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
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2004-5134
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.
Citation: Reynolds, R.J., 2004,
Hydrogeology and Water Quality of the Pepacton Reservoir Watershed
in Southeastern New York. Part 2. Hydrogeology, Stream Base Flow, and Ground-water Recharge:
U.S. Geological Survey Scientific Investigations Report 2004-5134, 31 p.
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