National Water-Quality Assessment Program

Environmental Setting of the 

Lower Merced River Basin, California

Revised February, 2007

Scientific Investigations Report 2006–5152

U.S. Department of the Interior

U.S. Geological Survey
Front cover: U.S. Geological Survey scientists taking measurements at the ground water and surface water interaction 

study site in the Merced River on April 3, 2003. The two scientists in the foreground on the left are measuring the 

direction of ground-water flow. The scientist in the foreground on the right is measuring the temperature below the 

streambed. The two scientists in the background are measuring the stream discharge.

(Photograph by Joseph L. Domagalski, U.S. Geological Survey). 
Environmental Setting of the 

Lower Merced River Basin, California

By Jo Ann M. Gronberg and Charles R. Kratzer

National Water-Quality Assessment Program

Scientific Investigations Report 2006–5152

U.S. Department of the Interior

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

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2006

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

Box 25286, Denver Federal Center

Denver, CO 80225

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

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Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

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

reproduce any copyrighted materials contained within this report.

Suggested reference:

Gronberg, J.M., and Kratzer, C.R., 2006, Environmental setting of the lower Merced River Basin, 

California: U.S. Geological Survey Scientific Investigations Report 2006–5152, 27 p.
Foreword

The U.S. Geological Survey (USGS) is committed to providing the Nation with credible scientific information that 

helps to enhance and protect the overall quality of life and that facilitates effective management of water, biological, energy, and mineral resources (http://www.usgs.gov/). Information on the Nation’s water resources is critical to 

ensuring long-term availability of water that is safe for drinking and recreation and is suitable for industry, irrigation, 

and fish and wildlife. Population growth and increasing demands for water make the availability of that water, now 

measured in terms of quantity and quality, even more essential to the long-term sustainability of our communities and 

ecosystems.

The USGS implemented the National Water-Quality Assessment (NAWQA) Program in 1991 to support national, 

regional, State, and local information needs and decisions related to water-quality management and policy (http://

water.usgs.gov/nawqa). The NAWQA Program is designed to answer: What is the condition of our Nation’s streams 

and ground water? How are conditions changing over time? How do natural features and human activities affect 

the quality of streams and ground water, and where are those effects most pronounced? By combining information 

on water chemistry, physical characteristics, stream habitat, and aquatic life, the NAWQA Program aims to provide 

science-based insights for current and emerging water issues and priorities. From 1991-2001, the NAWQA Program 

completed interdisciplinary assessments and established a baseline understanding of water-quality conditions in 51 

of the Nation’s river basins and aquifers, referred to as Study Units (http://water.usgs.gov/nawqa/studyu.html). 

Multiple national and regional assessments are ongoing in the second decade (2001–2012) of the NAWQA Program 

as 42 of the 51 Study Units are reassessed. These assessments extend the findings in the Study Units by determining 

status and trends at sites that have been consistently monitored for more than a decade, and filling critical gaps in 

characterizing the quality of surface water and ground water. For example, increased emphasis has been placed on 

assessing the quality of source water and finished water associated with many of the Nation’s largest community 

water systems. During the second decade, NAWQA is addressing five national priority topics that build an understanding of how natural features and human activities affect water quality, and establish links between sources

of contaminants, the transport of those contaminants through the hydrologic system, and the potential effects of 

contaminants on humans and aquatic ecosystems. Included are topics on the fate of agricultural chemicals, effects of 

urbanization on stream ecosystems, bioaccumulation of mercury in stream ecosystems, effects of nutrient enrichment 

on aquatic ecosystems, and transport of contaminants to public-supply wells. These topical studies are conducted 

in those Study Units most affected by these issues; they comprise a set of multi-Study-Unit designs for systematic 

national assessment. In addition, national syntheses of information on pesticides, volatile organic compounds (VOCs), 

nutrients, selected trace elements, and aquatic ecology are continuing. 

The USGS aims to disseminate credible, timely, and relevant science information to address practical and effective 

water-resource management and strategies that protect and restore water quality. We hope this NAWQA publication 

will provide you with insights and information to meet your needs, and will foster increased citizen awareness and 

involvement in the protection and restoration of our Nation’s waters. 

The USGS recognizes that a national assessment by a single program cannot address all water-resource issues of 

interest. External coordination at all levels is critical for cost-effective management, regulation, and conservation 

of our Nation’s water resources. The NAWQA Program, therefore, depends on advice and information from other 

agencies—Federal, State, regional, interstate, Tribal, and local—as well as nongovernmental organizations, industry, 

academia, and other stakeholder groups. Your assistance and suggestions are greatly appreciated.

 Robert M. Hirsch, Associate Director for Water
This page intentionally left blank.
Contents

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

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

Description of the Lower Merced River Basin......................................................... 3

Physical and Cultural Features .................................................................. 3

Physiography....................................................................................... 3

Geology ............................................................................................. 4

Soils ................................................................................................ 6

General Land Use ................................................................................. 6

Agricultural Land Use ........................................................................... 7

Pesticide Use....................................................................................... 8

Fertilizer Use ....................................................................................... 11

Urban Land Use .................................................................................... 12

Climate ............................................................................................. 13

Hydrology ................................................................................................ 19

Water Availability ................................................................................. 19

Surface Water .................................................................................... 19

Ground Water .................................................................................... 22

Summary ................................................................................................... 25

References Cited.......................................................................................... 26

Figures

1. Location of the lower Merced River Basin within the San Joaquin–Tulare Basins 

study unit, Cali­fornia........................................................................... 2

2. Physiography of the lower Merced River Basin, California ................................. 3

3. Generalized geologic section and view of the Central Valley, California .................. 4

4. Geology of the lower Merced River Basin, California ....................................... 5

5. Soil texture of the lower Merced River Basin, California .................................... 6

6. General land-use categories in the lower Merced River Basin, California ............... 7

7. Detailed agricultural land use in the lower Merced River Basin, Califor­nia ............... 8

8. Irrigation systems in the lower Merced River Basin, California.............................. 9

9. Areas responding to questionnaire on farming practices in Mustang Creek 

Subbasin, California ........................................................................... 11

10. Distribution of population density, lower Merced River Basin, California, 2000............ 12

11. Population changes by county for Merced, Stanislaus, and Mariposa Counties,

California, 1900-2000........................................................................... 13

12. Isopleths of mean temperature in San Joaquin–Tulare Basins study unit, 

California, 1980–1997 ........................................................................ 14

13A. Mean daily temperatures by month, Modesto, California, 1939–2004 and 

2003–2004 .................................................................................... 15

v
13B. Mean daily temperatures by month, average from U.S. Geological Survey 

weather stations at Turlock Airport at Turlock, California and near Monte Vista 

Avenue near Montpelier, California, January 2003–January 2005 ..................... 15

14. Locations of weather stations, streamflow gages, subbasins, and major 

discharges and diversions in the lower Merced River Basin, California ............... 16

15. Mean annual precipitation in lower Merced River Basin, California, 

1980–1997 .................................................................................... 17

16A. Annual precipitation, Modesto, California, water years 1889–2004 ........................ 17

16B. Mean monthly precipitation, Modesto, California, water years 1889–2004 and 

2003–2004....................................................................................... 18

16C. Monthly precipitation, average of U.S. Geological Survey weather stations at

Turlock Airport at Turlock, California and near Monte Vista Avenue near 

Montpelier, California, January 2003-January 2005 ....................................... 18

17. Water-year hydrologic classification for the San Joaquin Basin, California, 

1901-2004 ....................................................................................... 19

18. Mean annual streamflow, Merced River near Stevinson, California, 

water years 1941–2004........................................................................ 21

19. Mean daily streamflow for water years 2003-2004, showing mean long-term

monthly streamflow for water years 1941–2004 and mean monthly streamflow 

for water years 2003–2004, Merced River near Stevinson, California .................. 22

20A. Mean daily streamflow, Mustang Creek at Monte Vista Avenue near 

Montpelier, California, November 25, 2003–March 6, 2004 and 

January 8, 2005–March 17, 2005............................................................ 23

20B. Continuous streamflow (15-minute), Mustang Creek at Monte Vista Avenue 

near Montpelier, California, February 17–29, 2004.......................................... 23

21. Generalized conceptual model of northern San Joaquin–Tulare Basin, 

California aquifer .............................................................................. 24

22. Water-table altitude and extent of the Corcoran Clay, lower Merced 

River Basin, California, spring 2000 ......................................................... 25

Tables

1. Summary of crops and animals in the lower Merced River Basin and the 

Mustang Creek Subbasin, California ...................................................... 9

2. Pesticide use in the lower Merced River Basin and the Mustang Creek 

Subbasin, California ........................................................................... 10

3. Summary of daily mean streamflow characteristics at selected streamflowgaging stations in the lower Merced River Basin, California, water years 

1975–2004....................................................................................... 20

vi
Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 4,047 square meter (m2)

acre 0.4047 hectare (ha)

acre 0.4047 square hectometer (hm2) 

acre 0.004047 square kilometer (km2)

square foot (ft2) 929.0 square centimeter (cm2)

square foot (ft2) 0.09290 square meter (m2)

square mile (mi2) 259.0 hectare (ha)

square mile (mi2) 2.590 square kilometer (km2) 

Volume

cubic foot (ft3) 28.32 cubic decimeter (dm3) 

cubic foot (ft3) 0.02832 cubic meter (m3) 

acre-foot (acre-ft) 1,233 cubic meter (m3)

acre-foot (acre-ft) 0.001233 cubic hectometer (hm3) 

Flow rate

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

Mass

pound, avoirdupois (lb) 0.4536 kilogram (kg) 

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

 °C=(°F-32)/1.8

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

(NAVD 88).

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

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

Conversion Factors

vii
Abbreviations and Acronyms

CDWR, California Department of Water Resources 

MID, Modesto Irrigation District

NLCD, National Land-Cover Dataset 

NAWQA, National Water-Quality Assessment (Program)

SSURGO, Soil Survey Geographic database

STATSGO, State Soil Geographic database for California

SWAT, Soil Water Assessment Tool

TID, Turlock Irrigation District 

USGS, U.S. Geological Survey

VAMP, Vernalis Adaptive Management Plan 

viii
Abstract 

In 1991, the U.S. Geological Survey began to study the 

effects of natural and anthropogenic influences on the quality 

of ground water, surface water, biology, and ecology as part 

of the National Water-Quality Assessment (NAWQA) Program. As part of this program, the San Joaquin–Tulare Basins 

study unit is assessing parts of the lower Merced River Basin, 

California. This report provides descriptions of natural and 

anthropogenic fea­tures of this basin as background information to assess the influence of these and other factors on water 

quality. The lower Merced River Basin, which encompasses 

the Mustang Creek Subbasin, gently slopes from the northeast 

to the southwest toward the San Joaquin River.

The arid to semiarid climate is characterized by hot summers (highs of mid 90 degrees Fahrenheit) and mild winters 

(lows of mid 30 degrees Fahrenheit). Annual precipitation 

is highly variable, with long periods of drought and above 

normal precipitation. Population is estimated at about 39,230 

for 2000. The watershed is predominately agricultural on the 

valley floor. Approximately 2.2 million pounds active ingredient of pesticides and an estimated 17.6 million pounds active 

ingredient of nitrogen and phosphorus fertilizer is applied 

annually to the agricultural land.

Introduction

In 1991, the U.S. Geological Survey began implementation of the National Water-Quality Assessment (NAWQA) 

Program in 59 study units across the Nation. The long-term 

goals of the NAWQA Program are to assess the status of the 

quality of freshwater streams and aquifers, to describe trends 

or changes in water quality over time, and to provide a sound 

understanding of the natural and human factors that affect the 

quality of these resources (Hirsch and others, 1988).

In 2001, the NAWQA Program began its second decade 

of intensive water-quality assess­ments in 42 of the original 

59 study units. The San Joaquin–Tulare Basins study unit 

was part of the first decadal cycle of NAWQA investigations 

and remains in the second cycle. The three long-term goals 

upon which NAWQA was based remain the foundation of the 

national assessment in the second cycle. During the second 

cycle, however, more emphasis is placed on status assessments 

of geographic regions that were not sampled during the first 

cycle and on analyzing selected constituents that were not 

analyzed during the first cycle. More emphasis is also placed 

on describing long-term trends and on understanding human 

and natural fac­tors that control water quality. Five priority topics are being studied by NAWQA to better understand factors 

controlling water quality. The activities of the second cycle 

(2001–2010) of NAWQA are described in Gronberg and 

others (2004).

A study on the sources, transport, and fate of agricultural 

chemicals is one of the five priority topics being addressed by 

NAWQA. The San Joaquin–Tulare Basins study unit is one of 

seven study units across the Nation participating in this study. 

The San Joaquin–Tulare Basins study is located within the 

lower Merced River Basin.

The San Joaquin–Tulare Basins study unit covers 

approximately 31,200 mi2 (square miles) in cen­tral California. The study unit includes the western slope of the Sierra 

Nevada to the east, the San Joaquin Valley, and the eastern 

slope of the Coast Ranges to the west (fig. 1). The study unit 

can be separated into the San Joaquin Basin to the north and 

the hydrologically closed Tulare Basin to the south. The lower 

Merced River Basin lies on the east side of the San Joaquin 

Valley in the San Joaquin Basin. Within the lower Merced 

River Basin (321 mi2), two areas were studied inten­sively in 

2003 and 2004: the Mustang Creek Subbasin (21 mi2), and a 

ground-water flow path located in an agricultural setting that 

terminates at the Merced River.

The Mustang Creek Subbasin is one of five significant 

subbasins in the lower Merced River Basin. It was chosen 

for the surface water component of this study because it is 

the least hydrologically manipulated of the subbasins that fit 

the size criteria of the national study. The ground-water flow 

path area was chosen for this study because it fit the national 

criteria for land use and it terminates in a section of river 

where ground water is generally flowing into the river instead 

of vice-versa.

Environmental Setting of the 

Lower Merced River Basin, California

By Jo Ann M. Gronberg and Charles R. Kratzer
Yosemite

National

Park

New Exchequer

Dam

Turlock

Lake Lake

McClure

Reservoir

McSwain

Dam

TID Main

Canal

!

!

!

!

!

!

!

! !

!

!

!

!

!

!

SAN BENITO CO

KERN CO

KINGS CO

TULARE CO

TULARE CO

FRESNO CO

FRESNO CO

FRESNO CO

MERCED CO

ALPINE CO

CALAVERAS CO

TUOLUMNE CO

SAN JOAQUIN CO

SACRAMENTO

CO

EL DORADO CO

AMADOR CO

STANISLAUS CO

MARIPOSA CO

MADERA CO

MADERA CO

South Fork Kern River

Kern River

Tule River

Kaweah River

Kings River

San Joaquin River

Chowchilla River

Cosumnes River

Mokelumne River

Calaveras River

Stanislaus River

Tuolumne River

Los

Banos

Creek

Merced River

Los Gatos Creek

Kern River

Visalia Hanford

Fresno Clovis

Madera

Merced

Atwater

Turlock

Modesto

Manteca

Lodi

Tulare

Porterville

Delano

Bakersfield

121

120

35

37

38

36

120

121

N

EXPLANATION

MUSTANG CREEK

SUBBASIN

LOWER MERCED RIVER 

BASIN (INCLUDING MUSTANG 

CREEK SUBBASIN)

SAN JOAQUIN-TULARE BASINS

0 20 40 80 MILES

0 20 40 80 KILOMETERS

San Joaquin

Valley

SacramentoSan Joaquin

Delta

SAN

JOAQUIN

BASIN

TULARE

BASIN

CALIFORNIA

SIERRA NEVADA

COAST RANGES

TEHACHAPI MTNS

Figure 1. Location of the lower Merced River Basin within the San Joaquin–Tulare Basins study unit, Cali­fornia.

TID, Turlock Irrigation District. 

 Environmental Setting of the Lower Merced River Basin, California
Gronberg and others (1998) described the natural and 

anthropogenic features of the San Joaquin–Tulare Basins 

and factors that affect water quality. Their report provided 

background information to describe and explain water-quality 

conditions and to link the study unit to the national program. 

This current report will focus on describing the lower Merced 

River Basin.

Description of the Lower Merced River 

Basin

The physical and geological setting, land use, agricultural 

chemical use, and climate of the lower Merced River Basin 

(fig. 1) will be described in this section.

Physical and Cultural Features

Land use in the lower Merced River Basin is predominately agricultural on the valley floor. In addition to the towns 

of Livingston, Delhi, Winton, and parts of Atwater, the river 

basin includes the Castle Airport, Aviation, and Development 

Center, formerly the Castle Air Force Base, which is now 

remodeled into an industrial park (fig. 2). The University of 

California, Merced campus (which opened in Fall 2005) is in 

Merced County, but outside the lower Merced River Basin. 

Yosemite National Park is located in the upper Merced River 

Basin in the Sierra Nevada (fig. 1).

Physiography

Most of the lower Merced River Basin lies within the flat 

structural basin of the San Joaquin Valley, on the east side of 

the San Joaquin River (fig. 1). The upstream side of the basin 

extends eastward into the lower foothills of the Sierra Nevada, 

and is defined by the downstream boundary of the watershed 

contributing to the Merced River below McSwain Dam. The 

San Joaquin Valley is bounded by the Sierra Nevada to the 

east, the Coast Ranges to the west, the Tehachapi Mountains to 

the south, and the Sacramento–San Joaquin Delta to the north 

(fig. 1). The boundary of the basin is defined by the topographic drainage divides and in some areas, by the canals and 

laterals that serve this area. Altitude ranges from 72 ft (feet) in 

Winton

Atwater

Livingston

Delhi

Castle Airport, Aviation,

and Development Center

12030

N

0 4 8 16 MILES

0 4 8 16 KILOMETERS

EXPLANATION

ALTITUDE IN METERS

SUBBASIN

HIGH: 547 (1,794 FEET)

LOW: 22 (72 FEET)

MUSTANG CREEK SUBBASIN

FLOWPATH

Highline Canal

Lateral 8

Lower

Stevinson

Lateral

Garibaldi

Lateral

Merced River

Mustang Creek

Merced River

Ingalsbe Slough

North Side

Canal

Dry Creek

Main Canal

Livingston

Canal

Sand Creek

Basin

12045

3730

MARIPOSA CO

STANISLAUS CO

MERCED CO

Figure 2. Physiography of the lower Merced River Basin, California.

Description of the Lower Merced River Basin 
the San Joaquin Valley to 1,794 ft above sea level in the Sierra 

Nevada foothills (fig. 2). Elevation gradients average about 

13 ft/mi (feet per mile) on the valley floor and 141 ft/mi in the 

foothills.

Mustang Creek Subbasin is located on the north side of 

the lower Merced River Basin. It is bounded on the northwest 

by the Sand Creek Basin, which siphons under Highline Canal 

and thus flows out of the lower Merced River Basin and is not 

shown as part of the basin, and on the southeast by the drainage divide for Dry Creek. Mustang Creek flows into Highline 

Canal before it flows into the Merced River. Mustang Creek 

Subbasin is gently sloping from the northeast to the southwest, 

with altitudes ranging from 160 ft to 339 ft above sea level. 

The subbasin is relatively flat near the creek outlet and hilly in 

the upper part of the subbasin.

The ground-water flow path, about 0.7 mi long, is located 

on the north side of the Merced River (fig. 2), about 16 river 

mi upstream of the confluence of the Merced River with the 

San Joaquin River (fig. 1). The flow path area is flat, with a 

relief of about 20 ft over a distance of 340 ft, sloping gently 

toward the river. The river channel cuts another 20 ft to the 

river bed, which is at an altitude of about 70 ft above sea level.

Geology

The San Joaquin Valley is part of the Central Valley, 

which is a large, northwest-trending, asymmetric structural 

trough, filled with marine and continental sediments (Bartow, 

1991) (fig. 3). To the east of the valley, the Sierra Nevada 

is composed primarily of pre-Tertiary granitic rocks and is 

separated from the valley by a foothill belt of marine and 

metavolcanic rocks. The Coast Ranges west of the valley are 

a complex assemblage of rocks, including marine and continental sediments of Creta­ceous to Quaternary age (Page, 1977, 

1986). 

Alluvial deposits of the eastern part of the valley were 

derived primarily from the weathering of granitic intrusive 

rocks of the Sierra Nevada, and consist of highly permeable, 

medium- to coarse-grained sands with low total organic carbon. The deposits form broad alluvial fans where the streams 

enter the valley. These deposits generally are coarsest near 

the upper parts of the alluvial fans and finest near the valley 

trough. Dune sand, derived from the alluvial deposits, consists 

of well-sorted medium- to-fine sand, as much as 140 ft thick 

(Page, 1986). Stream-channel deposits along the Merced River 

consist of coarse sand.

Consolidated rocks and deposits exposed along the margin of the valley floor include Tertiary and Quaternary continental deposits, Cretaceous and Tertiary marine sedimentary 

rocks, and the pre-Tertiary Sierra Nevada basement complex 

(Davis and Hall, 1959; Croft, 1972; Page and Balding, 1973). 

The majority of the unconsolidated deposits in the study area 

are contained within the Pliocene-Pleistocene Laguna (not 

mapped at the surface in the study area), Turlock Lake, Riverbank, and Modesto Formations, plus minor amounts of Holocene stream channel and flood-basin deposits. The Turlock 

Lake, Riverbank and Modesto Formations form a sequence of 

overlapping terrace and alluvial fan systems indicating cycles 

of alluviation, soil formation, and channel incision that were 

influenced by climatic fluctuations, and resultant glacial stages 

in the Sierra Nevada (Bartow, 1991) (fig. 4).

SIERRA NEVADA

CENTRAL

Marine sediments Crystaline rock

Alluvial fan

deposits

C

O A S

T

R

A

N G E S

Flood

overflow

Buena Vista Lake Bed

Flood-basin, lake, 

and marsh deposits

Tulare

Lake Bed VALLEY

PACIFIC OCEAN

San Joaquin River

Kings River

Figure 3. Generalized geologic section and view of the Central Valley, California (from Page, 1986).

 Environmental Setting of the Lower Merced River Basin, California
The Corcoran Clay, at the base of the upper Turlock Lake 

Formation, is a widespread lacustrine deposit that is a key subsurface feature in the San Joaquin Valley. Page (1986) mapped 

the areal extent of this regional aquitard on the basis of a 

limited number of well logs and geophysical logs. Additional 

lithologic data recently were used to modify the extent of this 

important unit in the study area (Burow and others, 2004). The 

eastern extent of the Corcoran Clay approximately parallels 

the San Joaquin River valley axis. The Corcoran Clay ranges 

in depth from 92 to 279 ft below land surface, and in thickness 

from 0 to 187 ft in the study area.

The Mehrten Formation is a key subsurface feature 

tapped by wells in the eastern part of the study area. The 

Mehrten Formation reflects a change in lithology and texture 

from overlying sed­iments of primarily unconsolidated coarsegrained sediments of arkosic composition to Mehrten Formation sediments of primarily consolidated sediments of 

volcanic-derived mafic materials (Davis and Hall, 1959).

Winton

Livingston

Delhi

12030

N

0 4 8 16 MILES

0 4 8 16 KILOMETERS

EXPLANATION

GEOLOGIC SYMBOL

DUNES

ALLUVIAL DEPOSITS AND DEDGE

STREAM CHANNEL DEPOSITS

MODESTO FORMATION

RIVERBANK FORMATION

TURLOCK FORMATION

MEHRTEN FORMATION

VALLEY SPRINGS FORMATION

IONE FORMATION

NONMARINE SEDIMENTARY

MARIPOSA FORMATION

METAVOLCANICS

MUSTANG CREEK SUBBASIN

Highline Canal

Lateral 8

Lower

Stevinson

Lateral

Garibaldi

Lateral

Merced River

Mustang Creek

Merced River

Ingalsbe Slough

North Side

Canal

Dry Creek

Main Canal

Livingston

Canal

Holocene

Pleistocene

Pliocene

Miocene

Eocene

Undivided

Pre-cretaceous Tertiary Quaternary

CENOZOIC MESOZOIC

Sand Creek

Basin

12045

3730

!

!

!

MARIPOSA CO

STANISLAUS CO

MERCED CO

Figure 4. Geology of the lower Merced River Basin, California (modified from California Division of Mines and Geology, 1966).

Description of the Lower Merced River Basin 
Soils

A soil texture map derived from the State Soil Geographic (STATSGO) database for California is shown in 

figure 5 (U.S. Department of Agriculture, Natural Resources 

Conservation Service, 1994). The finer silt loam soils are 

located in the upper reaches of the Dry Creek Basin. The 

coarser sandy loam and loam soils are located in the lower 

part of the basin with the loam soils predominantly along the 

Merced River and Dry Creek stream channels.

In the Mustang Creek Subbasin, more detailed investigation of the soils using the Soil Survey Geographic (SSURGO) 

database revealed information on the location of a soil hardpan 

(U.S. Department of Agriculture, 2005a, b). A hardpan layer 

exists close to the surface on the east side of the basin (Dina 

Saleh, U.S. Geological Survey, written commun., 2005). This 

was supported by field observations during a February 2004 

storm, where sheet flooding occurred on the east side of Mustang Creek. On the west side of the basin, the hardpan layer is 

much less prevalent; water was observed to move more slowly 

and was able to infiltrate the soil.

General Land Use

Land use was interpreted using the National Land-Cover 

Dataset (NLCD)(30-meter resolution), which is a product of 

the Multi-Resolution Land Characteristics Program (Vogelmann and others, 2001). The land cover was interpreted from 

Landsat Thematic Mapper data acquired between 1990 and 

1994, using a classification system modified from the Anderson land-use and land-cover classification. Land use for the 

lower Merced River Basin is shown in figure 6. Approximately 

55 percent of the lower Merced River Basin is covered by agricultural land, 39 percent is forest, shrubland, and grassland, 

over 4 percent is urban and transitional land, and less than 

2 percent is water (Vogelmann and others, 2001). The forest, 

shrubland, and grassland are predominantly in the foothills of 

the Sierra Nevada. Urban areas and agricultural land are predominantly on the valley floor. The Mustang Creek Subbasin 

is dominated by agricultural land use.

Winton

Livingston

Delhi

12030

N

0 4 8 16 MILES

0 4 8 16 KILOMETERS

EXPLANATION

SOIL TEXTURE

LOAM

SANDY LOAM

SILT LOAM

MUSTANG CREEK SUBBASIN

Highline Canal

Lateral 8

Lower

Stevinson

Lateral

Garibaldi

Lateral

Merced River

Mustang Creek

Merced River

Ingalsbe Slough

North Side

Canal

Dry Creek

Main Canal

Livingston

Canal

Sand Creek

Basin

12045

3730

!

!

!

MARIPOSA CO

STANISLAUS CO

MERCED CO

Figure 5. Soil texture of the lower Merced River Basin, California (U.S. Department of Agricul­ture, Natural Resources 

Conservation Service, 1994).

 Environmental Setting of the Lower Merced River Basin, California
Agricultural Land Use

About 27.7 million acres, more than one-quarter of the 

State of California, is used for agriculture. In 1999, California agriculture generated about $24.8 billion in cash receipts. 

Agricultural use accounts for about one-third (by weight) of 

all pesticides sold in California and about 43 percent of the 

total annual ground and surface water used in the state (Kuminoff and others, 2000). In the San Joaquin–Tulare Basins, 

agriculture accounts for about 74 percent of total water use, 

with 5 percent used by urban areas and 21 percent used by 

environmental water uses (defined by the California Department of Water Resources, CDWR, as wild and scenic river 

flows, instream flows, and water use in managed wetlands) 

(California Department of Water Resources, 1998).

The NLCD data are sufficient for describing the general 

land use and its distribution. Maps of agricultural land use on 

the valley floor from the CDWR are more detailed, providing 

crop (fig. 7) and irrigation information (California Department 

of Water Resources, 1997, 1999, 2003). In the lower Merced 

River Basin, the dominant agricultural land use is almond 

orchards (45 percent of agricultural land), followed by corn 

and grain (16 percent), and vineyards (12 percent) (table 1). 

In the Mustang Creek Subbasin, these percentages are 42, 20, 

and 20, respectively. The distri­bution of different irrigation 

methods used in the study area is shown in figure 8 (California 

Department of Water Resources, 1997, 1999, 2003). 

Winton

Livingston

Delhi

12030

N

0 4 8 16 MILES

0 4 8 16 KILOMETERS

EXPLANATION

LAND-USE CATEGORY

WATER

URBAN LAND

TRANSITIONAL LAND

FOREST

SHRUBLAND

GRASSLAND

AGRICULTURAL LAND

MUSTANG CREEK SUBBASIN

Highline Canal

Lateral 8

Lower

Stevinson

Lateral

Garibaldi

Lateral

Merced River

Mustang Creek

Merced River

Ingalsbe Slough

North Side

Canal

Dry Creek

Main Canal

Livingston

Canal

Sand Creek

Basin

12045

3730

!

!

!

MARIPOSA CO

STANISLAUS CO

MERCED CO

Figure 6. General land-use categories in the lower Merced River Basin, California (Vogelmann and others, 2001).

Description of the Lower Merced River Basin 
Pesticide Use

California’s pesticide use data is maintained by the 

California Department of Pesticide Regula­tion. Prior to 1990, 

the State of California required reporting of all applications 

of restricted-use pes­ticides and all pesticides applied by 

licensed pesticide applicators. Applications of nonrestricteduse pesticides by private farming operations were not reported. 

Since 1990, all pesticide applica­tions must be reported to the 

California Department of Pesticide Regulation. There are two 

levels of report­ing—urban applications are reported by month 

by county, and agricultural applications are reported daily by 

section according to the Public Land Survey System. 

In 2003, 285 different chemicals were applied to the 

lower Merced River Basin and the counties overlapping the 

basin. Pesticides applied for agricultural uses in this area 

totaled about 2.2 million pounds. The 50 most heavily applied 

pesticides, which account for approximately 99 percent (by 

weight) of the total pesticides used for agricultural pur­poses, 

are listed in table 2. In addition to the 2.2 million pounds, 

0.4 million pounds were applied to rights-of-way and reported 

at the county level (Cal­ifornia Department of Pesticide 

Regulation, 2004).

Winton

Livingston

Delhi

12030

N

0 4 8 16 MILES

0 4 8 16 KILOMETERS

EXPLANATION

LAND-USE CATEGORY

NATIVE VEGETATION

ALMONDS

VINEYARD

OTHER ORCHARDS

CORN AND GRAIN

ALFALFA

OTHER ROW CROPS

PASTURE

CONFINED ANIMAL-FEEDING OPERATION

URBAN

WATER

MUSTANG CREEK SUBBASIN

Highline Canal

Lateral 8

Lower

Stevinson

Lateral

Garibaldi

Lateral

Merced River

Mustang Creek

Merced River

Ingalsbe Slough

North Side

Canal

Dry Creek

Main Canal

Livingston

Canal

Sand Creek

Basin

12045

3730

!

!

!

MARIPOSA CO

STANISLAUS CO

MERCED CO

Figure 7. Detailed agricultural land use in the lower Merced River Basin, Califor­nia (from California Department of Water 

Resources, 1997, 1999, 2003).

 Environmental Setting of the Lower Merced River Basin, California
Activity

Lower Merced River Basin Mustang Creek Subbasin

Area

 (acres)

Percent of 

agricultural 

land

Percent of 

total basin 

area

Area

 (acres)

Percent of 

agricultural 

land

Percent of total 

basin area

ORCHARDS AND VINEYARDS

Almonds 50,883 45 25 5,436 42 41

Vineyards 13,057 12 6 2,614 20 20

Other 6,642 6 3 94 <1 <1

ROW CROPS

Corn and grain 16,267 16 8 2,641 20 20

Alfalfa 4,974 4 2 0 0 0

Other (includes idle) 9,620 9 5 786 6 6

ANIMALS

Pasture 7,650 7 4 1,164 9 9

Confined animal feeding operations 2,617 2 1 188 1 1

Table 1. Summary of crops and animals in the lower Merced River Basin and the Mustang Creek Subbasin, California.

[<, actual value is less than value shown]

Figure 8. Irrigation systems in the lower Merced River Basin, California (from California Depart­ment of Water Resources, 

1997, 1999, 2003).

Winton

Livingston

Delhi

12030

N

0 4 8 16 MILES

0 4 8 16 KILOMETERS

EXPLANATION

IRRIGATION SYSTEM

SPRINKLER

MICRO SPRINKLER 

DRIP IRRIGATION SYSTEM

SURFACE IRRIGATION SYSTEM

UNKOWN OR NOT MAPPED

NOT IRRIGATED

MUSTANG CREEK SUBBASIN

Highline Canal

Lateral 8

Lower

Stevinson

Lateral

Garibaldi

Lateral

Merced River

Mustang Creek

Merced River

Ingalsbe Slough

North Side

Canal

Dry Creek

Main Canal

Livingston

Canal

Sand Creek

Basin

12045

3730

!

!

!

MARIPOSA CO

STANISLAUS CO

MERCED CO

Description of the Lower Merced River Basin 
Table 2. Pesticide use in the lower Merced River Basin and the Mustang Creek Subbasin, California.

[Data from California Department of Pesticide Regulation, 2004. <, actual vaue less than value shown]

Chemical [in parenthesis:

F, fungicide; I, insecticide; H, herbicide] Crops treated

Lower Merced River Basin Mustang Creek Subbasin

Area of 

application

(acres)

Total mass

(pounds)

Area of 

application

(acres)

Total mass

(pounds)

Sulfur (F) vineyard, peach, almond, corn 19,869 873,017 4,024 139,439

Mineral Oil almond, peach 14,830 226,132 495 13,508

1,3 Dichloropro­pene (F) sweet potato, almond, peach 1,889 193,925 0 0

Methyl Bromide (H, I, F) nursery outdoor plants, almond, peach 5,010 112,966 29 88

Petroleum Oil (I) almond, peach 5,771 109,326 657 19,742

Glyphosate, Isopro­pylamine Salt (H) almond, vineyard, peach 54,594 91,356 7,377 14,667

Chloropicrin (H, I, F) nursery outdoor plants, sweet potato 507 54,394 0 0

Copper Sulfate (Basic) peach, almond, walnut 5,086 53,823 117 1,019

Copper Hydroxide almond, peach, walnut, vineyard 14,893 50,009 3,910 8,402

Ziram (F) almond, peach, vine­yard 12,629 49,716 1,479 5,637

Metam-Sodium (H, I, F) sweet potato, vineyard, strawberry 152 31,405 0 0

Propargite (I) almond, corn, vineyard 20,780 29,617 1,540 3,075

Chlorpyrifos (I) almond, vineyard, wal­nut, alfalfa, corn 17,690 24,449 4,702 8,175

Paraquat Dichloride (H) vineyard, almond, peach, alfalfa 22,527 22,035 3,644 3,552

Copper Sulfate (Pen­tahydrate) almond, peach 605 21,045 0 0

Captan (F) almond, nursery out­door plants 8,359 18,186 2,200 3,851

Maneb (F) almond, walnut 5,209 17,117 436 1,173

Lime-Sulfur (I, F) vineyard, almond, boy­senberry 1,333 12,280 0 0

Oryzalin (H) almond, peach 10,018 11,770 298 249

2,4-D, Dimethy­lamine Salt (H) almond, peach, walnut, vine­yard 16,084 11,065 1,713 1,074

Simazine (H) almond, vineyard, peach, walnut 24,568 10,690 2,949 1,587

Trifluralin (H) almond, alfalfa 8,232 10,137 1,604 1,221

Oxyfluorfen (H) almond, vineyard, walnut, peach 34,751 9,844 4,112 1,598

Iprodione (F) almond, peach 20,074 9,050 3,138 1,596

Copper Oxide (OUS) (F, I) almond, walnut, peach 2,471 7,747 21 121

Glyphosate (H) almond, corn, peach 2,801 6,967 115 68

Cyprodinil (F) almond, vineyard, peach 26,980 6,365 1,783 357

MCPA, Dimethylamine Salt (H) oat, wheat 7,933 5,207 1,562 926

Thiophanate-Methyl (F) almond, peach, nursery outdoor plants 7,220 4,650 1,961 1,367

Norflurazon (H) almond, peach, alfalfa, vineyard, walnut 8,489 4,630 698 366

Cryolite (I) peach 316 3,580 0 0

Diuron (H) alfalfa, vineyard, walnut 2,710 3,411 0 0

Glyphosate, Monoammonium Salt (H) almond, walnut 3,330 2,624 1,007 637

Dicloran (F) vineyard 1,628 2,590 0 0

Pendimethalin (H) almond, walnut, bean 3,747 2,564 656 507

Permethrin (I) almond, peach, corn 14,134 2,434 3,249 650

Tebufenozide (I) almond, vineyard, walnut 12,513 2,381 2,574 448

Glyphosate, Diammonium Salt (H) almond, walnut, apple, corn 2,997 2,154 181 256

Phosmet (I) apple, walnut, peach, almond 621 2,061 30 119

Methidathion (I) peach, almond, apple 1,581 2,033 119 101

Potash Soap (H, I) vineyard 587 1,911 393 1,499

Dimethoate (I) corn, vineyard, bean 4,595 1,728 433 183

Azoxystrobin (F) almond, vineyard, peach 17,323 1,642 1,331 306

Diazinon (I) peach, almond, sweet potato, strawberry 899 1,550 2 <1

(S)-Metolachlor (H) corn, bean 1,186 1,508 556 617

Malathion (I) walnut, alfalfa, sweet potato 540 1,279 0 0

Fenhexamid (F) vineyard 1,845 1,221 0 0

Diglycolamine Salt of 3,6-Dichloro-oanisic Acid (H)corn, wheat, oat, barley, pasture 4,616 1,131 1,265 229

Fenbutatin-oxide (I) almond, peach, walnut 4,104 996 403 64

Mancozeb (F) vineyard, apple 461 957 5 4

10 Environmental Setting of the Lower Merced River Basin, California
Fertilizer Use

Fertilizer application data are not maintained in any 

database. Fertilizer sales are sometimes used as a surrogate 

for fertilizer application. Distribution to the county level may 

not accu­rately reflect actual use, however, because fertilizer sales recorded in one county may be used in another. In 

addition, recorded sales include commercial landscape and 

other non-agricultural uses. Ruddy and others (2006) used 

Census of Agriculture farm fertilizer expenditures to allocate 

farm fertilizer use to the county level. In this study, fertil­izer 

application for the lower Merced River Basin was estimated 

from crop acreage (Califor­nia Department of Water Resources, 

1997, 1999, 2003). Application rates were determined primarily from local reports and county experts, and in some cases, 

from ques­tionnaires on farming practices. Nitrogen application 

was estimated to be 16.1 million lb/yr (pounds per year), and 

phosphorus application was estimated to be 0.94 million lb/yr. 

These estimates may be high because the effect of organic 

farming, while recognized, was not quantified.

Nutrients from manure were estimated from the 2002 

Census of Agriculture data (U.S. Department of Agriculture, 

2004), which provided the number of animals by county. 

Nutrient content of manure was determined using methods 

established by Ruddy and others (2006). The percent­age of 

agricultural land within the basin for each county was used to 

estimate the percent­age of nutrients within the basin. Nitrogen 

from manure was estimated to be 13.0 million lb/yr, and 

phosphorus was estimated to be 2.9 million lb/yr.

Estimates of nutrients from fertilizer use in the Mustang Creek Subbasin were made using data collected from 

questionnaires sent to farmers in the area (fig. 9). Application amounts were calcu­lated from the cropped area and the 

application rates provided by farmers. When information on 

application rates was not available, estimates were derived 

using information from farming practices at nearby farms, 

Winton

Livingston

Delhi

12030

N

0 4 8 16 MILES

0 4 8 16 KILOMETERS

EXPLANATION

REPLY

NOT SENT

NO REPLY

REPLIED

MUSTANG CREEK SUBBASIN

0 1 2 MILES

0 1 2 KILOMETERS

Highline Canal

Lateral 8

Lower

Stevinson

Lateral

Garibaldi

Lateral

Merced River

Mustang Creek

Merced River

Ingalsbe Slough

North Side

Canal

Dry Creek

Main Canal

Livingston

Canal

Sand Creek

Basin

12045

3730

!

!

!

MARIPOSA CO

STANISLAUS CO

MERCED CO

Figure 9. Areas responding to questionnaire on farming practices in Mustang Creek Subbasin, California.

Description of the Lower Merced River Basin 11
local reports, and county experts. Nitrogen application was 

estimated to be 1.4 million lb/yr, and phosphorus application 

was estimated to be 0.07 million lb/yr. 

Estimates of nutrients from manure in the Mustang Creek 

Subbasin were calculated from the number of animals present 

(deter­mined from farm surveys when possible or estimated 

from building capacity when animal numbers were not available) and from the nutrient content of the manure (from Ruddy 

and others, 2006). Nitrogen from manure was estimated to be 

0.73 million lb/yr, and phosphorus from manure was estimated 

to be 0.11 million lb/yr. 

Urban Land Use

Population in the lower Merced River Basin was estimated to be 39,230 at the time of the 2000 Census (U.S. 

Bureau of the Census, 2001). This represents a 19-percent 

increase in population since the 1990 Census. The areas of 

highest population density are the communities of Delhi 

(8,022), Livingston (10,473), and Winton (8,832). The city 

centers of Hilmar (4,807; southwest of Delhi), and Atwater 

(23,113; south of Winton) lie outside the basin, but the higher 

density populations associated with these towns are along the 

edges of the basin (fig. 10). These higher 

density areas are located on the valley floor. The Sierra 

Nevada foothills adjacent to the valley are sparsely populated. 

The majority of the lower Merced River Basin (85 percent) 

lies within Merced County, with smaller parts within Stanislaus and Mariposa Counties. The overall increase in population between 1900 and 2000 for Merced, Stanislaus, and 

Mariposa Counties is shown in figure 11. The majority of the 

population in these three counties is outside of the lower 

Merced River Basin, however.

Winton

Livingston

Delhi

12030

N

0 4 8 16 MILES

0 4 8 16 KILOMETERS

EXPLANATION

PEOPLE PER SQUARE MILE

< 130

130 – 1,295 

1,295 – 6,475

6,475 – 12,950

> 12,950

MUSTANG CREEK SUBBASIN

Highline Canal

Lateral 8

Lower

Stevinson

Lateral

Garibaldi

Lateral

Merced River

Mustang Creek

Merced River

Ingalsbe Slough

North Side

Canal

Dry Creek

Main Canal

Livingston

Canal

Sand Creek

Basin

12045

3730

!

!

!

MARIPOSA CO

STANISLAUS CO

MERCED CO

Figure 10. Distribution of population density, lower Merced River Basin, California, 2000 (U.S. Bureau of the Census, 2001).

12  Environmental Setting of the Lower Merced River Basin, California
Climate

The San Joaquin Valley has an arid to semiarid climate 

that is characterized by hot summers and mild winters. Average temperatures are fairly uniform over the valley floor. Temperature decreases with increasing altitude in the foothills and 

mountains of the Sierra Nevada (fig. 12). Long-term records 

for temperature do not exist for sites within the lower Merced 

River Basin; however, the Modesto Irrigation District (MID) 

has temperature data for downtown Modesto (fig. 1) from 

1939 to 2004 (Modesto Irrigation District, 2005) (fig. 13A). 

Mean low temperatures in degrees Fahrenheit (F) range from 

the mid 30s in the winter to the upper 50s in the summer. 

Mean high temperatures in degrees F range from the mid 50s 

in the winter to the mid 90s in the summer. USGS weather stations installed for this study at Turlock Airport and near Monte 

Vista Avenue, located within the Mustang Creek Subbasin, 

have a recent but much shorter data record (figs. 13B and 14). 

The data for the Mustang Creek Subbasin and Modesto are 

very similar for 2003–2004, and show very little deviation 

from the long-term record.

The eastern slopes of the Coast Ranges and the valley 

are in the rain shadow of the Coast Ranges. Warm, moist air 

masses from the Pacific Ocean are forced aloft by the Sierra 

Nevada. The air masses cool, and the moisture condenses, 

resulting in heavy precipitation on the western slopes (Gronberg and others, 1998). In general, precipitation is less on the 

valley floor and increases with elevation in the foothills and 

mountains of the Sierra Nevada (fig. 15). No long-term records 

of precipitation are available for the lower Merced River 

Basin. The MID does have a long-term precipitation record for 

Modesto from water years 1889 to 2004, however (fig. 16A).

Mean annual precipitation (1889–2004) in Modesto is 

about 12.2 inches, but annual precipi­tation is highly variable. 

Seasonal distribution of precipitation is shown by the mean 

monthly pre­cipitation for 1889–2004 (fig. 16B). Eighty percent of the precipitation falls from November through March, 

with maximum precipitation in December through March. 

The recent (2003–2004) mean monthly data show significant 

deviations from the long-term seasonal pattern. December 

2003 was wetter and January 2004 was drier than the usual 

long-term pattern, however, the majority of the precipitation still fell during the winter months. Data recorded at the 

Turlock Airport and near Monte Vista Avenue weather stations 

also show this pattern (fig. 16C).

Figure 11. Population changes by county for Merced, Stanislaus, and Mariposa Counties, California, 1900-2000 (from 

Forstall, 1995, and U.S. Bureau of the Census, 2001).

0

100,000

50,000

0

150,000

200,000

250,000

300,000

350,000

400,000

450,000

500,000

1890 1920 1950 1980 2010

YEAR

POPULATION

MERCED COUNTY

STANISLAUS COUNTY

MARIPOSA COUNTY

Description of the Lower Merced River Basin 13
!

!

!

!

!

!

!

! !

!

!

!

!

!

!

SAN BENITO CO

KERN CO

KINGS CO

KINGS CO

TULARE CO

TULARE CO

FRESNO CO

FRESNO CO

FRESNO CO

FRESNO CO

MERCED CO

ALPINE CO

CALAVERAS CO

TUOLUMNE CO

SAN JOAQUIN CO

SACRAMENTO

CO

EL DORADO CO

AMADOR CO

STANISLAUS CO

MARIPOSA CO

MADERA CO

MADERA CO

Visalia Hanford

Fresno Clovis

Madera

Merced

Atwater

Turlock

Modesto

Manteca

Lodi

Tulare

Porterville

Delano

Bakersfield

121

120

35

35

36

37

37

38

38

36

119

119

120

121

N

EXPLANATION

TEMPERATURE, IN DEGREES FAHRENHEIT

< 30

30 – 40 

40 – 50

50 – 60

> 60

LOWER MERCED RIVER BASIN AND 

MUSTANG CREEK SUBBASIN

0 20 40 80 MILES

0 20 40 80 KILOMETERS

Figure 12. Isopleths of mean temperature in San Joaquin–Tulare Basins study unit, California, 1980–1997 (from National 

Center for Atmospheric Research, 2003).

14  Environmental Setting of the Lower Merced River Basin, California
A.

10

0

20

30

40

50

60

70

80

90

100

JAN. FEB. MAR. APR. MAY JUNE JULY AUG. SEPT. OCT. NOV. DEC.

1939-2004

MEAN DAILY TEMPERATURE, 

IN DEGREES FAHRENHEIT 

MEAN LOW, 1939-2004

EXPLANATION

MEAN DAILY, 1939-2004

MEAN HIGH, 1939-2004

MEAN LOW, 2003-2004

MEAN DAILY, 2003-2004

MEAN HIGH, 2003-2004

10

0

20

30

40

50

60

70

80

90

100

MEAN DAILY TEMPERATURE,

INDEGREES FAHRENHEIT 

DATA

MISSING

DATA

MISSING

B.

2003 2004 2005

AVERAGE LOW

AVERAGE DAILY

AVERAGE HIGH

JAN. MAR. MAY JULY SEPT. NOV. JAN. MAR. MAY JULY SEPT. NOV. JAN.

Figure 13. Mean daily temperatures, by month, for Modesto and Turlock areas, California. A. Modesto, water years 

1939–2004 (from Modesto Irrigation District, 2005).

Figure 13. Continued. B. Average from U.S. Geological Survey weather stations at Turlock Airport at Turlock, California and near 

Monte Vista Avenue near Montpelier, California, January 2003–January 2005.

Description of the Lower Merced River Basin 15
Figure 14. Locations of subbasins, streamflow gages, major discharges and diversions, and weather stations in the lower Merced River 

Basin, California.

Winton

Livingston

Delhi

N

0 4 8 16 MILES

0 4 8 16 KILOMETERS

EXPLANATION

DRY CREEK SUBBASIN

MUSTANG CREEK SUBBASIN

HIGHLINE CANAL SUBBASIN

(INCLUDES MUSTANG CREEK)

LIVINGSTON CANAL SUBBASIN

LOWER STEVINSON LATERAL SUBBASIN

LOWER MERCED RIVER BASIN (INCLUDES ALL)

STREAMFLOW GAGES

A MERCED RIVER BELOW MERCED FALLS DAM

 B MERCED RIVER BELOW SNELLING

 C MERCED RIVER AT CRESSEY

 D MERCED RIVER NEAR STEVINSON

 E MUSTANG CREEK AT MONTE VISTA AVENUE

 F MUSTANG CREEK AT BIFURCATION STRUCTURE

MAJOR DISCHARGES AND DIVERSIONS

1 MERCED IRRIGATION DISTRICT NORTH SIDE CANAL

 2 MERCED IRRIGATION DISTRICT MAIN CANAL

 3 INGALSBE SLOUGH

 4 DRY CREEK

 5 LIVINGSTON CANAL

 6 GARIBALDI LATERAL

 7 HIGHLINE CANAL

 8 LOWER STEVINSON LATERAL

WEATHER STATION

#

X

Highline Canal

Lateral 8

Lower

Stevinson

Lateral

Garibaldi

Lateral

Merced River

Mustang Creek

Merced River

Ingalsbe Slough

North Side

Canal

Dry Creek

Main Canal

Livingston

Canal

Sand Creek

Basin

CrockerHuffman

Dam

Merced

Falls Dam

McSwain

Dam

12045

12030

3730

!

!

!

MARIPOSA CO

STANISLAUS CO

MERCED CO

(

(

(

(

(

(

X X

# # #

#

# #

#

#

8

7 6 5

4

3 2 1

B A

D C

E

F

16  Environmental Setting of the Lower Merced River Basin, California
Figure 15. Mean annual precipitation in lower Merced River Basin, California, 1980–1997 (from DAYMET program; National Center for 

Atmospheric Research, 2003).

0

5

10

15

20

25

30

ANNUAL PRECIPITATION, IN INCHES

1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

WATER YEARS 1889-2004

Modesto Irrigation District Annual Precipitation

Mean Annual Precipitation, 1889-2004 (12.18 inches)

Figure 16A. Annual precipitation, Modesto, California, water years 1889–2004 (from Modesto Irrigation District, 2005).

Description of the Lower Merced River Basin 17

Winton

Livingston

Delhi

12030

N

0 4 8 16 MILES

0 4 8 16 KILOMETERS

Highline Canal

Lateral 8

Lower

Stevinson

Lateral

Garibaldi

Lateral

Merced River

Mustang Creek

Merced River

Ingalsbe Slough

North Side

Canal

Dry Creek

Main Canal

Livingston

Canal

EXPLANATION

ANNUAL PRECIPITATION, IN INCHES

0 – 11

11 – 12

12 – 13

13 – 14

14 – 15

15 – 16

16 – 17

17 – 18

18 – 19

19 – 20

20 – 21

21 – 22

Sand Creek

Basin

12045

3730

!

!

!

MARIPOSA CO

STANISLAUS CO

MERCED CO
Figure 16B. Mean monthly precipitation, Modesto, California, water years 1889–2004 and 2003–2004 (from Modesto Irrigation District, 

2005).

Figure 16C. Monthly precipitation, average of U.S. Geological Survey weather stations at Turlock Airport at Turlock, California and near 

Monte Vista Avenue near Montpelier, California, January 2003-January 2005.

.0

.5

1

1.5

2

2.5

3

3.5

4

OCT. NOV. DEC. JAN. FEB. MAR. APR. MAY JUNE JULY AUG. SEPT.

WATER YEARS 1889-2004, 2003-2004

MEAN MONTHLY PRECIPITATION, IN INCHES 

MEAN MONTHLY, WY 1889-2004

MEAN MONTHLY, WY 2003-2004

18  Environmental Setting of the Lower Merced River Basin, California

0

1

2

3

4

5

MONTHLY PRECIPITATION, IN INCHES 

DATA MISSING

2003 2004 2005

DATA MISSING

JAN. MAR. MAY JULY SEPT. NOV. JAN. MAR. MAY JULY SEPT. NOV. JAN.
Hydrology

The hydrologic setting of the lower Merced River Basin 

is described in this section in terms of water availability, 

surface water, and ground water. Water availability is used to 

illustrate the long-term variability in climate and streamflow in 

the entire San Joaquin Basin.

Water Availability

Water availability in the San Joaquin Basin can be characterized by the water-year classifica­tion system used by the 

State of California for water allocation and regulation (fig. 17). 

The index used for the basin is known as the 60-20-20 

water-year index (California Department of Water Resources, 

2004a). Sixty percent of the forecasted unimpaired flow from 

April through July, 20 percent of the forecasted unimpaired 

flow from October through March, and 20 percent of the previous water year’s index (with a cap) are summed. From wet 

to dry conditions, the water years are classified as wet, above 

normal, below normal, dry, or critical. The classifications for 

1901–2004 are shown in figure 17. This record shows high 

variability with periods of wetter con­ditions alternating with 

periods of drier conditions.

From 1901–1922, overall conditions were above normal, 

with 11 wet years, 5 above nor­mal, 4 below normal, 1 dry, 

and 1 critical year. Following that period, from 1923–1934, 

conditions were on the drier side, with 3 years above normal, 

2 below normal, 2 dry, and 5 critical years. From 1935–1946, 

5 wet and 5 above normal years dominated the 1 below normal 

and 1 dry year. From 1947–1961, 6 years below normal, 3 dry, 

and 2 critical years dominated the 1 above normal and 3 wet 

years. From 1962–1977, 5 wet and 3 above normal years balanced the 3 below normal, 3 dry, and 2 critical years over the 

period. The drought of 1976–1977 was followed by a 9-year 

period, 1978–1986, dominated by wetter conditions, including 5 wet, 2 above normal, and 2 dry. The 1987–1994 period 

had 1 wet year and 7 critical years, including the longest 

continuous dry period from 1987–1992. This was followed by 

6 continuous years of above normal and wet conditions from 

1995–2000, and 4 years of below normal and dry conditions in 

2001–2004.

Surface Water

The San Joaquin River receives water from tributaries 

draining the Sierra Nevada and Coast Ranges. The water quality of the San Joaquin River is of critical interest because it 

flows into the Sacramento–San Joaquin Delta, which supplies 

drinking water for southern California, as well as irrigation 

water for the western San Joaquin Valley. The Merced River is 

one of the main east-side tribu­taries, originating in the Sierra 

Nevada and flowing to the San Joaquin River (fig. 1).

Figure 17. Water-year hydrologic classification for the San Joaquin Basin, California, 1901–2004 (California Department of Water 

Resources, 2004b).

1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 0

2

4

6

8

Wet

Dry

Above Normal

Below Normal

Critical

INDEX

WATER YEAR INDEX, IN MILLION ACRE-FEET

Hydrology 19
The surface-water hydrology of the Merced River Basin 

has been significantly modified by the development of water 

resources. From the 1870s to the early 1900s, miles of canals 

were constructed to convey water to the land. Exchequer Dam 

was completed in 1926 to provide flood control and water 

for irrigation and power generation. In 1967, New Exchequer Dam was completed to expand Lake McClure Reservoir capacity to about 1 million acre-feet. In the same year, 

McSwain Dam was completed downstream as a regulating reservoir (fig. 1). Down­stream of the McSwain Dam, the Merced 

Falls Dam diverts flow into the Merced Irrigation District’s 

North Side Canal to pro­vide irrigation water to areas north 

of the Merced River. Farther downstream, Crocker–Huffman 

Dam diverts flow into the Merced Irrigation District’s Main 

Canal. Five major irrigation and drainage canals discharge to 

the Merced River below New Exchequer Dam (fig. 14). The 

Merced Irrigation District is responsible for three of these 

discharges: North Side Canal, originating from McSwain 

Dam, discharges upstream of Cressey through Ingalsbe 

Slough; Livingston Canal discharges upstream from Highway 99; and Garibaldi Lateral discharges downstream from 

Highway 99. Turlock Irrigation District (TID) is responsible 

for the other two canal discharges: Highline Canal discharges 

just downstream from the flow-path study area; and Lower 

Stevinson Lateral discharges just upstream of the Merced 

River near Stevinson gaging station (Stillwater Sciences and 

EDAW, 2001).

The area of focus for this study, the lower Merced River 

Basin, starts at New Exchequer Dam. Water quality above this 

point, for the most part, is unaffected by agricultural activities. The lower Merced River receives water from Dry Creek, 

and Mustang Creek by way of Highline Canal. Highline Canal 

(constructed in 1911) diverts water from the Tuolumne River 

by way of the TID Main Canal about 6 miles below Turlock 

Lake (see fig. 1), bringing irrigation water to farms in the 

Delhi area. It eventually turns into Lateral 8 and then into 

Lower Stevinson Lateral (Turlock Irrigation District, 2004). 

Sand Creek, north of Mustang Creek, flows under Highline 

Canal by way of a siphon and thus out of the lower Merced 

River Basin.

The percentage of days that the specified streamflows 

were equaled or exceeded during water years 1975–2004 for 

four gaging stations within the lower Merced River Basin is 

shown in table 3. These stations, in downstream order, are the 

Merced River below Merced Falls Dam, Merced River below 

Snelling, Merced River at Cressey, and Merced River near 

Stevinson (fig. 14). The variability and amount of streamflow 

are also shown in the table. The Merced Irrigation District 

Main Canal diversion is between the Merced Falls and the 

Snelling stations (fig. 14). The three sta­tions below this diversion, Merced River below Snelling, Merced River at Cressey, 

and Merced River near Stevinson, have similar streamflow, 

which is generally much less than streamflow at the Merced 

Falls site.

Gaging station

(see fig. 14 for locations) Station number

Drainage 

area

(square 

Percentage of days that daily mean streamflow was greater than or equal to 

value shown, in cubic feet per second

99 95 90 75 50 25 10 5 1

Merced River below Merced 

Falls Dam

11270900 (USGS) 1,061 88 159 189 256 1,140 1,940 2,840 3,750 6,140

Merced River below Snelling B05170 (CDWR) NA 34 76 106 154 213 486 1,950 3,106 5,272

Merced River at Cressey B05155 (CDWR) NA 9 42 74 140 221 539 2,010 3,205 5,300

Merced River near Stevinson 11272500 (USGS); 

B05125 (CDWR)

1,273 10 37 88 175 267 623 2,010 3,200 5,370

Table 3. Summary of daily mean streamflow characteristics at selected streamflow-gaging stations in the lower Merced River Basin, 

California, water years 1975–2004.

[Data from California Department of Water Resources, 2006. USGS, U.S. Geological Survey; CDWR, California Department of Water Resources; NA, not 

available] 

20 Environmental Setting of the Lower Merced River Basin, California
Another pattern illustrated in table 3 is the overall amount 

of streamflow from upstream stations to downstream stations. 

In more natural basins, the usual trend is to see higher streamflows downstream as the area of contribution to the river 

increases. The Merced River, which is highly engineered and 

utilized for agricultural irrigation, shows an over­all decrease in 

streamflow from the upper basin to the mouth.

Mean annual streamflow measured at the Merced River 

near Stevinson station is about 686 ft3/s (cubic feet per second). Mean annual streamflow for water years 1941–2004 

varies greatly from year to year. Water years 2003–2004 had 

below normal streamflow (fig. 18). Mean daily streamflow 

for Merced River near Stevinson (fig. 19) during water years 

2003–2004 was highest in May. This was due to relatively 

large reservoir releases in May as part of the Vernalis Adaptive Management Plan (VAMP). The objective of these May 

releases was to help move salmon smolt out to the Sacramento–San Joaquin Delta. The objective of smaller VAMP 

releases in October was to attract spawning salmon to the 

tributaries. The average monthly streamflows for water years 

2003–2004 were substantially lower than the average monthly 

streamflows in the long-term record.

The water year hydrologic classification (fig. 17) closely 

resembles the pattern of the annual streamflow at Merced 

River near Stevinson (fig. 18) and the pattern of precipitation 

at Modesto (fig. 16A). Periods of high and low streamflow and 

precipitation correspond to high and low periods shown on 

the water-year hydrologic classification graph (fig. 17). Mean 

annual streamflows for water years 2003 and 2004 were low 

compared with the mean annual streamflow for the 1941–2004 

period. This is also reflected in the hydrologic clas­sification 

graph (fig. 17). The precipitation graph shows that water 

years 2003 and 2004 were below average for the valley floor 

(fig. 16A).

Mustang Creek at Monte Vista Avenue is usually dry, 

except during prolonged winter storms (fig. 20A). Although 

not visible in figure 20A, the creek is essentially dry all summer, as the irrigation systems (see fig. 8) in the Mustang Creek 

Basin do not create significant runoff. Mustang Creek is flashy 

in reponse to storms, as during a storm on February 23, 2004 

that produced a peak streamflow of 207 ft3/s (fig. 20B). The 

flows in Mustang Creek are being modeled for this study using 

the Soil Water Assessment Tool (SWAT) model (Arnold and 

others, 1998).

1,000

0

2,000

3,000

4,000

1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

WATER YEAR

MEAN ANNUAL STREAMFLOW, IN CUBIC FEET PER SECOND

U.S. GEOLOGICAL SURVEY

CALIFORNIA DEPARTMENT OF WATER RESOURCES

MEAN ANNUAL STREAMFLOW, 1941–2004 (686 cubic feet per second)

Figure 18. Mean annual streamflow, Merced River near Stevinson, California, water years 1941–2004 

(from California Department of Water Resources, 2006).

Hydrology  21
Ground Water

Ground water occurs primarily in the unconfined to semiconfined aquifer above and east of the Corcorcan Clay and in 

the confined aquifer beneath the Corcoran Clay. The unconfined to semi-confined aquifer above the Corcoran Clay ranges 

in thickness from about 131 to 230 ft. The unconfined to 

semi-confined aquifer east of the Corcoran Clay is composed 

primarily of alluvial sediments, but includes the upper part 

of the Mehrten Formation, which is more consolidated than 

the overlying formations. The confined aquifer is composed 

of alluvial sediments and upper Mehrten Formation sediments from beneath the Corcoran Clay to the deepest extent of 

freshwater (Steven P. Phillips, U.S. Geological Survey, written 

commun., 2005).

Under pre-development conditions, ground-water 

recharge primarily was at the upper parts of the allu­vial fans 

from streams entering the valley. Ground-water flow followed 

the southwest slope of the subsurface bedrock and the dip 

of the overlying sedimentary deposits toward the southwest 

(Davis and Hall, 1959). Most ground water dis­charged as 

evapotranspiration in the central trough of the valley; to a 

lesser extent, it discharged to streams.

Development in the basin changed ground-water flow. 

Pumping for agricultural irrigation, and irrigation return flows, 

are much greater than natural recharge and discharge, and 

caused an increase in vertical flow in the ground-water flow 

system. Ground-water flows generally toward the southwest, 

as occurred prior to development. However, ground water 

moving along a horizontal flow path is extracted by wells 

and reapplied at the surface several times before reaching the 

valley trough (Steven P. Phillips, U.S. Geological Survey, written commun., 2005) (fig. 21). In general, ground-water flow 

is toward the San Joaquin River to the west. In the upslope 

areas (near the Mustang Creek Subbasin) where surface-water 

supplies are not available for irrigation, there is a pumping 

depression (fig. 22). 

Figure 19. Mean daily streamflow for water years 2003–2004, mean long-term monthly streamflow for water years 

1941–2004, and mean monthly streamflow for water years 2003–2004, Merced River near Stevinson, California.

200

 0

400

600

800

1,000

1,200

1,400

1,600

1,800

2,000

MEAN STREAMFLOW, IN CUBIC FEET PER SECOND

MEAN MONTHLY STREAMFLOW, WATER YEAR 1941–2004

MEAN MONTHLY STREAMFLOW, WATER YEAR 2003–2004

U.S. GEOLOGICAL SURVEY,

MEAN DAILY STREAMFLOW, WATER YEAR 2003–2004

2003 2004

WATER YEAR

OCTOBER

AUGUST

OCTOBER

AUGUST

APRIL

JUNE

APRIL

JUNE

DECEMBERDECEMBER

FEBRUARYFEBRUARY

22  Environmental Setting of the Lower Merced River Basin, California
A

MEAN DAILY STREAMFLOW, IN CUBIC FEET PER SECOND

0

BREAK

IN

RECORD

2003 2004 2005

10

15

20

25

30

35

40

45

5

NOVEMBER 25

DECEMBER 25DECEMBER 24

JANUARY 24JANUARY 23

FEBRUARY 23FEBRUARY 22

MARCH 24MARCH 24

APRIL 23

Figure 20A. Mean daily streamflow, Mustang Creek at Monte Vista Avenue near Montpelier, California, 

November 25, 2003–March 6, 2004, and January 8, 2005–March 17, 2005.

Figure 20B. Continuous streamflow (15-minute), Mustang Creek at Monte Vista Avenue near Montpelier, 

California, February 17–29, 2004.

250

200

150

50

17 19 21

FEBRUARY, 2004

STREAMFLOW, IN CUBIC FEET PER SECOND

23 25 27 29

0

100

B

Hydrology  23
Figure 21. Generalized conceptual model of northern San Joaquin–Tulare Basin, California aquifer. A, Map view showing flow toward 

axis of basin, overlain with B, geohydrologic section showing regional flow and vertical components of flow resulting from agricultural 

pumping and recharge and discharge to floodplain deposits (modified from Eberts and others, 2005).

Modesto

A

B

Merced

0 20 40 MILES

0 20 40 KILOMETERS

N

Sierra Nevada

Coast Ranges

Merced River

Tuolumne River

Stanislaus River

San

Joaquin

River

!

!

Regional groundwater flow

SIERRA NEVADA

COAST

RANGES

Floodplain Deposits

Alluvial Fan Deposits

121

120

119

38

37

36

35

water table

More Consolidated Deposits

Fluvially Deposited Alluvial Fan Deposits

REGIONAL FLOW

Floodplain

Deposits

CONFINING UNIT

Agricultural Recharge

irrigation wells

river

NOT TO SCALE

AQUIFER

24  Environmental Setting of the Lower Merced River Basin, California
Summary

In 2001, the U.S. Geological Survey National WaterQuality Assessment (NAWQA) Program began its second 

decade of intensive water-quality assessments. The staff of the 

San Joaquin–Tulare Basins study unit is focusing on understanding human and natural factors that control water quality 

in the lower Merced River Basin, California, as part of the 

NAWQA topical study on the sources, transport, and fate of 

agricultural chemicals. The lower Merced River Basin lies 

on the east side of the San Joaquin Valley in the San Joaquin 

Basin. The Mustang Creek Subbasin and a ground-water flow 

path in an agricultural setting that terminates in the Merced 

River are areas of interest within the basin. Most of the lower 

Merced River Basin lies within the flat structural basin of the 

San Joaquin Valley filled with marine and continental sediments. Altitude ranges from 72 feet above sea level on the 

valley floor to 1,794 feet in the Sierra Nevada foothills, and 

slopes gently from northeast to southwest.

The San Joaquin Valley has an arid to semiarid climate 

characterized by hot summers and mild winters. Temperature decreases with increasing altitude in the foothills and 

mountains of the Sierra Nevada. Mean daily low temperatures 

on the valley floor range from the 30s to the 50s (degrees 

Fahrenheit) and mean daily high temperatures range from the 

50s to the 90s (degrees Fahrenheit). Precipitation is less on 

the valley floor, and increases with altitude. Annual precipitation is highly variable, with an average of about12.2 inches at 

Modesto. Most of the precipitation falls during the winter.

The Merced River flows into the San Joaquin River and 

is typical of east-side tributaries. The lower Merced River 

receives water from Dry Creek, and from Mustang Creek by 

way of Highline Canal. Sand Creek flows under Highline 

Canal and out of the basin. The surface-water hydrology of the 

basin has been significantly modified by the development of 

water resources that included the building of dams, canals, and 

diversion structures.

Ground water occurs primarily in the unconfined to semiconfined aquifer above, and east of, the Corcoran Clay, and 

in the confined aquifer beneath the Corcoran Clay. In general, 

ground-water flow is toward the San Joaquin River in the west 

and toward the pumping depression in the Mustang Creek 

Subbasin.

Figure 22. Water-table altitude and extent of the Corcoran Clay, lower Merced River Basin, California, spring 2000 (from 

California Department of Water Resources, 2000).

12030

N

0 4 8 16 MILES

0 4 8 16 KILOMETERS

EXPLANATION

MUSTANG CREEK SUBBASIN

LINES OF EQUAL WATER LEVEL

ALTITUDE (DASHED WHERE

INFERRED), IN FEET ABOVE

NAVD 88. HACHURES INDICATE

DEPRESSION. INTERVAL 10 FEET.

BOUNDARY OF CORCORAN 

CLAY - EXISTS WEST OF LINE 

(FROM PAGE, 1986)

12045

3730

!

!

!

Sand Creek

Basin

Winton

Livingston

Delhi

100

80

80

60

70

80

90

100

100

110

110

120

60

60

40

40

MARIPOSA CO

STANISLAUS CO

MERCED CO

Highline Canal

Lateral 8

Lower

Stevinson

Lateral

Garibaldi

Lateral

Merced River

Mustang Creek

Merced River

Ingalsbe Slough

North Side

Canal

Dry Creek

Main Canal

Livingston

Canal

Summary  25
Approximately 55 percent of the lower Merced River 

Basin, predominantly on the valley floor, is covered by agricultural land. The dominant agricultural land use is almond 

orchards (45 percent of agricultural land), corn and grain (16 

percent), and vineyards (12 percent). Population in the lower 

Merced River Basin was estimated to be 39,230 during the 

2000 Census, which represents a 19-percent increase since the 

1990 Census. In 2003, 285 different chemicals were applied 

to the basin and counties overlapping the basin. Pesticides 

applied for agricultural uses totaled almost 2.2 million pounds. 

Fertilizer applications were estimated to be 16.1 million 

pounds for nitrogen and 0.94 million pounds for phosphorus. 

Nutrients from manure were estimated to be 13.0 million 

pounds for nitrogen and 2.9 million pounds for phosphorus. 

References Cited

Arnold, J.G., Srinivasan, R., Muttiah, R.R., and Williams, 

J.R., 1998, Large area hydrologic modeling and assessment 

part I: model development: Journal of the American Water 

Resources Association, v. 34, p. 73–89.

Bartow, J.A., 1991, The Cenozoic evolution of the San Joaquin 

Valley, California: U.S. Geological Survey Professional 

Paper 1501, 40 p.

Burow, K.R., Shelton, J.L., Hevesi, J.A., and Weissmann, 

G.S., 2004, Hydrogeologic characterization of the Modesto 

area, San Joaquin Valley, California: U.S. Geological Survey Scientific Investigations Report 2004–5232, 54 p.

California Department of Pesticide Regulation, 2004, Pesticide use data for 2003: Sacramento, California, California 

Environmental Protection Agency, Department of Pesticide 

Regulation, [digital data].

California Department of Water Resources, 1997, Land use 

data for Stanislaus County, California, for 1996: Sacramento, California, California Department of Water 

Resources, Division of Plan­ning, Statewide Planning 

Branch, Land and Water Use, [digital data].

California Department of Water Resources, 1998, California water plan update: California Department of Water 

Resources Bulletin 160-98, 2 v., [variously paged].

California Department of Water Resources, 1999, Land use 

data for Mariposa County, California, for 1998: Sacramento, 

California, California Department of Water Resources, 

Division of Plan­ning, Statewide Planning Branch, Land and 

Water Use, [digital data].

California Department of Water Resources, 2000, Merced 

groundwater basin, Spring 2000, Lines of equal elevation 

of water in wells, unconfined aquifer. Accessed February 7, 

2006, at http://www.sjd.water.ca.gov/groundwater/basin_

maps/maps/merc_e00/index.cfm.

California Department of Water Resources, 2003, Land use 

data for Merced County, California, for 2002: Sacramento, 

California, California Department of Water Resources, 

Division of Plan­ning, Statewide Planning Branch, Land and 

Water Use, [digital data].

California Department of Water Resources, 2004a, Drought 

preparedness, hydrologic and water supply conditions: an 

example of water year classifications. Accessed February 

18, 2005, at http://watersupplyconditions.water.ca.gov/

hydrologic.cfm.

California Department of Water Resources, 2004b, WSIHIST 

(11/30/04 1136), Chronological reconstructed Sacramento 

and San Joaquin Valley water year hydrologic classification 

indices. Accessed January 24, 2005, at http://cdec.water.

ca.gov/cgi-progs/iodir/WSHIST. 

California Department of Water Resources, 2006, Water 

data library: Daily discharge data for Merced River near 

Stevinson (B05125), Merced River at Cressey (B05155), 

and Merced River below Snelling (B05170). Accessed 

February 7, 2006, at http://wdl.water.ca.gov/hydstra/index.

cfm?site=B05125.

California Division of Mines and Geology, 1966, Geologic 

map of California, San Jose sheet: Department of Conservation, scale 1:250,000, 2 sheets.

Croft, M.G., 1972, Subsurface geology of the late Tertiary and 

Quaternary water-bearing deposits of the southern part of 

the San Joaquin Valley, California: U.S. Geological Survey 

Water-Supply Paper 1999-H, 29 p.

Davis, S.N., and Hall, F.R., 1959, Water quality of eastern 

Stanislaus and northern Merced Counties, California: 

Stanford University Publications, Geological Sciences, v. 6, 

no. 1, 112 p.

Eberts, S.M., Erwin, M.L., and Hamilton, P.A., 2005, Assessing the vulnerability of public supply wells to contamination 

from urban, agricultural, and natural sources: U.S. Geological Survey Fact Sheet 2005–3022, 4 p.

Forstall, R.L. (comp./ed.), 1995, California population of 

counties by decennial census: 1900 to 1990: Washington, 

D.C., U.S. Bureau of the Census, Population Division. 

Accessed January 17, 2005, at http://www.census.gov/

population/cencounts/ca190090.txt.

26  Environmental Setting of the Lower Merced River Basin, California
Gronberg, J.M., Dubrovsky, N.M., Kratzer, C.R., Domagalski, 

J.L., Brown, L.R., and Burow, K.R., 1998, Environmental 

setting of the San Joaquin–Tulare Basins, California: U.S. 

Geological Survey Water-Resources Investigations Report 

97-4205, 45 p.

Gronberg, J.M., Kratzer, C.R., Burow, K.R., Domagalski, J.L., 

and Phillips, S.P., 2004, Water-quality assessment of the San 

Joaquin–Tulare Basins — Entering a new decade: U.S. 

Geological Survey Fact Sheet 2004-3012, 6 p.

Hirsch, R.M., Alley, W.M., and Wilber, W.G., 1988, Concepts 

for a national water-quality assessment program: U.S. 

Geological Survey Circular 1021, 42 p.

Kuminoff, N.V., Sumner, D.A., and Goldman, George, 2000, 

The measure of California agricul­ture, 2000: University of 

California Agricultural Issues Center, 114 p.

Modesto Irrigation District, 2005, Precipitation and temperature data for 1888-2005 for Modesto. Data obtained from 

customer service. Recent data accessed at http://www.mid.

org/wthr.html.

National Center for Atmospheric Research, 2003, DAYMET 

climate data, average precipitation and temperature data for 

1980-1997. Accessed February 7, 2006, at 

http://www.daymet.org.

Page, R.W., 1977, Appraisal of ground-water conditions in 

Merced, California, and vicinity: U.S. Geological Survey 

Open-File Report 77-454, 43 p.

Page, R.W., 1986, Geology of the fresh ground-water basin of 

the Central Valley, California, with tex­ture maps and sections: U.S. Geological Survey Professional Paper 1401-C, 

54 p., 5 pls in pocket.

Page, R.W., and Balding, G.N., 1973, Geology and quality 

of water in the Modesto-Merced area, San Joaquin Valley, 

California: U.S. Geological Survey Water-Resources Investigations Report 73-6, 85 p.

Ruddy, B.C., Lorenz, D.L., and Mueller, D.K., 2006, Countylevel estimates of nutrient inputs to the land surface of the 

conterminous United States, 1982-2001: U.S. Geological 

Survey Scientific Investigations Report 2006–5012, 17 p.

 Stillwater Sciences and EDAW Inc., 2001, Merced River 

corridor restoration plan baseline studies, vol. I: identification of social, institutional, and infrastructural opportunities 

and constraints, final report, 54 p. Accessed May 7, 2006, at 

http://www.stillwatersci.com/publications/VolumeI.pdf

Turlock Irrigation District, 2004, “Highline Canal breaks 

disrupt service”, June 29, 2004, TID Head­lines, Accessed 

April 29, 2005, at http://www.tid.org/news/news_

archive/2004_JUN_29.htm.

U.S. Bureau of the Census, 2001, Census CD 2000 short 

form blocks, 2000, Census of population and housing: East 

Brunswick, New Jersey, Geolytics, Inc., [digital data]. 

U.S. Department of Agriculture, 1994, State Soil Geographic 

(STATSGO) database for Califor­nia: Natural Resources 

Conservation Service, [digital data].

U.S. Department of Agriculture, 2004, 2002 Census of agriculture, California state and county data, National Agricultural Statistics Service. Accessed March 23, 2006, at 

http://www.nass.usda.gov/Census_of_Agriculture/index.asp.

U.S. Department of Agriculture, 2005a, Soil survey 

(SSURGO) database for eastern Stanislaus area, California, 

Natural Resources Conservation Service. Accessed April 

29, 2005, at http://soildatamart.nrcs.usda.gov/.

U.S. Department of Agriculture, 2005b, Soil survey 

(SSURGO) database for Merced area, California, Natural 

Resources Conservation Service. Accessed April 29, 2005, 

at http://soildatamart.nrcs.usda.gov/.

Vogelmann, J.E., Howard, S.M., Yang, L., Larson, C.R., 

Wylie, B.K., and Van Driel, N., 2001, Completion of the 

1990s national land cover data set for the conterminous 

United States from Landsat Thematic Mapper data and 

ancillary data sources: Photogrammetric Engineering and 

Remote Sensing, vol. 67, p. 650–662.

References Cited  27