.3

T.B.2

T.B.1

BEAVERKILL

0 500 1000

0 250 METERS

FEET

C C'

NW SE

Geology from

T.W. Fluhr, 1949

VERTICAL EXAGGERATION X10

Figure 5. Geologic section C-C across the Beaver Kill valley at Jersey Brook, Sullivan County, N.Y. Trace of section shown 

in figure 2 (Modified from Fluhr, 1948).

Geology
10 Hydrogeology of the Beaver Kill Basin, in Sullivan County, Delaware, and Ulster Counties, New York

As at the other two sites, this thick embankment of 

ablation till incorporates some isolated lenses of sand 

and gravel and lacustrine silt and clay.

Ground moraine 

Ground moraine is found mostly in the upland 

areas of the Beaver Kill basin and occurs as a veneer 

of lodgment till over bedrock. The thickness of this till 

ranges to 6 ft in most upland areas but can exceed 60 ft 

in upland hollows and on south-facing slopes (Soren, 

1963). Ground moraine in this area is a massive, 

matrix-supported diamicton with random gravel-sized 

clasts. The matrix ranges from sandy silt to silty loam 

with minor amounts of clay and is poorly sorted. 

Ground moraine was transported and deposited 

beneath the active glacier and, therefore, is much more 

compact than ablation till and is difficult to excavate. 

The thickest accumulations of ground moraine in 

upland tributary valleys and hollows are the result of 

several glacial processes: (1) the dropout of subglacial 

debris as glacial ice moved over lee side depressions, 

(2) the meltout of basal debris from active ice, and (3) 

the meltout of debris from beneath stagnant, basal ice 

(Ozvath, 1985, p. 54). Ground moraine is absent 

where bedrock is exposed, such as in cols, notches, 

and ridges. 

Colluvium 

Colluvium constitutes one of the largest surficial 

geologic units in the Beaver Kill basin and covers 

about 7.4 percent of the basin (fig. 2). Colluvium 

commonly overlies valley deposits at the base of steep 

hillsides. Ozvath (1985) lists three types of colluvium 

in the western Catskills: (1) remobilized till, (2) talus, 

and (3) landslides. Remobilized till is present at the 

base of steep hillsides and forms a hummocky deposit, 

derived from lodgment till, that has moved downslope 

under gravity during periglacial conditions. Talus 

consists of angular, boulder-sized slabs of local 

bedrock that dislodged from steep valley walls or cliffs 

and accumulated at the base of the slope. Landslides 

are associated with the mass downslope movement of 

earth material, and form convex mounds of colluvium 

at the base of steep slopes with a corresponding 

concave depression in the hillside, directly upslope, 

where the material was removed.

Ice-contact stratified drift 

Geomorphic features within the Beaver Kill 

basin that are composed 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 (valley diamicton). 

Most of these deposits are within the main Beaver Kill 

and Willowemoc Creek valleys. 

Kames are small mounded hills that occupy the 

valley floor or lower hillslopes and range to 60 ft in 

height and 800 ft in width. They consist of englacial or 

supraglacial debris that accumulated during ice 

melting and typically exhibit 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. 

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 Beaver 

Kill 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 1/2 mi long. Many kame terraces 

have been partly eroded by postdepositional slumping 

or floods. Kame-terrace sediments within the Beaver 

Kill basin range from silt to cobble-sized gravel and 

are cross-bedded 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 are terracelike 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 
11

deposits. Kame deltas typically have flat-topped 

terrace surfaces that developed at some depth below a 

former lake level and consist of three units of 

sedimentation--topset beds, foreset beds, and 

bottomset beds. Topset beds were the result of the 

deposition of coarse sediment in a nearshore, shallowwater environment by meltwater streams; foreset beds 

were the result of meltwater stream deposition of 

sediment at the leading edge of a delta advancing into 

a proglacial lake; and bottomset beds were the result 

of the deposition of fine-grained sediment in deeper 

parts of the proglacial lake. Although both foreset and 

topset beds of kame deltas can contain considerable 

amounts of coarse sand and gravel, the elevations of 

these beds are typically above the modern stream 

grade and, therefore, these deposits are largely 

unsaturated, and generally do not represent areas of 

large ground-water potential. They may, however, 

represent potential ground-water recharge areas for 

saturated ice-contact deposits at depth.

Kame moraines are ice-contact landforms 

closely associated with a retreating ice front within a 

valley and also are known as “outwash heads.” Kame 

moraines generally consist of extremely coarse 

grained (up to boulder size), well-sorted material 

which indicates deposition by fast-moving meltwater 

streams close to the ice. These deposits generally 

exhibit 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 represent 

potential areas for ground-water development because 

they are typically buried beneath later outwash and 

alluvium and, thus, are mostly saturated.

Outwash and Alluvium 

Outwash consists of well-sorted, coarse-grained 

sediments deposited by meltwater streams issuing 

from the ice front. The relatively steep stream 

gradients within the Beaver Kill basin, coupled with 

variable meltwater flows and an abundance of coarsegrained 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 commonly reworked 

and redistributed the outwash and other material 

during postglacial time, to form a veneer of alluvium 

over the outwash. Sheets of outwash and alluvium in 

the major river valleys to the west of the Beaver Kill, 

such as the East and West Branch Delaware River 

valleys (fig. 2), are generally less than 2,000 ft wide 

but can be as much as 1/2 mi wide at major valley 

confluences. Outwash deposits on the valley floors of 

tributaries such as the Beaver Kill rarely exceed 1,000 

ft in width, however, outwash blankets the main valley 

of the Beaver Kill and Willowemoc Creek but is absent 

in the upper reaches of the basin, especially in the 

small tributary valleys along the northern flank of the 

Beaver Kill. 

The thickness and horizontal continuity of 

outwash within the Beaver Kill basin is highly 

variable, as a result of the various deglaciation 

processes that occurred in the basin. The retreat of 

stagnant ice, as mentioned earlier, resulted in the 

deposition of large amounts of ablation till during 

early stages of deglaciation; thus, most of the valleyfill sediments within the Beaver Kill and Willowemoc 

Creek valleys are ablation till or a combination of 

ablation till, ice-contact stratified drift, and finegrained lacustrine sediments. Where outwash is 

present, it overlies these deposits as a veneer but is as 

much as 100 ft thick in some places. Test borings that 

were made in the late 1940’s for the New York City 

Board of Water Supply in conjunction with the 

eventual construction of the East Branch Delaware 

Aqueduct revealed wide variability in outwash 

thickness. Geologic section D-D (fig. 6), at the East 

Branch Delaware aqueduct crossing under the Beaver 

Kill at Lewbeach, shows that the bulk of the valley fill 

here consists of till and lacustrine sand and silt, with 

only a 10-ft-thick veneer of outwash and alluvium on 

the surface. In contrast, geologic section E-E (fig. 7), 

at the East Branch Delaware aqueduct crossing under 

Willowemoc Creek, reveals a narrow deposit of 

outwash as much as 95 ft thick that fills a narrow 

valley incised into the till, although most of the valley 

fill consists of till and fine-grained lacustrine sediment.

Bedrock

The bedrock that underlies the Beaver Kill basin 

not only provides ground water to bedrock wells but is 

a major contributor of ground-water flow (base flow) 

to the Beaver Kill and Willowemoc Creek.

′

′

Geology
12 Hydrogeology of the Beaver Kill Basin, in Sullivan County, Delaware, and Ulster Counties, New York

2100

2000

1900

1800

1700

1600

1500

1400

1300

ALTITUDE, IN FEET ABOVE SEA LEVEL

SV-116

SV-117

SV-118

SV-121

SV-122

SV123

AIRSHAFT #1

BEAVERKILL

ROAD

NW DD' SE

VERTICAL EXAGGERATION X10 Hydrogeology By R.J. Reynolds, 1998

0

0 500

250

1000 FEET

METERS

EXPLANATION

TILL -- thin till over bedrock and 

 thicker deposits of ablation

 till in valleys

OUTWASH -- out wash sand and gravel

 deposited as valley train

LACUSTRINE SAND AND SILT -- bodies of

 sand and silt deposited in temporary

 proglacial lakes

ROCK -- Catskill Formation rocks

TEST BORING -- test borings drilled in conjunction with

 construction of East Branch Delaware Aqueduct.

 Number is county number, assigned by

 the U.S. Geological Survey

POTENTIOMETRIC SURFACE -- approximate potentiometric

 surface of underlying Catskill Formation rocks, 

 as defined by head measurements in test borings

 at the time of drilling

GEOLOGIC CONTACT -- dashed where inferred

SV-122

▼

▼

Figure 6. Geologic section D-D' along the East Branch Delaware aqueduct crossing of the Beaver Kill 

valley at Lewbeach, Sullivan County, N.Y. (Trace of section shown in figure 2.)
13

0 500 1000 FEET

0 250METERS

2100

2000

1900

1800

1700

1600

1500

E E'

NW SE

ALTITUDE, IN FEET ABOVE SEA LEVEL

WILLOWEMOC CREEK

WILLOWEMOC CREEK

SV-137

SV-111

SV-112

SV-113

ALLUVIUM -- modern and post-glacial flood plain

alluvium of silt, clay, sand and gravel

TILL -- thin till over bedrock; includes thicker

deposits of ablation till within valleys

TILL, THICK -- lateral embankment of ablation till

TILL MORAINE -- till deposited as distinct

morainal landforms

OUTWASH -- outwash of sand and gravel

deposited as valley train

LACUSTRINE SAND AND SILT -- bodies of lacustrine sand

and silt deposited in temporary proglacial lakes

ROCK -- Catskill Formation rocks

TEST BORING -- test boring drilled in conjunction with

the construction of the East Branch Delaware

Aqueduct. Number is county number, assigned

by USGS

GEOLOGIC CONTACT -- dashed where inferred

EXPLANATION

VERTICAL EXAGGERATION X10 Hydrogeology by R.J. Reynolds (1998) SV-113

Figure 7. Geologic section E-E' along the East Branch Delaware aqueduct crossing of Willowemoc Creek, Sullivan County, 

N.Y. (Trace of section shown in figure 2.)

Geology
14 Hydrogeology of the Beaver Kill Basin, in Sullivan County, Delaware, and Ulster Counties, New York

Stratigraphy

The entire Catskill Region, including the Beaver 

Kill basin, is underlain by the Catskill Formation of 

Upper Devonian age, which is as much as 6,000 ft 

thick and consists primarily of a sequence of 

nonmarine sandstones, shales, and conglomerates. 

Beds within the formation are nearly flat lying and dip 

slightly to the northwest. The Catskill Mountains were 

formed by the dissection of a bedrock plateau by 

streams and glacial erosion. The highest hilltops and 

ridges, which form the principal drainage divides, are 

underlain by a siliceous conglomerate that is highly 

resistant to erosion, the lower hilltops and ridges are 

capped by sandstone units, which also are highly 

resistant. The valleys are developed along the strike of 

less competent siltstones and shales and along fracture 

zones (Soren, 1961, p. 8). 

Fisher and others (1970) and Rickard (1975) 

have divided the Catskill Formation (or Catskill 

Facies) in the Beaver Kill basin into three major 

formations--the Honesdale, Slide Mountain, and 

Upper Walton Formations--all of which are part of the 

upper Devonian West Falls Group (Rickard, 1975, pl. 

3). Within the Beaver Kill basin, the stratigraphically 

higher (and younger) Honesdale and Slide Mountain 

Formations occupy the hilltops and hillsides, and the 

upper Walton Formation generally underlies the lower 

slopes and valleys (Fisher and others, 1970). Most of 

the beds of the Catskill Formation are cut by three 

intersecting planes of fracture (joints), one of which is 

parallel to the bedding and two of which are vertical. 

The two vertical joint sets are roughly perpendicular, 

which 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). 

The joint sets also serve to increase the secondary 

permeability of the rock and, thus, can increase the 

yield of bedrock wells that intercept one or more joints 

that transmit ground water. Parker and others (1964) 

characterize the beds of the Catskill Formation as 

being poor to moderately good aquifers whose well 

yields can vary widely within short distances. In 

general, the sandstone beds are much more permeable 

than the shale or conglomerate beds, although some 

sandstone units are so massive and completely 

cemented (lacking in fractures) that they transmit little 

water (Parker and others, 1964, p. 82). 

Ground Water

Records of 371 wells drilled into the Catskill 

Formation within the Appalachian Plateau 

physiographic province of the Delaware River basin 

indicate well depths ranging from 5 to 960 ft, and 

yields ranging from 0 to 600 gal/min, with an average 

yield of 25 gal/min (Parker and others, 1964). Wells 

that are completed in the Catskill Formation and are 

situated on valley floors typically exhibit artesian flow 

because potentiometric heads are above land surface. 

Data from exploratory borings made in the 1930’s and 

1940’s along the routes of the East and West Delaware 

Aqueducts show that several borings in valley floor 

and lower hillside settings produced artesian flow 

because the potentiometric surface was more than 10 

feet above land surface.

Ground water from the Catskill Formation 

generally is of excellent quality and is typically used 

for domestic, industrial, and municipal supplies 

without treatment. Hardness ranges from very soft to 

moderately hard, and dissolved solids concentrations 

are generally low. Iron concentrations are typically low, 

but can be elevated locally (Parker and others, 1964).

Ground-Water Flow System

The sequence of permeable sandstone units 

alternating with less permeable shale units within the 

Catskill Formation forms a series of stacked aquifers 

separated by confining units of varying thickness. 

Ground water within each of the sandstone units can 

be locally perched near the mountainsides, but is part 

of a larger saturated zone within each mountain or 

ridge. Weathering and glacial erosional processes such 

as scouring, plucking, and glacial unloading, have 

made exposed rock and rock nearest to the 

mountainsides typically more jointed and fractured 

than the unexposed rock type located deep within each 

mountain or ridge. This jointing increases the 

secondary permeability in these areas and, in turn, 

enhances the movement of ground water within these 

jointed zones, such that a water table is established. 

The water table within each saturated aquifer unit 

slopes downward toward its respective discharge point 

(spring) along the mountainside, as illustrated in figure 

8. Thus, the jointed and weathered zone of each 

bedrock unit that lies immediately beneath the 

mountainside gives rise to a stepwise pattern of 

ground-water discharge from the ridgetop to the valley 

bottom (fig. 8). Horizontal joints and permeable 

bedding-plane fractures typically form at the contact 
15

EXPLANATION

SANDSTONE -- fractured, saturated where shaded

CONFINING BED -- shales

TILL -- lateral embankment of ablation till; saturated where shaded

OUTWASH -- valley train outwash and alluvium; saturated where shaded

WATER TABLE

SPRING DISCHARGE -- contact springs at the base of sandstone members

GROUND WATER FLOW -- generalized direction of ground water flow within zone of sa

UNSATURATED FLOW -- generalized direction of unsaturated flow in fractured bedrock

DOWN-VALLEY FLOW -- indicated flow into the page (down-valley)

RUNOFF RUNOFF

TILL LATERALEMBANKMENT

22002100

20001900

1800

1700

16001500

ALTITUDE, IN FEET ABOVE SEA LEVEL

NOT TO SCALE

OUTWASH

STREAM

Modified 

Figure 8. Conceptual ground-water flow within the Beaver Kill Basin, New York.

Geology
16 Hydrogeology of the Beaver Kill Basin, in Sullivan County, Delaware, and Ulster Counties, New York

between lithologically dissimilar rock units; therefore, 

ground water within a saturated sandstone member 

typically discharges along the contact with the 

underlying shale unit as a contact spring. These 

springs can be ephemeral features that may flow only 

during the spring wet season (March and April), but, if 

the saturated zones above the springs are thick enough, 

they may flow all year long. 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 them moves mainly as slow, 

diffuse flow through small intergranular spaces and 

small, discontinuous fractures and, therefore, probably 

contributes little to contact springs.

Ground water within saturated sandstone in 

valley segments that contain thick deposits of ablation 

till (lateral embankments) occurs generally under 

confined conditions, with heads typically above land 

surface. 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 ((SV-122, 123) in section 

E-E , fig. 6) indicate that artesian flow from a 3-indiameter casing ranged to as much as 18 gal/min, and 

heads ranged from 12 to 13 ft above the valley floor. 

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.

HYDROLOGY

The hydrologic characteristics of 25 drainage 

basins within the Susquehanna and Delaware River 

basins were analyzed in a study by Coates (1971), in 

an effort to relate base-flow discharges to geomorphic, 

geologic, and hydrologic variables and to identify 

which variables (1) are most closely correlated with 

low-flow characteristics of streams and (2) yield the 

most reliable prediction of low-flow characteristics. 

Coates’ study evaluated and compared 25 drainage 

basins, each with a drainage area smaller than 400 

mi2

; 13 of which are in the western Catskill 

Mountains, and 12 of which are in the northern 

Susquehanna River Basin. Although the focus of 

Coates’ 1971 study was to compare the predominantly 

shale Susquehanna basins with the predominantly 

sandstone Catskill basins, some of the statistics that 

Coates presented for the Catskill basins can be used to 

compare the three Beaver Kill subbasins with the other 

10 Catskill basins in his study. Selected hydrogeologic 

data from the 13 western Catskill basins studied by 

Coates’ (1971) are given in table 1, which treats the 

Beaver Kill and Willowemoc Creek (1) as two separate 

basins upstream from their confluence (fig. 1), and 

(2) as a combined basin downstream from their 

confluence. Stream-flow gages were historically 

maintained on the Beaver Kill at Craigie Claire 

(station 01418500) and on Willowemoc Creek near 

Livingston Manor (station 01419500) in addition to 

the currently maintained Beaver Kill at Cooks Falls 

gage (station 01420500). Station locations are shown 

in fig. 1.

In addition to Coates’ 1971 study, Hely and 

Olmsted (1963) analyzed data from 123 basins within 

the Delaware River basin for the period 1921-1950; 

among these were 23 streams in the Catskill 

Mountain region, including the Beaver Kill and 

Willowemoc Creek.

Precipitation, Runoff, and 

Evapotranspiration

Of the 13 Catskill basins studied by Coates 

(1971), the Beaver Kill basin (at Cooks Falls) has the 

greatest relief--a maximum of 2,727 ft. The 

Willowemoc Creek and Beaver Kill (at Craigie Claire) 

subbasins receive the greatest average precipitation of 

the 13 basins--approximately 52 in/yr and have, 

correspondingly, the highest mean annual discharges--

2.42 and 2.55 (ft3/s)/mi2 (table 1). These high 

discharges result from (1) the large amount of 

precipitation that falls in these two subbasins, and 

(2) the large amount of runoff, which is a result of the 

steep topography and the geology of each basin. A 

recently published map by Randall (1996) showing 

lines of equal precipitation, runoff, and 

evapotranspiration in the glaciated Northeast for 

1951-80 indicates the wide range of values for these 

three hydrologic variables within the Beaver Kill 

basin (fig. 9). 

Mean annual precipitation within the Beaver 

Kill basin ranges from 42 in. at the lowest elevations 

(downstream western end of the basin) to 55 in. at the 

highest elevations (eastern end of the basin). Coates 

(1971) used an average value of 50.1 in/yr for the 

entire Beaver Kill basin, and 52 and 51.8 in/yr for 

Willowemoc Creek and for Beaver Kill at Cragie 

Claire, respectively, based on data available in 1958. 

These average precipitation values are slightly higher 

′
17

Table 1. Hydrogeologic characteristics of 13 Catskill drainage basins within the Delaware River Basin in southeastern New York [mi2, square mile; in., inches; ft, feet; (ft3/s)/mi2, cubic feet per second per square mile; Q90, flow at 90 percent duration. Data from Coates, 1971. Locations are sho

Streams within the Beaver Kill basin shown in boldface.]

a) Sandstone index - the average percentage of sandstone found within measured stratigraphic sections with each basin (Coates, 1971).b) Massiveness index - the percentage of rock units in observable outcrops that exhibit joint-free exposures of greater than 1-ft in thickness (Coat* Includes Willowemoc Creek drainage. Beaver Kill and Willowemoc Creek are treated as separate basins above their confluence at Roscoe 

and as a combined basin downstream from their confluence.

† Q90 - the stream discharge that is exceeded 90 percent of the time.

Basin

Basin 

area 

(mi2)

Max. 

basin 

relief

(ft)

Mean 

annual 

precip. 

(in.)

Mean 

annual 

discharge 

[(ft3/s)/mi2)]

Flow duration (Q90)† Baseflow recession (days)

Area 

stratified 

(percen

basin

Percent of 

mean 

annual 

discharge

Normalized 

to

basin size 

[(ft3/s)/mi2)]

To 10 percent 

of mean 

annual 

discharge

Normalized to 

decline from 

1.0 to 0.1 

(ft3/s)/mi2

Terry Clove 14.1 1290 44.0 1.88 10.0 .188 13.5 16.5 4.3 Mill Brook 25.0 2420 48.8 2.27 12.0 .272 26.5 36.1 2.9 Coles Cloves 28.0 1460 44.0 2.03 10.0 .203 17.7 21.0 5.7 Tremper Kill 33.0 2045 43.7 1.88 10.0 .188 13.2 25.2 5.5 Platte Kill 34.7 2055 43.0 1.82 9.0 .164 17.7 18.5 4.5 Trout Creek 49.5 1280 43.4 1.83 11.0 .201 22.5 29.7 7.6Little Delaware River 49.8 1965 42.7 1.81 7.8 .141 22.7 28.0 3.6 *Willowemoc Creek near 

Livingston Manor

63 1960 52.0 2.42 16.0 .387 22.1 32.0 4.4 

Oquaga Creek 66 1040 43.0 1.78 8.5 .151 17.5 22.9 5.2 Beaver Kill at 

Craigie Claire

82 2495 51.8 2.55 14.0 .357 30.3 37.6 5.0 

W. Branch Delaware River 142 1835 41.5 1.70 8.5 .145 27.5 33.0 12.2 E. Branch Delaware River 163 2595 44.0 1.89 10.0 .189 27.1 34.7 5.0 *Beaver Kill at Cooks 

Falls 241 2727 50.1 2.34 16.0 .374 31.5 36.7 6.0 

Hydrology

Mean 76.5 1930 45.5 2.01 11.0 .228 23.1 28.6 5.5 
18 Hydrogeology of the Beaver Kill Basin, in Sullivan County, Delaware, and Ulster Counties, New York

than those indicated by Randall (1996), but are 

uniformly so and, thus, are suitable for purposes of 

basin comparison. 

Runoff increases from less than 24 in/yr at the 

western end of the Beaver Kill basin to 40 in/yr at the 

extreme eastern end (Fig. 9). This increase 

corresponds to the general eastward increase in 

elevation and relief within the Beaver Kill basin. The 

maximum basin relief, for example, increases from 

1,960 ft in the Willowemoc Creek basin (station 

01419500) to 2,495 ft in the Beaver Kill subbasin to 

the north (station 01418500), and to 2,727 ft in the 

entire Beaver Kill basin (station 01420500). 

Hely and Olmsted (1963) calculated average 

annual runoff for the 123 basins in the Delaware River 

Basin for the period 1921-1950; their results indicated 

that the Beaver Kill and Willowemoc Creek subbasins 

had two of the highest values of average annual 

runoff—33.7 and 32.9 in/yr, respectively. 

Evapotranspiration rates within the Beaver Kill basin 

were calculated by Randall (1996) to range from 19 in. 

over the western two-thirds of the basin to 18 in. in the 

eastern third.

Sources of Stream Base Flow

Several of the low-flow statistics listed in table 1 

indicate that the Beaver Kill and Willowemoc Creek 

have, in proportion to their size, the highest base flows 

per square mile of the 13 Catskill basins studied by 

Coates (1971). The discharge that is exceeded 90 

percent of the time (Q90) is a commonly used measure 

of base flow. Values of Q90 for the 13 Catskill basins 

are shown in table 1 in two formats--first as a 

percentage of mean annual discharge, then as 

normalized to basin size and expressed as cubic feet 

per second per square mile. The Q90 for Willowemoc 

Creek is 16 percent of the mean annual discharge, and 

the Q90 for the Beaver Kill subbasin is 14 percent; 

similarly, the Q90 normalized to basin size is 

0 1 2 3 4 5 MILES

0 1 2 34 5 KILOMETERS

75°07'30"

41°

52'

30"

42°

75° 74°52'30" 30' 45' 37'30"

Base from New York State Department of Transportation

1:250,000 series, 1983

01420500

01418500

01429500

Precipitation (inches)

Runoff (inches)

Evapotranspiration (inches)

Streams

Basin boundary

Subbasin boundary

Stream gaging station and USGS station number

(station names given in Table 2)

EXPLANATION

01420500

48

30

18

19

NEW YORK

40

55

50 50

48

46

44

42

42

30

35

19

18

30

28

26

24

Figure 9. Precipitation, runoff, and zones of evapotranspiration within the Beaver Kill basin, New York 

(Modified from Randall, 1996).
19

equivalent to 0.39 (ft3/s)/mi2 in the Willowemoc basin 

and 0.36 (ft3

/s)/mi2 in the Beaver Kill subbasin. These 

relatively high base-flow values indicate that these 

basins contain a large amount of ground water in 

storage, presumably in saturated stratified drift along 

the valley floor. Another statistic that implies large 

ground water storage within a given basin is the 

duration of base-flow recession. The Beaver Kill at 

Cooks Falls (station 01420500) has an average 

baseflow recession period of 31.5 days (before 

declining to 10 percent of the mean annual discharge), 

which is the highest of all 13 basins studied by Coates 

(1971). A long base-flow recession generally indicates 

that ground water is supplying the base flow during 

long precipitation-free periods. Coates (1971) 

delineated the percentage of land area occupied by 

stratified drift within each basin and found that 

stratified drift comprises only 5 percent of the Beaver 

Kill subbasin above Roscoe, 4.4 percent of the 

Willowemoc basin, and 6 percent of the Beaver Kill 

basin above Cooks Falls. The other 10 Catskill basins 

listed in table 1 contain higher percentages of stratified 

drift, yet produce smaller sustained base flows than the 

Willowemoc and Beaver Kill subbasins. Therefore, 

other hydrogeologic units within the Beaver Kill and 

Willowemoc basins must be the source of the 

relatively large contribution of ground water, under 

base flow conditions, to these two streams. 

A possible source may be the bedrock that 

underlies both basins. Bedrock in the Catskills consists 

of sandstone, shale, and siltstone, of which sandstone 

is the most permeable because it contains large, 

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 he studied (table 1). His “sandstone 

index” is the average percentage of sandstone found 

within measured stratigraphic sections within each 

basin, while the “massiveness index” is a measure of 

sandstone competency and fabric and is used to 

quantify the percentage of rock units in observable 

outcrops with joint-free exposures greater than 1-ft in 

thickness. Coates observed that joints in the sandstone 

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 (fig. 1) and 

that most of the sandstone within the 13 Catskill basins 

studied is massive and, therefore, would tend to have 

large, continuous joints that could serve to transmit 

ground water. Of the 13 Catskill basins studied by 

Coates, the Beaver Kill and Willowemoc Creek 

subbasins have the highest sandstone indices--98 

percent for the Beaver Kill and 91 percent for the 

Willowemoc, and two of the highest massiveness 

indices--96 percent for the Beaver Kill subbasin and 88 

percent for Willowemoc Creek (table 1).

Coates performed regression analysis of values 

for these two indices against 90-percent flow duration 

(Q90), expressed (1) as a percentage of mean annual 

discharge, and (2) adjusted to 1 (ft3

/s)/mi2, to test 

whether these two indices are related to certain lowflow statistics for each of the 13 Catskill basins. The 

results for all 13 Catskill basins indicated a moderately 

good correlation (r2

 = 0.76) between the sandstone 

index and Q90, and a similar correlation (r2

 = 0.79) 

between the “massiveness index” and Q90 (Coates, 

1971, table 6). Similarly, his regression analysis of 

precipitation against Q90 for the 13 Catskill basins 

shows a high correlation (r2

 = 0.90) for Q90 expressed 

as a percentage of mean annual discharge, and a 

slightly higher correlation (r2

 = 0.95) for Q90 adjusted 

to 1 ft3

/s/mi2. Coates’ regression analyses of Q90

against other geomorphologic factors for the 13 

Catskill basins showed relatively poor correlation 

coefficients, however. For example, a regression of 

Q90 against maximum basin relief yielded an r2 of 

only 0.49 for Q90 expressed as a percentage of mean 

annual discharge, and an r2 of 0.53 for Q90 adjusted to 

1 (ft3

/s)/mi2. The results of Coates’ study seem to 

indicate that the sustained high base flows in the 

Beaver Kill and Willowemoc subbasins are the result 

of considerable ground-water seepage, presumably 

from the jointed sandstone aquifer. 

The thick till that covers most of the upland 

parts of the Beaver Kill and Willowemoc subbasins 

might seem to be a potential source of base flow, but 

recent studies in New York and New England have 

shown that ground-water flow from thick till deposits 

does not significantly increase the base flow of streams 

in glaciated basins. Randall and Johnson (1987) 

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 of 

stratified drift, the area of 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 coefficient for the area covered 

by stratified drift was from 9 to 25 times larger than 

Hydrology
20 Hydrogeology of the Beaver Kill Basin, in Sullivan County, Delaware, and Ulster Counties, New York

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 coefficients indicate that the area of till within a 

basin has only minor effect on the contribution of 

ground water to stream base flow. 

A similar study by Wandle and Randall (1994) 

of the effects of surficial geology, lakes, swamps, and 

annual water availability on low flows of streams in 

central New England reached similar conclusions 

regarding the effect of till and stratified drift on stream 

base flow. The regression equations developed in that 

study to estimate 7Q10 for both high-relief and lowrelief areas of central New England indicated that 

ground-water discharge from coarse stratified drift is 4 

to 8 times greater than the discharge from till. These 

results provide additional evidence to suggest that the 

high sustained base flows within the Beaver Kill and 

Willowemoc Creek subbasins may be the result of 

ground-water discharge from the underlying fractured 

and jointed sandstone, not from the large areas of thick 

till that blanket the uplands of both basins.

Sub-basin runoff

A study of the entire Delaware River Basin by 

Hely and Olmsted (1963) presents runoff statistics for 

each of 23 Catskill Mountain streams under two 

separate summer low-flow conditions:

1. The amount of base flow from ground-water storage 

(Ra) for 30 days following a discharge equal to 

mean annual discharge (expressed in inches), and

2. The amount of base flow from ground-water 

storage (R1.0) for 30 days following a discharge 

equal to (1.0 ft3/s)/mi2. This discharge, for any of 

the streams studied, would be the numerical 

equivalent of the drainage area as measured at the 

gaging station, in cubic feet per second and 

expressed in inches. 

Hely and Olmsted plotted both values of base 

flow against the respective mean annual precipitation 

for each of the 23 Catskill streams (fig. 10). Best-fit 

regression lines through both sets of data indicate that 

base flow increases as mean annual precipitation 

increases, but at a much greater rate when calculated 

from Ra data than from R1.0 data. The base-flow value 

Ra increases rapidly as mean annual precipitation 

increases because high mean precipitation produces 

high mean runoff; therefore, the 30-day segment of the 

recession curve used to calculate the value (Ra) will be 

located relatively high on the recession curve. 

Therefore, the Ra values for each basin are a weaker 

indication of ground-water contributions during dry 

conditions than are R1.0 values. If the 23 basins have 

similar geologic characteristics, values of R1.0 for 

these streams should not vary with precipitation 

because areal variations in mean annual precipitation 

are independent of the physical characteristics of a 

given basin. The R1.0 data in figure 10 show a 

significant upward trend, however, which indicates 

that R1.0 does increase with increasing mean annual 

precipitation. Hely and Olmsted (1963) attributed this 

upward slope to local variations in geology, 

topography, and(or) ground-water evapotranspiration, 

and plotted winter values of R1.0 for these streams 

against mean annual precipitation. The resulting data 

showed essentially the same slope, suggesting that the 

relationship does not involve ground-water 

evapotranspiration, which is absent in winter, but only 

local differences in bedrock, surficial geology, and 

topography.

More importantly, figure 10 shows that values of 

both Ra and R1.0 for the Beaver Kill at Cragie Claire 

(01418500) plot above both trend lines, while the 

same two runoff values for Willowemoc Creek near 

Livingston Manor (01419500) plot consistently below 

both trend lines. This discrepancy indicates a large 

difference in the amount of ground-water runoff 

produced from each of these subbasins from virtually 

the same mean annual precipitation (51.8 in. for the 

Beaver Kill and 52 in. for Willowemoc Creek). For 

example, the Ra and R1.0 values for Beaver Kill at 

Craigie Claire (1.3 in. and 0.56 in., respectively), are 

significantly larger than the Ra and R1.0 values 

Willowemoc Creek (0.80 and 0.44 in., respectively). 

These data suggests that some geologic difference 

between the two subbasins is producing a much 

greater amount of ground-water runoff (base flow) in 

the Beaver Kill than in Willowemoc Creek. Moreover, 

the larger numerical difference between the two Ra

values (0.50 in.) than for R1.0 values (0.12 inches) 

suggests that the greatest difference in base flow 

between these two subbasins occurs during periods of 

average precipitation and average flow (Ra data), as 

opposed to dry conditions, represented by the R1.0

data. Although these data are not normalized to basin 

size, the two basins are of comparable in size--82 mi2

for the Beaver Kill basin at Craigie Claire and 63 mi2

for the Willowemoc Creek basin.

These data indicate that the Beaver Kill 

subbasin releases much more ground water than does 
21

the Willowemoc Creek subbasin during periods of 

average precipitation and flow, conditions, as well as 

during low-flow (dry) conditions, even though the 

Beaver Kill R1.0 data is closer to the trend line in fig. 

10 (i.e., normality with comparable basins) than that 

for the Willowemoc Creek subbasin.

Hydrogeologic differences between the two 

subbasins that might account for these differences in 

ground-water discharge may include:

1. Surficial geology--The Beaver Kill subbasin, which 

drains the northern rim of the Beaver Kill basin 

divide, contains much larger areas of thick 

upland ablation and lodgment till than does the 

Willowemoc Creek subbasin. An analysis of the 

digital data used to produce the surficial 

geologic map in figure 2 indicates that the 

Beaver Kill subbasin contains 14.3 mi2 of thicktill units (17.5 percent of the subbasin area), 

whereas the Willowemoc Creek subbasin 

contains only 6.1 mi2 of these units (9.8 percent 

of the subbasin area). 

2. Bedrock--An analysis of published bedrock maps 

(Fisher and others, 1970) indicates that the three 

bedrock formations that underlie these two 

subbasins are present in similar proportions. For 

example, the upper Walton Formation underlies 

approximately 47 percent of the Beaver Kill 

subbasin and 53 percent of the Willowemoc 

subbasin, and similarly, the Slide Mountain 

Formation underlies 38 percent of the Beaver Kill 

and 37 percent of the Willowemoc subbasins. The 

percentage of sandstone in the Beaver Kill 

subbasin, as measured in exposed rock faces, is a 

slightly higher (98 percent) than in the 

Willowemoc creek subbasin (91 percent) (Coates, 

1971), and the sandstone in the Beaver Kill 

subbasin is slightly more massive than that in the 

Willowemoc Creek subbasin (table 1). The more 

massive sandstone is characterized by longer, 

more continuous fractures and joints, which could 

allow for greater storage and more rapid release 

of ground water following recharge events. This 

greater storage and more rapid release is probably 

the main reason for the higher base flows in the 

Beaver Kill than in Willowemoc Creek.

1

1 1

1 1 1

1 1

1

1 1

1

1 11

1

111

1 1

1 1

RUNOFF FROM GROUND WATER STORAGE (BASE FLOW), IN INCHES

1.5

1.0

0.5

0

EXPLANATION

Runoff in 30 days following

mean discharge (Ra)

'

Streams used in this study

01418500 Beaver Kill at Cragie Claire

01419500 Willowemoc Creek near

Livingston Manor

01420500 Beaver Kill at Cooks Falls

1

1

Runoff in 30 days following

a discharge of 1.0 cubic foot

per second per square mile

01420500

01420500 01418500

01418500

01419500

01419500

(R1.0) Catskill Mountains (Ra)

Catskill Mountains (R1.0)

3 5 40 45 50 55 58

MEAN ANNUAL PRECIPITATION , IN INCHES, 1921-50

1

1

1

1

1 1

1 1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

Figure 10. Relations between summer baseflow (ground-water runoff from storage) and mean annual 

precipitation for 23 Catskill Mountain streams, studied in Hely and Olmsted (1963).

Hydrology
22 Hydrogeology of the Beaver Kill Basin, in Sullivan County, Delaware, and Ulster Counties, New York

SUMMARY

The Beaver Kill basin, which emcompasses the 

Beaver Kill and Willowemoc Creek subbasins, drains 

nearly 300 mi2

 of the southwestern Catskill 

Mountains. The most widespread surficial geologic 

unit, is till, being a direct result of ice-stagnation 

retreat. Large masses of ablation till occupy the main 

valleys of the Beaver Kill and Willowemoc Creek and 

locally reach thicknesses greater than 450 ft. These 

lateral embankments of till typically fill the bedrock 

valley such that the modern valley axis is laterally 

displaced as much as 1,000 ft from the preglacial 

(bedrock) valley axis. Ice-contact stratified drift in the 

form of kame moraines, kame deltas, and kame 

terraces are closely associated with, and interfinger 

with, deposits of ablation till. A veneer of coarse 

outwash sand and gravel overlies till on the floor of the 

Beaver Kill and Willowemoc Creek valleys. The 

outwash is of highly variable thickness; it is typically 

10 feet thick but can reach thicknesses of as much as 

100 ft locally. Saturated outwash and ice-contact 

stratified drift are the most productive ground-water 

source within the basin.

Mean annual precipitation within the Beaver 

Kill basin varies with elevation and ranges from 

42 in/yr at the downstream (western) end of the basin 

to 55 in/yr at the upstream (eastern) end. Runoff also 

varies with basin elevation and ranges from 24 in/yr at 

the western end of the basin to 40 in/yr at the extreme 

eastern end.

A 1971 study of hydrogeologic factors that 

affect streamflow in the Susquehanna River basin and 

in Catskill (Delaware basin) streams indicates that the 

Beaver Kill and Willowemoc Creek have the highest 

mean annual discharges of 13 Catskill streams studied. 

The low flow that is exceeded 90 percent of the time, 

normalized to basin area, (Q90) is 0.39 (ft3

/s)/mi2 for 

the Willowemoc Creek subbasin and 0.36 (ft3

/s)/mi2

for the Beaver Kill subbasin. These high values 

indicate that the dry-weather flow of both streams is 

sustained by ground water, discharged primarily from 

sandstone members of the underlying Catskill 

Formation, rather than from stratified drift (sand and 

gravel) which represents less than 5 percent of the area 

of both subbasins.

SELECTED REFERENCES

Coates, D.R., 1971, Hydrogeomorphology of Susquehanna 

and Delaware Basins, in Morisawa, Marie, 

Quantitative geomorphology - some aspects and 

applications: Proceedings of the Second Annual 

Geomorphology Symposia Series, Binghamton, N.Y., 

Oct. 15-16, 1971, State University of New York at 

Binghamton, p. 273-306.

Fisher, D.W., Isachsen, Y.W., and Rickard, L.V., 1970, 

Geologic map of New York: New York Museum and 

Science Service Map and Chart Series No. 15, 5 sheets, 

1:250,000 scale.

Fluhr, T.W., 1948a, Memorandum on the results of a 

preliminary test-boring program at a proposed damsite 

on the Beaver Kill near Craigie Clair, Sullivan County: 

Unpublished report, New York City Board of Water 

Supply, 6 p.

———, 1948b, Memorandum on the results of preliminary 

test-borings at the Roundtop damsite on Willowemoc 

Creek, Sullivan County: Unpublished report, New York 

City Board of Water Supply, 8 p.

———, 1949, Memorandum on the results of preliminary 

borings at a proposed damsite on the Beaver Kill near 

Jersey Brook: Unpublished report, New York City 

Board of Water Supply, 6 p.

———, 1953, Geology of New York City’s water supply 

system, a progress report: The Municipal Engineers 

Journal, v. 39, 4th quarter, p. 125-145.

Fluhr, T.W., and Terenzio, P.E., 1984, Engineering geology 

of the New York City water supply system: New York 

State Geological Survey Open-File Report 05.08.001, 

184 p. 

Gubitosa, Matthew, 1980, Glacial geology of the Hancock 

area, western Catskills, New York: State University of 

New York at Binghamton, Master’s thesis, 102 p.

Hely, A.G., and Olmsted, F.H., 1963, Some relations 

between streamflow characteristics and the 

environment in the Delaware River Region: U.S. 

Geological Survey Professional Paper 417B, 25 p.

Kipp, J.A., Lawrence, F.W., and Dinger, J.S., 1983, A 

conceptual model of ground-water flow in the Eastern 

Kentucky Coal Field, in Graves, D.H., (ed.), 

Symposium on Surface Mining, Hydrology, 

Sedimentology, and Reclamation: Lexington, Ky, 

University of Kentucky, Nov. 27-Dec. 2, 1983, 

p. 543-548.

Kirkland, J.T., 1973, Glacial geology of the western 

Catskills: State University of New York at Binghamton, 

Doctoral dissertation, 104 p.

Ozvath, D.L., 1985, Glacial geomorphology and late 

Wisconsinan deglaciation of the western Catskill 

Mountains, New York: State University of New York at 

Binghamton, Doctoral dissertation, 181 p.
23

Ozvath, D.L., and Coates, D.R., 1986, Woodfordian 

stratigraphy in the western Catskill mountains, in

Cadwell, D.H., The Wisconsinan stage of the First 

Geologic District, eastern New York: New York State 

Museum Bulletin No. 455, p. 109-120.

Parker, G.G., Hely, A.G., Keighton, W.B., Olmsted, F.H., 

and others, 1964, Water resources of the Delaware 

River basin: U.S. Geological Survey Professional 

Paper 381, 200 p.

Randall, A.D., 1996, Mean annual runoff, precipitation, and 

evapotranspiration in the glaciated northeastern United 

States, 1951-80: U.S. Geological Survey Open-File 

Report 96-395, 2 sheets, scale 1:1,000,000.

Randall, A.D., and Johnson, A.I., 1988, The northeast 

glacial aquifers RASA project--an overview of results 

through 1987, in Randall, A.D., and Johnson, A.I. 

(eds.), Regional Aquifer Systems of the United StatesThe Northeast Glacial Aquifers: American Water 

Resources Association Monograph Series, No. 11, 

p. 1-15.

Reed, G.W., and Fluhr, T.W., 1948, Memorandum on a 

preliminary investigation of damsites on the Beaver 

Kill in Sullivan County, vicinity of Craigie Claire: New 

York City Board of Water Supply, 6 p.

Rich, J.L., 1935, Glacial geology of the Catskills: New York 

State Museum Bulletin No. 299, 180 p.

Rickard, L.V., 1975, Correlation of the Siluvian and 

Devonian rocks in New York State: New York State 

Museum an Science Service Map and Chart Series, 

No. 24, 16 p.

Soren, Julian, 1961, The ground-water resources of Sullivan 

County, New York: New York State Department of 

Conservation Bulletin GW-46, 66 p.

———, 1963, The ground-water resources of Delaware 

County, New York: New York State Department of 

Conservation Bulletin GW-50, 59 p.

Wandle, S.W., and Randall, A.D., 1994, Effects of surficial 

geology, lakes and swamps, and annual water 

availability on low flows of streams in central New 

England, and their use in low-flow estimation: U.S. 

Geological Survey Water Resources Investigations 

Report 93-4092, 57 p.

Selected References