Scientific Investigations Report 2014–5155

Prepared in cooperation with Albuquerque Bernalillo County Water Utility Authority

Water Chemistry, Seepage Investigation, Streamflow, 

Reservoir Storage, and Annual Availability of Water for the 

San Juan-Chama Project, Northern New Mexico, 1942–2010

U.S. Department of the Interior

U.S. Geological Survey
Front cover:

Background, Heron Reservoir looking west from the road over the dam, December 13, 2007. Photograph by Sarah E. McKean.
Water Chemistry, Seepage Investigation, 

Streamflow, Reservoir Storage, and Annual 

Availability of Water for the San Juan-Chama 

Project, Northern New Mexico, 1942–2010

By Sarah E. McKean and Scott K. Anderholm

Prepared in cooperation with Albuquerque Bernalillo County

Water Utility Authority

Scientific Investigations Report 2014–5155

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2014

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Suggested citation:

McKean, S.E., and Anderholm, S.K., 2014, Water chemistry, seepage investigation, streamflow, reservoir storage, 

and annual availability of water for the San Juan-Chama Project, northern New Mexico, 1942–2010: U.S. Geological 

Survey Scientific Investigations Report 2014–5155, 52 p., http://dx.doi.org/10.3133/sir20145155.

ISSN 2328-031X (print)

ISSN 2328-0328 (online)

ISBN 978-1-4113-3854-8
iii

Acknowledgments

The authors thank John Stomp and David Price of the Albuquerque Bernalillo County Water 

Utility Authority for providing guidance and support for this investigation to improve understanding of the water resources available to the Albuquerque and Bernalillo County area. The authors 

thank the Jicarilla Apache Nation and personnel from the Jicarilla Apache Nation including 

Andrea LeFevre for access for sampling activities along Willow Creek and for assistance with 

data-collection activities. The authors thank personnel from the Bureau of Reclamation Albuquerque Area Office and the Bureau of Reclamation Chama Field Division office including Victor 

Salazar and Ed Wilcox for access to the San Juan-Chama Project infrastructure for sampling 

activities and for assistance with data-collection activities.
iv
v

Contents

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

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

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

Description of Study Area ...................................................................................................................3

San Juan-Chama Project ....................................................................................................................5

Previous Studies ..........................................................................................................................9

Methods of Analysis.............................................................................................................................9

Water-Chemistry Data Collection..............................................................................................9

Seepage Investigation ..............................................................................................................10

Analysis of Streamflow.............................................................................................................15

Water Chemistry .........................................................................................................................................16

Azotea Tunnel Outlet and Willow Creek above Heron Reservoir...............................................19

Heron Reservoir and Willow Creek below Heron Reservoir.......................................................21

Seepage Investigation ................................................................................................................................25

Streamflow, Reservoir Storage, and Annual Availability of Water for the

San Juan-Chama Project..............................................................................................................25

Variation and Trends in Streamflow for the San Juan-Chama Project......................................28

Heron Reservoir Storage...................................................................................................................36

Seasonal and Annual Variation in Streamflow for Rio Grande Water.......................................37

Annual Availability of Water for the San Juan-Chama Project...................................................40

Summary........................................................................................................................................................48

References Cited..........................................................................................................................................49

Appendix 1. Water-chemistry data for the San Juan-Chama Project, southern

Colorado and northern New Mexico, 1973–2010 ................................................. served online

Figures

1. Map showing location of the study area, hydrographic areas in the

study area, selected geographic features, and climate stations,

southern Colorado and northern New Mexico........................................................................2

2. Map showing schematic of the San Juan-Chama Project and location

of stations in the study area, southern Colorado and northern New Mexico....................4

3. Graphs showing temperature and precipitation data for climate

station Chama in the northern New Mexico part of the study area, 1905–2009 ................7

4. Map showing surface geology of the study area, location of selected

streamflow-gaging stations, and the boundary of selected watersheds,

southern Colorado and northern New Mexico........................................................................8

5. Graphs showing streamflow measurements from Azotea Tunnel Outlet

and the high-flow seepage investigation, northern New Mexico,

June 2010 .....................................................................................................................................12

6. Trilinear diagram of representative water composition from selected

stations in the study area, northern New Mexico and representative

water composition from selected stations from Rio Blanco and

Navajo River, southern Colorado .............................................................................................17
vi

7. Graphs showing variation of water-chemistry at selected sampling stations

in the study area, northern New Mexico, 1973–2009............................................................18

8. Graphs showing monthly variation in specific-conductance values for

selected stations and comparison of the variation of the average

monthly specific conductance to the median monthly streamflow,

with monthly streamflow categorized as high flow or low flow, for selected

stations in the study area, northern New Mexico, 1973–2009 ............................................20

9. Graphs showing vertical profiles of water chemistry parameters in

Heron Reservoir, 2008–9 ............................................................................................................24

10. Graph showing comparison of the total inflows to the reach and the

total outflow from the reach with fitted regression line.......................................................26

11. Graphs showing distribution of annual streamflow, median annual streamflow,

and annual flow-duration curves for selected streamflow-gaging stations

in the study area, 1971–2010 .....................................................................................................27

12. Bar graph showing annual storage in Heron Reservoir, in acre-feet,

in northern New Mexico, 1970–2010 .......................................................................................28

13. Graphs showing annual and seasonal variation in streamflow at the

streamflow-gaging stations Azotea Tunnel Outlet and Willow Creek

above Heron Reservoir, in northern New Mexico, 1971–2010 ............................................30

14. Graphs showing variation in streamflow at the streamflow-gaging station

Azotea Tunnel Outlet, northern New Mexico, 1970–2010..............................................................31

15. Graphs showing annual and seasonal variation in streamflow at the

streamflow-gaging station Willow Creek below Heron Reservoir, northern

New Mexico, 1971–2010 ............................................................................................................35

16. Graph showing Pacific Decadal Oscillation index for 1900–2012, defined

as the median annual monthly North Pacific sea-surface temperature

anomaly with a 5-year moving average..................................................................................36

17. Box-and-whisker diagram of median monthly change in the volume

of water in storage at Heron Reservoir, northern New Mexico, 1970–2010 .....................38

18. Graphs showing annual variation in storage in Heron Reservoir, in

northern New Mexico, 1970–2010............................................................................................38

19. Graphs showing annual and seasonal variation in Rio Grande streamflow

at streamflow-gaging stations Willow Creek near Park View and Willow

Creek above Heron Reservoir, 1942–2010...............................................................................39

20. Graphs showing change in the available streamflow for the San Juan-Chama

Project from variation in the structural and capacity limitations .......................................42

21. Graph showing operationally limited available streamflow for the

San Juan-Chama Project for four scenarios of structural and capacity

limitations, 1974–2010.................................................................................................................44

22. Bar graph showing annual available streamflow for the San Juan-Chama

Project, 1975–2010, northern New Mexico.............................................................................45

23. Bar graph showing percentage of annual available streamflow not

diverted for the San Juan-Chama Project for structure, capacity, and

operations limitations, percentage of annual streamflow not diverted

for reasons not accounted for in this analysis, and the total annual

streamflow not diverted for the San Juan-Chama Project, northern

New Mexico, 1974–2010 ............................................................................................................47

24. Graph showing measured annual streamflow at Azotea Tunnel Outlet

compared to estimated annual operationally limited available streamflow

for the San Juan-Chama Project, northern New Mexico, 1974–2010................................47
vii

Tables

1. List of the entities that have contracts for water from the San Juan-Chama

Project and the annual amount of water contracted .............................................................5

2. Descriptions of stations in the study area for the San Juan-Chama Project,

northern New Mexico..................................................................................................................6

3. Measurements from the high-flow seepage investigation at Azotea

Tunnel Outlet, Willow Creek above Heron Reservoir, and Willow Creek

above Azotea Creek and the calculated streamflow resulting from

groundwater interaction and associated uncertainty, northern New Mexico,

June 24, 2010 ...............................................................................................................................13

4. Low-flow seepage investigation results including the discharge

measurements and associated uncertainty for Reach 1 from 1/4 mile

downstream from the outlet of Azotea Tunnel to Willow Creek above

the confluence with Willow Creek above Azotea Creek and Reach 2

from Willow Creek above the confluence with Willow Creek above Azotea

Creek to 1/10 mile upstream from Willow Creek above Heron Reservoir

station and the calculated streamflow resulting from groundwater

interaction and associated uncertainty, northern New Mexico,

August 19–21, 2009 .....................................................................................................................14

5. Selected water-chemistry data for Azotea Tunnel Outlet, Willow Creek

above Azotea Creek, Willow Creek above Heron Reservoir, Heron

Reservoir, and Willow Creek below Heron Reservoir, northern

New Mexico, 2007–9 ..................................................................................................................22

6. Selected statistics for streamflow at selected streamflow-gaging stations

in the study area, northern New Mexico................................................................................26

7. Selected statistics for characterization of streamflow at Azotea

Tunnel Outlet, northern New Mexico, 1971–2010. Statistics include the

ordinal day of the year on which selected percentiles of annual

streamflow occurred and the number of days between selected percentiles................32

8. Results of the Mann-Kendall trend test for monthly streamflow and

the monthly percentage of annual streamflow for streamflow-gaging

station Azotea Tunnel Outlet, in northern New Mexico, 1971–2010...................................33

9. Average rates of precipitation and pan evaporation for Heron Resrvoir,

northern New Mexico, 1975–2005............................................................................................37

10. Average annual available streamflow for selected limitations of the

San Juan-Chama Project ..........................................................................................................46
viii

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 4,047 square meter (m2

)

Volume

cubic foot (ft3

) 0.02832 cubic meter (m3) 

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

)

Flow rate

acre-foot per day (acre-ft/d) 0.01427 cubic meter per second (m3

/s)

acre-foot per year (acre-ft/yr) 1,233 cubic meter per year (m3

/yr)

cubic foot per second (ft3

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

cubic foot per day (ft3

/d) 0.02832 cubic meter per day (m3/d)

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

°F=(1.8×°C)+32

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

°C=(°F-32)/1.8

Unless otherwise noted, the datums used in this report for vertical coordinate information are 

referenced to the North American Vertical Datum of 1988 (NAVD 88).

Unless otherwise noted, the datums used in this report for horizontal coordinate information are 

referenced to North American Datum of 1983 (NAD 83).

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25 °C).

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L).
Water Chemistry, Seepage Investigation, Streamflow, 

Reservoir Storage, and Annual Availability of Water

for the San Juan-Chama Project, Northern New Mexico, 

1942–2010

By Sarah E. McKean and Scott K. Anderholm

Abstract

The Albuquerque Bernalillo County Water Utility 

Authority supplements the municipal water supply for the 

Albuquerque metropolitan area, in central New Mexico, 

with surface water diverted from the Rio Grande. The U.S. 

Geological Survey, in cooperation with the Albuquerque 

Bernalillo County Water Utility Authority, undertook this 

study in which water-chemistry data and historical streamflow 

were compiled and new water-chemistry data were collected 

to characterize the water chemistry and streamflow of the San 

Juan-Chama Project (SJCP). Characterization of streamflow 

included analysis of the variability of annual streamflow and 

comparison of the theoretical amount of water that could 

have been diverted into the SJCP to the actual amount of 

water that was diverted for the SJCP. Additionally, a seepage 

investigation was conducted along the channel between Azotea 

Tunnel Outlet and the streamflow-gaging station at Willow 

Creek above Heron Reservoir to estimate the magnitude of 

the gain or loss in streamflow resulting from groundwater 

interaction over the approximately 10-mile reach. 

Generally, surface-water chemistry varied with 

streamflow throughout the year. Streamflow ranged from high 

flow to low flow on the basis of the quantity of water diverted 

from the Rio Blanco, Little Navajo River, and Navajo River 

for the SJCP. Vertical profiles of the water temperature over 

the depth of the water column at Heron Reservoir indicated 

that the reservoir is seasonally stratified. The results from the 

seepage investigations indicated a small amount of loss of 

streamflow along the channel. 

Annual variability in streamflow for the SJCP was an 

indication of the variation in the climate parameters that 

interact to contribute to streamflow in the Rio Blanco, Little 

Navajo River, Navajo River, and Willow Creek watersheds. 

For most years, streamflow at Azotea Tunnel Outlet started 

in March and continued for approximately 3 months until the 

middle of July. The majority of annual streamflow at Azotea 

Tunnel Outlet occurred from May through June, with a median 

duration of slightly longer than a month. Years with higher 

maximum daily streamflow generally are associated with 

higher annual streamflow than years with lower maximum 

daily streamflow.

The amount of water that can be diverted for the SJCP is 

controlled by the availability of streamflow and is limited by 

several factors including legal limits for diversion, limits from 

the SJCP infrastructure including the size of the diversion 

dams and tunnels, the capacity of Heron Reservoir, and 

operational constraints that limit when water can be diverted. 

The average annual streamflow at Azotea Tunnel Outlet was 

94,710 acre-feet, and the annual streamflow at Azotea Tunnel 

Outlet was approximately 75 percent of the annual streamflow 

available for the SJCP. The average annual percentage 

of available streamflow not diverted for the SJCP was 14 

percent because of structural limitations of the capacity of 

infrastructure, 1 percent because of limitations of the reservoir 

storage capacity, and 29 percent because of the limitations 

from operations. For most years, the annual available 

streamflow not diverted for unknown reasons exceeded the 

sum of the water not diverted because of structural, capacity, 

and operational limitations.

Introduction

The Albuquerque Bernalillo County Water Utility 

Authority (ABCWUA) supplements the municipal water 

supply for the Albuquerque metropolitan area, in central New 

Mexico, with surface water diverted from the Rio Grande. 

ABCWUA’s allotment of surface water diverted from the 

Rio Grande is derived from the San Juan-Chama Project 

(SJCP), which delivers water from streams in the southern 

San Juan Mountains in the Colorado River Basin in southern 

Colorado to the Rio Chama in the Rio Grande Basin in 

northern New Mexico (fig. 1). SJCP water is diverted from the 

upper tributaries of the San Juan River, in southern Colorado, 

across the Continental Divide to Heron Reservoir, in northern 
2 Water Chemistry, Seepage, Streamflow, Reservoir Storage, and Annual Availability of Water for the San Juan-Chama Project

Figure 1. Location of the study area, hydrographic areas in the study area, selected geographic features, and climate stations, 

southern Colorado and northern New Mexico.

107°10' 107°00' 106°50' 106°40' 106°30' 37°30'

37°20'

37°10'

37°00'

36°50'

36°40'

0 3 6 9 12 MILES

0 3 6 9 12 KILOMETERS

Horse Lake Cree k

Upper Willow

Creek

Heron Reservoir

Rio Chama

Navajo River

Sa n Juan River

Rio Blanco

COLORADO

NEW MEXICO

Little Navajo River S A N

J U A N

M

O

U

N

T A

I

N

S

RIO GRANDE COUNTY

CONEJOS COUNTY

MINERAL COUNTY

ARCHULETA COUNTY

RIO ARRIBA COUNTY

Continental

Divide

Colorado River Basin

Colorado River Basin

Rio Grande Basin

Rio Grande Basin

Continental

Divide

TUSAS MOUNTAINS

Rio Chama

Valley

Chama

Chama

EXPLANATION

14,350 feet

6,088 feet

Elevation

Watershed boundary

Weather station and identifier—Historical Climatology

 Network, National Weather Service Cooperative

 Network

Base map from U.S. Government digital data

Geographic projection

North American Datum of 1983 (NAD 83)

Elevation base map from U.S. Geological Survey

National Elevation Dataset 

Geographic projection

North American Datum of 1983 (NAD 83)

Figure 4

Figure 3

Azotea

Creek

Willow

Creek

COLORADO

Index map

NEW MEXICO

Study area

Rio Grande

Rio Chama

Albuquerque
Introduction 3

New Mexico. The water from Heron Reservoir is routed 

to Albuquerque through the Rio Chama, in northern New 

Mexico, and the Rio Grande. The distribution of surface 

water for municipal supply has increased interest in the water 

chemistry, including the concentrations of salinity, trace 

elements, and nutrients in water imported from the San Juan 

River watershed and the timing and availability of water 

for diversion. 

Review of previous investigations has indicated that 

limited information exists about the chemistry of SJCP water 

flowing into and stored in Heron Reservoir. Additionally, 

little is known about groundwater/surface-water interactions 

along the approximately 11-mile (mi)-long channel used to 

convey water from Azotea Tunnel Outlet to Heron Reservoir 

(fig. 2). In an effort to provide more information about water 

chemistry and quantity, the U.S. Geological Survey (USGS), 

in cooperation with the ABCWUA, undertook this study in 

which water-chemistry data and historical streamflow were 

compiled and new water-chemistry data were collected 

to characterize the water chemistry and streamflow of the 

SJCP. Characterization of streamflow included analysis of 

the variability of annual streamflow and comparison of the 

theoretical amount of water that could have been diverted 

into the SJCP to the actual amount of water that was diverted 

for the SJCP. Additionally, a seepage investigation was 

conducted along the channel between Azotea Tunnel Outlet 

and the streamflow-gaging station (station) at Willow Creek 

above Heron Reservoir to estimate the magnitude of the gain 

or loss in streamflow resulting from groundwater interaction 

over the approximately 10-mi reach (fig. 2). 

The SJCP water, approximately 96,200 acre-feet per 

year (acre-ft/yr), is divided among various entities that have 

contracts for the water, including irrigation districts and 

municipal, domestic, and industrial water users (generally 

referred to as “SJCP contractors′′) (table 1). The SJCP 

infrastructure consists of diversion dams constructed in 

southern Colorado on the Rio Blanco, Navajo River, and 

Little Navajo River; a conduit and tunnel system; and Heron 

Dam. The conduit and tunnel system conveys the water 

approximately 26 mi across the Continental Divide and 

discharges it into Willow Creek above Heron Reservoir (figs. 

1 and 2). Heron Reservoir and Heron Dam, constructed on 

Willow Creek just upstream from the confluence with the 

Rio Chama, provide storage of water diverted from the San 

Juan River watershed and allow for controlled releases to 

SJCP contractors and for downstream use including fish and 

wildlife and recreational purposes (Bureau of Reclamation, 

2011a).

Purpose and Scope

This report describes the results of a study to 

characterize the water chemistry and streamflow including 

annual streamflow variability and availability of water from 

the SJCP including the channel between Azotea Tunnel Outlet 

and Heron Reservoir, Heron Reservoir, and the outflow from 

Heron Reservoir. This report also describes the results of 

seepage investigations conducted in August 2009 and June 

2010 along the channel between Azotea Tunnel Outlet and 

the station at Willow Creek above Heron Reservoir. The 

study area included the Willow Creek watershed in northern 

New Mexico and the SJCP infrastructure from Azotea Tunnel 

Outlet to the station at Willow Creek below Heron Reservoir. 

Existing surface-water chemistry and streamflow data from 

1943 to 2010 were compiled, and water-chemistry samples 

were collected in spring, summer, and fall of water years 2009 

and 2010 (a water year is the 12-month period from October 

1 through September 30 designated by the calendar year in 

which it ends). Vertical profiles of selected water-chemistry 

parameters were collected over the depth of the water column 

in Heron Reservoir coincident with sampling. 

Description of Study Area

The study area is located in the Rio Chama Valley in 

northern New Mexico and southern Colorado (fig. 1). The 

southern San Juan Mountains are a southern subrange of the 

Rocky Mountains. In the study area, they decline in elevation 

to the south into the Rio Chama Valley, where they are 

bounded to the east by the Rio Chama (Atwood and Mather, 

1932). The peaks of the southern San Juan Mountains form 

the Continental Divide; watersheds east of the divide drain 

to the Rio Grande, and watersheds west of the divide drain to 

the Colorado River. In the study area, Willow Creek, Azotea 

Creek, and Horse Lake Creek are located in the Rio Chama 

Valley east of the Continental Divide and are tributaries of 

the Rio Chama (fig. 1). The Rio Chama Valley is a shallow 

physiographic basin that is bounded on the west by anticlines 

that form the eastern edge of the San Juan Basin and the Tusas 

Mountains to the east (Muehlberger, 1967). 

Land-surface elevations in the Rio Chama Valley in 

the Willow Creek watershed of Heron Reservoir range from 

approximately 6,088 to 9,900 feet (ft). The National Weather 

Service Cooperative Observer Program station 291664 

located at Chama, N. Mex. (table 2 and fig. 1), at an elevation 

of 7,850 ft, indicated that average annual precipitation was 

21.7 inches (in.) and average annual temperature was 42.5 

degrees Fahrenheit (°F) during 1905–2009 (U.S. Historical 

Climatology Network, 2011). The annual average temperature 

at this station ranged from 39.5 to 47.3 °F, and the annual 

precipitation ranged from 11.3 to 32.34 in. for 1905–2009 

(figs. 3A and 3B). Average monthly temperatures were greater 

than 32 °F in the months of April through November, and 

the average monthly temperatures were less than 32 °F in the 

months of December through March (fig. 3C). Slightly more 

than half (56 percent) of the monthly precipitation occurred 

from October through April, and 44 percent of monthly 

precipitation occurred from May through September (fig. 3D).
4 Water Chemistry, Seepage, Streamflow, Reservoir Storage, and Annual Availability of Water for the San Juan-Chama Project

0 2 4 6 MILES

0 2 4 6 KILOMETERS

EXPLANATION

14,350 feet

6,088 feet

Elevation

Channel

Heron Reservoir water-quality sample site

Streamflow-gaging station

Water diversion gaging station

Tunnel

Base map from U.S. Government digital data

Geographic projection

North American Datum of 1983 (NAD 83)

Elevation base map from U.S. Geological Survey

National Elevation Dataset 10-meter digital

elevation model, 1/3 arc-second

Geographic projection

North American Datum of 1983 (NAD 83)

CONEJOS COUNTY

ARCHULETA COUNTY

RIO ARRIBA COUNTY

COLORADO

NEW MEXICO

Continental

Divide

Heron Reservoir

Horse Lake Creek

RioChama

Upper Willow

Creek

Rio Chama

Navajo River

Little Navajo River

Navajo River

Rio Blanco

106°55' 106°50' 106°45' 106°40' 106°35'

37°15'

37°10'

37°5'

37°00'

36°55'

36°50'

36°45'

36°40'

Rio Blanco near Pagosa Springs Rio Blanco below Blanco Diversion

Little Navajo River below

Little Oso Diversion

Navajo River below

Oso Diversion

Willow Creek above Azotea Creek

Willow Creek

near Park View

Heron Reservoir near dam

Willow Creek below Heron Reservoir Heron Dam

Horse Lake Creek 

Willow Creek above

Heron Reservoir

Little Navajo River at Chromo

Navajo River at Banded

Peak Ranch

Blanco Tunnel

Oso Tunnel

Azotea Tunnel Blanco Diversion

Little Oso Diversion

Oso Diversion

Navajo River

near Chromo

Willow

Creek

Azotea Creek

Azotea Tunnel Outlet

Figure 2. Schematic of the San Juan-Chama Project and location of stations in the study area, southern Colorado and northern New 

Mexico.
Introduction 5

The surficial geology near Heron Reservoir includes 

the Jurassic Morrison Formation, the Cretaceous Dakota 

Sandstone and Burro Canyon Formation, and the Cretaceous 

Mancos Shale (fig. 4). The Brushy Basin Member of the 

Morrison Formation is exposed in the gorge below Heron 

Dam (Owen, 2005) and at stream level downstream from 

the confluence of Willow Creek with Rio Chama (Lucas and 

others, 2005). The Morrison Formation is overlain by the 

Lower Cretaceous Burro Canyon Formation and the Upper 

Cretaceous Dakota Sandstone (Varney, 2005). Heron Dam is 

set in the Paguate Tongue of the Dakota Sandstone, which also 

forms the surficial exposures of the nearby southern shoreline 

(Owen, 2005). The Dakota Sandstone is composed of 

shallow and deepwater marine deposits including interbedded 

sandstone and carbonaceous shale and siltstone (Varney, 2005; 

Kelley, 2011). The Graneros Member of the Mancos Shale 

outcrops along most of the shore of Heron Reservoir, except 

along the southern and northeastern shoreline where the 

Paguate Sandstone Member of the Dakota Sandstone outcrops 

(Owen, 2005). The Mancos Shale is generally composed of 

sea-floor deposits of limy mud (Kelley, 2011). 

The surficial geology along Willow Creek and the 

channel north of Heron Reservoir is mostly composed of 

Mancos Shale with outcroppings of Dakota Sandstone (fig. 4; 

Muehlberger, 1967; Green and Jones, 1997). Large outcrops of 

Dakota Sandstone and smaller outcrops of Dakota Sandstone 

overlain by the Graneros Shale Member of the Mancos Shale 

and the Greenhorn Limestone Member of the Mancos Shale 

occur along Willow Creek north of the confluence with 

the channel (Muehlberger and others, 1963). In addition, 

Quaternary alluvium and terrace deposits occur along upper 

Willow Creek and the channel (Muehlberger and others, 1963). 

Generally, Late Cretaceous-age to early Quaternary-age rocks 

have been eroded from this area (Kelley, 2011).

San Juan-Chama Project

The U.S. Congress initially authorized the SJCP in 1962 

under Public Law (P.L.) 87-483, an act that authorized the 

Secretary of the Interior to construct, operate, and maintain the 

Navajo Indian Irrigation Project and the initial stage of the San 

Juan-Chama Project as participating projects of the Colorado 

River Storage Project and for other purposes (Section 8, Public 

Law 87-483, June 13, 1962 [S.107] 76 Stat. 96), which allowed 

diversion of a portion of the New Mexico allocation of water 

from the Colorado River Basin under the Upper Colorado 

River Compact to the Rio Grande Basin. Water diverted for the 

SJCP is stored in Heron Reservoir until the water is delivered 

on the basis of annual water contracts to the SJCP contractors 

downstream from Heron Reservoir (table 1). Generally, SJCP 

contractors cannot store water in Heron Reservoir from one 

year to the next and are obligated to schedule delivery of their 

full annual allotment of SJCP water by the end of the calendar 

year. Water from the Rio Grande Basin, which is any water 

from a source in the Rio Grande Basin, cannot be stored in 

Heron Reservoir; therefore, any Rio Grande water that flows 

into Heron Reservoir from the upstream watersheds must be 

accounted for and released. In P.L. 87-483, Congress stipulated 

that diversions from the San Juan River watershed be limited 

to a maximum of 1,350,000 acre-feet (acre-ft) of water in any 

10 consecutive years and 270,000 acre-ft in a single year and 

that the reservoir is not allowed to spill. Additionally, diversions 

from the Rio Blanco and Navajo River are limited, such that 

streamflow in the rivers cannot be depleted below minimum 

monthly bypass requirements (Bureau of Reclamation, 1955). 

The minimum monthly bypass requirements for the Little 

Navajo River were not set in P.L. 87-483 but were listed in the 

1964 Bureau of Reclamation (Reclamation) definite plan report 

(Bureau of Reclamation, 1964) and set by memorandum in 

1977 (Bureau of Reclamation, 1986).

The SJCP infrastructure consists of three diversion dams 

(Blanco Diversion, Little Oso Diversion, and Oso Diversion), a 

tunnel and conduit system, and Heron Dam (schematic shown 

on fig. 2). The diversion dams were constructed on the Rio 

Blanco, Navajo River, and Little Navajo River in the southern 

San Juan Mountains (fig. 1). The tunnel system is gravity 

driven and includes siphons under the stream segments. The 

conduits connect the diversions to the tunnel system. The tunnel 

Table 1. List of the entities that have contracts for water from 

the San Juan-Chama Project and the annual amount of water 

contracted.

San Juan-Chama Project 

water contractors

Annual amount of 

water contracted 

(acre-feet)

Irrigation supply

Middle Rio Grande Conservancy District 20,900

Pojoaque Valley Irrigation District 1,030

Municipal, domestic, and industrial

City of Albuquerque (now the Albuquerque 

Bernalillo County Water Utility Authority)

48,200

Jicarilla Apache Nation 6,500

City and county of Santa Fe 5,605

Los Alamos County 1,200

City of Española 1,000

City of Belen 500

Village of Los Lunas 400

Village of Taos Ski Valley 400

Town of Bernalillo 400

Town of Red River 60

Twining Water and Sanitation District 15

Total 86,210

Cochiti Lake for fish and wildlife, pool reserve 

of 1,200 surface acres

5,000

Allocated but uncontracted 4,990

Total 96,200
6 Water Chemistry, Seepage, Streamflow, Reservoir Storage, and Annual Availability of Water for the San Juan-Chama Project

Table 2. Descriptions of stations in the study area for the San Juan-Chama Project, northern New Mexico.

[USGS, U.S. Geological Survey; CLIMATE, climate station; WC, water chemistry; SF, streamflow; STORAGE, volume of water in storage in reservoir;

--, data not available; NWS Coop, National Weather Service Cooperative Observer Program; Reclamation, U.S. Bureau of Reclamation; NAD 83, North 

American Datum of 1983; NGVD 29, National Geodetic Vertical Datum of 1929; NAD 27, North American Datum of 1927; CDWR, Colorado Division

of Water Resources]

Site type USGS site identifier Site name Site name for report

CLIMATE Chama (Historical Climatology Network, National Weather 

Service Cooperative Network site 291664)

Chama

WC/SF 09343300 Rio Blanco below Blanco Diversion Dam near Pagosa

Springs, Colo.

Rio Blanco below Blanco Diversion

WC/SF 09344400 Navajo River below Oso Diversion Dam near Chromo, Colo. Navajo River below Oso Diversion

WC/SF 08284160 Azotea Tunnel Outlet near Chama, N. Mex. Azotea Tunnel Outlet

WC 08284150 Willow Creek above Azotea Creek near Park View, N. Mex. Willow Creek above Azotea Creek

WC/SF 08284200 Willow Creek above Heron Reservoir, near Los Ojos, N. Mex. Willow Creek above Heron Reservoir

SF 08284300 Horse Lake Creek above Heron Reservoir, near Los Ojos, N. Mex. Horse Lake Creek

WC/SF 08284500 Willow Creek near Park View, N. Mex. Willow Creek near Park View

WC 364107106421710 Heron Reservoir near dam Heron Reservoir

STORAGE 08284510 Heron Reservoir near Los Ojos, N. Mex. Heron Reservoir storage

WC/SF 08284520 Willow Creek below Heron Dam, N. Mex. Willow Creek below Heron Reservoir

Site name for report Latitude Longitude Datum Elevation Datum

Chama 36° 55′ 00′′ 106° 35′ 00′′ -- 7,850 --

Rio Blanco below Blanco Diversion 37° 12′ 13′′ 106° 48′ 44′′ NAD 83 7,858 NGVD 29

Navajo River below Oso Diversion 37° 01′ 49′′ 106° 44′ 14′′ NAD 27 7,648 NGVD 29

Azotea Tunnel Outlet 36° 51′ 12′′ 106° 40′ 18′′ NAD 27 7,520 NGVD 29

Willow Creek above Azotea Creek 36° 48′ 15′′ 106° 39′ 30′′ NAD 27 7,404 NGVD 29

Willow Creek above Heron Reservoir 36° 44′ 33′′ 106° 37′ 34′′ NAD 27 7,196 NGVD 29

Horse Lake Creek 36° 42′ 24.05′′ 106° 44′ 44.14′′ NAD 83 7,187 NGVD 29

Willow Creek near Park View 36° 40′ 05′′ 106° 42′ 15′′ NAD 27 6,945 NGVD 29

Heron Reservoir 36° 41′ 07′′ 106° 42′ 17′′ NAD 83 -- --

Heron Reservoir storage 36° 39′ 55.56′′ 106° 42′ 19.51′′ NAD 83 -- --

Willow Creek below Heron Reservoir 36° 39′ 46′′ 106° 42′ 20′′ NAD 83 6,935 NGVD 29

Site name for report Parameters

Start of 

period of 

record

End of 

period of 

record

Data collection 

agency

Data reporting 

agency

Number of 

waterchemistry 

samples

Chama Precipitation 1935 2010 NWS Coop NWS Coop

Temperature 1935 2010 NWS Coop NWS Coop

Rio Blanco below Blanco Diversion Discharge 1971 2010 CDWR CDWR

Water chemistry 1973 2009 USGS USGS 74

Navajo River below Oso Diversion Discharge 1971 1971 CDWR CDWR

Water chemistry 1971 2009 USGS USGS 40

Azotea Tunnel Outlet Discharge 1971 2010 Reclamation USGS

Water chemistry 1974 2009 USGS USGS 21

Willow Creek above Azotea Creek Water chemistry USGS USGS 3

Willow Creek above Heron Reservoir Discharge 1961 2010 USGS/Reclamation USGS/Reclamation

Water qaulity 1973 2009 USGS USGS 6

Horse Lake Creek Discharge 1962 2009 USGS USGS

Water chemistry 2008 2009 USGS USGS 14

Willow Creek near Park View Discharge 1942 1971 USGS USGS

Water chemistry 1961 1965 USGS USGS 76

Heron Reservoir Water chemistry 2008 2009 USGS USGS 4

Heron Reservoir storage Discharge 1971 2010 Reclamation USGS

Willow Creek below Heron Reservoir Discharge 1971 2008 Reclamation USGS

Water chemistry 2007 2009 USGS USGS 3
Introduction 7

1905191019151920192519301935194019451950195519601965197019751980198519901995200020052010

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

40

48

47

45

43

41

40

37

34

25

19

16

10

70

60

50

40

30

20

10

0

3

2.5

2

1.5

1

0.5

0

Year

1905191019151920192519301935194019451950195519601965197019751980198519901995200020052010

Year

Month

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Month

39

38

42

44

46

13

22

31

28

A. Annual average temperature

B. Annual precipitation

C. Average monthly temperature

D. Average monthly precipitation

Total annual precipitation,

in inches

Total monthly precipitation,

in inches

Annual average temperature,

in degrees Fahrenheit

Average monthly temperature,

in degrees Fahrenheit

Figure 3. Temperature and precipitation data for climate station Chama (National Weather Service Cooperative Observer Program 

weather station), in the northern New Mexico part of the study area, 1905–2009. A, Annual average temperature. B, Annual 

precipitation. C, Average monthly temperature. D, Average monthly precipitation.
8 Water Chemistry, Seepage, Streamflow, Reservoir Storage, and Annual Availability of Water for the San Juan-Chama Project

Base map from U.S. Government digital data

Geographic projection

North American Datum of 1983 (NAD 83)

Geology from Stoeser and others (2007)

106°55' 106°50' 106°45' 106°40'

37°20'

36°40'

37°00'

37°15'

37°10'

36°55'

36°50'

36°45'

37°5'

Channel

Tunnel

Unconsolidated surficial deposits and rocks of

 Quaternary Age

Sedimentary rocks of Tertiary Age

Igneous rocks of Tertiary Age

Sedimentary and Igneous rocks of Early Tertiary 

and Late Cretaceous Age

Cretaceous

Jurassic

Geologic units

Qa Modern alluvium

Qdo Older glacial drift

Ql Landslide deposits

Qb Basalt flows

Tsj San Jose Formation

Te Eocene prevolcanic sedimentary rocks

Tps Sedimentary units, Paleocene

Tbb Basalt flows and associated tuff, breccia, and

 conglomerate of late-volcanic bimodal suite

Taf Ash-flow tuff of main volcanic sequence

Tial Intra-ash flow andesitic lavas

Tpl Pre-ash-flow andesitic lavas, breccias, tuffs,

 and conglomerates

Tmi Middle tertiary intrusive rock

Tlv Volcanic rocks, lower Oligocene and Eocene

Animas Formation

Kd Dakota Sandstone and Burro Canyon Formation

Kls Lewis Shale

Km Mancos Shale

Kmv Mesaverde Group, undivided

Kpcl Picture Cliffs Sandstone and Lewis Shale

Jm Morrison Formation

Watershed boundary

EXPLANATION

Water diversion site and identifier—

U.S. Geological Survey streamflow-gaging station and

 identifier—

1

2

3

5

8

9

6

7

10

4

5

3

1

2

3

1. Rio Blanco near Pagosa Springs

1. Blanco Diversion

2. Little Oso Diversion

3. Oso Diversion

2. Navajo River at Banded Peak Ranch

3. Navajo River near Chromo

4. Little Navajo River at Chromo

5. Willow Creek above Azotea Creek

6. Horse Lake Creek above Heron Reservoir

7. Willow Creek near Park View

8. Azotea Tunnel Outlet

9. Willow Creek above Heron Reservoir

10. Willow Creek below Heron Reservoir

Navajo River

Little Navajo River Rio Blanco

Heron

Reservoir

Rio Chama

Willow

Creek

Azotea

Creek

Horse Lake Creek Azotea Tunnel Outlet

 

Taf

Tpl

Tpl

Tpl

Tpl

Tlv

Tps

Tbb

Taf

Taf

Tial

Ql

Ql

Ql

Te

Te

Te

Te

Te

Tpl

Te

Te

Tmi

Tmi

Tmi

Ql

Ql

Ql

Ql

Ql

Ql

Qb

Te TKa

TKa

TKa

TKa

Qdo

Qdo

Qdo

Kpcl

Kpcl Kpcl

Kpcl

Kpcl

Kpcl

Kmv

Kmv

Kmv

Km

Km

Kd

Kd

Kd

Kd

Kd Kd

Kd

Jm

Kd Kd

Km

Km

Km

Km

Km

Kpcl

Kmv

Km

Kmv

Kmv

Kmv

Kmv Kmv

Kls

Kls

Kls

Kls

Kls TKa

TKa Kls

Kmv

Kmv

Taf

Tmi

Tial Taf Taf

Qa

Qa

Qa

Qa

Kmv Ql

0 2 4 MILES

0 2 4 KILOMETERS

Figure 4. Surface geology of the study area, location of selected streamflow-gaging stations, and the boundary of selected 

watersheds, southern Colorado and northern New Mexico.
Introduction 9

and conduit system conveys water across the Continental 

Divide and discharges into Azotea and Willow Creeks (fig. 2). 

Azotea Creek and sections of Willow Creek between Azotea 

Tunnel Outlet and Heron Dam were channelized to prevent 

erosion caused by increased streamflow within the channel 

from the water diverted for the SJCP (Cannon, 1969). Heron 

Dam, constructed on Willow Creek just upstream from its 

confluence with the Rio Chama, and Heron Reservoir provide 

storage of SJCP water and allow for controlled releases to 

SJCP contractors.

The capacity of the infrastructure limits the amount 

of water that can be diverted from the Rio Blanco, Little 

Navajo River, and Navajo River to Heron Reservoir. Water 

from the Rio Blanco is diverted at Blanco Diversion into the 

Blanco Tunnel, which has a capacity of 520 cubic feet per 

second (ft3

/s) and extends approximately 9 mi to the Little 

Navajo River (Bureau of Reclamation, 2011a). Water from 

the Blanco Tunnel is combined with water diverted from the 

Little Navajo River through the Little Oso Feeder Conduit 

(capacity of 150 ft3

/s; location not shown on fig. 2) into the 

Oso Tunnel (Bureau of Reclamation, 2011a). The Oso Tunnel 

has a capacity of 550 ft3

/s and extends approximately 5 mi 

to the Navajo River (Bureau of Reclamation, 2011a). Water 

from the Oso Tunnel is combined with water diverted from 

the Navajo River through the Oso Feeder Conduit (capacity of 

650 ft3

/s; location not shown on fig. 2) into the Azotea Tunnel 

(Bureau of Reclamation, 2011a). Azotea Tunnel has a capacity 

of 950 ft3

/s and extends approximately 13 mi to Azotea Creek 

in the Rio Grande Basin (Bureau of Reclamation, 2011a). 

Heron Dam is an earthfill structure that is 269 ft high (Bureau 

of Reclamation, 2011b). The reported reservoir capacity is 

401,320 acre-ft (Bureau of Reclamation, 2011b); however, the 

storage capacity of Heron Reservoir has changed over time as 

the result of sediment accumulation. A bathymetry analysis 

of Heron Reservoir completed in 2010 calculated the total 

storage capacity of the reservoir as 428,355 acre-ft, with a 

surface area of 6,148 acres at a surface elevation of 7,190.8 ft 

(above the National American Vertical Datum of 1988 [NAVD 

88]) and a normal storage capacity of 400,031 acre-ft at a 

surface elevation of 7,186.1 (above NAVD 88) (Ferrari, 2011). 

Ferrari (2011) also reported a reduction of storage capacity 

from sediment accumulation of 2,151 acre-ft from October 

21, 1970, to July 2010. The dam outlet works, constructed on 

Willow Creek above the confluence with the Rio Chama, have 

a capacity of 4,160 ft3

/s (Bureau of Reclamation, 2011b).

Previous Studies

A review of previous investigations of water chemistry in 

the Rio Chama watershed indicated that limited information 

exists about the chemistry of SJCP water that flows into 

Heron Reservoir and the chemistry of water stored in Heron 

Reservoir. Additionally, little is known about groundwater/

surface-water interactions along the channel used to convey 

water from the San Juan River watershed to Heron Reservoir. 

Although the New Mexico Environment Department 

(NMED) conducts studies of water quality throughout 

New Mexico, NMED does not include the Willow Creek 

watershed above Heron Reservoir in its cyclic total 

maximum daily load sampling (New Mexico Environment 

Department, 2003). Langman and Anderholm (2004) 

studied the effects of reservoir installation and operation and 

introduction of SJCP water into the Rio Grande Basin on 

the streamflow and water chemistry of the Rio Chama and 

the Rio Grande in New Mexico. Langman and Anderholm 

(2004) reported a median specific-conductance value 

for water in Heron Reservoir of 312 microsiemens per 

centimeter (μS/cm) on the basis of four sampling events 

from 1987 and 1991. 

Reclamation prepared an update of the calculated yield 

of the SJCP in 1986. An operations model for the SJCP was 

developed by utilizing streamflow data, project infrastructure 

limitations, and estimates of scheduled water deliveries to 

determine the amount of water that could be diverted from 

the San Juan watershed and the amount of water available 

to be released from Heron Reservoir for the period from 

1935 to 1984. The average amount of water diverted from 

the streams of the SJCP to Heron Reservoir was calculated 

to be 108,900 acre-ft/yr, and the firm yield (the maximum 

attainable yield without allowing shortages in supply) 

from Heron Reservoir was calculated to be 94,200 acre-ft/

yr (Bureau of Reclamation, written commun., 1986). The 

report specified that in 1986 there were water contracts and 

commitments of 95,475 acre-ft/yr for the SJCP.

The firm yield estimate for Heron Reservoir was 

updated by Reclamation in 1999 to cover the interval from 

1935 to 1997, including an additional 13 years of data 

from 1984 to 1997 (Bureau of Reclamation, 1999). The 

firm yield was defined as “the optimal amount of water 

that could be diverted from the three tributary streams...

that would maintain regulated flows in the tributaries and 

meet the demands of the entities that had contracts for 

SJCP [sic] water without drying up the reservoir nor cause 

diverted water to be spilled from the reservoir′′ (Bureau of 

Reclamation, 1999). The revised firm yield estimate was 

96,500 acre-ft/yr; it was noted that this firm yield estimate 

was limited by flow conditions in 1978 and that the firm 

yield could not be increased without causing shortages in 

that year (Bureau of Reclamation, 1999). The average annual 

inflow to Heron Reservoir over the interval from 1935 to 

1997 was 119,650 acre-ft (Bureau of Reclamation, 1999). 

Methods of Analysis

Various methods were used to collect data for the study 

and to analyze the data. Water samples were collected to 

characterize the water chemistry. Discharge measurements 

were collected for the seepage investigation. Statistical 

methods were used for analysis of streamflow. 
10 Water Chemistry, Seepage, Streamflow, Reservoir Storage, and Annual Availability of Water for the San Juan-Chama Project

Water-Chemistry Data Collection

Water-chemistry data collected for this project were 

from 5 stations in the study area (table 2) and 2 stations in 

southern Colorado (Falk and others, 2013). Samples were 

collected from Azotea Tunnel Outlet, Willow Creek above 

Azotea Creek, Willow Creek above Heron Reservoir, Heron 

Reservoir, and Willow Creek below Heron Reservoir to 

characterize the chemistry of the diverted water. Samples 

were collected three times per year for 2 water years 

during low flow in October 2007 and November 2008 and 

during high flow (when SJCP water was diverted from the 

Rio Blanco, Little Navajo River, and Navajo River and 

streamflow is primarily composed of snowmelt runoff) 

in April and June of 2008 and May and June of 2009. 

Physical properties of the water (dissolved oxygen, pH, 

specific conductance, and temperature) were measured in 

the field. Samples were collected either as width-integrated 

samples if the water at the sampling station was not well 

mixed across the width of the section or as grab samples 

if the water at the sampling station was determined to be 

well mixed by observation of upstream conditions. The 

water at the sampling station was well mixed at stations 

that were downstream from flumes or the outlet works from 

the diversion dams that created turbulent flow. Samples 

from Heron Reservoir were collected with a Van Dorn 

water sampler, which is designed to collect a sample from 

a discrete depth interval. Samples were collected from a 

single depth interval if the reservoir was determined to be 

unstratified by observation of vertical profiles of physical 

parameters including temperature, specific conductance, 

and dissolved oxygen. If the reservoir was determined to be 

stratified, samples were collected from two depth intervals - 

a shallow sample collected above the thermocline and a deep 

sample collected below the thermocline. Samples collected 

from all stations were analyzed for major ions, alkalinity, 

trace elements, dissolved solids, and nutrients at the USGS 

National Water Quality Laboratory (NWQL) in Lakewood, 

Colo. The sample collected at Willow Creek below Heron 

Reservoir in August 2009 was also analyzed for a suite of 

organic compounds. 

The methods used for the analysis of major ions, 

nutrients, and semivolatile compounds are outlined in 

Fishman (1993) and Patton and Kryskalla (2011). The 

methods used for the analysis of trace elements are outlined 

in Fishman and Friedman (1989), Garbarino (1999), and 

Garbarino and others (2006). The method for analysis of 

dissolved organic carbon is outlined in Brenton and Arnett 

(1993). The method for analysis of mercury is outlined 

in Garbarino and Damrau (2001). The methods used for 

analysis of volatile organic compounds are outlined in 

Connor and others (1998). The methods for organochlorine 

pesticides and gross polychlorinated biphenyls (PCBs) are 

outlined in Wershaw and others (1987). The methods for 

waste-indicator compounds are outlined in Zaugg and others 

(2006). The methods for human-health pharmaceuticals 

are outlined in Furlong and others (2008). Water-chemistry 

data obtained for this study were collected, processed, and 

preserved in accordance with established USGS methods as 

outlined in the USGS National Field Manual for the Collection 

of Water-Quality Data (U.S. Geological Survey, variously 

dated). The quality-assurance procedures employed in this 

study included the collection of two field blanks to assess 

potential contamination during sample collection, processing, 

transport, or analysis. No notable contamination was observed 

in the field blanks. 

The USGS NWQL reports concentrations as quantitative, 

estimated, or censored (Childress and others, 1999). Results 

for analyte concentrations in a sample that are equal to or 

greater than the laboratory reporting level (LRL) are reported as 

quantitative values. The LRL is defined as two times the longterm method detection level (LT-MDL), where the LT-MDL is 

set to limit the occurrence of a false positive result in which an 

analyte is reported as a detection when it is not actually present. 

Results for analyte concentrations that are below the LT-MDL 

or not detected in the sample are censored and reported as 

less than (remark code “<′′) the LRL. Results for analyte 

concentrations that are below the LRL but greater than the 

LT-MDL are reported as estimated (remark code “E′′). Analytes 

that were analyzed for but not detected were reported as “U,′′ 

and analytes that were detected but not quantified were reported 

as “M.′′

This report also includes historical data available in the 

USGS National Water Information System database (U.S. 

Geological Survey, 2014). Historical water-chemistry data 

were checked for ion balance but were otherwise presumed to 

be correct. Unless stated otherwise, major-ion concentrations 

presented in this report refer to dissolved concentrations.

Results from particular water-chemistry samples were 

selected for evaluation because the samples were sequentially 

collected following a Lagrangian sampling scheme. Comparison 

of nutrient concentrations in water samples from Azotea Tunnel 

Outlet, Willow Creek above Azotea Creek, and Willow Creek 

above Heron Reservoir focused on the results from samples 

collected on May 5, 2009, and June 24, 2009. Evaluation of 

nutrient concentrations in water samples from Heron Reservoir 

focused on the results from samples collected in 2009. Analysis 

of nutrient concentrations in water samples from Willow Creek 

below Heron Reservoir focused on the results from samples 

collected in 2009 because these samples were closest in time to 

the samples collected on Heron Reservoir.

Surveys of water-chemistry along vertical profiles in Heron 

Reservoir were collected with a multimeter sonde instrumented 

with sensors for pH, specific conductance, dissolved oxygen, 

and temperature. The depth of the sonde was measured with a 

cloth tape attached along the length of the instrument cable, and 

measurements were collected at 5- to 10-ft intervals. Waterchemistry data obtained for this study with the sonde were 

collected in accordance with USGS methods described in the 

National Field Manual for the Collection of Water-Quality Data 

(U.S. Geological Survey, variously dated).
Introduction 11

Seepage Investigation

All discharge measurements collected during this study 

were made by using standard USGS discharge measurement 

protocols (Rantz, 1982a; Rantz, 1982b; and Nolan and 

Shields, 2000). Discharge measurements for the seepage 

investigation were collected with an acoustic Doppler current 

profile (ADCP) (Mueller and others, 2009) system mounted 

on a small trimaran or a temporary Parshall flume with a 

2-in. throat. Discharge measurements were also collected 

from an artesian well and from a discontinued streamflow 

gage at Willow Creek above Azotea Creek by using the 

existing Parshall flume with a 2-ft throat. All measurement 

locations were selected to optimize favorable flow conditions 

including uniform cross-section and nonturbulent flow. 

Discharge measurements during the seepage investigation 

were conducted in downstream order, with the amount of 

time between measurements at the different stations estimated 

from stream velocity and the distance between the stations 

so that the same water was measured throughout the reach. 

Seepage investigations were conducted in June 2010 during 

high flow and in August 2009 during low flow. High flow was 

defined as the interval during which SJCP water was diverted 

from the Rio Blanco, Little Navajo River, and Navajo River 

and generally occurred from April to August. Low flow was 

defined as the interval during which SJCP water was not 

diverted to Azotea Tunnel. Streamflow at Azotea Tunnel 

Outlet generally decreased from July through October because 

the amount of water available for diversion for the SJCP 

declined. There was no streamflow at Azotea Tunnel Outlet 

from November to February because the gates on the SJCP 

diversion structures were closed for maintenance and safety 

reasons (V. Salazar, Manager of the Chama New Mexico 

Field Division, Bureau of Reclamation, oral commun., 2009). 

Analysis of streamflow at Azotea Tunnel Outlet for water 

year 2009 indicated that diversions ended by mid-August. 

The majority of streamflow at Willow Creek above Heron 

Reservoir is from Azotea Tunnel Outlet because there is little 

to no flow from tributaries between these two stations.

The gain or loss in streamflow resulting from interaction 

with groundwater can be calculated as the residual difference 

of all other gains and losses of streamflow in the reach 

(modified from Simonds and Sinclair, 2002) by using the 

following equation:

Qs = Qds – Qus – Qin – Qout (1)

where

Qs is the net gain or loss in streamflow resulting 

from groundwater interaction in cubic feet per 

second;

Qds is streamflow measured at the downstream end of 

the reach in cubic feet per second;

Qus is streamflow measured at the upstream end of the 

reach in cubic feet per second;

Qin is the sum of inflows along in the reach such 

as inflow from tributaries or bank storage in 

cubic feet per second; and

Qout is the sum of outflows along the reach such as 

outflow to diversions and bank storage or 

evaporative losses from the water surface in 

cubic feet per second. 

Streamflow at Azotea Tunnel Outlet and Willow Creek 

above Heron Reservoir, primarily a function of the operation 

of the SJCP, is generally not steady. Over the interval of 

a day, streamflow at Azotea Tunnel Outlet is generally 

unsteady with a diurnal variation. Seasonally, streamflow 

at Azotea Tunnel Outlet is unsteady and varies daily such 

that streamflow is either increasing or decreasing without 

an interval of nearly constant flow (fig. 5A). Additionally, 

diversion of water for the SJCP generally occurs from early 

spring to early fall, and there is not an interval of natural 

base flow at Azotea Tunnel Outlet. Streamflow at Willow 

Creek above Heron Reservoir is primarily composed of 

water from Azotea Tunnel Outlet, with an additional small 

component of intermittent streamflow derived from the 

Willow Creek watershed; therefore, streamflow at Willow 

Creek above Heron Reservoir exhibits similar daily and 

seasonal variability in streamflow as streamflow at Azotea 

Tunnel Outlet. 

Because of variable streamflow at Azotea Tunnel 

Outlet and Willow Creek above Heron Reservoir, discharge 

measurements for the high-flow seepage investigation were 

collected over an interval of 8 hours (table 3). Data from the 

upstream and downstream stations were correlated to account 

for the water traveltime along the channel and the change 

in streamflow during the interval of measurement (fig. 5B). 

Adjustment for the water traveltime ensured that the same 

packet of water was being compared between the stations. 

For the high-flow seepage investigation, discharge 

measurements were made at four stations with appreciable 

flow on June 24, 2010. Discharge measurements at Azotea 

Tunnel Outlet and Willow Creek above Heron Reservoir 

were collected at 30-minute intervals over 7 hours (fig. 

5B). Two ADCP systems were used to make concurrent 

measurements at the upstream and downstream stations. 

The ADCPs were connected to draglines spanning the 

stream width and were manually maneuvered across the 

stream. The two ADCP systems also were used to make 

concurrent measurements at the same stream section so 

that the measurements could be compared to determine 

the uncertainty between the two systems. The discharge 

measurements made with the two instruments were 

within the standard deviation (Helsel and Hirsch, 2002) 

of the repeated measurements; therefore, the discharge 

measurements made with the two instruments were not 

significantly different. The measured discharge was 

calculated as the average of multiple sequential discharge 

measurements (at least 3), and the uncertainty was calculated 

as 5 percent of the measurement. Because of the age and 

condition of the flume at the discontinued station at Willow 
12 Water Chemistry, Seepage, Streamflow, Reservoir Storage, and Annual Availability of Water for the San Juan-Chama Project

200

0

400

600

800

1,000

1,200

8 10 12 14 16 18 20

Time, in hours

Date

160

150

152

154

156

158

148

146

144

142

140

Discharge, in cubic feet per second Streamflow at Azotea Tunnel Outlet, in cubic feet per second

21 3 4 5 6 7 8

June 24, 2010

9 10 11 12 13 14 15

June

16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

EXPLANATION

B. Discharge measurements from near Azotea Tunnel Outlet and near the Willow Creek

 above Heron Reservoir station over time, June 24, 2010

EXPLANATION

Measured streamflow at Azotea Tunnel Outlet

Near the Willow Creek above Heron Reservoir station

Near Azotea Tunnel Outlet

A. Streamflow at Azotea Tunnel Outlet, June 2010

Figure 5. Streamflow measurements from Azotea Tunnel Outlet and the high-flow seepage investigation, northern New Mexico, June 

2010. A, Streamflow at Azotea Tunnel Outlet, June 2010. B, Discharge measurements from near Azotea Tunnel Outlet and near the 

Willow Creek above Heron Reservoir station over time, June 24, 2010.
Introduction 13

Creek above Azotea Creek, the accuracy was estimated to be 

greater than 10 percent, and the estimated uncertainty was 

plus or minus 0.5 ft3

/s at a measured discharge of 5 ft3/s or 

less. Flow from an artesian well that flows into the channel 

was measured several times over the day and was determined 

to be less than 0.1 ft3

/s; thus, flow from the artesian well was 

not included in the calculations for the high-flow seepage 

investigation because it was 2 orders of magnitude less 

than the calculated streamflow and associated uncertainty. 

Streamflow at the discontinued station at Willow Creek 

above Azotea Creek was measured three times during the 

day. Evaporative losses were assumed to be minimal over the 

short distance between the measurement stations and the short 

interval of time over which measurements were collected. 

Accounting for evaporative losses would decrease a calculated 

gain of streamflow to the stream and increase a calculated loss 

of streamflow from the stream. Change in bank storage during 

the seepage investigation also was assumed to be minimal 

along the channel because the change in gage height at the 

upstream station was less than 1 ft, and the change in gage 

height at the downstream station was approximately 0.1 ft.

For the low-flow seepage investigation, discharge 

measurements were made at six stations with measurable 

flow on August 19, 2009, through August 21, 2009, when 

streamflow at Azotea Tunnel Outlet and Willow Creek above 

Heron Reservoir was extremely low. Streamflow resulting 

from groundwater interaction was determined for two reaches: 

Reach 1 covered the interval from a quarter mile downstream 

from Azotea Tunnel Outlet (referred to as “LF-Upstream′′) to 

Willow Creek above the confluence with Willow Creek above 

Azotea Creek (referred to as “LF-Midpoint′′), and Reach 

2 covered the interval from LF-Midpoint to one-tenth of a 

mile upstream from Willow Creek above Heron Reservoir 

station (referred to as “LF-Downstream′′). The discharge 

measurements for the low-flow seepage investigation included 

measurements collected with a 2-in. modified Parshall 

flume and measurement of the flow from an artesian well 

that flows into the channel. The estimated uncertainty for 

discharge measurements collected at the LF-Upstream and 

LF-Midpoint stations is 30 percent because of the extremely 

low streamflow quantity and poor channel conditions for 

the installation of the flume including a wide channel and 

sandy sediments. The estimated uncertainty for discharge 

measurements at the LF-Downstream station was 50 percent 

because of the extremely low streamflow quantity and poor 

channel conditions for installation of the flume that resulted 

in some portion of the streamflow bypassing the flume. The 

estimated uncertainty for the artesian well was 50 percent 

because the rate of flow was estimated by measuring the time 

required to fill a bucket, a method that lacks precision. It was 

assumed that evaporative losses were minimal over the short 

distance between the measurement stations; accounting for 

evaporative losses would decrease a computed gain to the 

stream system and increase a computed loss from the stream. 

It was also assumed that the change in bank storage was 

minimal along the channel because streamflow was steady 

during the investigation. Several measurements were made at 

the LF-Midpoint station over a 24-hour period to determine 

the variation in streamflow; measured discharge ranged from 

0.03 to 0.02 ft3

/s (table 4). 

Table 3. Measurements from the high-flow seepage investigation at Azotea Tunnel Outlet, Willow Creek above Heron Reservoir, and 

Willow Creek above Azotea Creek and the calculated streamflow resulting from groundwater interaction and associated uncertainty, 

northern New Mexico, June 24, 2010.

[Measurement time, time of the discharge measurement; ft3/s, cubic feet per second; --, no discharge at measurement location]

Pair

Azotea Tunnel Outlet Willow Creek above Azotea Creek Willow Creek above Heron Reservoir

Measurement 

time

Discharge 

(ft3

/s)

Measurement 

time

Discharge 

(ft3/s)

Measurement 

time

Discharge 

(ft3/s)

1 10:04 153 10:18 3.9 13:02 153

2 10:32 152 -- -- 13:37 153

3 11:01 153 -- -- 14:00 151

4 11:32 149 -- -- 14:37 151

5 12:01 150 -- -- 15:09 148

6 12:33 146 -- -- 15:40 145

7 13:01 147 -- -- 16:07 148

8 13:33 146 -- -- 16:37 145

9 14:02 143 15:38 3.5 17:09 141

10 14:33 144 -- -- 17:37 142

11 15:01 142 16:38 3.5 18:08 139
14 Water Chemistry, Seepage, Streamflow, Reservoir Storage, and Annual Availability of Water for the San Juan-Chama Project

Table 4. Low-flow seepage investigation results including the discharge measurements and associated uncertainty for Reach 1 from 

1/4 mile downstream from the outlet of Azotea Tunnel to Willow Creek above the confluence with Willow Creek above Azotea Creek 

and Reach 2 from Willow Creek above the confluence with Willow Creek above Azotea Creek to 1/10 mile upstream from Willow Creek

above Heron Reservoir station and the calculated streamflow resulting from groundwater interaction and associated uncertainty, 

northern New Mexico, August 19–21, 2009.

[ft3/s, cubic feet per second; --, no discharge at measurement location]

Reach 1

Willow Creek above the 

confluence with Willow 

Creek above Azotea Creek 

(LF-Midpoint) 

1/4 mile downstream 

from Azotea Tunnel Outlet 

(LF-Upstream) 

Artesian well

Gain or loss in streamflow 

resulting from groundwater interaction Remark

Streamflow 

(ft3

/s)

Uncertainty

(ft3/s)

Streamflow

(ft3/s)

Uncertainty

(ft3/s)

Streamflow 

(ft3/s)

Uncertainty 

(ft3/s)

Streamflow

(ft3/s)

Uncertainty 

(ft3/s)

0.03 0.02 0.02 0.01 0.04 0.02 -0.03 0.03 Maximum downstream 

discharge during 

interval

0.02 0.01 0.02 0.01 0.04 0.02 -0.04 0.02 Minimum downstream 

discharge during 

interval

Reach 2

1/10 mile upstream 

from Willow Creek above 

Heron Reservoir station 

(LF-Downstream) 

Willow Creek above the 

confluence with Willow 

Creek above Azotea Creek 

(LF-Midpoint) 

Willow Creek above 

Azotea Creek

Gain or loss in streamflow 

resulting from groundwater interaction Remark

Streamflow

(ft3

/s)

Uncertainty 

(ft3/s)

Streamflow

(ft3/s)

Uncertainty

(ft3/s)

Streamflow 

(ft3/s)

Uncertainty

(ft3/s)

Streamflow 

(ft3/s)

Uncertainty 

(ft3/s)

0.08 0.04 0.03 0.02 -- 0.05 0.04 Maximum upstream 

discharge during 

interval

0.08 0.04 0.02 0.01 -- 0.06 0.04 Minimum upstream 

discharge during 

interval

Groundwater interaction for Reach 1 and Reach 2

1/10 mile upstream 

from Willow Creek above 

Heron Reservoir station 

(LF-Downstream) 

1/4 mile downstream 

from Azotea Tunnel Outlet 

(LF-Upstream) 

Artesian well

Gain or loss in 

streamflow resulting from 

groundwater interaction Remark

Streamflow 

(ft3

/s)

Uncertainty 

(ft3/s)

Streamflow

(ft3/s)

Uncertainty

(ft3/s)

Streamflow

(ft3/s)

Uncertainty 

(ft3/s)

Streamflow 

(ft3/s)

Uncertainty 

(ft3/s)

0.08 0.04 0.02 0.01 0.04 0.02 0.02 0.05 Maximum downstream 

discharge during 

interval
Introduction 15

Estimates of uncertainty for the seepage investigations 

include the uncertainty between the upstream and downstream 

measurement equipment and the uncertainty associated with 

each discharge measurement. The estimated uncertainty for 

the calculated streamflow resulting from groundwater/surfacewater interactions was calculated by using the formula for 

the propagation of uncertainty for independent uncertainty 

(Taylor, 1997):

(2)

where

s is the cumulative uncertainty for the calculated 

gain or loss in streamflow resulting from 

groundwater/surface-water interaction; and 

A, B, and C are the estimated uncertainty for the individual 

discharge measurements. 

For the high-flow seepage investigation, the estimated 

cumulative uncertainty was calculated as plus or minus 10 

ft3

/s. For the low-flow seepage investigation, the estimated 

cumulative uncertainty ranged from 0.02 ft3

/s to 0.05 ft3/s 

depending on the reach.

Analysis of Streamflow

The streamflow data compiled for this report were 

collected by the USGS, the Colorado Division of Water 

Resources (CDWR), and Reclamation. Data were either 

requested from the collecting agency or obtained from an 

agency-supported Web-accessible database. It was assumed 

that all data were reviewed for accuracy and correctness, 

and no attempt was made to evaluate the quality of the data. 

Information for all stations is presented in table 2.

Prior to 1971, the USGS collected streamflow data at 

stations on Willow Creek. Mean daily streamflow values for 

the stations at Willow Creek above Heron Reservoir near Los 

Ojos, N. Mex. (Willow Creek above Heron Reservoir), Willow 

Creek near Park View, N. Mex. (Willow Creek near Park 

View), and Horse Lake Creek above Heron Reservoir near 

Los Ojos, N. Mex. (Horse Lake Creek) (fig. 2) were calculated 

by following USGS streamflow measurement protocols 

described by Rantz (1982a, b). After 1971, streamflow data for 

Azotea Tunnel at Outlet near Chama, N. Mex. (Azotea Tunnel 

Outlet), Willow Creek above Heron Reservoir, and Willow 

Creek below Heron Dam, N. Mex. (Willow Creek below 

Heron Reservoir) were provided to the USGS by Reclamation 

as calculated mean daily streamflow. Stage data for Heron 

Reservoir near Los Ojos, N. Mex. (Heron Reservoir), were 

provided to the USGS by Reclamation as the daily observation 

at 2400 hours (midnight). The mean daily streamflow values 

and daily observations of reservoir stage for the stations 

reported to the USGS were retrieved from the USGS National 

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

2014). Additional data used for computations in this report 

were retrieved from the CDWR online database (Colorado 

Division of Water Resources, 2011) and are detailed in Falk 

and others (2013).

Nonparametric statistical methods, which are dependent 

on the relative position of numerically ranked data (Helsel and 

Hirsch, 2002), were applied to calculate annual and monthly 

summary statistics for streamflow at selected stations. Median 

annual streamflow was calculated for all stations on the basis 

of the calendar year for all years with complete records. 

Median monthly streamflow was calculated for each month 

with a complete record of streamflow.

Nonparametric statistical methods, including calculation 

of percentiles, were applied to describe streamflow. The 50th 

percentile annual streamflow duration (Q50, the median) is 

the volume of streamflow exceeded 50 percent of the time 

over the period of record being analyzed. Similarly, the 

75th percentile (Q75) means that 75 percent of the annual 

streamflow for the period of record is less than or equal to 

the streamflow at the 75th percentile, and the 25th percentile 

(Q25) means that 25 percent of the annual streamflow for 

the period of record is less than or equal to the streamflow 

at the 25th percentile. The range of flows between the 25th 

and 75th percentiles, or interquartile range (IQR), represents 

50 percent of the annual streamflow duration and indicates 

the distribution statistical dispersion of the data. Variation in 

streamflow can be characterized as the range of values in the 

IQR: the larger the range of values in the IQR, the greater the 

variation in streamflow. The IQR can be compared between 

streams after normalizing the IQR to the median streamflow 

to determine the coefficient of variation (COV), which is a 

measure of the distribution of the annual streamflow. 

Annual variation in streamflow was evaluated with 

annual flow-duration curves. Flow-duration curves, or 

cumulative frequency curves, show the percentage of time 

that a specific streamflow is equaled or exceeded during a 

given period (Searcy, 1959). Flow-duration curves based on 

streamflow data that are representative of long-term flow 

can be used to indicate future streamflow and to estimate 

the probability that a specific streamflow will be equaled or 

exceeded in the future (Searcy, 1959). Annual flow-duration 

curves were constructed for this report by ranking the annual 

streamflow over the period of record from largest to smallest 

and calculating the exceedance probability by using the 

Weibull formula for calculating plotting positions (Helsel and 

Hirsch, 2002).

The length of time that water was diverted and the 

amount of water that was diverted each day were calculated 

with nonparametric statistical methods from daily streamflow 

data from Azotea Tunnel Outlet. The ordinal day of the year 

on which selected percentiles of annual flow occurred was 

determined for Azotea Tunnel Outlet. The selected percentiles 

included the first day of measurable streamflow at Azotea 

Tunnel Outlet, the ordinal day of the 25th percentile of annual 

streamflow, the ordinal day of the 75th percentile of annual 

streamflow, and the ordinal day of the 90th percentile of 

annual streamflow. The length of time that water was diverted 

to Azotea Tunnel Outlet was determined for two intervals:

s = + A B +C 2 2 2
16 Water Chemistry, Seepage, Streamflow, Reservoir Storage, and Annual Availability of Water for the San Juan-Chama Project

(1) the duration for the IQR of annual streamflow, defined as the 

number of days between the ordinal day on which 25 percent of 

the annual streamflow occurred to the ordinal day on which 75 

percent of the annual streamflow occurred, and (2) the duration 

for 90 percent of the annual streamflow, defined as the number 

of days between the start of streamflow and the ordinal day on 

which 90 percent of the annual streamflow occurred.

Trends in streamflow were evaluated with the MannKendall trend test, a nonparametric rank-based method that 

determines if a parameter monotonically increases or decreases 

compared to another parameter (Helsel and Hirsch, 2002). 

Monotonic increases or decreases (trends) were considered 

statistically significant at a p-value less than or equal to 0.05. 

The p-value is “the probability of obtaining the calculated 

test statistic or one even less likely, when the null hypothesis 

is true... and the lower the p-value, the stronger is the case 

against the null hypothesis′′ (Helsel and Hirsch, 2002, p. 108). 

The null hypothesis is “that there is no trend′′ (Helsel and 

Hirsch, 2002, p. 324). Kendall’s tau “measures the strength of 

the monotonic relationship′′ (Helsel and Hirsh, 2002, p. 212) 

between two parameters. The strength of the monotonic relation 

was determined from the magnitude of tau and was classified as 

very weak for tau values less than 0.20, weak for tau values of 

0.21–0.40, moderate for tau values of 0.41–0.60, strong for tau 

values of 0.61–0.80, and very strong for tau values of 0.81 or 

greater. The sign of tau indicated whether the trend was negative 

or positive. These data were also tested for autocorrelation by 

regressing the lagged residuals of the variable over time for a 

significant trend, as described by Helsel and Hirsch (2002). All 

data included in the trend analysis were determined not to be 

significantly autocorrelated. 

Water Chemistry 

Water chemistry in the streams, channels, and reservoirs 

in the study area likely is influenced by the geology in the 

watersheds. The water chemistry, as represented by the ion 

composition, is a function of the source of the water, which 

is generally precipitation, and will evolve as a function of the 

mineral assemblages that are present in the rocks and ions 

already present in the water (Hem, 1989). The ion composition 

is affected by the mineral assemblage of the rocks and 

sediments with which the water comes into contact and the 

occurrence of biologic and biochemical processes (Hem, 1989). 

The specific conductance, a measure of the ability of a fluid 

to conduct electrical current, increases as minerals from rocks 

and sediments dissolve into the water (Hem, 1989). Generally, 

the most abundant cations present in water are calcium (Ca), 

magnesium (Mg), sodium (Na), and potassium (K), and the 

most abundant anions present in water are chloride (Cl), fluoride 

(F), bicarbonate (HCO3

), and sulfate (SO4) (Hem, 1989). The 

specific conductance typically is representative of the dissolvedsolids concentration of the water (Hem, 1989). Water that is 

diverted to the SJCP from the Rio Blanco, Little Navajo River, 

and Navajo River likely is also influenced by the geology of 

these watersheds. The chemical composition of precipitation 

across the study area is likely to be equivalent because the 

topography across the region is similar, and the precipitation 

generally is derived from the same storm systems (Ingersoll 

and others, 2008). As the precipitation makes contact with 

the ground, either as surface runoff or as infiltration into 

the subsurface, it is exposed to the geologic material in the 

watershed. Concentrations of dissolved solids and SO4

 in 

water in contact with Cretaceous-age marine shale generally 

increase along the groundwater flow path (Azimi-Zonooz and 

Duffy, 1993). Apodaca (1998) noted that the geochemical 

composition of water from the Mancos Shale near the study 

area is dominated by Ca and SO4

.

The chemistry of water in the study area varies by the 

geology along the flow path. A water type was determined 

for samples from Azotea Tunnel Outlet, Willow Creek above 

Azotea Creek, Willow Creek above Heron Reservoir, Heron 

Reservoir, and Willow Creek below Heron Reservoir on the 

basis of the concentrations of the major ions. Representative 

water compositions for each sample station were selected 

because the composition is typical of the chemical 

composition of all samples from the station and were plotted 

on a trilinear diagram along with the chemical composition 

of representative samples from Rio Blanco below Diversion, 

Navajo River below Diversion, and Horse Lake Creek 

(data from Falk and others, 2013) (fig. 6). The predominant 

composition, expressed in milliequivalents per liter (the 

concentration of the ion species expressed as the molar 

concentration normalized by the ionic charge; Hem, 1989), 

must be greater than 40 percent of the total. If no cation or 

anion was predominant, the water was classified either as the 

two most common ions or as “mixed′′ if the ions were present 

in nearly equal portions. Water from Azotea Tunnel Outlet was 

either HCO3

-Ca/Na/K type or HCO3-Na/K type depending 

on flow. Water from Willow Creek above Azotea Creek was 

HCO3/SO4

-Ca/Mg type, and water from Willow Creek above 

Heron Reservoir was either HCO3

-Ca/Na/K type or SO4-Ca/

Mg type depending on flow. Water from Heron Reservoir and 

Willow Creek below Heron Reservoir was HCO3

-Ca type for 

both high flow and low flow (fig. 6). Major- and trace-element 

concentrations in samples collected for this study were less 

than the U.S. Environmental Protection Agency’s primary and 

secondary drinking-water standards and less than the NMED 

surface-water standards for Heron Reservoir and perennial 

reaches of tributaries to the Rio Chama in the study area 

(U.S. Environmental Protection Agency, 2011; New Mexico 

Environment Department, 2011). All water-chemistry data are 

presented in appendix 1.

The specific conductance of water is proportional to 

dissolved-solids concentration in water (Hem, 1989). The 

specific conductance for all stations ranged from 71 to 

1,060 microsiemens per centimeter (μS/cm) (fig. 7A). The 

largest range in values of specific conductance was in water 

from Azotea Tunnel Outlet, Willow Creek above Azotea 

Creek, and Willow Creek above Heron Reservoir. Water from 
Water Chemistry 17

Calcium (Ca)

Percentage of total milliequivalents per liter

Magnesium (Mg)

Calcium (Ca) + magnesium (Mg)

Percentage of total milliequivalents per liter

Chloride (Cl) + fluoride (F) + sulfate (SO4

)

Percentage of total milliequivalents per liter

Chloride (Cl) + fluoride (F)

100

80

60

40

20

0

0 20 40 60 80 100

0

20

40

60

80

100

Sodium (Na) + potassium (K)

Sulfate (SO4

)

Bicarbonate (HCO3

) + carbonate (CO3

)

0

20

40

60

80

100

0

0

20

40

60

80

100 100

80

60

40

20

0

0

20

40

60

80

80

60

40

20

0

100

100

20

40

60

80

100

MIXED MIXED

SO4

HCO Cl-F 3

HCO3

 - SO4

Mg

Ca

Ca - Mg

Na - K

Ca - Na - K

100

80

60

40

20

0

EXPLANATION

Willow Creek above Heron Reservoir - low flow

Willow Creek below Heron Reservoir

Horse Lake Creek

Willow Creek above Azotea Creek

Willow Creek above Heron Reservoir - high flow

Navajo River below Oso Diversion

Rio Blanco below Blanco Diversion

Azotea Tunnel Outlet - high flow Heron Reservoir

Azotea Tunnel Outlet - low flow

Mixing line for water at

Azotea during high flow

Figure 6. Representative water composition from selected stations in the study area, northern New Mexico and representative water 

composition from selected stations from Rio Blanco and Navajo River, southern Colorado.
18 Water Chemistry, Seepage, Streamflow, Reservoir Storage, and Annual Availability of Water for the San Juan-Chama Project Concentration, in milligrams per liter Concentration, in milligrams per liter

Sample site

Sample site

2 3 4 5

1

10

100

1,000

1

10

1,000

10

100

1,000

10,000

1 3 2 4 5

Bicarbonate (HCO3) Chloride (Cl)

Calcium (Ca) Magnesium (Mg)

1

10

100

1 1 2 3 4 5

0.01

0.1

1

Fluoride (F)

10

10

1,000

Sulfate (SO4)

100

100

Sodium (Na)

1

10

100 Potassium (K)

0.1

1

10

B. Major-ion concentrations

1 2 3 4 5

Specific conductance, in microsiemens per centimeter

Specific conductance value during

 low-flow

Specific conductance value during

 high-flow

Sample site

1. Azotea Tunnel Outlet

2. Willow Creek above Azotea Creek

3. Willow Creek above Heron Reservoir

4. Heron Reservoir

5. Willow Creek below Heron Reservoir

EXPLANATION

Sample site

Low-flow

High-flow

EXPLANATION

1 3 2 4 5 1 3 2 4 5

0.1

1

100

10

1 3 2 4 5 1 3 2 4 5

1 2 3 4 5

A. Specific conductance values

Figure 7. Variation of water-chemistry at selected sampling stations in the study area, northern New Mexico, 1973– 2009. A, Specificconductance values. B, Major-ion concentrations.
Water Chemistry 19

Azotea Tunnel Outlet ranged from 71 to 551 μS/cm (fig. 7A), 

water from Willow Creek above Azotea Creek ranged from 

110 to 746 μS/cm (fig. 7A), and water from Willow Creek 

above Heron Reservoir ranged from 84 to 1,060 μS/cm (fig. 

7A). The specific conductance value of water from Heron 

Reservoir and Willow Creek below Heron Reservoir was 

approximately 210 μS/cm (fig. 7A). 

Azotea Tunnel Outlet and Willow Creek above 

Heron Reservoir

The chemistry of water from Azotea Tunnel Outlet 

and Willow Creek above Heron Reservoir varied with 

streamflow. Water at Azotea Tunnel Outlet during high 

flow was a mixture of water diverted from Rio Blanco, 

Little Navajo River, and Navajo River. The proportion of 

water derived from each stream varied on the basis of water 

availability and the legal, logistical, and structural limitations 

of the SJCP. Water at Azotea Tunnel Outlet during low flow 

was discharged at Azotea Tunnel Outlet after diversions for 

the SJCP were terminated and was generally a small volume 

typically less than 2 ft3

/s. The source of streamflow at Azotea 

Tunnel Outlet during low flow when water is not diverted 

from the SJCP is unknown; however, it likely originates 

in Azotea Tunnel. The majority of streamflow at Willow 

Creek above Heron Reservoir is from Azotea Tunnel Outlet 

because there is little to no flow from tributaries between 

these two stations. Water at Willow Creek above Heron 

Reservoir during high flow and low flow was a mixture of 

water from Azotea Tunnel Outlet and inflow from tributaries. 

Additional analyses of seasonal variation in streamflow are 

presented in later sections (see section “Variation and Trends 

in Streamflow for the San Juan-Chama Project′′).

During high flow, the specific conductance and ionic 

composition of water from Azotea Tunnel Outlet were 

similar to water from streams that contribute to the SJCP 

(figs. 7A and 8A). The variation in specific conductance 

of water from Azotea Tunnel Outlet was similar to water 

from Rio Blanco and Navajo River (fig. 8A). The ionic 

composition of water from Azotea Tunnel Outlet during high 

flow was similar to the composition of a mixture of water 

from Rio Blanco and Navajo River (Falk and others, 2013), 

such that the ionic composition of Azotea Tunnel Outlet fell 

on a line between the ionic composition of water from Rio 

Blanco and Navajo River (fig. 6). The similarity of the ionic 

composition of water from Azotea Tunnel Outlet to the ionic 

composition of a mixture of water from the Rio Blanco and 

Navajo River indicated that no substantial chemical changes 

or mixing with water from other sources occurred during 

transit through the SJCP infrastructure during high flow. 

Water from Azotea Tunnel Outlet during low flow 

was compositionally different from water from streams 

that contribute to the SJCP and waters from the streams 

in the Willow Creek Basin including Horse Lake Creek 

and Willow Creek above Azotea Creek (fig. 6). Higher 

specific conductance values in water from Azotea Tunnel 

Outlet generally occurred during low flow than during high 

flow (figs. 7A and 8B). Concentrations of Ca, Na, HCO3

, 

and Cl were highest during low flow (fig. 7B). There was 

proportionately more Na, K, and Cl and proportionately less 

SO4

 and Mg in water from Azotea Tunnel Outlet during low 

flow than in water from any other station in the study area 

(fig. 6). Analysis of water-chemistry data from Azotea Tunnel 

Outlet during low flow suggests the existence of either 

another source of water other than the streams that contribute 

to the SJCP or the occurrence of a chemical process within 

Azotea Tunnel that changes the chemistry of the water. There 

is insufficient data to determine what that source of water 

might be or what chemical effect could occur; however, a 

possible source of water may be seepage of water along the 

13-mi length of the concrete-lined Azotea Tunnel. 

During high flow, small but distinct differences in 

ion composition and specific conductance existed in water 

from Willow Creek above Heron Reservoir compared 

to water from Azotea Tunnel Outlet. There was larger 

proportion of HCO3

 (fig. 6) and a slightly larger range of 

specific conductance values in water from Willow Creek 

above Heron Reservoir during high flow than in water 

from Azotea Tunnel Outlet (fig. 7A). Comparison of paired 

samples collected from Azotea Tunnel Outlet and Willow 

Creek above Heron Reservoir on the same day in 2008 

and 2009 indicated that the concentrations of major ions, 

with the exception of F for all dates and alkalinity for May 

2009, increased downstream, such that the concentrations 

of major ions were greater in water from Willow Creek 

above Heron Reservoir relative to water from Azotea 

Tunnel Outlet. Although the volume of surface-water 

inflows along the channel between Azotea Tunnel Outlet 

and Willow Creek above Heron Reservoir were generally 

small (approximately 2 orders of magnitude) relative to 

the volume of water in the channel, the dissolved-solids 

concentrations and major-ion concentrations of inflows 

derived from the Willow Creek watershed, such as Willow 

Creek above Azotea Creek, were generally much greater 

than concentrations at Willow Creek above Heron Reservoir 

(figs. 7A and 7B). There were increased concentrations of 

Mg and SO4

 in water sampled from the Willow Creek above 

Heron Reservoir relative to water from Azotea Tunnel Outlet 

(fig. 7B).The Willow Creek watershed is underlain mostly 

by the Mancos Shale (Cretaceous-age marine shale; fig. 4). 

It is likely that the dissolved-solids concentration of water 

at stations in the Willow Creek watershed was elevated 

relative to the dissolved-solids concentration in water from 

other watersheds not underlain by Mancos Shale (figs. 7A

and 7B). During high flow, water at Willow Creek above 

Heron Reservoir was likely a mixture of water from Azotea 

Tunnel Outlet and a small volume of surface-water inflows 

from tributaries with large concentrations of dissolved solids 

such as Willow Creek above Azotea Creek. Simple mixing 

calculations indicated that small (less than 5 ft3

/s) volumes 

of water from Willow Creek above Azotea Creek could 
20 Water Chemistry, Seepage, Streamflow, Reservoir Storage, and Annual Availability of Water for the San Juan-Chama Project

A. Average specific conductance by month for stations Rio Blanco, Navajo River, Azotea

 Tunnel Outlet, Willow Creek above Heron Reservoir, and Willow Creek near Park View 

B. Median monthly streamflow versus average monthly specific conductance for stations

 Azotea Tunnel Outlet and Willow Creek above Heron Reservoir

5,000

500

50 Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Month

Monthly average specific conductance, in

microsiemens per centimeter 

2,000

200

20

10 100 1,000 10,000 100,000

Median monthly streamflow, in acre-feet, axis is log scale

Monthly average specific conductance, in

microsiemens per centimeter 

EXPLANATION

EXPLANATION

Azotea Tunnel Outlet low flow

Willow Creek above Heron

 Reservoir low flow

Azotea Tunnel Outlet high flow

Willow Creek near

 Park View

Rio Blanco

Azotea Tunnel Outlet

Navajo River Willow Creek above

 Heron Reservoir 

Willow Creek above Heron

 Reservoir high flow

Figure 8. Monthly variation in specific-conductance values for selected stations and comparison of the variation of the average 

monthly specific conductance to the median monthly streamflow, with monthly streamflow categorized as high flow or low flow, for 

selected stations in the study area, northern New Mexico, 1973–2009. A, Average specific conductance by month for stations Rio 

Blanco, Navajo River, Azotea Tunnel Outlet, Willow Creek above Heron Reservoir, and Willow Creek near Park View. B, Median monthly 

streamflow as a function of monthly average specific conductance for stations Azotea Tunnel Outlet and Willow Creek above Heron 

Reservoir.
Water Chemistry 21

increase major-ion concentrations in water from Willow Creek 

above Heron Reservoir relative to water from Azotea Tunnel 

Outlet and could result in the difference in the ion composition 

observed between water from Azotea Tunnel Outlet and Willow 

Creek above Heron Reservoir.

Water from Willow Creek above Heron Reservoir during 

low flow was compositionally similar, with the exception of 

increased proportions of Cl and F and a decreased proportion 

of HCO3

, to water from streams in the Willow Creek Basin, 

including Horse Lake Creek and Willow Creek above Azotea 

Creek (fig. 6). Generally, the specific conductance was 

increased relative to high flow, and the concentrations of most 

major ions were highest during low flow (figs. 7A and 7B). 

During low flow, water at Willow Creek above Heron Reservoir 

was likely derived primarily from surface-water inflows from 

tributaries with large concentrations of dissolved solids such as 

Willow Creek above Azotea Creek and a small volume of water 

from Azotea Tunnel Outlet. The monthly variation in average 

specific conductance of water from Willow Creek near Park 

View is likely representative of water derived from the Willow 

Creek watershed (fig. 8A). Additionally, water at Willow Creek 

above Heron Reservoir was possibly affected by evaporative 

and chemical processes during transit from Azotea Tunnel 

Outlet that altered the proportion of ions, increasing Cl and 

decreasing HCO3

 concentrations relative to the concentrations 

of other ions. 

In general, surface-water chemistry at Azotea Tunnel 

Outlet and Willow Creek above Heron Reservoir varied 

throughout the year as a function of streamflow. Streamflow 

varied from high flow to low flow as the result of the quantity 

of water diverted from the Rio Blanco, Little Navajo River, 

and Navajo River for the SJCP. During high flow, the ionic 

composition of water from Azotea Tunnel Outlet was similar 

to the ionic composition of a mixture of water from Rio Blanco 

and Navajo River. Also, there were small but distinct differences 

in the ionic composition of water from Willow Creek above 

Heron Reservoir compared to water from Azotea Tunnel Outlet 

(fig. 6). Water at Willow Creek above Heron Reservoir was 

likely a mixture of water from Azotea Tunnel Outlet and a 

small volume of surface-water inflows from tributaries such as 

Willow Creek above Azotea Creek with large concentrations 

of dissolved solids. During low flow, the ionic composition 

of water from Azotea Tunnel Outlet was compositionally 

different from water from streams that contribute to the SJCP 

and waters from streams in the Willow Creek Basin; the source 

of the water is currently unknown. The ionic composition of 

water at Willow Creek above Heron Reservoir during low 

flow was compositionally similar to water from streams in the 

Willow Creek Basin and was primarily derived from tributary 

inflow to Willow Creek, with a minor contribution of flow 

from the SJCP. The increase in specific conductance and ion 

concentrations from Azotea Tunnel Outlet to Willow Creek 

above Heron Reservoir likely was a function of the tributary 

inflow along the channel including Willow Creek above Azotea 

Creek and physical and chemical processes affecting the water 

along the channel. The increase in specific conductance and ion 

concentrations is inversely proportional to streamflow, such 

that a smaller volume of streamflow would be more affected 

by tributary inflow and physical and chemical processes than a 

larger volume of streamflow.

An evaluation of nutrient concentrations in water samples 

from Azotea Tunnel Outlet, Willow Creek above Azotea 

Creek, and Willow Creek above Heron Reservoir focused 

on the results from samples collected on May 5, 2009, and 

June 24, 2009, because the three stations were sampled 

sequentially. Results from other samples are included in 

the analysis if needed but were not the focus of the analysis 

because the relation of water chemistry and streamflow causes 

variation in water chemistry at different flow conditions. The 

predominant form of total nitrogen reported at these stations 

was total ammonia plus organic nitrogen, the predominant 

form of dissolved nitrogen reported at these stations was 

ammonia plus organic nitrogen, and the predominant form 

of dissolved phosphorus was orthophosphate (table 5). The 

dissolved nitrogen component generally accounted for more 

than 50 percent of the total nitrogen, except for the June 

2009 sampling at Willow Creek above Heron Reservoir 

in which the dissolved ammonia plus organic nitrogen 

component accounted for 34 percent of the total ammonia 

plus organic nitrogen. The dissolved phosphorus component 

generally accounted for less than 30 percent of the total 

phosphorus. Total organic carbon concentrations ranged from 

3.3 to 11.8 milligrams per liter (table 5). In general, nutrient 

concentrations and organic carbon concentrations increased 

from Azotea Tunnel Outlet to Willow Creek above Heron 

Reservoir, likely because of higher concentrations of nutrients 

contributed by inflow of Willow Creek above Azotea Creek 

(table 5). 

Heron Reservoir and Willow Creek below

Heron Reservoir

The chemistry of water from Heron Reservoir and 

Willow Creek below Heron Reservoir are compositionally 

similar to the chemistry of water at Willow Creek above 

Heron Reservoir during high flow (fig. 6). In general, the 

concentration of major ions with the exception of K and Cl in 

water from Heron Reservoir and Willow Creek below Heron 

Reservoir was greater than the concentration of major ions 

in water from Azotea Tunnel Outlet during high flow. The 

specific conductance and concentration of major ions at Heron 

Reservoir and Willow Creek below Heron Reservoir are in 

the range of values for Willow Creek above Heron Reservoir 

during high flow (figs. 7A and 7B). The ionic composition 

and the concentration of major ions in samples from Heron 

Reservoir and Willow Creek below Heron Reservoir indicated 

that the chemistry of water for the SJCP is altered during 

transit along the channel from the Azotea Tunnel Outlet to 

Heron Reservoir, likely as a result of mixing with inflows 

along the channel. 
Table 5. Selected water-chemistry data for Azotea Tunnel Outlet, Willow Creek above Azotea Creek, Willow Creek above Heron Reservoir, Heron Rebelow Heron Reservoir, northern New Mexico, 2007–9. [mg/L, milligrams per liter; <, less than; --, data not available; E, estimated value] Station name Date Sam- pling depth Total nitrogen, water, unfil- tered Total ammo- nia plus organic nitrogen, water, unfiltered Total organic nitrogen, water, unfil- tered Ammonia plus organic nitrogen, water, filtered Am- monia, water, filtered Nitrate plus nitrite, water, filtered Nitrate, water, filtered Nitrite, water, filtered Ortho- phos- phate, water, filtered Ph

pho

wa

filte

feet All concentrations reported as mg/L nitrogen All concentrat

as mg/L ph

Azotea Tunnel Outlet 5/5/2009 -- 0.27 0.21 <0.21 0.12 <0.020 0.06 0.057 <0.002 0.028 0.

Willow Creek above Azotea 5/5/2009 -- <0.90 0.86 <0.86 0.51 <0.020 <0.04 <0.040 <0.002 0.021 0.

Willow Creek above Heron Reservoir 5/5/2009 -- 0.28 0.23 <0.23 0.14 <0.020 0.05 0.051 <0.002 0.028 0.

Azotea Tunnel Outlet 6/24/2009 -- <0.30 0.26 <0.26 0.13 <0.020 <0.04 <0.040 <0.002 0.026 0.

Willow Creek above Azotea 6/24/2009 -- <0.77 0.73 <0.73 0.47 <0.020 <0.04 <0.040 <0.002 E0.008 0.

Willow Creek above Heron Reservoir 6/24/2009 -- <0.33 0.29 <0.29 0.10 <0.020 <0.04 <0.040 <0.002 0.026 0.

Heron Reservoir 4/8/2008 10 -- -- <0.16 -- <0.020 0.04 0.04 <0.002 0.01 -

Heron Reservoir 10/8/2008 11 -- -- <0.13 -- <0.020 <0.04 <0.040 <0.002 <0.008 -

Heron Reservoir 10/8/2008 75 -- -- <0.11 -- <0.020 0.11 0.107 <0.002 0.034 -

Heron Reservoir 5/6/2009 8 <0.37 0.33 <0.33 0.21 <0.020 <0.04 <0.040 <0.002 0.009 E0.

Heron Reservoir 9/24/2009 12 <0.20 0.16 <0.16 0.14 <0.020 <0.04 <0.040 <0.002 E0.006 E0.

Heron Reservoir 9/24/2009 70 0.26 0.16 <0.16 0.11 <0.020 0.09 0.092 <0.002 0.02 0.

Willow Creek below Heron Reservoir 12/13/2007 -- -- -- <0.13 -- <0.020 0.05 E0.044 E0.001 0.013 -

Willow Creek below Heron Reservoir 11/17/2008 -- -- -- <0.17 -- <0.020 E0.03 E0.030 0.002 0.014 -

Willow Creek below Heron Reservoir 8/18/2009 -- 0.26 0.17 <0.17 0.18 <0.020 0.09 0.09 <0.002 0.024 0.
Water Chemistry 23

Vertical profiles of the water temperature over the depth 

of the water column collected in April 2008, October 2008, 

July 2009, August 2009, and September 2009 indicated that 

Heron Reservoir is thermally stratified in the summer and 

early fall and thermally mixed in early spring. Stratification 

of reservoirs occurs because of the variation of the density 

of water with temperature; the density of water increases as 

the temperature decreases to a maximum density at 4 degrees 

Celsius (°C), and water colder than 4 °C decreases in density 

(Nevers and Whitman, 2005). Stratification of reservoirs 

can occur in the summer as water that is near the surface is 

warmed relative to deeper water and in the winter as water that 

is near the surface is cooled and sinks downward (Nevers and 

Whitman, 2005). Because water colder than 4 °C is less dense 

than warmer water, stratification in the winter can result in the 

coldest water near the surface (Nevers and Whitman, 2005). 

Mixing of the stratified layers can occur from wind shear 

and during seasonal air temperature transitions that affect the 

temperature of water near the surface. The region between the 

upper warm zone and the lower cold zone is referred to as the 

“thermocline′′ and is generally defined as the region where 

the temperature changes at least 1 °C per meter (Graham and 

others, 2008). 

Temperature-dependent density differences in a stratified 

water column can prevent mixing and can result in the 

formation of chemical variations between the layers. Typically, 

the dissolved-oxygen concentration of water in the deeper 

layer of a stratified reservoir will decrease, and the carbon 

dioxide and nutrient concentrations will increase because of 

the decomposition of organic material (Nevers and Whitman, 

2005). Depending on the amount of organic material available 

for decomposition, the lower layers can become devoid 

of oxygen, or anoxic, and can be depleted enough to be 

insufficient to support aquatic life (Nevers and Whitman, 

2005). Additionally, during summer stratification, the water 

near the surface can become depleted in oxygen because of 

decreased solubility of oxygen at increased temperatures, 

resulting in the highest oxygen concentrations at a middle zone 

where phytoplankton continues to produce oxygen (Nevers 

and Whitman, 2005). The inflow of water to a reservoir from 

upstream sources can mix with lower layers, increase oxygen 

concentrations at depth, and decrease stratification effects 

during the runoff season (Petts, 1986). 

During turnover, when the stratified layers are mixed, the 

products of decomposition from the lower layer, including the 

increased concentrations of carbon dioxide and nutrients, are 

cycled into the upper layers, and the higher concentrations of 

dissolved oxygen in the upper layers are cycled into the lower 

layers (Nevers and Whitman, 2005). The chemistry of water 

released from reservoirs can be affected by the stratification 

of a reservoir if the dam outlet preferentially discharges water 

from a lower layer with decreased oxygen and increased 

carbon dioxide and nutrient concentrations. 

Vertical profiles of the water temperature over the depth 

of the water column at Heron Reservoir indicated that the 

reservoir is seasonally stratified (fig. 9A). Heron Reservoir 

was not stratified in the spring of 2008 (April 2008); however, 

the reservoir was stratified in the summer (July and August 

2009) and early fall of 2008 and 2009 (October 2008 and 

September 2009) (fig. 9A). Summer stratification of Heron 

Reservoir resulted in warmer water near the surface and cooler 

water at lower depths (fig. 9A). The top of the thermocline at 

Heron Reservoir increased in depth from approximately 25 ft 

below the water surface in July 2009 to approximately 45 ft 

below the water surface in October 2008 (fig. 9B). In 2008 and 

2009, the temperature of the water surface increased through 

the spring from April to July and then decreased through late 

summer from July to October (fig. 9A). The seasonal shift 

in the temperature profile and the change in the depth of the 

thermocline suggest that the stratification of water in the 

reservoir transitions from unstratified to stratified through 

the summer, likely after the largest inflows of water from the 

SJCP have declined. It is likely that the reservoir turns over in 

the fall as the water near the surface cools in response to air 

temperatures and sinks; however, there are no data available to 

determine if the reservoir is thermally stratified in the winter. 

Vertical profiles of the dissolved-oxygen concentration 

over the depth of the water column at Heron Reservoir 

indicated that the thermal stratification results in chemical 

stratification. The concentration of dissolved oxygen was 

nearly constant at all depths during the spring (fig. 9B); 

however, the concentration of dissolved oxygen varied with 

depth during the summer and fall (fig. 9B). The dissolvedoxygen concentration was greatest during the spring when 

the water temperature was lowest, which is consistent with 

increased oxygen solubility at lower water temperatures. 

When the reservoir was stratified, the dissolved-oxygen 

concentration was highest above and below the thermocline, 

and the lowest dissolved-oxygen concentration generally 

occurred at the top of the thermocline (fig. 9B). The dissolvedoxygen concentration in the reservoir below the thermocline 

declined through the summer with the lowest dissolvedoxygen concentrations in October. 

Evaluation of nutrient concentrations in water samples 

from Heron Reservoir focused on the results from samples 

collected in 2009 because the samples collected on May 5 

and June 24 were closest in time to the sample collection 

at Azotea Tunnel Outlet and Willow Creek. Results from 

other samples are included in the analysis if needed but were 

not the focus of the analysis because the relation of water 

chemistry and streamflow causes variation in water chemistry 

at different flow conditions. The predominant form of total 

nitrogen reported for Heron Reservoir was total ammonia 

plus organic nitrogen, the predominant form of dissolved 

nitrogen reported at these stations was ammonia plus organic 

nitrogen, and the predominant form of dissolved phosphorus 

was orthophosphate (table 5). The dissolved ammonia plus 

organic nitrogen component generally accounted for more 

than 50 percent of the total ammonia plus organic nitrogen. 

The dissolved phosphorus component of water from above 

the thermocline for May 2009 and September 2009 accounted 

for less than 30 percent of the total phosphorus; the dissolved 
24 Water Chemistry, Seepage, Streamflow, Reservoir Storage, and Annual Availability of Water for the San Juan-Chama Project

0

100

80

60

40

20

120

140

160

180

200

0 2 4 6 8 10 12 14

0 5 10 15 20 25

Dissolved-oxygen concentration, in milligrams per liter

Temperature, in Celsius

Depth below the water surface

of Heron Reservoir, in feet

0

100

80

60

40

20

120

140

160

180

200

Depth below the surface

of the reservoir, in feet

EXPLANATION

April 2008

October 2008

July 2009

August 2009

Identified depth of the center

 of the thermocline layer

September 2009

EXPLANATION

April 2008

October 2008

July 2009

August 2009

September 2009

Center of thermocline

October 2008

July 2009

August 2009

September 2009

A. Water temperature

B. Dissolved-oxygen concentration

Figure 9. Vertical profiles of water chemistry parameters in Heron Reservoir, 2008–9. A, Water temperature. B, Dissolved-oxygen 

concentration.
Streamflow, Reservoir Storage, and Annual Availability of Water for the San Juan-Chama Project 25

phosphorus component of water from below the thermocline 

on September 2009 accounted for approximately 50 percent of 

the total phosphorus (table 5). 

In general, nutrient concentrations and organic carbon 

concentrations varied above and below the thermocline 

if the reservoir was stratified. Nitrogen and phosphorus 

concentrations were equal or higher and organic carbon was 

slightly lower in samples from below the thermocline than 

in samples from above the thermocline (except for dissolved 

ammonia plus organic nitrogen in September 2009). It is likely 

that the variation in nutrient concentrations are the result of 

increased decomposition of organic material and less uptake 

of nutrients by algae below the thermocline. The concentration 

of dissolved solids and major ions did not vary substantially 

across the thermocline; there was a slightly higher 

concentration of dissolved solids in water from above the 

thermocline, which likely resulted from increased evaporation 

near the surface. 

Analysis of nutrient concentrations in water samples 

from Willow Creek below Heron Reservoir focused on 

the results from samples collected in 2009 because these 

samples were closest in time to the samples collected on 

Heron Reservoir. Results from other samples are included in 

the analysis if needed but were not the focus of the analysis 

because the relation of water chemistry and streamflow causes 

variation in water chemistry at different flow conditions. The 

concentrations of nutrients in water from Willow Creek below 

Heron Reservoir were similar to concentrations of nutrients in 

water below the thermocline in Heron Reservoir (table 5). It is 

likely that water released from the dam is pulled from lower 

in the reservoir and that the water chemistry could be affected 

by nutrient cycling in the reservoir. The samples collected at 

Willow Creek below Heron Reservoir on August 18, 2009, 

included analysis of the water sample for volatile organic 

compounds, semivolatile organic compounds, organochlorine 

pesticides, polychlorinated biphenyl, wastewater indicator 

compounds, human-health pharmaceuticals, and mercury. 

The concentrations of these compounds were below the 

reporting level (reporting level varied by analyte) except for 

N,N-Diethyl-meta-toluamide (DEET), with an estimated 

concentration of 0.01 micrograms per liter in an unfiltered 

water sample. 

Seepage Investigation

The gain or loss along the channel between Azotea 

Tunnel Outlet and the station at Willow Creek above Heron 

Reservoir was calculated for high flow and low flow. During 

high flow, there was a median calculated loss of 5±10 ft3

/s at 

a median streamflow of 148 ft3

/s, or an estimated loss of 3±7 

percent for the reach of the channel between Azotea Tunnel 

Outlet and the Willow Creek above Heron Reservoir station. 

The results of a regression analysis (Helsel and Hirsch, 2002) 

of the total inflows to the reach (measured discharge near 

Azotea Tunnel Outlet and at Willow Creek above Azotea 

Creek) compared to the total outflow from the reach (measured 

discharge near the Willow Creek above Heron Reservoir 

station) also indicated an estimated loss of 3 percent, with 

a consistent trend of a loss for all the measurements (linear 

regression fit with a slope of 0.97 and a root mean square 

error of 1.6 ft3

/s; fig. 10). The results for the high seepage 

investigation indicated that a small but significant loss occurs 

along the reach between Azotea Tunnel Outlet and Willow 

Creek above Heron Reservoir. 

During low flow, the estimated streamflow resulting 

from groundwater/surface-water interactions was calculated 

for Reach 1 and Reach 2 for a range of streamflow at the 

LF-Midpoint station. The calculated loss in streamflow for 

Reach 1 from the LF-Upstream station to the LF-Midpoint 

ranged from 0.03±0.03 ft3

/s to 0.04±0.02 ft3/s, and the 

calculated gain in streamflow for Reach 2 from the LF-Midpoint 

to the LF-Downstream ranged from 0.05±0.04 ft3

/s to 0.06 

±0.04 ft3

/s (table 4). The estimated streamflow resulting from 

groundwater/surface-water interactions for the interval from 

the LF-Upstream station to the LF-Downstream station was 

0.02±0.05 ft3

/s (table 4).

The results from the seepage investigations indicated a 

small amount of loss of streamflow along the channel. During 

high flow, the loss was estimated at 3 percent; however, 

the uncertainty in the measured streamflow measurements 

precludes an exact determination of the loss. During low flow, 

the estimated loss was extremely small, and the uncertainty 

generally was close to or exceeded the estimated loss. The 

possible loss of streamflow resulting from groundwater/surfacewater interactions during low flow was not a significant volume 

compared to streamflow volumes during high flow. 

Streamflow, Reservoir Storage, and 

Annual Availability of Water for the San 

Juan-Chama Project

Streamflow for stations in the SJCP were assessed with 

measured or estimated streamflow data. Streamflow statistics 

presented are for the period of record for each station; annual 

statistics are reported on the basis of calendar years (table 6). 

The Azotea Tunnel Outlet station has been operated by 

Reclamation since 1970. The streamflow gage at Azotea Tunnel 

Outlet is a concrete flume in the tunnel structure. Median (50th 

percentile) annual streamflow from 1971 to 2010 was 89,290 

acre-ft (table 6 and fig. 11A). Annual streamflow at Azotea 

Tunnel Outlet has been variable over the period of record. The 

annual streamflow was asymmetrically distributed around the 

median, such that values greater than the median cover a larger 

range than do values that are less than the median. Additionally, 

the range of values of annual streamflow that occurred most 

frequently, from 75,000 to 87,500 acre-ft, were below the 

median annual streamflow (fig. 11A). Streamflow at Azotea 

Tunnel Outlet is generally close to median annual streamflow 

with only two years less than 25,000 acre-ft (fig. 11A). 
26 Water Chemistry, Seepage, Streamflow, Reservoir Storage, and Annual Availability of Water for the San Juan-Chama Project

170

150

155

160

165

145

140

135

130

130 135 140 145 150 155 160 165 170

Total inflows, in cubic feet per second

Total outflows, in cubic feet per second

Linear regression

Equation of the line:

Total outflows = 0.97 (total inflows)

1:1 line

EXPLANATION

Discharge measurement

Table 6. Selected statistics for streamflow at selected streamflow-gaging stations in the study area, northern New Mexico.

[IQR, interquartile range]

Azotea 

Tunnel 

Outlet

Willow Creek 

above Heron 

Reservoir

Willow Creek 

above Heron 

Reservoir

Horse 

Lake Creek

Willow Creek 

below Heron 

Reservoir

Willow Creek 

near Park 

View

Period of record included in the 

statistical analysis

1971–2010 1962–70 1971–2010 1963–2009 1971–2010 1943–69

Annual streamflow percentiles, in 

acre-feet

10th percentile 47,520 2,460 51,442 6 49,730 1,750

25th percentile 62,920 4,430 66,436 37 72,600 2,700

50th percentile 89,290 5,710 102,570 185 95,540 9,170

75th percentile 117,580 11,100 125,824 679 118,220 15,120

90th percentile 144,930 14,680 161,530 969 127,960 20,480

IQR (75th percentile–25th percentile) 54,660 6,670 59,388 642 45,620 12,420

Coefficient of variation (IQR/50th 

percentile)

0.61 1.17 0.58 3.47 0.48 1.35

Figure 10. Comparison of the total inflows to the reach (measured discharge near Azotea Tunnel Outlet and at Willow Creek above 

Azotea Creek) and the total outflow from the reach (measured discharge near the Willow Creek above Heron Reservoir station) with 

fitted regression line.
Streamflow, Reservoir Storage, and Annual Availability of Water for the San Juan-Chama Project 27

Streamflow on Willow Creek was measured at two 

stations: Willow Creek near Park View and Willow Creek 

above Heron Reservoir. The Willow Creek near Park View 

station was operated by the USGS from 1936 to 1971 (there 

was no winter record for 1936 through 1942) (table 2). The 

location of the station is detailed in Falk and others (2013). 

Median (50th percentile) annual streamflow from 1943 to 

1969 at Willow Creek near Park View was 9,170 acre-ft 

(table 6).

Streamflow recorded at the Willow Creek above Heron 

Reservoir station for the period before the SJCP diversions 

(1962–70) is the natural streamflow on Willow Creek. 

Streamflow recorded at Willow Creek above Heron Reservoir 

for the period of the SJCP diversions (1971–2010) is the 

combined flow from Azotea Tunnel Outlet and the natural 

streamflow on Willow Creek. The station at Willow Creek 

above Heron Reservoir was operated by the USGS from 

1962 to 1971 and has been operated by Reclamation since 

1971. The location of the station is detailed in Falk and others 

(2013). Median (50th percentile) annual streamflow from 

1962 to 1970 at Willow Creek above Heron Reservoir was 

5,710 acre-ft (table 6), and median annual streamflow from 

1971 to 2010 was 102,570 acre-ft (table 6 and fig. 11B). 

Because the majority of the streamflow at Willow Creek above 

Heron Reservoir is derived from the SJCP, the distribution of 

the annual streamflow is similar to Azotea Tunnel Outlet (fig. 

11B). The COV for Willow Creek above Heron Reservoir 

decreased when diversions for the SJCP started and changed 

from 1.17 over the interval 1962–70 to 0.58 over the interval 

1971–2010; this change indicates that annual variation in 

streamflow at Willow Creek above Heron Reservoir declined 

after the start of diversion of water for the SJCP.

The Horse Lake Creek station was operated by the USGS 

from 1962 to June 1971, by Reclamation from July 1971 to 

September 1998, and then again by the USGS since October 

1998. The location of the station is detailed in Falk and others 

(2013). Median (50th percentile) annual streamflow over the 

period of record at Horse Lake Creek was 185 acre-ft (table 6). 

Seasonal and annual variations in streamflow for Horse Lake 

Creek are reported in Falk and others (2013).

Releases from Heron Reservoir are reported at the 

Willow Creek below Heron Reservoir gage. The streamflow 

at Willow Creek below Heron Reservoir has been reported 

by Reclamation and published by the USGS since 1971. 

Releases from Heron Reservoir were measured from 1971 

to 2000 with mercury manometers located in the conduits 

of Heron Dam and were measured from 2000 to 2012 with 

rating tables for the radial gates (Jeff Albertson, Chama New 

Mexico Field Division, Bureau of Reclamation, oral commun., 

2014). Median annual streamflow from 1971 to 2010 was 

95,540 acre-ft (table 6 and fig. 11C). The annual volume of 

water released from Heron Reservoir includes SJCP water and 

Rio Grande water. Accounting for the Rio Grande water is 

required by the Rio Grande Compact and is reported annually 

0

125,000

100,000

75,000

25,000

50,000

0

175,000

150,000

200,000

6543210

B. Willow Creek above Heron Reservoir

7

Median annual streamflow

89,290 acre-feet

Median annual streamflow

95,540 acre-feet

Median annual streamflow

102,570 acre-feet

0 20 40 60 80 100

Annual streamflow, in acre-feet Annual streamflow, in acre-feet

150,000

125,000

100,000

75,000

50,000

25,000

0

175,000

200,000

6543210

A. Azotea Tunnel Outlet

7 8

0 20 40 60 80 100

0 20 40 60 80 100

Percentage of time indicated value

was equalled or exceeded

Annual streamflow, in acre-feet 150,000

125,000

100,000

75,000

50,000

25,000

0

175,000

200,000

1086420

C. Willow Creek below Heron Reservoir

Number of years

with values in

the indicated

range

Figure 11. Distribution of annual streamflow, median annual 

streamflow, and annual flow-duration curves for selected 

streamflow-gaging stations in the study area, 1971–2010. A, 

Azotea Tunnel Outlet. B, Willow Creek above Heron Reservoir. C, 

Willow Creek below Heron Reservoir.
28 Water Chemistry, Seepage, Streamflow, Reservoir Storage, and Annual Availability of Water for the San Juan-Chama Project

by Reclamation to the Rio Grande Compact Commission. 

Accounting for the SJCP water diversions and deliveries 

and the Rio Grande water is calculated by using the Upper 

Rio Grande Water Operations Model (U.S. Army Corps of 

Engineers, 2013). The accounting for Rio Grande water is 

calculated by using several different methods. These methods 

include (1) mass balance equation for the reservoir, (2) the 

ratio inflow method that calculates the Rio Grande water as 

the difference between streamflow at Azotea Tunnel Outlet 

and Willow Creek above Heron Reservoir with an adjustment 

of the resulting Rio Grande flow by a ratio factor to account 

for inflow below Willow Creek above Heron Reservoir, and 

(3) the net end-of-month gain method that is a variation of 

the mass balance equation (U.S. Army Corps of Engineers, 

2005). The accounting for Rio Grande water also includes a 

method for estimating seepage from Heron Reservoir such 

that the seepage from Heron Reservoir is considered Rio 

Grande water for accounting purposes and the amount of Rio 

Grande water released from the reservoir is decreased by the 

amount of seepage (U.S. Army Corps of Engineers, 2005).

The annual streamflow is asymmetrically distributed around 

the median, such that values greater than the median cover 

a smaller range than do values that are less than the median 

(fig. 11A); however, the range of values of annual streamflows 

that occurred most frequently, from 87,500 to 100,000 acre-ft, 

included the median streamflow.

The stage gage for Heron Reservoir is a water-stage 

recorder that has been operated by Reclamation since 1970. 

The stage data are recorded as an elevation of the water 

surface above mean sea level and are converted to storage 

capacity by using a capacity table that relates the elevation to 

storage contents in acre-feet. The volume of water in storage 

in Heron Reservoir on January 1 varied over time (fig. 12), 

generally depending on the inflow of water from upstream and 

the release of water for downstream uses. 

The difference in the COV and the streamflow yield 

per square mile (Falk and others, 2013) for various streams 

in the study area showed that there was greater yield and 

less variability in annual streamflow in streams from large 

watersheds at high elevations than in streams from small 

watersheds at high elevations or streams at low-elevation 

watersheds. Falk and others (2013) concluded that the COVs 

for stations on the Rio Blanco and Navajo River, streams 

located at high elevation, were 0.61 or less and that the 

dispersion of the annual streamflow over the period of record 

was small relative to other streams in the study area. The study 

also concluded that the COVs for stations on Willow Creek 

and Horse Lake Creek, streams located at low elevation, were 

greater than 1 and that the dispersion of the annual streamflow 

over the period of record was large relative to other streams in 

the study area (Falk and others, 2013). The COVs for Azotea 

Tunnel Outlet, Willow Creek above Heron Reservoir (1971–

2010), and Willow Creek below Heron Reservoir were 0.61 or 

less, and the COVs for Willow Creek near Park View, Willow 

Creek above Heron Reservoir (1962–70), and Horse Lake 

Creek were greater than 1 (table 6). The annual variability of 

streamflow at Azotea Tunnel Outlet and Willow Creek above 

Heron Reservoir (1971–2010) is controlled by the variability 

in the high-elevation streams from which water is diverted 

for operation of the SJCP. Differences in air temperature and 

precipitation, generally related to changes in elevation, were 

likely the predominant cause of variation in streamflow yield 

among these stations.

Annual storage in Heron Reservoir, in acre-feet

450,000

250,000

200,000

150,000

100,000

50,000

400,000

350,000

300,000

0

197019751980198519901995200020052010

Year

Heron Reservoir maximum capacity

401,320 acre-feet

Figure 12. Annual storage in Heron Reservoir, in acre-feet, in northern New Mexico, 1970–2010.
Streamflow, Reservoir Storage, and Annual Availability of Water for the San Juan-Chama Project 29

Variation and Trends in Streamflow for the

San Juan-Chama Project

Annual variability in streamflow for the SJCP was an 

indication of the variation in climate parameters that interact 

to contribute to streamflow in the Rio Blanco, Little Navajo 

River, Navajo River, and Willow Creek watersheds. For these 

watersheds, the variation in annual streamflow generally 

coincides with the timing of Pacific Decadal Oscillation 

(PDO) cycles, defined by using the North Pacific monthly 

sea-surface temperature anomaly (Mantua and Hare, 2002). 

A negative PDO index for an interval is associated with 

decreased precipitation in the Southwestern United States, 

and a positive PDO index for an interval is associated with 

increased precipitation in the Southwestern United States 

(Hanson and others, 2004). Annual streamflow for the 

SJCP has been lower during intervals when the PDO index 

is negative (correlated to drier conditions for the study 

area) than during intervals when the PDO index is positive 

(correlated to wetter conditions in the study area) (Falk and 

others, 2013). Annual streamflow at Azotea Tunnel Outlet 

ranged from 164,130 acre-ft (1979) to 6,360 acre-ft (2002) 

(fig. 13A), and annual streamflow at Willow Creek above 

Heron Reservoir ranged from 179,340 acre-ft (1979) to 

5,540 acre-ft (2002) (fig. 13A).

The median monthly streamflow for Azotea Tunnel 

Outlet and Willow Creek above Heron Reservoir indicated that 

streamflow at these stations was typical of watersheds that are 

snowmelt-dominated systems, with the majority of streamflow 

occurring in April through June (fig. 13B). The median 

monthly streamflow at Azotea Tunnel Outlet and Willow 

Creek above Heron Reservoir did not show the bimodal 

variation that is characteristic of streamflow for stations on 

Willow Creek and Horse Lake Creek above Heron Reservoir 

(Falk and others, 2013). 

The volume of water diverted for the SJCP during a 

year depended on the length of time water was diverted and 

the amount of water that was diverted each day. The first day 

of measurable streamflow at Azotea Tunnel Outlet for the 

period of record started between day 60 (March 1) and day 

105 (April 15) with a few exceptions. The start of streamflow 

was earlier than day 60 in 1972 and 1973 when the reservoir 

was filling and in 2007 and 2009, and streamflow started after 

day 105 in 1993 (fig. 14A). The start of streamflow at Azotea 

Tunnel Outlet has recently occurred earlier in the year than in 

previous years; since 1997, the start of streamflow at Azotea 

Tunnel Outlet occurred on day 80 (March 21) or earlier and 

in 2009 started on day 35 (February 4) (fig. 14A). Streamflow 

at Azotea Tunnel Outlet has decreased substantially over the 

period of record after day 198 (July 17), as determined by the 

75th percentile of the ordinal day on which 90 percent of the 

annual streamflow occurred (table 7). The median duration 

of days for the IQR of annual streamflow was 37 days, and 

the median duration for 90 percent of the annual streamflow 

to occur was 99 days (table 7 and figs. 14B and 14C). For 

most years, streamflow at Azotea Tunnel Outlet started by 

March and continued for approximately 3 months until the 

middle of July. Streamflow that occurred at Azotea Tunnel 

Outlet after July generally accounted for a minor percentile 

of annual streamflow. The majority of annual streamflow at 

Azotea Tunnel Outlet generally occurred from May through 

June, with a median duration of slightly longer than a month. 

In general, larger annual streamflow was associated with a 

longer duration for the IQR but not with a longer duration 

for 90 percent of the annual streamflow (figs. 14B and 14C). 

Additionally, higher annual streamflow occurred in years with 

higher maximum daily streamflow (fig. 14D). 

Trends in the seasonal distribution of streamflow at 

Azotea Tunnel Outlet over time can indicate changes in the 

timing of snowmelt runoff on the streams that are diverted for 

the SJCP and changes in the availability of water for diversion. 

The monthly percentage of annual streamflow and the monthly 

streamflow from March through October for 1971–2010 were 

tested for trends compared to time to determine if the amount 

of streamflow that occurred in these months changed over 

time. The monthly percentage of annual streamflow and the 

monthly streamflow for March through October for 1971–

2010 was tested for trends compared to the volume of annual 

streamflow to determine if the volume of annual streamflow 

affected the monthly distribution of streamflow. Annual 

streamflow for 1971–2010 was tested for trends compared 

to time to determine if the volume of annual streamflow 

monotonically changed since 1971. 

The amount of streamflow at Azotea Tunnel Outlet in 

April, May, and June has changed over time. Significant trends 

were detected for the monthly streamflow in April (weakly 

positive) and the monthly percentage of annual streamflow for 

May (weakly positive) and June (weakly negative) (table 8). 

These trends indicated that from 1971 to 2010, the amount of 

streamflow that occurred in April increased, the percentage 

of annual streamflow that occurred in May increased, and 

the percentage of annual streamflow that occurred in June 

decreased. These trends indicated that the streamflow at 

Azotea Tunnel Outlet has shifted to occur earlier in the year 

since 1971. 

The amount of streamflow at Azotea Tunnel Outlet 

from April through August also is significantly correlated 

with annual streamflow. Significant trends for the monthly 

streamflow compared to annual streamflow at Azotea Tunnel 

Outlet were detected for April (weakly positive), May through 

July (strongly positive), and August (weakly positive) (table 

8). Additionally, significant trends for the monthly percentage 

of annual streamflow compared to annual streamflow at Azotea 

Tunnel Outlet were detected for April (weakly negative), June 

(weakly positive), and July (moderately positive) (table 8). 

These trends indicated that more streamflow occurred from 

April through August for years with greater annual streamflow 

than during years with less annual streamflow.

Statistically significant trends for the monthly percentage 

of annual streamflow compared to annual streamflow were 

detected for April, June, and July (table 8). The significant 

trends for the monthly percentage of annual streamflow 
30 Water Chemistry, Seepage, Streamflow, Reservoir Storage, and Annual Availability of Water for the San Juan-Chama Project

1,600,000

1,400,000

1,200,000

1,000,000

800,000

600,000

400,000

200,000

0

C. Cumulative streamflow over 10-consecutive-year intervals at Azotea Tunnel Outlet

1971−1980

1972−1981

1973−1982

1974−1983

1975−1984

1976−1985

1977−1986

1978−1987

1979−1988

1980−19891981−1990

1982−1991

1983−1992

1984−1993

1985−1994

1986−1995

1987−1996

1988−1997

1989−1998

1990−1999

1991−2000

1992−2001

1993−2002

1994−2003

1995−2004

1996−2005

1997−2006

1998−2007

1999−2008

2000−2009

2001−2010

10-consecutive-year period

1,350,000 acre-feet - limit of any 10-consecutive-year periods

Cumulative streamflow over 10-consecutiveyear period, in acre-feet Annual streamflow, in acre-feet

A. Annual streamflow at Azotea Tunnel Outlet and Willow Creek above Heron Reservoir 200,000

100,000

80,000

60,000

40,000

20,000

180,000

160,000

140,000

120,000

0

1970 1975 1980 1985 1990 1995 2000 2005 2010

Azotea Tunnel Outlet median

annual streamflow

89,290 acre-feet

Willow Creek above Heron Reservoir median annual streamflow

102,570 acre-feet

Note: 270,000 acre-feet limit for any one year

B. Median monthly streamflow at Azotea Tunnel Outlet and Willow Creek above Heron Reservoir 40,000

20,000

15,000

10,000

5,000

35,000

30,000

25,000

0

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Month

Year

Median monthly streamflow, in acre-feet

Azotea Tunnel Outlet

Azotea Tunnel Outlet

Willow Creek above Heron Reservoir

Annual streamflow Azotea Tunnel Outlet

Annual streamflow Willow Creek above Heron Reservoir

Figure 13. Annual and seasonal variation in streamflow at the streamflow-gaging stations Azotea Tunnel Outlet and Willow Creek above 

Heron Reservoir, in northern New Mexico, 1971–2010. A, Annual streamflow at Azotea Tunnel Outlet and Willow Creek above Heron 

Reservoir. B, Median monthly streamflow at Azotea Tunnel Outlet and Willow Creek above Heron Reservoir. C, Cumulative streamflow over 

10-consecutive-year periods at Azotea Tunnel Outlet. 
Streamflow, Reservoir Storage, and Annual Availability of Water for the San Juan-Chama Project 31

C. Duration for the interquartile range (IQR) of annual streamflow at Azotea Tunnel Outlet 180,000

100,000

80,000

60,000

40,000

20,000

160,000

140,000

120,000

0

14 21 28 35 42 49

Annual streamflow, in acre-feet Ordinal day of the year

A. Ordinal day of the start of streamflow at Azotea Tunnel Outlet

1970 1975 1980 1985 1990 1995 2000 2005 2010

Year

April 15

March 21

March 1

Ordinal day of the start of

streamflow at Azotea

Tunnel Outlet

B. Duration for the interquartile range (IQR), or the middle 50 percent, of annual streamflow

 and the duration for 90 percent of the annual streamflow at Azotea Tunnel Outlet 180,000

100,000

80,000

60,000

40,000

20,000

160,000

140,000

120,000

0

0 30 60 90 120 150 180 210 240 270 300 330 360

Number of days

Number of days

Annual streamflow, in acre-feet

EXPLANATION

January 20 EXPLANATION

day 20

January 1 day 0

February 9 day 40

March 1

day 60

March 21

day 80

April 10

day 100

April 30

day 120

Duration for the IQR, or the middle

 50 percent, of annual streamflow

 at Azotea Tunnel Outlet

Duration for 90 percent of the

 annual streamflow at Azotea

 Tunnel Outlet

EXPLANATION

Duration for the IQR, or the middle

 50 percent, of annual streamflow

 at Azotea Tunnel Outlet

Figure 14. Variation in streamflow at the streamflow-gaging station Azotea Tunnel Outlet, northern New Mexico, 1970–2010. A, Ordinal day 

of the start of streamflow at Azotea Tunnel Outlet. B, Duration for the interquartile range, or the middle 50 percent, of annual streamflow and 

the duration for 90 percent of the annual streamflow at Azotea Tunnel Outlet Tunnel Outlet. C, Duration for the interquartile range of annual 

streamflow at Azotea Tunnel Outlet. D, Annual streamflow compared to the maximum daily streamflow at Azotea Tunnel Outlet.
32 Water Chemistry, Seepage, Streamflow, Reservoir Storage, and Annual Availability of Water for the San Juan-Chama Project

D. Annual streamflow compared to the maximum daily streamflow at Azotea Tunnel Outlet 180,000

100,000

80,000

60,000

40,000

20,000

160,000

140,000

120,000

0

0 500 1,000 1,500 2,000 2,500

Maximum daily streamflow, in acre-feet

Annual streamflow, in acre-feet

EXPLANATION

Measured discharge at

Azotea Tunnel Outlet

Table 7. Selected statistics for characterization of streamflow at Azotea Tunnel Outlet, northern New Mexico, 1971–2010. Statistics 

include the ordinal day of the year on which selected percentiles of annual streamflow occurred and the number of days between 

selected percentiles.

[Midspread of annual streamflow is defined as the number of days between the ordinal day on which 25 percent of the annual streamflow had occurred to the 

ordinal day on which 75 percent of the annual streamflow had occurred. Duration for 90 percent of the annual streamflow is defined as the number of days 

duration between the start of streamflow and the ordinal day on which 90 percent of the annual streamflow had occurred. IQR, interquartile range]

First day of 

measureable 

streamflow

Ordinal day of 

25th percentile of annual 

streamflow

Ordinal day of 

75th percentile of annual 

streamflow

Ordinal day of 

90th percentile of annual 

streamflow

Midspread of 

annual streamflow 

(IQR)

Duration for 

90 percent of 

the annual 

streamflow

Ordinal day percentiles, in ordinal day 

of the year

25th percentile 70 120 157 172 33 85

50th percentile (median) 79 125 164 180 37 99

75th percentile 94 133 174 198 43 133

IQR (75th percentile–25th percentile) 24 13 17 26 10 48

Figure 14. Variation in streamflow at the streamflow-gaging station Azotea Tunnel Outlet, northern New Mexico, 1970–2010. A, Ordinal day 

of the start of streamflow at Azotea Tunnel Outlet. B, Duration for the interquartile range, or the middle 50 percent, of annual streamflow and 

the duration for 90 percent of the annual streamflow at Azotea Tunnel Outlet Tunnel Outlet. C, Duration for the interquartile range of annual 

streamflow at Azotea Tunnel Outlet. D, Annual streamflow compared to the maximum daily streamflow at Azotea Tunnel Outlet.—Continued
Streamflow, Reservoir Storage, and Annual Availability of Water for the San Juan-Chama Project 33

Table 8. Results of the Mann-Kendall trend test for monthly streamflow and the monthly percentage of annual streamflow for 

streamflow-gaging station Azotea Tunnel Outlet, in northern New Mexico, 1971–2010.

[Strength of correlation: 0.00–0.20 very weak (VW), 0.21–0.40 weak (W), 0.41–0.60 moderate (M), 0.61–0.80 strong (S); bold, p-value below the significance 

value of 0.05; --, not statistically significant] 

Trends in the time series 

of the monthly streamflow 

Trends in the monthly streamflow 

compared to annual streamflow

Kendall’s tau p-value Strength of correlation Kendall’s tau p-value Strength of correlation

March 0.1922 0.0920 -- 0.1058 0.3535 --

April 0.2282 0.0381 W 0.3231 0.0033 W

May 0.1026 0.3513 -- 0.6179 <0.0001 S

June -0.1193 0.2785 -- 0.7094 <0.0001 S

July -0.0974 0.3759 -- 0.6590 <0.0001 S

August -0.0347 0.7530 -- 0.3610 0.0011 W

September 0.0867 0.4341 -- 0.2162 0.0512 --

October -0.1286 0.2650 -- 0.0029 0.9798 --

Trends in the time series of the monthly 

percentage of annual streamflow 

Trends in the monthly percentage of annual 

streamflow compared to annual streamflow

Kendall’s tau p-value Strength of correlation Kendall’s tau p-value Strength of correlation

March 0.1902 0.0920 -- -0.0412 0.7149 --

April 0.1846 0.0934 -- -0.3872 0.0004 W

May 0.2205 0.0451 W -0.1359 0.2168 --

June -0.2308 0.0360 W 0.3462 0.0017 W

July -0.1923 0.0805 -- 0.4410 <0.0001 M

August -0.0629 0.5680 -- 0.1477 0.1802 --

September 0.0505 0.6489 -- 0.0712 0.5208 --

October -0.1316 0.2434 -- -0.0838 0.4579 --
34 Water Chemistry, Seepage, Streamflow, Reservoir Storage, and Annual Availability of Water for the San Juan-Chama Project

were weakly negative for April, weakly positive for June, 

and moderately positive for July. The positive trends for 

the monthly percentage of annual streamflow in June and 

July indicated that increased annual streamflow was also 

correlated to proportionally more streamflow in these months 

than in other months (table 8). The relative amount of the 

streamflow increase in June and July was greater than the 

increase in other months, such that more of the increase in 

annual streamflow occurred in June and July. The weakly 

negative trend for the percentage of annual streamflow in 

April indicated that the proportion of streamflow in April 

decreased with increased annual streamflow. 

The results from the Mann-Kendall trend test showed 

that there has not been a significant trend in the annual 

streamflow at Azotea Tunnel Outlet over time since 1971; 

however, the amount of streamflow during April has 

increased, the percentage of annual streamflow in May has 

increased, and the percentage of annual streamflow in June 

has decreased (table 8). These trends indicated that snowmelt 

may be occurring earlier in the year; however, the trends are 

only weakly correlated. Additionally, the trends in streamflow 

over time are not as strongly correlated as the trends in 

streamflow compared to annual streamflow. 

The difference between the occurrence and the strength 

of significant trends between the time series of the seasonal 

distribution of streamflow and the seasonal streamflow 

compared to annual streamflow indicated that the seasonal 

distribution of streamflow was more strongly controlled by 

the change in the annual streamflow than by time. Similar 

results were obtained for the seasonal distribution of 

streamflow for Navajo River at Banded Peak Ranch, such that 

there was not a significant trend in the seasonal distribution of 

streamflow over time (Falk and others, 2013). The similarity 

of the trends for the streams of the SJCP and Azotea Tunnel 

Outlet indicates that the temporal changes in the seasonal 

distribution of annual streamflow for the streams of the SJCP 

are controlling the changes in streamflow at Azotea Tunnel 

Outlet. Additionally, Falk and others (2013) determined that 

increased annual streamflow for streams of the SJCP resulted 

in the snowmelt runoff occurring later in the year and that 

there was generally a longer duration of runoff. 

Annual streamflow at Willow Creek below Heron 

Reservoir ranged from 19,880 to 154,180 acre-ft (figs. 11C

and 15A). Annual streamflow at this station significantly 

increased from 1971 to 2010, with a Kendall’s tau of 0.2846 

and a p-value of 0.0097 that made it statistically significant. 

The average annual streamflow at Willow Creek below 

Heron Reservoir by decade was 70,320 acre-ft for 1970–79; 

93,440 acre-ft for 1980–89; 104,490 acre-ft for 1990–99; and 

105,690 acre-ft for 2000–10. Increased release of water from 

Heron Reservoir is likely related to changes in downstream 

use of the water. 

The median monthly streamflow for Willow Creek 

below Heron Reservoir indicated that from 1971 to 2010 the 

largest streamflows generally occurred in March, April, and 

December and that smaller streamflows occurred in July, 

August, and September (fig. 15B). The timing of release of 

water from Heron Reservoir likely is related to the schedule 

of delivery for water by SJCP, such that more water is 

scheduled for delivery in the early spring and later summer. 

The large monthly streamflow at Willow Creek below Heron 

Reservoir in December is likely related to scheduled delivery 

of water remaining in storage prior to the end of the calendar 

year. Historically, the timing of large monthly releases from 

Heron Reservoir has varied, likely because of variations in 

downstream use and the total annual release. The majority 

of releases from Heron Reservoir during the 1970s and early 

1980s occurred in December, and the majority of releases in the 

middle and late 1980s and 1990s occurred in March and April 

(fig. 15C). More recently in the 2000s, releases have been more 

evenly distributed through the year with some recent increases 

in releases in December (2007 and 2008) and September (2009 

and 2010) (fig. 15C). 

The annual variations in streamflow at Azotea Tunnel 

Outlet and Willow Creek above Heron Reservoir are regulated 

by climate patterns such as the PDO that control the amount 

of precipitation in the watersheds that contribute to streamflow 

in the Rio Blanco, Little Navajo River, and Navajo River. 

Although there is not a significant monotonic trend in annual 

streamflow for streams that contribute to the SJCP over the 

period of record, annual streamflow has been lower during 

intervals when the PDO index is negative (indicating drier 

conditions) than during intervals when the PDO index is 

positive (indicating wetter conditions) (Falk and others, 2013). 

In general, streamflow at Azotea Tunnel Outlet will be lower 

during negative PDO index intervals and higher during positive 

PDO index intervals. The annual median of the PDO index, 

defined by following Falk and others (2013), has been negative 

since 2007 (fg. 16).

Changes in the seasonal distribution of streamflow for the 

Rio Blanco, Little Navajo River, and Navajo River could affect 

the amount of water diverted and the seasonal distribution of 

streamflow at Azotea Tunnel Outlet and Willow Creek above 

Heron Reservoir. It is possible that a shortened duration for 

snowmelt runoff could result in higher daily streamflow than 

an equivalent snowmelt runoff with a longer duration. Higher 

daily streamflow could result in more days with streamflow 

volumes that exceed the capacity of the diversion structures and 

a reduction in the proportion of water that could be diverted for 

the SJCP. Less total annual snowmelt runoff will result in less 

total water available for diversion for the SJCP. Additionally, 

changes in the timing could affect the amount of streamflow 

available for diversions because the minimum bypass flow 

requirements for the Rio Blanco, Little Navajo River, and 

Navajo River vary by month. Determination of the possible 

effects of changes in the seasonal distribution of streamflow for 

the SJCP was beyond the scope of this report.

For the Rio Blanco, Little Navajo River, and Navajo 

River, longer duration of runoff generally occurred later in the 

year, a smaller percentage of streamflow generally occurred 

in March, and a larger percentage of streamflow and larger 

monthly streamflow generally occurred in June during years 
Streamflow, Reservoir Storage, and Annual Availability of Water for the San Juan-Chama Project 35

C. Monthly percentage of annual streamflow

0

10

20

30

40

50

60

70

80

90

100

Monthly percentage of annual streamflow

Willow Creek below

 Heron Reservoir

B. Median monthly streamflow

Year

Median monthly streamflow, in acre-feet

A. Annual streamflow 180,000

100,000

80,000

60,000

40,000

20,000

160,000

140,000

120,000

0

Annual streamflow, in acre-feet

EXPLANATION

Willow Creek below

 Heron Reservoir

EXPLANATION

March

EXPLANATION

18,000

6,000

4,000

2,000

14,000

16,000

12,000

10,000

8,000

0

Month

Year

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

1970

19751980198519901995200020052010

1975 1980 1985 1990 1995 2000 2005 2010

April

September

December

Figure 15. Annual and seasonal variation in streamflow at the streamflow-gaging station Willow Creek below Heron Reservoir, 

northern New Mexico, 1971–2010. A, Annual streamflow. B, Median monthly streamflow. C, Monthly percentage of annual streamflow 

for March, April, September, and December over the period of record.
36 Water Chemistry, Seepage, Streamflow, Reservoir Storage, and Annual Availability of Water for the San Juan-Chama Project

with more annual streamflow (Falk and others, 2013). The 

majority of annual streamflow at Azotea Tunnel Outlet 

generally occurred from May through June with a median 

duration of slightly longer than a month. In general, a longer 

duration for the middle 50 percent of annual streamflow; 

a larger streamflow in May, June, and July; and a larger 

percentage of streamflow in June and July occurred during 

years with larger annual streamflow at Azotea Tunnel Outlet. 

Additionally, years with higher maximum daily streamflow 

were also years with higher annual streamflow. These 

results indicated that the timing and duration of streamflow 

at Azotea Tunnel Outlet occurred later and that higher 

daily streamflow occurred during years with more annual 

streamflow than during years with less annual streamflow. 

The seasonal and annual streamflows at Willow Creek 

below Heron Reservoir are controlled by releases from 

Heron Reservoir to meet the demand from downstream 

users. Annual streamflow at Willow Creek below Heron 

Reservoir has increased over the period of record. Generally, 

streamflow at Willow Creek below Heron Reservoir was 

greatest during early spring with smaller flows in later 

summer; however, releases for recent years have been more 

evenly distributed through the year, with increases in releases 

in December and September.

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

-2.5

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010 2020

Year

Annual median of the monthly North Pacific seasurface temperature anomaly

EXPLANATION

 Annual median of the monthly North Pacific

 sea-surface temperature anomaly 5-year

 moving average

 Annual median of the monthly North Pacific

 sea-surface temperature anomaly

Pacific Decadal Oscillation Index

Figure 16. Pacific Decadal Oscillation index for 1900–2012, defined as the median annual monthly North Pacific sea-surface 

temperature anomaly with a 5-year moving average (data modified from Mantua and Hare, 2011).

Heron Reservoir Storage

For reservoir systems like Heron Reservoir, the seasonal 

and annual variations in the amount of water in storage in 

the reservoir are predominantly controlled by the volume of 

inflows and the demands for releases from the downstream 

water users. Storage in a reservoir will increase when the 

inflows exceed the outflows and seepage. For the analysis 

of change in reservoir storage, inflows to Heron Reservoir 

included gaged streamflows at Willow Creek above Heron 

Reservoir and Horse Lake Creek, ungaged streamflow and 

direct runoff into the reservoir, and precipitation onto the 

water surface. Outflows from Heron Reservoir included the 

gaged streamflow at Willow Creek below Heron Reservoir, 

evaporation from the water surface, and leakage from the 

reservoir. Leakage from Heron Reservoir was estimated in 

1988 to “average between 2 and 3 cfs [cubic feet per second] 

at current storage levels′′ (Bureau of Reclamation, 1988) 

(2 ft3

/s for 1 year is equivalent to 1,448 acre-ft/yr). Average 

monthly pan evaporation rates for Heron Reservoir for 1975 

to 2005 ranged from 3.6 to 8.5 in., with a maximum average 

monthly rate of 8.49 in. in June (table 9). The estimated 

annual evaporation from Heron Reservoir, calculated from the 
Streamflow, Reservoir Storage, and Annual Availability of Water for the San Juan-Chama Project 37

sum of the average monthly pan evaporation rates for April 

through October over the maximum surface area of 6,148 

acres, was 22,940 acre-ft/yr. Average monthly precipitation 

rates for Heron Reservoir for 1975 to 2005 ranged from 0.8 to 

2.5 in. (table 9). In general, the monthly average precipitation 

rates for 1975 to 2005 were slightly less than the monthly 

average precipitation rates for 1905 to 2009 (fig. 3). The 

estimated annual direct precipitation to Heron Reservoir, 

calculated from the sum of the average monthly precipitation 

rates for January through December for 1975–2005 over the 

maximum surface area of 6,148 acres, was 8,820 acre-ft/yr. 

For this analysis, the change in storage was calculated from 

the reported storage in Heron Reservoir.

The amount of water in storage at Heron Reservoir on 

January 1 of each year during the period 1970–2010 ranged 

from 42,630 acre-ft in 1971 to 394,800 acre-ft in 1985. For 

most years of the 1970s, the amount of water in storage 

increased annually (fig. 12). The reservoir was near capacity 

for most of the 1980s and 1990s (fig. 12). The amount of water 

in storage in the reservoir decreased from 2000 to 2004 and 

generally increased from 2005 to 2010 (fig. 12). 

In general, the median monthly change in the volume of 

water in storage at Heron Reservoir was positive April through 

June and negative January through March and August through 

December (fig. 17). The largest median monthly increases in 

storage occurred in April, May, and June (fig. 17), the months 

when the largest median monthly streamflow occurred on 

Willow Creek above Heron Reservoir (fig. 13B). The largest 

individual monthly decreases in storage occurred in March, 

April, September, and December (fig. 17). Large decreases in 

storage in March, April, and December likely are associated 

with releases from Heron Reservoir for SJCP contractors, as 

measured at Willow Creek below Heron Reservoir (figs. 15B

and 15C). The largest decreases in storage in June occurred 

in 1977, 2000, and 2002, when there was below average 

streamflow at Willow Creek above Heron Reservoir and above 

average streamflow at Willow Creek below Heron Reservoir. 

In general, storage changes at Heron Reservoir reflect the 

differences in streamflow at Willow Creek above Heron 

Reservoir and Willow Creek below Heron Reservoir. 

Table 9. Average rates of precipitation and pan evaporation for Heron Resrvoir, northern New Mexico, 1975–2005 (modified from 

Whipple, 2007).

[--, data not available]

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Pan evaporation, 

in inches

-- -- -- 4.8 6.9 8.5 8.4 7.1 5.5 3.6 -- --

Precipitation, in 

inches

1.3 1.3 1.5 1.2 1.3 0.8 1.8 2.5 1.7 1.5 1.5 0.99

Annual changes in the volume of water in storage at 

Heron Reservoir have ranged from a decrease in storage 

of 130,660 acre-ft in 2002 to an increase in storage of 

102,200 acre-ft in 1973 (fig. 18A). Over the period of record, 

the change in storage fluctuated between an increase of 

25,000 acre-ft and a decrease of 25,000 acre-ft (20 of the 

40 years of record) (figs. 18A and 18B). Reservoir storage 

during years with near maximum reservoir capacity generally 

fluctuated over a small range because inflows for these years 

have nearly equaled outflow. Annual variation in reservoir 

storage has increased since 1995 (fig. 18A) and is likely 

affected by the variability in both inflows and outflows since 

1995 (figs. 13B and 15A).

Seasonal and Annual Variation in Streamflow 

for Rio Grande Water

Rio Grande water (streamflow derived from water 

sources in the Rio Grande Basin) was measured on Willow 

Creek at two stations: (1) the station at Willow Creek near 

Park View for the period of record from 1942 to 1971 and 

(2) Willow Creek above Heron Reservoir for the period 

before the SJCP diversions from 1962 to 1970. Streamflow 

at Willow Creek near Park View exceeded the streamflow at 

Willow Creek above Heron Reservoir because the station at 

Willow Creek near Park View collected streamflow from a 

larger drainage area than did the Willow Creek above Heron 

Reservoir station (table 6); however, the streamflow at both 

gages followed the same annual trend from 1963 to 1970 (fig. 

19A). The amount of Rio Grande water was also estimated 

at Willow Creek above Heron Reservoir for the period from 

1971 to 2010 as the difference between measured streamflow 

at Azotea Tunnel Outlet and Willow Creek above Heron 

Reservoir (streamflow at Willow Creek above Heron Reservoir 

minus streamflow at Azotea Tunnel Outlet). Rio Grande water, 

reported as measured streamflow or estimated streamflow, 

presented in this report was not calculated by following the 

methods used for the Rio Grande Compact, detailed in the 

Upper Rio Grande Water Operations Model (U.S. Army Corps 
38 Water Chemistry, Seepage, Streamflow, Reservoir Storage, and Annual Availability of Water for the San Juan-Chama Project

60,000

-20,000

0

-40,000

-60,000

40,000

20,000

-80,000

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Month

Median monthly change in the volume of water

in storage at Heron Reservoir, in acre-feet

EXPLANATION

Whisker, 90th percentile

Whisker, 10th percentile

Outlier, individual observation outside

 -1.5 of the interquartile range

Upper quartile line, 75th percentile

Lower quartile line, 25th percentile

Median, 50th percentile

Observation

Outlier, individual observation outside

 +1.5 of the interquartile range

Annual change in storage in Heron Reservoir, in acre-feet

A. Annual storage in Heron Reservoir

150,000

100,000

50,000

-50,000

-100,000

-150,000

 0

1970 1975 1980 1985 1990 1995 2000 2005 2010

Year

Change in storage in Heron Reservoir, in acre-feet

B. Distribution of annual storage in Heron Reservoir

150,000

100,000

50,000

-50,000

-100,000

-150,000

 0

0 2 4 6 8 10 12

Number of years

Figure 17. Median monthly change in the volume of water in storage at Heron Reservoir, northern New Mexico, 1970–2010.

Figure 18. Annual variation in storage in Heron Reservoir, in northern New Mexico, 1970–2010. A, Annual storage in Heron Reservoir. 

B, Distribution of annual storage in Heron Reservoir.
Streamflow, Reservoir Storage, and Annual Availability of Water for the San Juan-Chama Project 39

40,000

45,000

0

35,000

30,000

25,000

20,000

15,000

10,000

-10,000

5,000

-5,000

Year

EXPLANATION

Willow Creek above Heron Reservoir—Gaged discharge from 1963 to 1970 

Estimated annual streamflow for Rio Grande water at Willow Creek

 above Heron Reservoir—Estimated as the difference between

 measured streamflow at Azotea Tunnel Outlet and Willow Creek above

 Heron Reservoir from 1971 to 2010

Willow Creek near Park View

Willow Creek above Heron Reservoir—Gaged discharge from

 1963 to 1970 

Estimated Rio Grande water at Willow Creek above

 Heron Reservoir—Estimated as the difference between

 measured streamflow at Azotea Tunnel Outlet and

 Willow Creek above Heron Reservoir from 1971 to 2010

Reported Rio Grande water for the San Juan-Chama Project

Willow Creek near Park View

EXPLANATION

Estimated Rio Grande water at Willow Creek above

 Heron Reservoir

Willow Creek above Heron Reservoir median annual

 streamflow

Willow Creek near Park View median annual streamflow

Reported Rio Grande water for the San Juan-Chama

 Project median annual streamflow

1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010

5,900 acre-feet

9,170 acre-feet

5,710 acre-feet

14,900 acre-feet

3,000

1,000

500

0

2,500

2,000

1,500

-500

Month

Median monthly streamflow, in acre-feet Annual streamflow, in acre-feet

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

A. Annual streamflow

B. Median monthly streamflow

Figure 19. Annual and seasonal variation in Rio Grande streamflow at streamflow-gaging stations Willow Creek near Park View and 

Willow Creek above Heron Reservoir, 1942–2010. A, Annual streamflow. B, Median monthly streamflow.
40 Water Chemistry, Seepage, Streamflow, Reservoir Storage, and Annual Availability of Water for the San Juan-Chama Project

of Engineers, 2005), but represents an estimate of the amount 

of Rio Grande water for the SJCP. The complex computational 

methods for calculating the Rio Grande water for the SJCP 

reported to the Rio Grande Compact Commission, shown in 

fig. 19A (N. Shafike, Interstate Stream Commission, written 

commun., 2014), are beyond the scope of this report.

Annual Rio Grande streamflow varied substantially over 

the period of record, and this variation was likely a function 

of climate including precipitation. The largest measured 

streamflow was 34,860 acre-ft on Willow Creek near Park 

View in 1958 (fig. 19A). Median annual streamflow from 

1943 to 1969 at Willow Creek near Park View was 9,170 

acre-ft (table 6). Median annual streamflow from 1963 to 

1971 at Willow Creek above Heron Reservoir was 5,710 

acre-ft (table 6). Median estimated annual streamflow for Rio 

Grande water at Willow Creek above Heron Reservoir from 

1971 to 2010 was 5,900 acre-ft. Median reported Rio Grande 

water for the San Juan-Chama Project was 14,900 acre-ft (fig. 

19A). The difference between measured streamflow at Azotea 

Tunnel Outlet and Willow Creek above Heron Reservoir was 

less than zero acre-ft for 1998, 1999, and 2002 (fig. 19A). 

The estimated annual Rio Grande water at Willow Creek 

above Heron Reservoir from 1971 to 2010 varied over a range 

of approximately 500 to 20,000 acre-ft/yr until 1996. After 

1996, the variability and magnitude of the estimated annual 

Rio Grande water changed noticeably; however, it was not 

possible to determine the cause from the available reported 

data for the stations (fig. 19A). 

The median monthly Rio Grande streamflow indicated 

that snowmelt runoff in March and April was the main source 

of streamflow (fig. 19B). Monthly streamflow data for the 

two stations with measured streamflow and one station with 

estimated streamflow, all with different periods of record, 

showed a similar general trend throughout the year (fig. 19B). 

Approximately 70 percent of the annual streamflow occurred 

in March and April. Summer monsoon derived streamflow 

in July, August, and September comprised approximately 

15 percent of the annual streamflow at these stations. From 

November to February when little to no water was diverted 

for the SJCP, most of the streamflow in Willow Creek was Rio 

Grande water. 

Annual Availability of Water for the San JuanChama Project

Existing data have been used to compare the theoretical 

amount of water that could have been diverted for the SJCP 

to the actual amount of water that was diverted. The amount 

of water diverted from the streams is affected by a number of 

factors including legal, structural, capacity, and operational 

limitations. The effects of these factors on the amount of water 

available for diversion for the SJCP have been estimated by 

using historical streamflow data to sequentially determine the 

amount of water that could be diverted after accounting for 

the legal limitations (legally limited available streamflow), 

structural limitations (structurally limited available 

streamflow), capacity limitations (capacity-limited available 

streamflow), and operational limitations (operationally limited 

available streamflow). Evaluation of the effects of the factors 

that limit diversions could provide information to water 

management agencies to maximize the diversion of water for 

the SJCP and to assess possible changes to infrastructure or 

operating procedure. 

The legal limitation that has the largest effect on the 

amount of streamflow available for diversion for the SJCP is 

the minimum monthly bypass requirements detailed by P.L. 

87-483. Streamflow in the Rio Blanco, Little Navajo River, 

and Navajo River cannot be depleted below the minimum 

monthly bypass requirement for each stream. The minimum 

monthly bypass requirements vary for each stream by month, 

such that the required volume of streamflow maintained in 

each stream generally is larger March through September 

than October through February (Falk and others, 2013). In 

general, a daily minimum bypass requirement is determined 

by equally dividing the monthly requirement by the days of 

the month. The diversion gates at the Blanco Diversion Dam, 

Little Oso Diversion Dam, and Oso Diversion Dam are set 

every day or every few days to ensure the daily minimum 

bypass requirement for streamflow is satisfied (E. Wilcox, 

Chama New Mexico Field Division, Bureau of Reclamation, 

oral commun., 2009). Other legal limitations, including the 

maximum diversion in any 10 consecutive years and the 

annual maximum diversion, have not been exceeded since 

diversions started in 1971 (figs. 13A and 13C). An additional 

legal limitation for diversion of water is that water cannot be 

spilled from Heron Reservoir; however, this has not occurred 

because diversions have been reduced before the capacity was 

exceeded. 

The legally limited available streamflow for the Rio 

Blanco, Little Navajo River, and Navajo River was calculated 

on the basis of the historical streamflow record for the period 

March 3, 1974, to December 31, 2010 (described in detail by 

Falk and others, 2013). The calculated daily streamflow above 

the minimum monthly bypass requirement was computed by 

subtracting the daily minimum bypass requirement from the 

daily streamflow for each stream. Daily calculations were not 

modified to ensure that the minimum monthly bypass was 

satisfied when daily streamflow was less than daily minimum 

bypass. The calculated daily legally limited available 

streamflow was aggregated to annual intervals to summarize 

the results. The legally limited available streamflow is greater 

than the actual water diverted because of the structural, 

capacity, and operational limitations. 

The structural limitations that affect diversion of water 

for the SJCP include the capacity of the tunnels through 

which the diverted water is routed. The daily streamflow 

that can be diverted from Rio Blanco was limited by the 

capacity of Blanco Tunnel (520 ft3

/s), and the amount that 

can be diverted from Little Navajo River was limited by 

the capacity of Little Oso Feeder (150 ft3

/s). The combined 

amount of water that can be diverted from Rio Blanco and 

Little Navajo River was also limited by the capacity of Oso 
Streamflow, Reservoir Storage, and Annual Availability of Water for the San Juan-Chama Project 41

Tunnel (550 ft3/s). The daily streamflow that can be diverted 

from Navajo River was limited by the capacity of Oso Feeder 

(650 ft3

/s). The combined amount of water that can be diverted 

from Rio Blanco, Little Navajo River, and Navajo River was 

also limited to the capacity of Azotea Tunnel. The reported 

capacity of Azotea Tunnel is 950 ft3

/s; however, analysis of 

the daily streamflow data at Azotea Tunnel Outlet indicated 

that streamflow volumes of 1,050 ft3

/s were common. The 

daily structurally limited available streamflow was calculated 

by subtracting the amount of water that could not be diverted 

because of structural limitations from the legally limited 

available streamflow. The calculated daily structurally limited 

available streamflow was aggregated to annual intervals to 

summarize the results.

Comparison of the structurally limited available 

streamflow at Azotea Tunnel capacities of 950 ft3

/s and 1,050 

ft3

/s indicated that the additional 100-ft3/s capacity results 

in an average annual increase of 2,500 acre-ft of available 

streamflow with the largest increases in the 1970s and 1980s 

(fig. 20A). Additionally, most of the increases occurred in 

years with more than 90,000 acre-ft of streamflow available 

for diversion for the SJCP and escalated during years with 

more annual available streamflow for diversion for the SJCP 

(fig. 20B). These results indicated that operating the SJCP 

at a tunnel capacity of 1,050 ft3

/s could capture additional 

streamflow during years with average to above average 

streamflow. Historical streamflow at Azotea Tunnel Outlet 

exceeded 950 ft3

/s during years of large runoff; therefore, a 

maximum capacity of 1,050 ft3

/s for Azotea Tunnel was used 

to estimate the structurally limited available streamflow for 

the SJCP. Use of 1,050 ft3

/s for the capacity of Azotea Tunnel 

could result in an overestimation of the amount of water that 

could have been diverted for the SJCP during times that the 

reported limit of 950 ft3

/s was observed. 

The storage capacity of Heron Reservoir is another 

limitation affecting the amount of water that can be diverted 

for the SJCP. The capacity-limited available streamflow is the 

amount of water that can be diverted for the SJCP, accounting 

for reductions in diversions to prevent exceeding the capacity 

of the reservoir. The reported storage in Heron Reservoir 

exceeded the maximum reported storage for several days over 

the period of record in the 1980s and 1990s to a maximum 

reported storage of 402,100 acre-ft. Management of reservoir 

operations, such as releases of water from storage during 

intervals when the reservoir was near maximum capacity, 

likely allows flexibility in the amount of inflows that can 

occur. Though analysis of streamflow and available storage 

in Heron Reservoir indicates that operation of the reservoir 

is more complicated, the simplifying assumption that the 

capacity-limited available streamflow could not exceed the 

reported capacity of Heron Reservoir was used for this report. 

In order to evaluate the effect of different storage capacities 

on the amount of streamflow that can be diverted for the SJCP, 

the capacity-limited available streamflow was calculated by 

using two different values: (1) the reported maximum storage 

capacity of 401,320 acre-ft and (2) a smaller storage capacity 

of 400,320 acre-ft. The smaller storage capacity provides an 

approximation of the effect that a change in the volume of 

storage in Heron Reservoir would have on the amount of water 

that could be diverted for the SJCP.

The capacity-limited available streamflow was calculated 

on a daily basis by subtracting water that could not be 

diverted because the reservoir capacity would be exceeded 

from the structurally limited available streamflow. A daily 

available storage in Heron Reservoir was calculated as the 

difference of the maximum reservoir storage capacity and 

the reported volume of water in storage in Heron Reservoir. 

If the daily available storage was greater than zero, the total 

daily structurally limited available streamflow was assumed 

to be divertible. The assumption that the storage capacity 

of the reservoir is decreased to 400,320 acre-ft will result 

in a maximum estimate of the amount of water not diverted 

and a minimum estimate for the capacity-limited available 

streamflow because it is likely that the reservoir was managed 

to allow inflow during intervals near capacity. The assumption 

that the maximum storage capacity of the reservoir is 401,320 

acre-ft will result in a minimum estimate of the amount of 

water not diverted and a maximum estimate for the capacitylimited available streamflow. Outflow from Heron Reservoir 

was not explicitly incorporated into the estimate of the 

capacity-limited available streamflow; however, the use of 

the reported volume of water in storage for Heron Reservoir 

implicitly accounts for outflow. The calculated daily capacity 

limited available streamflow was aggregated to annual 

intervals to summarize the results.

Comparison of the capacity-limited available streamflow 

at the two reservoir storage capacities of 401,320 acre-ft and 

400,320 acre-ft indicated that the difference of 1,000 acre-ft 

of storage capacity resulted in an average decrease of 7,700 

acre-ft of available streamflow, with the largest decrease in 

the 1980s (fig. 20A). Decreased available streamflow resulting 

from a difference in the estimated maximum storage capacity 

occurred in years when more than 300,000 acre-ft of water 

was stored in Heron Reservoir, but not all years with more 

than 300,000 acre-ft of stored water were limited by reservoir 

capacity (fig. 20B). During the late 1980s, Heron Reservoir 

was near the maximum capacity; however, annual streamflow 

for these years was below average, and therefore the diversion 

of water for the SJCP was not limited by reservoir capacity. 

These results indicate that the limitation of the capacity of 

Heron Reservoir can substantially decrease the streamflow 

that can be diverted for the SJCP; however, this limitation 

only affected diversions in years with high annual available 

streamflow and near-capacity storage of water in the reservoir 

(fig. 20C). A capacity of 401,320 acre-ft for Heron Reservoir 

was used to estimate the capacity-limited available streamflow 

for the SJCP. Use of the 401,320-acre-ft capacity for Heron 

Reservoir could result in an overestimation of the amount 

of water that could have been diverted for the SJCP if the 

reservoir generally was managed to maintain some amount of 

volume of storage to provide a buffer for additional inflow. 
42 Water Chemistry, Seepage, Streamflow, Reservoir Storage, and Annual Availability of Water for the San Juan-Chama Project Annual storage in Heron Reservoir, in acre-feet

Increase in available streamflow for assumed increased capacity of Azotea

 Tunnel Outlet from 950 cubic feet per second (ft3

/s) to 1,050 ft3/s , in acre-feet

Decrease in available streamflow for assumed decreased capacity of 1,000

 acre-feet for Heron Reservoir, in acre-feet

Heron Reservoir storage

Annual streamflow for diversion for the San Juan-Chama Project, in acre-feet

Estimated change in annual available

streamflow, in acre-feet

Estimated change in annual available

streamflow, in acre-feet

Estimated change in annual available

streamflow, in acre-feet

Year

C. Estimated decrease in annual capacity-limited available streamflow compared to storage in Heron Reservoir

A. Estimated change in annual structurally limited available streamflow and capacity-limited available streamflow

0

–10,000

–30,000

–50,000

–70,000

100,000

150,000

250,000

400,000

450,000

 –40,000

–80,000

1970 1975 1980 1985 1990 1995 2000 2005 2010

Year

EXPLANATION

12,000

10,000

8,000

4,000

2,000

0

 6,000

0 50,000 100,000 150,000 200,000 250,000 300,000

20,000

–10,000

0

10,000

–20,000

–30,000

–60,000

–70,000

–80,000

 –40,000

 –50,000

1970 1975 1980 1985 1990 1995 2000 2005 2010

B. Estimated increase in annual structurally limited available streamflow compared to annual streamflow

–20,000

–60,000

350,000

300,000

50,000

0

200,000

Figure 20. Change in the available streamflow for the San Juan-Chama Project from variation in the structural and capacity limitations. 

A, Comparison of the estimated increase in annual structurally limited available streamflow resulting from a larger estimated maximum 

capacity of Azotea Tunnel from 950 cubic feet per second (ft3

/s) to 1,050 ft3/s and the estimated decrease in annual capacity-limited 

available streamflow resulting from a smaller estimated maximum capacity of Heron Reservoir from 401,320 acre-feet (acre-ft) to 400,320 

acre-ft, 1974–2010. B, Comparison of the estimated increase in annual structurally limited available streamflow resulting from a larger 

estimated maximum capacity of Azotea Tunnel from 950 ft3

/s to 1,050 ft3/s to annual streamflow measured at Azotea Tunnel Outlet. C, 

Comparison of the estimated decrease in annual capacity-limited available streamflow resulting from a smaller estimated maximum

capacity of Heron Reservoir from 401,320 acre-ft to 400,320 acre-ft to annual streamflow measured at Azotea Tunnel Outlet, 1974–2010.

Annual storage in Heron Reservoir, in acre-feet

Increase in available streamflow for assumed increased capacity of Azotea

 Tunnel Outlet from 950 cubic feet per second (ft3

/s) to 1,050 ft3/s , in acre-feet

Decrease in available streamflow for assumed decreased capacity of 1,000

 acre-feet for Heron Reservoir, in acre-feet

Heron Reservoir storage

Annual streamflow for diversion for the San Juan-Chama Project, in acre-feet

Estimated 

stre

Estimated change in annual available

streamflow, in acre-feet

Estimated change in annual available

streamflow, in acre-feet

Year

C. Estimated decrease in annual capacity-limited available streamflow compared to storage in Heron Reservoir

0

–10,000

–30,000

–50,000

–70,000

100,000

150,000

250,000

400,000

450,000

 –40,000

–80,000

1970 1975 1980 1985 1990 1995 2000 2005 2010

Year

EXPLANATION

12,000

10,000

8,000

4,000

2,000

0

 6,000

0 50,000 100,000 150,000 200,000 250,000 300,000

–60,000

–70,000

–80,000

 –50,000

1970 1975 1980 1985 1990 1995 2000 2005 2010

B. Estimated increase in annual structurally limited available streamflow compared to annual streamflow

–20,000

–60,000

350,000

300,000

50,000

0

200,000
Streamflow, Reservoir Storage, and Annual Availability of Water for the San Juan-Chama Project 43 Annual storage in Heron Reservoir, in acre-feet

Increase in available streamflow for assumed increased capacity of Azotea

 Tunnel Outlet from 950 cubic feet per second (ft3

/s) to 1,050 ft3/s , in acre-feet

Decrease in available streamflow for assumed decreased capacity of 1,000

 acre-feet for Heron Reservoir, in acre-feet

Heron Reservoir storage

Annual streamflow for diversion for the San Juan-Chama Project, in acre-feet

Estimat

s

Estimated change in annual available

streamflow, in acre-feet

C. Estimated decrease in annual capacity-limited available streamflow compared to storage in Heron Reservoir

0

–10,000

–30,000

–50,000

–70,000

100,000

150,000

250,000

400,000

450,000

 –40,000

–80,000

1970 1975 1980 1985 1990 1995 2000 2005 2010

Year

EXPLANATION

2,000

0

0 50,000 100,000 150,000 200,000 250,000 300,000

–20,000

–60,000

350,000

300,000

50,000

0

200,000

Figure 20. Change in the available streamflow for the San Juan-Chama Project from variation in the structural and capacity limitations. 

A, Comparison of the estimated increase in annual structurally limited available streamflow resulting from a larger estimated maximum 

capacity of Azotea Tunnel from 950 cubic feet per second (ft3

/s) to 1,050 ft3/s and the estimated decrease in annual capacity-limited 

available streamflow resulting from a smaller estimated maximum capacity of Heron Reservoir from 401,320 acre-feet (acre-ft) to 

400,320 acre-ft, 1974–2010. B, Comparison of the estimated increase in annual structurally limited available streamflow resulting from 

a larger estimated maximum capacity of Azotea Tunnel from 950 ft3

/s to 1,050 ft3/s to annual streamflow measured at Azotea Tunnel 

Outlet. C, Comparison of the estimated decrease in annual capacity-limited available streamflow resulting from a smaller estimated 

maximum capacity of Heron Reservoir from 401,320 acre-ft to 400,320 acre-ft to annual streamflow measured at Azotea Tunnel Outlet, 

1974–2010.—Continued

The amount of water that can be diverted for the SJCP 

is also limited by operational constraints as a result of the 

management of the SJCP. Historically, management of the 

SJCP included closure of the gates on the diversion structures 

on the Rio Blanco, Little Navajo River, and Navajo River from 

November to February for maintenance and safety reasons. 

The operationally limited available streamflow was calculated 

by determining which days recorded diversions of zero and 

subtracting the daily legally limited available streamflow 

from the daily capacity-limited available streamflow. If the 

operationally limited available streamflow was computed as 

less than zero, a value of zero was assumed. For each year, 

the day of the start of diversion of streamflow was determined 

as the first day on which flow occurred at Azotea Tunnel 

Outlet, and the day of the end of diversion of streamflow 

was determined as the day on which 99.9 percent of annual 

streamflow occurred. The calculated daily operationally 

limited available streamflow was aggregated to annual 

intervals to summarize the results.

The effects of the variation in the structural and capacity 

limitations on the amount of water available for diversion 

for the SJCP was estimated by calculating the operationally 

limited available streamflow for four scenarios that span the 

structural and capacity limitations described previously. The 

four scenarios for operationally limited available streamflow 

included (1) Azotea Tunnel capacity of 950 ft3

/s and storage 

capacity of Heron Reservoir of 400,320 acre-ft, (2) Azotea 

Tunnel capacity of 1,050 ft3

/s and storage capacity of Heron 

Reservoir of 400,320 acre-ft, (3) Azotea Tunnel capacity of 

950 ft3

/s and storage capacity of Heron Reservoir of 401,320 

acre-ft, and (4) Azotea Tunnel capacity of 1,050 ft3

/s and 

storage capacity of Heron Reservoir of 401,320 acre-ft (fig. 

21). Comparison of the scenarios indicated that the variations 

in the structural and capacity limitations primarily affected 

the available streamflow during the 1980s and early 1990s 

(fig. 21). Additionally, the variations in the structural and 

capacity limitations was minor compared to the amount of 

operationally limited available streamflow for the SJCP during 

recent years from 1999. 

The calculations of the structurally limited available 

streamflow, capacity-limited available streamflow, and 

operationally limited available streamflow for the SJCP are 
44 Water Chemistry, Seepage, Streamflow, Reservoir Storage, and Annual Availability of Water for the San Juan-Chama Project

estimates and are subject to the limitations of the data and the 

simplifying assumptions that were made about management of 

the SJCP from 1971 to 2010. Consistent positive or negative 

errors in streamflow data would bias these estimates. Changes 

in operating procedures of the project would also affect the 

estimates of the structurally limited available streamflow 

and capacity-limited available streamflow. The estimated 

available streamflow is suitable for evaluating the significance 

of selected factors that affect the amount of water that can 

be diverted for the SJCP but is not suitable for accurate 

accounting of all factors that affect diversions for the SJCP. 

The estimated operationally limited available streamflow 

did not equal the actual diversion of streamflow measured at 

Azotea Tunnel Outlet, indicating that an amount of water was 

not diverted for reasons not accounted for in this analysis. 

Water is not diverted for many reasons, including higher 

bypass flows to ensure downstream requirements were met 

300,000

150,000

200,000

250,000

50,000

100,000

0

Year

Annual available streamflow, in acre-feet

EXPLANATION

Four scenarios of operationally limited available streamflow,

 in acre-feet

Measured streamflow at Azotea Tunnel Outlet, in acre-feet

Legally limited available streamflow, in acre-feet

Azotea Tunnel Outlet capacity of 950 cubic feet per second (ft3/s) and

 storage capacity of Heron Reservoir of 400,320 acre-feet

Azotea Tunnel Outlet capacity of 1,050 ft3

/s and storage capacity of

 Heron Reservoir of 400,320 acre-feet

Azotea Tunnel Outlet capacity of 950 ft3

/s and storage capacity of

 Heron Reservoir of 401,320 acre-feet

Azotea Tunnel Outlet capacity of 1,050 ft3

/s and storage capacity of

 Heron Reservoir of 401,320 acre-feet

1970 1975 1980 1985 1990 1995 2000 2005 2010

Figure 21. Operationally limited available streamflow for the San Juan-Chama Project for four scenarios of structural and capacity 

limitations, 1974–2010.

and times when ice or debris may block the intake structures 

such that flow cannot be diverted (C. Donnelly, Albuquerque 

Area Office, Bureau of Reclamation, written commun., 2013). 

Inclusion of additional reasons for decreased diversion of 

water for the SJCP was beyond the scope of this report (this 

volume of water is referred to as “available streamflow not 

diverted for unknown reasons′′).

The annual availability of streamflow for the SJCP 

varied over the period from 1975 to 2010, which includes 

all years with complete record of daily flow (fig. 22). In 

general, variation in the estimate of streamflow availability 

for the different limitations was bounded by the variability 

in the streamflow for the SJCP (computed as the sum of the 

streamflow from Rio Blanco, Little Navajo River, and Navajo 

River) (fig. 22). The streamflow for the SJCP was less than 

the annual water contracted (95,475 acre-ft) during several 

years and was substantially reduced after accounting for 
Streamflow, Reservoir Storage, and Annual Availability of Water for the San Juan-Chama Project 45

Year

Annual streamflow, in acre-feet

Legally limited available streamflow, in acre-feet

Structurally limited available streamflow in excess of

 capacity-limited streamflow, in acre-feet

Capacity-limited available streamflow in excess of

 operationally limited available streamflow, in acre-feet

Operationally limited available streamflow, in acre-feet

Measured streamflow at Azotea Tunnel Outlet, in acre-feet

Sum of streamflow from Rio Blanco, Little Navajo River,

 and Navajo River, in acre-feet

EXPLANATION

1970 1975 1980 1985 1990 1995 2000 2005 2010

0

50,000

100,000

200,000

250,000

300,000

350,000

150,000

Figure 22. Annual available streamflow for the San Juan-Chama Project, 1975–2010, northern New Mexico.
46 Water Chemistry, Seepage, Streamflow, Reservoir Storage, and Annual Availability of Water for the San Juan-Chama Project

the minimum monthly bypass requirement (legally limited 

available streamflow) (fig. 22). The reduction in available 

streamflow because of the structural limitations of the SJCP 

infrastructure (computed as the difference between legally 

limited streamflow available for diversion and the structurally 

limited available streamflow) was an annual average of 

7,000 acre-ft (table 10). The capacity-limited available 

streamflow was the same as the structurally limited available 

streamflow for most years, except for a few years in the 1980s 

and 1990s when Heron Reservoir was near capacity. The 

annual average reduction in available streamflow because of 

operational limitations (computed as the difference between 

the capacity-limited available streamflow and the operationally 

limited available streamflow) was 9,000 acre-ft (table 10) and 

generally was larger in the 1980s and 1990s than the 1970s 

and 2000s (fig. 22). 

The average annual legally limited available streamflow 

for the SJCP from 1975 to 2010 provided estimates of the 

expected amount of water available for diversion for the 

SJCP. The average annual legally limited available streamflow 

for the SJCP was 131,000 acre-ft, and the average annual 

streamflow at Azotea Tunnel Outlet was 94,710 acre-ft 

(table 10). On average, the annual streamflow at Azotea 

Tunnel Outlet was approximately 75 percent of the annual 

legally limited available streamflow for the SJCP. The 

annual streamflow not diverted for the SJCP, defined as 

the difference between the annual legally limited available 

streamflow for the SJCP and the measured annual streamflow 

at Azotea Tunnel Outlet, was an average of 35,000 acre-ft. 

The average annual percentage of available streamflow not 

diverted for the SJCP was 14 percent because of structural 

limitations of the capacity of infrastructure, 1 percent 

because of limitations of the reservoir storage capacity, and 

29 percent because of the limitations from operations. For 

most years, the annual available streamflow not diverted for 

unknown reasons exceeded the sum of the water not diverted 

because of structural, capacity, and operational limitations 

(fig. 23). From 1975 to 2010, the average annual measured 

streamflow at Azotea Tunnel Outlet was 85 percent of the 

annual operationally limited available streamflow for that 

year (fig. 23). During the period from 2000 to 2010, the 

measured streamflow at Azotea Tunnel Outlet was an average 

of 93 percent of the estimated operationally limited available 

streamflow (fig. 23). The difference between the measured 

annual streamflow at Azotea Tunnel Outlet and the annual 

operationally limited available streamflow was greater at 

higher streamflow than at lower streamflow (fig. 24). 

Table 10. Average annual available streamflow for selected limitations of the San Juan-Chama Project.

Streamflow for 

the San JuanChama Project 

(the sum of 

the streamflow from Rio 

Blanco, Little 

Navajo River, 

and Navajo 

River)

Legally 

limited 

available streamflow 

(acre-feet) 

Structurally 

limited available streamflow 

(acre-feet)

Capacitylimited 

available streamflow

(acre-feet)

Operationally 

limited 

available streamflow 

(acre-feet)

Measured 

streamflow 

at Azotea 

Tunnel 

Outlet 

(acre-feet)

Average annual 

streamflow for 1975–

2010, in acre-feet

180,000 131,000 124,000 123,000 114,000 94,710
Streamflow, Reservoir Storage, and Annual Availability of Water for the San Juan-Chama Project 47

Percentage of annual available

streamflow not diverted

Annual streamflow not diverted (difference

between streamflow available for the San

Juan-Chama Project and streamflow

at Azotea Tunnel Outlet), in acre-feet

0

100

180,000

200,000

20,000

0

10

197419751976

Year

1977197819791980198119821983198419851986198719881989199019911992199319941995199619971998199920002001200220032004200520062007200820092010

20

30

40

50

60

70

80

90

40,000

60,000

80,000

100,000

120,000

140,000

160,000

EXPLANATION

Operationally limited

Capacity limited

Structurally limited

Percentage of available streamflow not diverted because of

Percentage of available streamflow not diverted for reasons

 not accounted for in this analysis

Annual streamflow not diverted

50,000

100,000

150,000

200,000

250,000

300,000

0

0 50,000 100,000 150,000 200,000 250,000 300,000

Measured annual streamflow at Azotea Tunnel Outlet, in acre-feet

Estimated annual operationally limited available streamflow

for the San Juan-Chama Project, in acre-feet

Annual streamflow

1:1 line

Figure 23. Percentage of annual available streamflow not diverted for the San Juan-Chama Project for structure, capacity, and 

operations limitations, percentage of annual streamflow not diverted for reasons not accounted for in this analysis, and the total annual 

streamflow not diverted for the San Juan-Chama Project, northern New Mexico, 1974–2010.

Figure 24. Measured annual streamflow at Azotea Tunnel Outlet compared to estimated annual operationally limited available 

streamflow for the San Juan-Chama Project, northern New Mexico, 1974–2010.
48 Water Chemistry, Seepage, Streamflow, Reservoir Storage, and Annual Availability of Water for the San Juan-Chama Project

Summary

The Albuquerque Bernalillo County Water Utility 

Authority (ABCWUA) supplements the municipal water 

supply for the Albuquerque metropolitan area, in central New 

Mexico, with surface water diverted from the Rio Grande. 

ABCWUA’s allotment of surface water diverted from the Rio 

Grande is derived from the San Juan-Chama Project (SJCP), 

which delivers water from streams in the southern San Juan 

Mountains in the Colorado River Basin in southern Colorado 

to the Rio Chama in the Rio Grande Basin in northern New 

Mexico. In an effort to provide more information about water 

chemistry and quantity, the U.S. Geological Survey (USGS), 

in cooperation with the ABCWUA, undertook this study in 

which historical streamflow and water-chemistry data were 

compiled, and new water-chemistry data were collected 

to characterize the water chemistry and streamflow of the 

SJCP. Characterization of streamflow included analysis of 

the variability of annual streamflow and comparison of the 

theoretical amount of water that could have been diverted into 

the SJCP to the actual amount of water that was diverted for 

the SJCP. Additionally, a seepage investigation was conducted 

along the channel between Azotea Tunnel Outlet and the 

streamflow-gaging station (station) at Willow Creek above 

Heron Reservoir to estimate the magnitude of the gain or loss 

in streamflow resulting from groundwater interaction over the 

approximately 10-mile reach.

In general, surface-water chemistry at Azotea Tunnel 

Outlet and Willow Creek above Heron Reservoir varied 

throughout the year as a function of streamflow. Streamflow 

varied from high flow to low flow on the basis of the quantity 

of water diverted from the Rio Blanco, Little Navajo River, 

and Navajo River for the SJCP. During high flow, the ionic 

composition of water from Azotea Tunnel Outlet was similar 

to the composition of a mixture of water from Rio Blanco and 

Navajo River, and there were small but distinct differences 

in ion composition and specific conductance in water from 

Willow Creek above Heron Reservoir compared to water 

from Azotea Tunnel Outlet. Vertical profiles of the water 

temperature over the depth of the water column collected 

in April 2008, October 2008, July 2009, August 2009, and 

September 2009 indicated that Heron Reservoir is thermally 

stratified in the early fall and thermally mixed in early spring. 

Vertical profiles of the dissolved-oxygen concentration over 

the depth of the water column at Heron Reservoir indicated 

that the thermal stratification results in chemical stratification. 

When the reservoir was stratified, the dissolved-oxygen 

concentration was highest above and below the thermocline, 

and the lowest dissolved-oxygen concentration generally 

occurred at the top of the thermocline. Major- and traceelement concentrations in samples collected for this study 

were less than the U.S. Environmental Protection Agency’s 

primary and secondary drinking-water standards and less 

than the New Mexico Environment Department’s surfacewater standards for Heron Reservoir and perennial reaches of 

tributaries to the Rio Chama in the study area. 

The results from the seepage investigations indicated 

a small amount of loss of streamflow along the channel. 

During high flow, the loss was estimated at 3 percent; 

however, the uncertainty in the measured streamflow 

measurements precludes an exact determination of the loss. 

During low flow, the estimated loss was extremely small, 

and the uncertainty generally was close to or exceeded the 

estimated loss. The possible loss of streamflow resulting from 

groundwater/surface-water interactions during low flow was 

not a significant volume compared to streamflow volumes 

during high flow.

Annual variability in streamflow for the SJCP was an 

indication of the variation in the climate parameters that 

interact to contribute to streamflow in the Rio Blanco, Little 

Navajo River, Navajo River, and Willow Creek watersheds. 

The median monthly streamflow for Azotea Tunnel Outlet 

and Willow Creek above Heron Reservoir indicated that 

streamflow at these stations was typical of watersheds that 

are snowmelt-dominated systems. For most years, streamflow 

at Azotea Tunnel Outlet started by March and continued 

for approximately 3 months until the middle of July. The 

majority of annual streamflow at Azotea Tunnel Outlet 

generally occurred from May through June, with a median 

duration of slightly longer than a month. Additionally, higher 

annual streamflow occurred in years with higher maximum 

daily streamflow. The results from the Mann-Kendall trend 

test showed that there was not a significant trend in the 

annual streamflow at Azotea Tunnel Outlet over time from 

1971 to 2010. The difference between the occurrence and 

the strength of significant trends between the time series 

of the seasonal distribution of streamflow and the seasonal 

streamflow compared to annual streamflow indicated that 

the seasonal distribution of streamflow was more strongly 

controlled by the change in the annual streamflow than by 

time.

Historically, the timing of large monthly releases from 

Heron Reservoir has varied, likely because of variations in 

downstream use and the total annual release. The majority 

of releases from Heron Reservoir during the 1970s and early 

1980s occurred in December, and the majority of releases 

in the middle and late 1980s and 1990s occurred in March 

and April. More recently in the 2000s, releases have been 

more evenly distributed through the year with some recent 

increases in releases in December (2007 and 2008) and 

September (2009 and 2010).

Rio Grande water (streamflow derived from water 

sources in the Rio Grande Basin) varied substantially over 

the period of record, and this variation was likely a function 

of climate including precipitation. The median monthly Rio 

Grande water indicated that snowmelt runoff in March and 

April was the main source of streamflow and accounted for 

approximately 70 percent of the annual streamflow. From 

November to February, when little to no water was diverted 

for the SJCP, most of the streamflow in Willow Creek was 

Rio Grande water.
References Cited 49

The amount of water that can be diverted for the SJCP 

is controlled by the streamflow available for diversion and 

is limited by several factors including legal limitations for 

diversion, limitations from the SJCP infrastructure including 

the size of the diversion dams and tunnels, the capacity 

of Heron Reservoir, and operational constraints that limit 

when water can be diverted. Comparison of the structurally 

limited available streamflow at the two tunnel capacities of 

950 cubic feet per second (ft3

/s) and 1,050 ft3/s indicated that 

the additional 100 ft3

/s of capacity could capture additional 

streamflow during years with average to above average 

streamflow. Comparison of the capacity-limited available 

streamflow at the two reservoir storage capacities of 401,320 

acre-feet (acre-ft) and 400,320 acre-ft indicated that the 

difference of 1,000 acre-ft of storage capacity can substantially 

decrease the amount of streamflow that can be diverted for the 

SJCP; however, the effect is constrained to years with high 

annual available streamflow and near-capacity storage of water 

in the reservoir.

The average annual streamflow at Azotea Tunnel Outlet 

was 94,710 acre-ft, and the annual streamflow at Azotea 

Tunnel Outlet was approximately 75 percent of the annual 

streamflow available for diversion for the SJCP. The average 

annual percentage of available streamflow not diverted for the 

SJCP was 14 percent because of structural limitations of the 

capacity of infrastructure, 1 percent because of limitations of 

the reservoir storage capacity, and 29 percent because of the 

limitations from operations.

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ISBN 978-1-4113-3854-8

9 7 81411 338548

McKean—Water Chemistry, Seepage, Streamflow, Reservoir Storage, and Annual Availability of Water for the San Juan-Chama Project—SIR 2014–5155

ISSN 2328-031X (print)

ISSN 2328-0328 (online) Printed on recycled paper