SPATIAL AND TEMPORAL VARIATIONS IN

STREAMFLOW, DISSOLVED SOLIDS, NUTRIENTS,

AND SUSPENDED SEDIMENT IN THE RIO GRANDE

VALLEY STUDY UNIT, COLORADO, NEW MEXICO,

AND TEXAS, 1993-95

National Water-Quality Assessment Program

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 02-4224
U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 02-4224

SPATIAL AND TEMPORAL VARIATIONS IN

STREAMFLOW, DISSOLVED SOLIDS, NUTRIENTS,

AND SUSPENDED SEDIMENT IN THE RIO GRANDE

VALLEY STUDY UNIT, COLORADO, NEW MEXICO,

AND TEXAS, 1993-95

National Water-Quality Assessment Program

By Stephanie J. Moore and Scott K. Anderholm

Albuquerque, New Mexico

2002
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

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

not constitute endorsement by the U.S. Geological Survey.

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

U.S. Geological Survey

Information Services

Box 25286

Denver, CO 80225-0286

District Chief

U.S. Geological Survey

Water Resources Division

5338 Montgomery Blvd. NE, Suite 400

Albuquerque, NM 87109-1311

from:

Information regarding research and data-collection programs of the U.S. Geological Survey is ava

on the Internet via the World Wide Web. You may connect to the home page for the New Mexico D

Office using the URL http://nm.water.usgs.gov.
v

CONTENTS

P

Foreword................................................................................................................................................................................ 

Abstract.................................................................................................................................................................................. 

Introduction ........................................................................................................................................................................... 

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

Description of study unit ............................................................................................................................................. 

Surface water ..................................................................................................................................................... 

Surface-water and ground-water interactions.................................................................................................... 

Anthropogenic structures................................................................................................................................... 

Water quality...................................................................................................................................................... 

Study methods ....................................................................................................................................................................... 

Data collection............................................................................................................................................................. 

Box plots...................................................................................................................................................................... 

Loads ........................................................................................................................................................................... 

Load estimation ........................................................................................................................................................... 

Streamflow............................................................................................................................................................................. 

Spatial variations in streamflow................................................................................................................................... 

Temporal variations in streamflow .............................................................................................................................. 

Downstream temporal variations in streamflow .......................................................................................................... 

Dissolved solids..................................................................................................................................................................... 

Spatial variations in dissolved-solids concentrations .................................................................................................. 

Temporal variations in dissolved-solids concentrations .............................................................................................. 

Downstream temporal variations in dissolved-solids concentrations.......................................................................... 

Estimated dissolved-solids loads ................................................................................................................................. 

Nutrients ................................................................................................................................................................................ 

Dissolved nitrate concentrations.................................................................................................................................. 

Spatial variations in dissolved nitrate concentrations........................................................................................ 

Temporal variations in dissolved nitrate concentrations.................................................................................... 

Downstream temporal variations in dissolved nitrate concentrations ............................................................... 

Estimated dissolved nitrate loads....................................................................................................................... 

Total phosphorous concentrations ............................................................................................................................... 

Spatial variations in total phosphorous concentrations ..................................................................................... 

Temporal variations in total phosphorous concentrations ................................................................................. 

Downstream temporal variations in total phosphorous concentrations............................................................. 

Estimated total phosphorous loads .................................................................................................................... 

Suspended sediment .............................................................................................................................................................. 

Spatial variations in suspended-sediment concentrations............................................................................................ 

Suspended-sediment/streamflow relations................................................................................................................... 

Estimated suspended-sediment loads .......................................................................................................................... 

Effects of variations in streamflow and concentrations on loads in the Rio Grande Valley study unit ................................. 

Summary................................................................................................................................................................................ 

Selected references ................................................................................................................................................................ 

FIGURES

1. Map showing location of Rio Grande Valley study unit and selected sites ............................................................. 

2. Map showing areas of urban, agricultural, rangeland, and forest land-use and land-cover classifications for

the Rio Grande Valley study unit........................................................................................................................ 
vi

3-7. Graphs showing:

3. Annual mean streamflow at selected sites in the Rio Grande Valley study unit, water years 1993-95..

4. Temporal variation in instantaneous streamflow measurements at selected sites in the Rio Grande Va

study unit, water years 1993-95 .........................................................................................................

5. Daily mean streamflow at Rio Grande near Del Norte, Colo., and Rio Grande above the mouth of

Trinchera Creek, near Lasauses, Colo., water years 1993-95 ............................................................

6. Daily mean streamflow at Rio Grande below Leasburg Dam, near Leasburg, N. Mex., and Rio Grand

El Paso, Tex., water years 1993-95 ....................................................................................................

7. Daily mean streamflow at Rio Grande below Taos Junction Bridge, near Taos, N. Mex., Rio Chama

near Chamita, N. Mex., and Rio Grande at Otowi Bridge, near San Ildefonso, N. Mex., water year

1993-95 ..............................................................................................................................................

8. Box plots showing distribution of dissolved-solids concentrations at selected sites in the Rio Grande Val

study unit, water years 1993-95..............................................................................................................

9-11. Graphs showing:

9. Dissolved-solids concentrations at selected sites in the Rio Grande Valley study unit, water years 199

10. Temporal variation in dissolved-solids concentrations at selected sites in the Rio Grande Valley study

unit, water years 1993-95...................................................................................................................

11. Annual mean streamflow and estimated daily mean dissolved-solids load at selected sites in the

Rio Grande Valley study unit, water year 1994 .................................................................................

12. Box plots showing distribution of dissolved nitrite plus nitrate concentrations at selected sites in the

Rio Grande Valley study unit, water years 1993-95 ...............................................................................

13-15. Graphs showing:

13. Dissolved nitrite plus nitrate concentrations at selected sites in the Rio Grande Valley study unit,

water years 1993-95 ...........................................................................................................................

14. Temporal variation in dissolved nitrite plus nitrate concentrations at selected sites in the Rio Grande

Valley study unit, water years 1993-95 ..............................................................................................

15. Annual mean streamflow and estimated daily mean dissolved nitrite plus nitrate and total phosphoro

loads at selected sites in the Rio Grande Valley study unit, water year 1994 ....................................

16. Box plots showing distribution of total phosphorous concentrations at selected sites in the Rio Grande V

study unit, water years 1993-95..............................................................................................................

17. Graph showing total phosphorous concentrations at selected sites in the Rio Grande Valley study unit,

water years 1993-95................................................................................................................................

18. Graph showing temporal variation in total phosphorous concentrations at selected sites in the Rio Grand

Valley study unit, water years 1993-95...................................................................................................

19. Box plots showing distribution of suspended-sediment concentrations at selected sites in the Rio Grande

Valley study unit, water years 1993-95...................................................................................................

20. Graph showing relation of suspended-sediment concentration to instantaneous streamflow at selected site

in the Rio Grande Valley study unit, water years 1993-95 .....................................................................

21. Graph showing annual mean streamflow and estimated daily mean suspended-sediment load at selected

sites in the Rio Grande Valley study unit, water years 1993-95 .............................................................

22. Map showing spatial variations in streamflow, dissolved solids, dissolved nitrite plus nitrate, total

phosphorus, and suspended sediments throughout the Rio Grande Valley study unit............................

TABLES

1. Rio Grande Valley study unit sites ...............................................................................................................

2. Estimated daily mean dissolved-solids load and standard error of prediction, water years 1993-95...........

3. Estimated daily mean dissolved nitrite plus nitrate load and standard error of prediction, water years 199

4. Estimated daily mean total phosphorous load and standard error of prediction, water years 1993-95........

5. Estimated daily mean suspended-sediment load and standard error of prediction, water years 1993-95....
vii

CONVERSION FACTORS AND DATUMS

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

°F = (1.8 × °C) + 32.

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

25  ̊C).

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

Multiply By To obtain

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

mile (mi) 1.609 kilometer (km)

square mile (mi2) 2.590 square kilometer (km2)

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

ton per day (ton/d) 0.9072 metric ton per day (t/d)

cubic foot per second (ft3s) 0.02832 cubic meter per second

(m3/s)
1

ABSTRACT

Streamflow and water quality vary spatially and

temporally in the Rio Grande from Del Norte,

Colorado, to El Paso, Texas. The variations in

streamflow and in concentrations of selected waterquality constituents—dissolved solids, dissolved nitrite

plus nitrate as nitrogen, total phosphorus, and

suspended sediment—are described in this report. A

multivariate linear regression model,

ESTIMATOR2000, was used to estimate loads for

selected constituents.

Streamflow decreases in the downstream

direction throughout most of the basin because

outflows (due to agricultural use, leakage to ground

water, and evapotranspiration) are greater than inflows.

Streamflow increases between Rio Grande above the

mouth of Trinchera Creek, near Lasauses, Colorado, to

Rio Grande at Otowi Bridge, near San Ildefonso, New

Mexico, because ground-water and tributary inflow are

greater than outflow.

Concentrations of dissolved solids, dissolved

nitrite plus nitrate, total phosphorus, and suspended

sediment generally increase in the downstream

direction. Concentrations of dissolved solids, dissolved

nitrite plus nitrate, and total phosphorus decrease

between Rio Grande above the mouth of Trinchera

Creek, near Lasauses, Colorado, and Rio Grande at

Otowi Bridge, near San Ildefonso, New Mexico,

because of dilution by tributary inflow. Concentrations

of dissolved nitrite plus nitrate, total phosphorus, and

suspended sediment decrease between Rio Grande

Floodway at San Marcial, New Mexico, and Rio

Grande below Leasburg Dam, near Leasburg, New

Mexico, because of reservoir effects (nutrient uptake

and settling of sediment).

Several instances of decreasing streamflow and

increasing loads indicate the presence of inflows with

large constituent concentrations (relative to those of the

Rio Grande immediately upstream from that inflow);

this occurs (1) between Rio Grande near Del Norte,

Colorado, and Rio Grande above the mouth of

Trinchera Creek, near Lasauses, Colorado, for

dissolved solids, (2) between Rio Grande at Otowi

Bridge, near San Ildefonso, New Mexico, and Rio

Grande Floodway at San Marcial, New Mexico, for 

constituents, and (3) between Rio Grande below

Leasburg Dam, near Leasburg, New Mexico, and R

Grande at El Paso, Texas, for all constituents.

Streamflow increases along every reach of th

Rio Grande between the streamflow-gaging station 

Grande above the mouth of Trinchera Creek, near

Lasauses, Colorado, and the station Rio Grande at

Otowi Bridge, near San Ildefonso, New Mexico. Th

increases in streamflow result in increases in the lo

of dissolved solids, total phosphorus, and suspende

sediment regardless of changes in concentrations.

INTRODUCTION

In 1991, the U.S. Geological Survey began

implementation of the National Water-Quality

Assessment (NAWQA) Program. Goals of the

NAWQA Program are to describe the status of and

trends in the quality of the Nation’s surface- and

ground-water resources and to improve understand

of the natural and anthropogenic factors that affect

water-quality conditions. The NAWQA Program is

studying many of the Nation’s most important surfa

and ground-water systems, which are referred to as

study units. Assessment activities in the Rio Grand

Valley (RIOG) study unit (fig. 1) began in 1991, and 

high-intensity sampling phase began in 1993.

The quality of water in the Rio Grande and i

tributaries is of vital importance to the people that

depend on the water for its many uses. The quality 

water varies throughout the course of the Rio Gran

primarily as a result of inflows of ground water and

surface water to the river.

Purpose and Scope

This report describes the spatial and tempora

variations in streamflow and water quality in the R

SPATIAL AND TEMPORAL VARIATIONS IN STREAMFLOW,

DISSOLVED SOLIDS, NUTRIENTS, AND SUSPENDED SEDIMENT

IN THE RIO GRANDE VALLEY STUDY UNIT, COLORADO,

NEW MEXICO, AND TEXAS, 1993-95

By Stephanie J. Moore and Scott K. Anderholm
Figure 1. Rio Grande Valley study unit and selected sites.

Rio

Puerco

Rio

Jose

Chama

Rio

Rio Grande

Rio

San

Albuquerque

Saguache

Las Cruces

Alamosa

Caballo Res

Elephant Butte Res

San Ildefonso Pueblo

Taos

Conejos

River

Lobatos

Lasauses

Great Sand Dunes

Saguache Cr

San Luis Closed Basin

San Juan Mountains

Del Norte

Sangre de

Cristo Range

Sangre de Cristo Range

Medano Cr

Chama Basin

Jemez Mountains

Cochiti

Lake

Bandelier

NM

Santa Fe

Taos Plateau San Luis Valley

Isleta

Bernardo

Bosque del

Apache NWR

San

Marcial

Leasburg

Dam

Radium

Springs

Sunland Park

Study

unit

Colorado

New

Mexico

Texas

Rito de los Frijoles

Alamosa

NWR

RGDelN

Conejos

RGLob

RGTrinch

Chama RGTJB

RGOtowi

RGIsleta

RGConvey

RGFldwy

RGLeasb

RGELP El Paso

Chihuahuan

Desert

San Juan Mountains

Mesilla

Valley Rin

Va

con

lley

Grande

San Acacia

Rio Grande

Gorge

Creede

Red R

Trinchera Creek

Franklin

Eddy

Canal

Continental

Divide

Continental Divide

0 20 40 60

0 20 40 60

80 MILES

80 KILOMETERS

UNITED STATES OF AMERICA

REPUBLIC OF MEXICO

Base from U.S. Geological Survey digital data, 1:100,000, 1983 Albers Equal-Area Conic projection Standard parallels 29°30 ́ and 45°30 ́, central meridian -96°00 ́

Española

Chamita

COLORADO

NEW MEXICO

TEXAS

Abiquiu Res

Rio Pueblo de Taos

Embudo

Creek

Goose

Cr

S. Fork

Heron

Res

El

Vado

Res

Conveyance

Channel

Rio Grande

Floodway

Rio Salado

34°

35°

33°

32°

36°

37°

108°

38° 107°

106°

STUDY

UNIT

BOUNDARY

WWTP

WWTP

WWTP

Santa Fe R

Rio

Rancho

RGDelN

EXPLANATION

SOUTHERN BOUNDARY OF SAN LUIS

CLOSED BASIN

GAINING REACH DUE TO

GROUND-WATER INFLOW

AREA OF IRRIGATION

DIVERSIONS AND RETURN FLOWS

WASTEWATER TREATMENT PLANT

STREAMFLOW-GAGING STATION

AND SITE IDENTIFICATION (TABLE 1)

NATIONAL MONUMENT

NATIONAL WILDLIFE REFUGE

NM

NWR

WWTP

2

Jemez

River
3

Grande from Del Norte, Colorado, to El Paso, Texas.

The variations in water as it moves downstream reflect

the volume and composition of inflows to or outflows

from the Rio Grande. Many times the variations are

small in terms of actual concentration but large in terms

of percent difference between two sites; nevertheless,

determining the effects of inflows and outflows on

streamflow and water quality is useful in evaluating the

impact of natural and anthropogenic factors on the Rio

Grande. Streamflow and selected water-quality data for

12 sites in the Rio Grande Valley study unit of the

NAWQA Program are presented and discussed

(table 1).

Selected water-quality constituents described in

this report are dissolved solids, dissolved nitrite plus

nitrate as nitrogen, total phosphorus, and suspended

sediment. A multivariate linear regression model was

used to estimate loads for selected constituents. Spatial

and temporal variations in constituent concentrations

and constituent loads are presented and discussed for

water years (WY) 1993-95.

Description of Study Unit

The RIOG NAWQA study unit covers

approximately 45,700 mi2 in Colorado, New Mexico,

and Texas; it includes the entire Rio Grande drainage

basin upstream from the streamflow-gaging station Rio

Grande at El Paso, Texas (RGELP), and the closedbasin part of the San Luis Basin in Colorado (fig. 1). A

detailed description of the RIOG study unit is available

in Ellis and others (1993). Land use in the study unit is

predominantly rangeland (58 percent), forest (36

percent), and agriculture (4 percent) (fig. 2). Irrigation

is the predominant water-use category (89 percent) in

the study unit; public supply is the second largest

water-use category (9 percent) (Richey and Ellis,

1993). The climatological variations throughout the

study unit range from alpine tundra to Sonoran desert.

Annual precipitation can exceed 50 in. in the

headwaters of the Rio Grande, whereas other areas may

receive less than 6 in. (Ellis and others, 1993).

Surface Water

The headwaters of the Rio Grande are in the San

Juan Mountains, in south-central Colorado. Major

tributaries are the South Fork of the Rio Grande, the

Conejos River, and the Rio Chama (fig. 1). Other

perennial tributaries include Goose Creek, Red River,

Rio Pueblo de Taos, Embudo Creek, and the Jemez

River (fig. 1). With the exception of the Jemez River

these tributaries discharge to the Rio Grande upstre

from the gaging station Rio Grande at Otowi Bridg

near San Ildefonso, New Mexico (RGOtowi)—that 

most surface-water inflow to the Rio Grande is deri

from mountains adjacent to the Rio Grande upstrea

from RGOtowi; downstream from this gaging stati

the river receives little surface-water inflow (Ortiz a

Lange, 1996). Numerous intermittent and ephemer

streams discharge to the Rio Grande only during

periods of intense rainfall and (or) heavy snowmelt

Although many of these intermittent and ephemera

streams typically contribute little surface-water infl

to the Rio Grande (flow for 1 day or less per

precipitation event), the inflow can have large

concentrations of suspended sediment (as much as

185,000 mg/L) and nutrients (dissolved nitrite plus

nitrate concentration of 1.7 mg/L and total

phosphorous concentration of 45 mg/L) (Healy, 199

The Rio Puerco and Rio Salado are the largest

ephemeral channels that discharge to the Rio Gran

Surface-Water and Ground-Water Interactions

Surface water and ground water interact alon

the Rio Grande and its tributaries. Variations in the

ground-water flow systems adjacent to the river can

result in inflow of ground water to the Rio Grande 

recharge from the Rio Grande to the ground-water

system (outflow). Some reaches of the river receive

ground-water discharge; these are referred to as

gaining reaches. Winograd (1959) identified a gain

reach between Lobatos, Colorado, and the mouth o

Red River. During a seepage test in the mid-1970’s

Wilson and others (1981) identified a gaining reach

between Radium Springs and Las Cruces, New Mex

(fig. 1). Nickerson (1995) identified two distinct

gaining reaches in the Las Cruces, New Mexico–E

Paso, Texas, vicinity from 1988 to 1992.

In some reaches, water infiltrates from the ri

to adjacent aquifers; these are referred to as losing

reaches. Between Cochiti Lake and San Acacia, Ne

Mexico (fig. 1), the Rio Grande has become

channelized and in some places is higher than the

adjacent land surface, resulting in infiltration of surf

water to the adjacent shallow aquifer. To prevent th

water table from rising above land surface, riversid

drains were constructed in the 1930’s to intercept

shallow ground water and return it to the Rio Gran
Table 1. Rio Grande Valley study unit sites

Drainage Contributing

Site U.S. Geological Survey area drainage

identification Station (square area

(fig. 1) number Latitude Longitude miles) (square miles)

RGDelN 08220000 Rio Grande near Del Norte, Colo. 37o41'22" 106o27'38" 1,310 1,310

RGTrinch 08240000 Rio Grande above mouth of Trinchera Creek, near Lasauses, Colo. 37o18'58" 105o44'32" 5,680 2,750

Conejos 08249000 Conejos River near Lasauses, Colo. 37o18'01" 105o44'47" 789 789

RGLob 08251500 Rio Grande near Lobatos, Colo. 37o04'42" 105o45'22" 7,520 4,590

RGTJB 08276500 Rio Grande below Taos Junction Bridge, near Taos, N. Mex. 36o19'12" 105o45'14" 9,460 6,530

Chama 08290000 Rio Chama near Chamita, N. Mex. 36o04'26" 106o06'40" 3,140 3,040

RGOtowi 08313000 Rio Grande at Otowi Bridge, near San Ildefonso, N. Mex. 35o52'29" 106o08'30" 13,960 10,930

RGIsleta 08331000 Rio Grande at Isleta, N. Mex. 34o54'21" 106o41'04" 17,570 14,540

RGConvey 08358300 Rio Grande Conveyance Channel at San Marcial, N. Mex. 33o41'07" 106o59'40" 1-- 1--

RGFldwy 08358400 Rio Grande Floodway at San Marcial, N. Mex. 33o40'50" 106o59'30" 28,900 24,740

RGLeasb 08363500 Rio Grande below Leasburg Dam, near Leasburg, N. Mex. 32o28'36" 106o55'03" 38,500 29,000

RGELP 08364000 Rio Grande at El Paso, Tex. 31o48'10" 106o32'25" 39,580 30,080

1 Conveyance channel is an anthropogenic structure that was not assigned a drainage area 

Station name
Urban

Agricultural

Rangeland

Forest

EXPLANATION

Figure 2. Areas of urban, agricultural, rangeland, and forest land-use and land-cover classifications

for the Rio Grande Valley study unit.

0 20 40 60

0 20 40 60

80 MILES

80 KILOMETERS

34°

35°

33°

32°

36°

37°

108°

38° 107°

106°

5
6

Anthropogenic Structures

Many anthropogenic structures affect

streamflow in the Rio Grande. Eighteen reservoirs have

storage capacities greater than 5,000 acre-ft in the Rio

Grande study unit (Ellis and others, 1993). These

reservoirs alter the natural flow of the river, often

dampening seasonal effects and allowing for increased

evapotranspiration. Three reservoirs that affect the Rio

Grande are Cochiti Lake, Elephant Butte Reservoir,

and Caballo Reservoir (fig. 1). The Rio Chama, a major

tributary to the Rio Grande, is affected by three

reservoirs: Heron, El Vado, and Abiquiu.

Numerous irrigation diversions (outflows) and

return flows (inflows) are in agricultural areas (fig. 1).

In Colorado, surface water is diverted to irrigate crops

such as alfalfa, small grains, irrigated meadows, and

irrigated pastures. In the intensively farmed area north

of the Rio Grande, in the San Luis Closed Basin,

surface water is used to recharge the shallow groundwater aquifer and is subsequently withdrawn by

pumping through center-pivot sprinklers to irrigate

alfalfa, small grains, potatoes, and a variety of other

vegetables. In New Mexico, surface water is typically

diverted for irrigation and routed to the fields through

irrigation canals; return flows are routed back to the

Rio Grande through open drains. Return flows consist

of (1) water that was originally diverted for irrigation

purposes but was not used and (or) (2) any water

(generally water from the Rio Grande or the irrigation

system) that infiltrates to the shallow alluvial aquifer

and discharges to the drains.

The Rio Grande Conveyance Channel and the

Rio Grande Floodway are located between San Acacia,

New Mexico, and San Marcial, New Mexico; both

discharge to Elephant Butte Reservoir immediately

downstream from San Marcial. In this reach of the Rio

Grande, there is streamflow in either the Floodway,

which is the stabilized natural river channel, or the

Conveyance Channel, which was built in the 1950’s to

increase streamflow to Elephant Butte Reservoir during

periods of low flow. During the 1960’s and 1970’s,

most streamflow was diverted into the Conveyance

Channel. Since 1985, most streamflow is in the

Floodway, whereas streamflow in the Conveyance

Channel consists of return flows from agricultural areas

between San Acacia and San Marcial, New Mexico.

Effluent discharge from wastewater treatment

plants (WWTP’s) provides inflow to the Rio Grande

(WWTP inflow); any wastewater effluent can affect

water quality in the Rio Grande. Many WWTP’s are

located along the Rio Grande; the largest WW

in Rio Rancho, Albuquerque, and Las Cruce

The Franklin Eddy Canal, part of the 

Reclamation San Luis Valley Closed Basin P

discharges water pumped from the aquifer in 

basin part of the San Luis Valley to the Rio 

Water Quality

Many natural and anthropogenic facto

the quality of surface and ground water in th

study unit. Natural factors include the chem

composition of surface (tributary) water and 

water discharged to the Rio Grande, the che

composition of precipitation, dissolution of 

nutrient uptake, decay of organic matter, ero

evapotranspiration. Anthropogenic factors in

effects of irrigation, wastewater effluent, urb

septic-tank discharge, and leaching of fertili

addition to these anthropogenic factors, hum

activities may enhance natural processes suc

erosion and evapotranspiration.

Inflows to a river or stream can potenti

constituent concentrations and constituent lo

an inflow mixes with river water, the resultin

constituent concentration can increase, stay 

or decrease relative to the concentration in t

immediately upstream from the inflow. The c

concentration and volume of inflow relative 

the river determine the magnitude of change 

resulting concentration. When an inflow mix

river water, constituent loads almost always 

because of the addition of mass; however, if a 

constituent is completely absent from an infl

constituent concentration is zero), then the c

load will remain unchanged (although the re

concentration will have decreased because o

Outflows from a river or stream genera

affect constituent concentrations; however, s

sediment concentrations can be an exception

outflow causing a large decrease in streamfl

in a decrease in stream velocity, causing sed

settle out of suspension, which results in a d

suspended-sediment concentrations. Outflow

a decrease in constituent loads because of th

of constituent mass.
7

STUDY METHODS

Data Collection

Water samples generally were collected monthly

from April 1993 to September 1995. Depth- and widthintegrated samples were collected and processed

according to protocols described in Shelton (1994). A

detailed description of the RIOG-NAWQA waterquality sampling program and site-specific statistical

data summaries are available in Healy (1997).

Box Plots

Concentrations of selected water-quality

constituents are presented in the form of box plots. Box

plots provide a concise graphical representation of the

distribution of a data set. They provide a visual

representation of (1) the center of the data (the

median), (2) the variation or spread (the interquartile

range, defined as the 75th percentile minus the 25th

percentile), (3) the skewness or quartile skew

(represented by the relative size of the box halves), and

(4) the presence or absence of unusual values or

outliers (Helsel and Hirsch, 1992).

Loads

In a river system, the load of a particular

constituent is defined as the quantity (mass or weight)

of that constituent transported past a given point in a

given period of time. The instantaneous load

(sometimes referred to as a flux) is the product of the

constituent concentration and streamflow (discharge);

thus, the units are mass per time.

Loads are affected by changes in streamflow

(inflows and outflows) or changes in concentration. A

constituent load will increase only if constituent mass

is added to the river; this can occur either directly (for

example, resuspension or erosion of sediment during

high-velocity flow events) or indirectly, when water

containing some of that constituent is added to the river

(for example, tributary or ground-water inflow). A

constituent load will decrease only when some of that

constituent is removed from the river; this can occur

either directly (for example, the settling of suspended

sediment or nutrient uptake) or indirectly, when water

containing that constituent is diverted or removed (for

example, irrigation diversions).

Evapotranspiration will not affect loads of

conservative constituents simply because the quant

of water removed is always proportional to the incre

in constituent concentration. Return flows in

agricultural areas will not affect loads of conservat

constituents if there are no sources or sinks and no

change in ground-water storage in the surrounding

aquifer. Assuming there is no annual change in grou

water storage, then annual return flows equal annua

diversions minus evapotranspiration; therefore, the

load diverted is equal to the load returned. General

dissolved solids can be considered conservative,

whereas nutrients (dissolved nitrite plus nitrate and

total phosphorus) are not conservative because of

nutrient uptake and addition of nutrients due to the

leaching of fertilizers.

Load Estimation

Constituent loads were estimated using

ESTIMATOR2000 (Cohn and others, 1992).

ESTIMATOR2000 performs a multivariate linear

regression on the natural log of instantaneous

concentration and instantaneous streamflow data.

Streamflow, time, and seasonality are used as

explanatory (or predictor) variables. The regression 

then applied to the daily mean streamflow (arithme

mean of all continuous streamflow measurements f

that day) to compute daily constituent loads. The da

loads are summed to estimate the total monthly and

total annual loads. From the total annual load, an

estimated daily mean load (the average load on any

given day for a particular year) is calculated.

Two common problems with regression

applications are (1) retransformation bias and (2) la

amounts of censored data (concentrations less than 

minimum reporting level). ESTIMATOR2000 uses 

minimum variance unbiased estimator (MVUE) to

reduce bias introduced during retransformation; the

adjusted maximum likelihood estimator is a

generalization of the MVUE designed to deal with n

normal data distributions, such as those encountered 

data sets with large amounts of censored data (Coh

and others, 1992).

ESTIMATOR2000 requires the user to define 

model for each site and constituent. Cohn and othe

(1992) determined that this model satisfactorily

captures most of the variability in constituent

concentrations:
8

ln[C] = B0 + B1 ln[Q] + B2 ln[Q]2 + B3 T + B4 T2 (1)

+ B5 sin[2PiT] + B6 cos[2PiT] + E,

where

ln[ ] denotes the natural logarithm function;

C is the daily mean constituent concentration;

Q is the daily mean streamflow;

T is time, expressed in years; and

E is an independent and random error.

This model requires the estimation of seven

parameters: B0, a constant; B1 and B2, a quadratic fit to

the logarithm of streamflow (these parameters remove

the variability related to flow dependence); B3 and B4,

a quadratic fit to time (these parameters remove

variability related to time trends); B5 and B6, a

sinusoidal, first-order Fourier function (these

parameters remove the effects of annual seasonality).

For purposes of this report, the seven-parameter model

was used for each site and constituent. Not all

parameter estimates were statistically different from

zero. However, all explanatory variables were used in

every case, regardless of whether the parameter

estimates were statistically different from zero,

because “there was no a priori reason to believe that

the parameter values should be equal to zero” (Cohn

and others, 1992, p. 2358). In addition, if a parameter

estimate is not statistically different from zero, then its

regression coefficient will have little effect on

estimated values.

ESTIMATOR2000 assumes that the

instantaneous constituent concentration and the

instantaneous streamflow are representative of average

daily conditions (Robertson and Roerish, 1999). Large

standard error in load estimates at a particular site is in

part due to an instantaneous concentration or

instantaneous streamflow that is not representative of

average daily conditions.

Loads were estimated for all sites in the RIOG

study unit for WY 1993-95, with the following

exceptions. Loads were not estimated for Rio Grande at

Isleta, New Mexico (RGIsleta), because a continuous

streamflow record was not available at this site.

Dissolved nitrite plus nitrate loads were not estimated

upstream from San Marcial, New Mexico, because of

insufficient data (too much censored data) for model

calibration. Load estimates are presented in tables 2-5,

along with the standard errors of prediction 

estimated loads.

The standard error of prediction, whic

represents variability in the system in additi

variability related to uncertainty in the mode

parameters, is a measure of the root mean squ

of the difference between the estimated load 

true load. The standard errors of prediction 

presented in tons per day and as a percentag

estimated daily mean loads (tables 2-5). Gen

dissolved-solids model yielded estimates wi

lowest standard errors of prediction (1 to 16 

and the suspended-sediment model yielded 

with the highest standard errors of prediction 

percent). The authors have chosen to focus d

of dissolved-solids and nutrient-load estimat

1994 because this year generally has the mos

the lowest standard errors of prediction; how

discussion of suspended-sediment load estim

include WY 1993-95 because of the larger a

variability in estimated suspended-sediment 

STREAMFLOW

A general knowledge of the spatial and 

variations in streamflow of the Rio Grande is 

to understand the variations in constituent

concentrations and the downstream transpor

constituents. Temporal variations in streamfl

used to determine how inflows and outflows 

from year to year and (2) vary throughout th

examining spatial variations in streamflow, lo

major inflows and outflows along the Rio G

be determined.

Annual mean streamflow (the arithmet

the arithmetic mean monthly streamflows for 

year) allows for general comparison of strea

at a single site from year to year and (2) from 

throughout the basin. During the 3-year stud

WY 1995 was the wettest at all 12 sites; WY 

the driest at most sites on the Rio Grande (fi

the basis of long-term streamflow records, W

1993-95 were wetter than the typical year (C

and others, 1996; Ortiz and Lange, 1996).

10

Spatial Variations in Streamflow

During WY 1993-95, annual mean streamflow at

Rio Grande near Del Norte, Colorado (RGDelN),

ranged from 722 to 1,164 ft3

/s (fig. 3). Streamflow

decreased approximately 650 ft3

/s from RGDelN to

Rio Grande above the mouth of Trinchera Creek, near

Lasauses, Colorado (RGTrinch), indicating an outflow

between the two sites. Much of the outflow between

RGDelN and RGTrinch was due to irrigation

diversions to the San Luis Valley; from WY 1993 to

WY 1995, diversions to the San Luis Valley ranged

from approximately 530 to 840 ft3

/s, which is

approximately 73 percent of flow at RGDelN

(Colorado Division of Water Resources, 2002).

Downstream from RGTrinch, the Conejos River

discharges to the Rio Grande. Streamflow at Conejos

River near Lasauses, Colorado (Conejos), was

approximately equal to streamflow at RGTrinch

(fig. 3). Thus, the resulting streamflow (RGTrinch plus

Conejos), hereafter referred to as Sum1 streamflow,

was almost twice that at RGTrinch. Sum1 streamflow

was roughly equivalent to streamflow at Rio Grande

near Lobatos, Colorado (RGLob), indicating no

additional major inflows or outflows between

RGTrinch and RGLob. Between RGLob and Rio

Grande below Taos Junction Bridge, near Taos, New

Mexico (RGTJB), streamflow increased approximately

450 ft3

/s; this is considerably larger than surface-water

tributary inflow (approximately 390 ft3/s) between the

two sites (Ortiz and Lange, 1996), indicating the

presence of another source of inflow, most likely

ground water. Winograd (1959) indicated groundwater discharge to this section of the Rio Grande to be

approximately 90 ft3

/s. Downstream from RGTJB, the

Rio Chama discharged approximately 800 ft3/s to the

Rio Grande; the resulting streamflow (RGTJB plus

Chama), hereafter referred to as Sum2 streamflow, was

approximately twice that at RGTJB. Streamflow at

RGOtowi was slightly larger than Sum2 streamflow

(fig. 3), indicating inflow (probably tributary) between

these two sites. No annual mean streamflow data are

available for Rio Grande at Isleta, New Mexico

(RGIsleta). Streamflow at Rio Grande Conveyance

Channel at San Marcial, New Mexico (RGConvey),

remained relatively constant (approximately 350 ft3/s)

from WY 1993 to WY 1995. Streamflow at Rio Grande

Floodway at San Marcial, New Mexico (RGFldwy),

ranged from 1,279 to 1,560 ft3/s. Total discharge to

Elephant Butte Reservoir (RGConvey plus RGFldwy),

hereafter referred to as Sum3 streamflow, ranged from

1,624 to 1,914 ft3

/s. Streamflow decreased between

RGOtowi and Sum3 (fig. 3) because outflows (due to

irrigation diversions, ground-water outflow, 

evapotranspiration) were greater than groun

and surface-water inflows (from the Jemez R

Rio Puerco, the Rio Salado, the Albuquerqu

and the Rio Rancho WWTP). Elephant Butt

Reservoir, Caballo Reservoir, and numerous 

diversion structures control flow at Rio Gran

Leasburg Dam, near Leasburg, New Mexico

(RGLeasb). Streamflow at RGLeasb was sm

Sum3 streamflow (by approximately 790 ft3

the smaller flow was due to outflows for irri

diversions between Elephant Butte Reservoi

Leasburg, the change in storage in the two r

and evapotranspiration. Streamflow decrease

approximately 180 ft3/s between RGLeasb an

because outflows (due to irrigation diversion

evapotranspiration, and possibly leakage fro

Grande to the shallow ground-water system

greater than ground-water and surface-water 

Temporal Variations in Streamflow

Most sites in the RIOG study unit had 

flows during the snowmelt season (from abo

through July) and the smallest flows (base fl

is primarily ground-water discharge) during 

months (from about November through Feb

(figs. 4A-L). Annually, there can be large va

streamflow during the snowmelt season rela

variation in streamflow during the winter mo

At RGDelN (fig. 4A), large flows resu

snowmelt in the late spring and early summe

late summer thunderstorms (July, August) p

large flows of a lesser duration. Streamflow 

RGTrinch (fig. 4B) was largest during the sn

season and smallest from July to October, w

largest irrigation diversions occurred (Color

Division of Water Resources, 2002). Stream

Chama (fig. 4F) is regulated by upstream re

Some seasonal variation remained, however, 

these controls; the largest flow occurred from 

June. At RGConvey (fig. 4I), streamflow wa

related to the rate of ground-water discharge 

agricultural drains, which was indirectly con

the volume of water diverted and used for ir

purposes. Seasonal variations at RGLeasb (fi

RGELP (fig. 4L) were directly related to rele

Caballo and Elephant Butte Reservoirs. Thus

larger during the growing season, which ext

March through September, and are smaller w

or no water is released from Caballo Reserv

October through February.
Figure 4. Temporal variation in instantaneous streamflow measurements at selected sites in the Rio Grande

Valley study unit, water years 1993-95. Location of sites in figure 1.

4A

4B

4C

4D

6,000

4,000

2,000

0

800

600

400

200

0

2,000

1,500

1,000

500

0

2,000

1,500

1,000

500

0

S T R E A M F L 

O W, I N C U B I C F E E T P E R S E C O N 

D

Rio Grande near

Del Norte, Colo.

(RGDelN)

EXPLANATION

Rio Grande above

the mouth of

Trinchera Creek,

near Lasauses,

Colo.

(RGTrinch)

Conejos River near

Lasauses, Colo.

(Conejos)

Rio Grande near

Lobatos, Colo.

(RGLob)

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

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

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

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

11
4E EXPLANATION

4F

4G

4H

8,000

6,000

4,000

2,000

0

10,000

8,000

6,000

4,000

2,000

0

3,000

2,000

1,000

0

6,000

4,000

2,000

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

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

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

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

S T R E A M F L 

O W, I N C U B I C F E E T P E R S E C O N 

D

Rio Grande below

Taos Junction Bridge,

near Taos, N. Mex.

(RGTJB)

Rio Chama near

Chamita, N. Mex.

(Chama)

Rio Grande at

Otowi Bridge, near

San Ildefonso, N. Mex.

(RGOtowi)

Rio Grande at

Isleta, N. Mex.

(RGIsleta)

Figure 4. Temporal variation in instantaneous streamflow measurements at selected sites in the Rio Grande

Valley study unit, water years 1993-95--Continued. Location of sites in figure 1.

12
Figure 4. Temporal variation in instantaneous streamflow measurements at selected sites in the Rio Grande

Valley study unit, water years 1993-95--Concluded. Location of sites in figure 1.

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

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

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

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

4I

S T R E A M F L 

O W, I N C U B I C F E E T P E R S E C O N 

D

4J

4K

4L

EXPLANATION

Rio Grande

Conveyance Channel

at San Marcial, N. Mex.

(RGConvey)

Rio Grande Floodway

at San Marcial, N. Mex.

(RGFldwy)

Rio Grande below

Leasburg Dam,

near Leasburg, N. Mex.

(RGLeasb)

Rio Grande

at El Paso, Tex.

(RGELP)

600

400

200

0

6,000

4,000

2,000

0

3,000

2,000

1,000

0

4,000

3,000

2,000

1,000

0

13
14

Downstream Temporal Variations in

Streamflow

Downstream from RGDelN, diversions during

the growing season caused smaller flows at RGTrinch

(fig. 5). Except for the winter months, streamflow at

RGTrinch was less than that at RGDelN, indicating

diversions between the two sites. A similar situation

occurred between RGLeasb and RGELP (fig. 6),

although the diversions were smaller than those

between RGDelN and RGTrinch. During winter

months, water is not released from Caballo Reservoir;

ground-water discharge to the Rio Grande between

RGLeasb and RGELP contributes to the increasing

streamflow between the sites.

Sum2 streamflow was approximately equal to

streamflow at RGOtowi (fig. 7). In May and from

November through January, RGOtowi streamflow was

greater than Sum2 streamflow, indicating the presence

of tributary inflow (other than the Rio Chama) or

ground-water discharge to the Rio Grande between

RGTJB and RGOtowi. In August 1994 and 1995,

RGOtowi streamflow was less than Sum2 streamflow,

indicating that outflows were greater than inflows

between RGTJB and RGOtowi.

DISSOLVED SOLIDS

The concentration of dissolved solids is a general

indicator of water quality; it is a measure of the

dissolved ions in a water sample, including

bicarbonate, calcium, chloride, magnesium, potassium,

silica, sodium, and sulfate. Many natural and

anthropogenic factors affect dissolved-solids

concentrations in water. Natural factors include

composition of precipitation, evapotranspiration, and

the dissolution of minerals. Anthropogenic factors

include wastewater effluent, application and

subsequent leaching of agricultural chemicals, and

increased evapotranspiration due to reservoirs and

irrigation.

Spatial Variations in Dissolved-Solids

Concentrations

Median dissolved-solids concentrations ranged

from 73 mg/L at RGDelN to 652 mg/L at RGELP (fig.

8). An increase in the median dissolved-solids

concentration was fourfold between RGDelN and

RGTrinch. Possible causes of the increased 

solids concentrations at RGTrinch are (1) su

water inflow and (or) ground-water discharg

Rio Grande, (2) surface-water inflow of grou

pumped from the San Luis Valley Closed Ba

through the Franklin Eddy Canal, and (3) in

the WWTP in Alamosa, Colorado (Levings a

1998). Edelmann and Buckles (1984, p. 17) a

and others (1973, pl. 4) identified regions of 

salinity hazard” (regions with a specific con

greater than 2,250 μS/cm at 25 °C), which c

to regions of large dissolved-solids concentr

(greater than 1,500 mg/L) in the shallow unc

aquifer adjacent to the Rio Grande near Ala

Colorado (fig. 1); these regions may contribu

water discharge to the Rio Grande between 

and RGTrinch. The median dissolved-solids

concentration was smaller at RGLob (191 m

at RGTrinch (298 mg/L) because of dilution 

Conejos inflow (fig. 8). From RGLob to RG

dissolved-solids concentrations varied little 

indicating that concentrations of the inflow(

similar to those at RGLob. The median disso

solids concentration increased from RGOtow

mg/L) to RGFldwy (299 mg/L) as the result 

water inflow (return flows, WWTP inflow, a

tributaries) and ground-water discharge to th

Grande. Large concentrations at RGConvey 

mg/L) resulted from irrigation effects (the

concentration of solutes by evapotranspirati

Rio Grande water is diverted to fields for irr

evapotranspiration in riparian areas, and reg

ground-water discharge to agricultural drain

median dissolved-solids concentration at RG

(448 mg/L) was greater than that at RGFldw

mg/L) because of evapotranspiration (includ

evapotranspiration in riparian and agricultur

and evaporation from reservoirs), ground-w

discharge to the Rio Grande, and RGFldwy 

mixing with more concentrated water from R

in Elephant Butte Reservoir. From RGLeasb 

RGELP, the median dissolved-solids concen

increased by 204 mg/L (fig. 8) because of ir

effects and ground-water discharge.

Dissolved-solids concentrations at mo

the RIOG study unit exhibited little variatio

RGDelN, where concentrations ranged from 

mg/L (fig. 8).




19

 Concentrations at RGTrinch, RGLob,

RGFldwy, RGLeasb, and RGELP had the largest

variations (range greater than 260 mg/L) in the study

unit. Generally, large variations in dissolved-solids

concentrations at RGTrinch, RGFldwy, RGLeasb, and

RGELP were caused by large variations in the quantity

and quality of inflows.

Temporal Variations in Dissolved-Solids

Concentrations

Temporal variations of dissolved-solids

concentrations indicate four seasonal patterns (fig. 9).

Six of 12 sites in the RIOG study unit (fig. 9A) had (1)

larger dissolved-solids concentrations during the late

fall and winter months (September-February), when

streamflow is dominated by ground-water discharge,

and (2) smaller concentrations during the spring and

summer months (April through August), when flow is

dominated by snowmelt and rainfall runoff. RGLob,

RGTJB, and RGFldwy had the largest dissolved-solids

concentrations in August and September (fig. 9),

during the period of small flows (fig. 4) (except for the

occasional large flows on the Rio Puerco that resulted

in large flows at RGFldwy), and the smallest

concentrations during May and June (fig. 9) because of

large flows of snowmelt runoff (fig. 4). RGTrinch had

a unique seasonal pattern; the smallest dissolved-solids

concentrations occurred from November to February

(fig. 9C). RGLeasb and RGELP had the largest

concentrations from October to January (fig. 9D); at

this time, flow in the Rio Grande consists of return flow

and (or) ground-water discharge because water is not

being released from Caballo Reservoir.

Downstream Temporal Variations in

Dissolved-Solids Concentrations

Plots of instantaneous concentrations of

dissolved solids allow for close examination of

downstream temporal variations (fig. 10). In general,

dissolved-solids concentrations were largest between

RGDelN and RGTrinch (approximately 200 mg/L)

from May to August (fig. 10A); this coincides with the

largest diversions between RGDelN and RGTrinch

(Colorado Division of Water Resources, 2002, and fig.

5). In WY 1995, dissolved-solids increases between

RGDelN and RGTrinch (approximately 80 mg/L) were

smallest during the winter months (December through

February) when the smallest diversions occur (fig. 

the smaller concentrations at RGTrinch during win

months could be attributed to a larger percentage o

streamflow at RGTrinch from RGDelN. Water at

RGTrinch mixes with inflow from the Conejos Riv

resulting in smaller dissolved-solids concentrations 

RGLob relative to RGTrinch (fig. 10B). Generally, 

decreases were largest from May to August when (

large flows on the Conejos River result in a larger

percentage of Conejos River water at RGLob and (

dissolved-solids concentrations are largest at RGTri

and smallest at Conejos (fig. 10B); the decreases w

smallest from October to February when small flows 

the Conejos River resulted in a smaller percentage 

Conejos River water at RGLob. Dissolved-solids

concentrations were approximately equal at RGLo

and RGTJB (fig. 10C). Concentrations generally

increased less than 100 mg/L between RGOtowi an

RGFldwy (fig. 10D) because the amount of inflow w

small and water quality of the inflow was very sim

to that of the Rio Grande. Dissolved-solids

concentrations increased 300 to 500 mg/L between

RGOtowi and RGFldwy during late summer (Augu

and September) because of large inflows from the R

Puerco and the Rio Salado, ephemeral streams kno

for their large dissolved-solids concentrations. From

RGLeasb to RGELP, concentrations increased

approximately 200 mg/L throughout the year

(fig. 10E).

Estimated Dissolved-Solids Loads

Given the large streamflow at RGDelN (fig. 

the estimated daily mean dissolved-solids load was

small (106 tons/d) because of the small dissolvedsolids concentrations (fig. 8). Downstream at

RGTrinch, streamflow decreased by more than a fac

of two, yet the estimated daily mean dissolved-soli

load increased by 31 tons/d (table 2). Decreasing

streamflow indicates a net loss, yet increasing

dissolved-solids load indicates a source with larger

dissolved-solids concentrations than those at RGDe

or RGTrinch. Possible sources of dissolved solids a

ground water from the shallow unconfined aquifer

adjacent to the Rio Grande near Alamosa, Colorad

and surface-water inflow from the San Luis Closed

Basin Project through the Franklin Eddy Canal. Fr

WY 1993 to WY 1995, approximately 16 percent o

the load at RGTrinch was from the Franklin Eddy

Canal; however, from December through February


Figure 10. Temporal variation in dissolved-solids concentrations at selected sites in the Rio Grande Valley

study unit, water years 1993-95. Location of sites in figure 1.

10A

DISSOLVED-SOLIDS CONCENTRATION, IN MILLIGRAMS PER LITER

600

400

200

0

A M J J A S O N D J F M A M J J A S O N D J F M A M J J A S

1993 1994 1995

A M J J A S O N D J F M A M J J A S O N D J F M A M J J A S

1993 1994 1995

A M J J A S O N D J F M A M J J A S O N D J F M A M J J A S

1993 1994 1995

600

400

200

0

500

400

300

200

100

0

10C

10B Rio Grande above

the mouth of

Trinchera Creek

near Lasauses, Colo.

(RGTrinch)

Conejos River near

Lasauses, Colo.

(Conejos)

Rio Grande near

Lobatos, Colo.

(RGLob)

EXPLANATION

Rio Grande near

Lobatos, Colo.

(RGLob)

Rio Grande below

Taos Junction

Bridge, near Taos, N. Mex.

(RGTJB)

Rio Grande near

Del Norte, Colo.

(RGDelN)

Rio Grande above

the mouth of

Trinchera Creek,

near Lasauses, Colo.

(RGTrinch)

22
10D EXPLANATION

Rio Grande at

Otowi Bridge,

near San

Ildefonso, N. Mex.

(RGOtowi)

Rio Grande

Floodway at

San Marcial,

N. Mex.

(RGFldwy)

DISSOLVED-SOLIDS CONCENTRATION, IN MILLIGRAMS PER LITER

1,000

800

600

400

200

0

2,000

1,500

1,000

500

0

Rio Grande below

Leasburg Dam,

near Leasburg, N. Mex.

(RGLeasb)

Rio Grande at

El Paso, Tex.

(RGELP)

Figure 10. Temporal variation in dissolved-solids concentrations at selected sites in the Rio Grande Valley

study unit, water years 1993-95--Concluded. Location of sites in figure 1.

10E

A M J J A S O N D J F M M A J J A S O N D J F M A M J A J S

A M J J A S O N D J F M M A J J A S O N D J F M A M J A J S

1993 1994 1995

1993 1994 1995

23
Table 2. Estimated daily mean dissolved-solids load and standard error of prediction, water years 1993-95

[Station names in table 1.

Sum1 = RGTrinch + Conejos; Sum2 = RGTJB + Chama; Sum3 = RGConvey

 + RGFldwy; NA, not applicable]

Dissolved solids 

Site USGS Estimated daily Standard error Standard error

identification station Water mean load of prediction of prediction

(table 1) number year (tons/day) (tons/day) (percent)

RGDelN 08220000 1993 143 4.82 3.38

RGDelN 08220000 1994 106 2.74 2.59

RGDelN 08220000 1995 173 5.08 2.94

RGTrinch 08240000 1993 157 13.63 8.69

RGTrinch 08240000 1994 137 6.53 4.78

RGTrinch 08240000 1995 249 23.20 9.31

Conejos 08249000 1993 58 2.43 4.18

Conejos 08249000 1994 40 1.45 3.58

Conejos 08249000 1995 67 2.25 3.37

Sum1 NA 1993 215 NA NA

Sum1 NA 1994 177 NA NA

Sum1 NA 1995 316 NA NA

RGLob 08251500 1993 297 27.76 9.34

RGLob 08251500 1994 184 8.87 4.83

RGLob 08251500 1995 345 27.67 8.02

RGTJB 08276500 1993 456 18.66 4.09

RGTJB 08276500 1994 402 8.39 2.09

RGTJB 08276500 1995 544 14.69 2.70

Chama 08290000 1993 419 16.91 4.04

Chama 08290000 1994 425 9.79 2.30

Chama 08290000 1995 426 11.54 2.71

Sum2 NA 1993 875 NA NA

Sum2 NA 1994 827 NA NA

Sum2 NA 1995 970 NA NA

RGOtowi 08313000 1993 946 43.17 4.57

RGOtowi 08313000 1994 885 18.74 2.12

RGOtowi 08313000 1995 1,091 28.12 2.58

RGConvey 08358300 1993 479 16.16 3.37

RGConvey 08358300 1994 504 6.58 1.30

RGConvey 08358300 1995 529 8.26 1.56

RGFldwy 08358400 1993 932 146.70 15.73

RGFldwy 08358400 1994 943 69.19 7.33

RGFldwy 08358400 1995 1,192 91.25 7.65

Sum3 NA 1993 1,412 NA NA

Sum3 NA 1994 1,448 NA NA

Sum3 NA 1995 1,722 NA NA

RGLeasb 08363500 1993 985 43.52 4.42

RGLeasb 08363500 1994 1,076 28.20 2.62

RGLeasb 08363500 1995 1,309 43.34 3.31

RGELP 08364000 1993 1,180 44.49 3.77

RGELP 08364000 1994 1,214 23.09 1.90

RGELP 08364000 1995 1,550 44.50 2.87
25

as much as 45 percent of the dissolved-solids load at

RGTrinch was from the Franklin Eddy Canal (E.M.

Herrera, Bureau of Reclamation, written commun.,

2001).

From RGTrinch to RGLob, the estimated daily

mean dissolved-solids load increased from 137 to 184

tons/d (table 2), annual mean streamflow doubled

(fig. 11), and the median dissolved-solids

concentration decreased by one-third (fig. 8). Despite

the large increase in streamflow, the load increased by

only 47 tons/d because the dissolved-solids

concentrations at Conejos were small relative to those

at RGTrinch (fig. 8).

From RGLob to RGOtowi, changes in dissolvedsolids load corresponded to changes in streamflow (fig.

11), and dissolved-solids concentrations showed only

slight variations (fig. 8). In this reach, the load was

affected primarily by increasing streamflow resulting

from surface-water inflow and ground-water discharge

to the Rio Grande. From RGTJB to RGOtowi, the

estimated daily mean dissolved-solids load more than

doubled because of an incoming load from Chama that

was approximately equal to the load at RGTJB

(table 2).

From RGOtowi to Sum3 (RGFldwy plus

RGConvey), the estimated daily mean dissolved-so

load increased from 885 to 1,448 tons/d because of

larger dissolved-solids concentrations (fig. 8) and

smaller streamflow (fig. 3); that is, concentrations

increased more than streamflow decreased. From Su

to RGLeasb, the dissolved-solids load decreased fr

1,448 to 1,076 tons/d because of changes in storage 

Elephant Butte and Caballo Reservoirs.

From RGLeasb to RGELP, streamflow

decreased by 213 ft3

/s and the estimated daily mea

load increased by 138 tons/d. Decreasing streamflo

indicates a net loss, yet the increasing load indicate

source(s) of larger dissolved-solids concentrations t

those at RGLeasb or RGELP. Regional ground-wat

discharge with large concentrations in the area (Wil

and others, 1981, p. 77) is the most likely cause of 

increasing load.

Figure 11. Annual mean streamflow and estimated daily mean dissolved-solids load at selected sites in the Rio 

Valley study unit, water year 1994. Station names in table 1; location of sites in figure 1.

(Sum1 = RGTrinch + Conejos; Sum2 = RGTJB + Chama; Sum3 = RGConvey + RGFldwy)

2,400

2,100

1,800

1,500

1,200

900

600

300

0

ANNUAL MEAN STREAMFLOW, IN CUBIC FEET PER SECOND

RGDelN RGTrinch Conejos Sum1 RGLob RGTJB Chama Sum2 RGOtowi RGConvey RGFldwy Sum3 RGLeasb RGELP

Annual mean streamflow

Estimated daily mean dissolved-solids load

106

106

137 177 184

402 425

827 885

504

943

1,450

1,080

1,210

404
26

NUTRIENTS

Nutrients are elements that are essential to living

organisms. Because of the scarcity of nitrogen and

phosphorus relative to other essential elements,

nitrogen and phosphorus are considered to regulate or

“limit” plant productivity. This report limits discussion

of nutrients to nitrogen and phosphorus.

Natural nutrient sources include atmospheric

deposition, decaying organic material, and weathering

of rocks and minerals. Anthropogenic nutrient sources

include human, animal, and domestic waste (for

example, wastewater effluent and runoff from animal

feedlot facilities), phosphate detergents, fertilizers,

septic-tank discharge, industrial waste, and urban

runoff.

Nitrate and nitrite are soluble forms of nitrogen

commonly detected in surface and ground water of the

RIOG study unit. For this report, the authors limit the

discussion to dissolved nitrite plus nitrate. In the RIOG

study unit, dissolved nitrite concentrations were

typically less than 0.01 mg/L, indicating that dissolved

nitrite plus nitrate concentrations are essentially

dissolved nitrate; thus, dissolved nitrite plus nitrate is

hereafter referred to as dissolved nitrate.

Concentrations of total phosphorus are generally

much larger than concentrations of dissolved

phosphorus (Healy, 1997) because phosphorus is

relatively insoluble and is sorbed by sediment (Hem,

1985, p. 126). Total phosphorous and suspendedsediment concentrations generally exhibit a

statistically significant positive correlation on the Rio

Grande (Anderholm and others, 1995, p. 76). Part of

the phosphorus sorbed to sediment is available to plants

and could contribute to excessive plant growth.

Dissolved Nitrate Concentrations

Spatial Variations in Dissolved Nitrate

Concentrations

Concentrations of dissolved nitrate ranged from

less than the minimum reporting level (0.05 mg/L) to

2.0 mg/L throughout the Rio Grande (fig. 12).

Concentrations were less than 0.05 mg/L at RGDelN,

Conejos, RGLob, and Chama and probably resulted

from large surface-water inflow from undeveloped

areas, which generally yield dissolved nitrate

concentrations less than 0.04 mg/L (Clark and others,

2000). Upstream from RGIsleta, dissolved nitrate

concentrations were small (median concentrations of

0.12 mg/L or less) and showed only slight v

(fig. 12). The larger concentrations at RGTr

RGTJB, and RGOtowi relative to other sites 

from RGIsleta are due to ground-water and 

water inflows. The decreases in dissolved ni

concentrations at RGLob (relative to RGTri

RGOtowi (relative to RGTJB) are due to dilu

surface-water inflow water containing small 

nitrate concentrations, as seen in figure 12 b

inflow from the Conejos River and the Rio C

median dissolved nitrate concentration incre

0.06 to 0.66 mg/L between RGOtowi and R

because of WWTP inflow and, to a lesser ext

flows. The median concentration at RGConv

mg/L) was smaller than those at RGIsleta (0

and RGFldwy (0.71 mg/L), indicating that re

have small dissolved nitrate concentrations r

irrigation water (water at RGIsleta and RGFl

smaller dissolved nitrate concentrations in th

Conveyance Channel indicate that (1) nutrie

occurs during irrigation and (2) leaching of 

based fertilizers does not cause increased di

nitrate concentrations. The interquartile rang

concentrations at RGFldwy was similar to th

RGIsleta (fig. 12), indicating little effect on

concentrations from surface-water inflows (

Rio Puerco and Rio Salado). The median di

nitrate concentration decreased from 0.71 to 

between RGFldwy and RGLeasb because of 

uptake in Elephant Butte and Caballo Reser

median dissolved nitrate concentration incre

0.08 mg/L at RGLeasb to 0.31 mg/L at RGE

potential causes include WWTP inflow, grou

discharge to the Rio Grande, and (or) return 

Sites with small variability in dissolve

concentrations include sites that drain basin

urban or agricultural land uses (fig. 2): RGD

Conejos, and Chama. Sites with large variab

dissolved nitrate concentrations include RG

RGFldwy, and RGELP, all of which are affe

WWTP inflow. The large variability in conc

at sites affected by WWTP inflow is due to 

variations in streamflow, which results in va

degrees of dilution of wastewater effluent; fo

when streamflow is small, WWTP inflow co

larger percentage of total streamflow in the R

28

Temporal Variations in Dissolved Nitrate

Concentrations

Five sites (RGDelN, RGTrinch, RGLob,

RGOtowi, and RGELP) had their largest dissolved

nitrate concentrations during the fall and winter

(generally November–April) and their smallest

concentrations during the spring and summer

(generally March–October) (fig. 13A, 13B, 13D, 13G,

13L). Possible causes of smaller dissolved nitrate

concentrations during spring and summer months are

(1) dilution effects from large streamflows with small

dissolved nitrate concentrations (snowmelt runoff) and

(2) increased nutrient uptake resulting from longer

days and warmer temperatures.

At RGTJB, RGIsleta, and RGFldwy, dissolved

nitrate concentrations were smallest during May and

June when streamflow was greatest (fig. 13E, 13H,

13J). Dissolved nitrate concentrations generally were

largest during the winter months (fig. 13E, 13H, 13J)

because of (1) less dilution of WWTP inflow by

snowmelt runoff and (2) less nutrient uptake.

At Chama, dissolved nitrate concentrations were

greater than the minimum reporting level (0.05 mg/L)

only during the summer and early fall months (June–

October) (fig. 13F). Chama streamflow ranged from

250 to 1,960 ft3/s between June and October.

Therefore, detectable nitrate concentrations at Chama

are not related to a particular flow regime.

Dissolved nitrate concentrations at RGConvey

were generally largest in March and from August to

October (fig. 13I). Because streamflow at RGConvey

was relatively constant throughout the year (fig. 4I),

dissolved nitrate concentrations are probably not

related to streamflow but rather to agricultural

practices.

At RGLeasb, dissolved nitrate concentrations

generally were largest from October to January

(fig. 13K) when return flows and WWTP inflows

constitute a larger percentage of water in the Rio

Grande (because little or no water is released from

Caballo Reservoir). Dissolved nitrate concentrations

were generally smallest from March to September (less

than 0.10 mg/L), indicating that return flows and

WWTP inflows are diluted by large streamflows with

small dissolved nitrate concentrations. Dissolved

nitrate concentrations generally decreased from

January to April (fig. 13K) because of dilution as

discharge from Caballo Reservoir increased.

Downstream Temporal Variations in Diss

Nitrate Concentrations

Small but consistent increases in disso

nitrate concentrations (generally less than 0

occurred between RGLob and RGTJB throu

year, except during the snowmelt season (ge

May–July) when dissolved nitrate concentra

both sites were less than 0.1 mg/L (fig. 14A

indicates an inflow source (with larger dissol

concentrations) diluted by large streamflows 

snowmelt season and increased nutrient upta

the growing season.

Dissolved nitrate concentrations incre

much as 1.6 mg/L between RGOtowi and R

most likely because of WWTP inflow (fig. 1

increases were smallest during the snowmel

when WWTP inflow is diluted by large flow

Generally, dissolved nitrate concentra

similar at RGIsleta and RGFldwy, indicating 

effect from inflows and little nutrient uptake 

moves downstream in the Rio Grande (fig. 1

However, dissolved nitrate concentrations at

RGConvey were smaller than at RGIsleta (fi

Water in the Conveyance Channel consists o

flows. Water at RGIsleta is representative of w

for irrigation. The smaller dissolved nitrate

concentrations at RGConvey relative to thos

irrigation water are indicative of nutrient up

water is applied to fields and moves through 

ground-water system to agricultural drains a

eventually to the Conveyance Channel.

Estimated Dissolved Nitrate Loads

As already stated, dissolved nitrate lo

not estimated upstream from San Marcial be

insufficient data for model calibration. In W

estimated daily mean dissolved nitrate loads 

between Sum3 (2.58 tons/d) and RGLeasb (0

then increased between RGLeasb and RGEL

ton/d) (fig. 15 and table 3). Dissolved nitrate 

RGFldwy (2.40 tons/d) were greater than th

RGConvey (0.18 ton/d) because streamflow 

RGFldwy was greater than that at RGConve

and, most importantly, because dissolved ni

concentrations at RGFldwy were larger than 

RGConvey (fig. 12). The dissolved nitrate lo

decreased from Sum3 to RGLeasb because s

decreased (due to storage in Elephant Butte 

Caballo Reservoirs) and dissolved nitrate

concentrations decreased (due to nutrient up

reservoirs). The estimated daily mean dissol
Figure 13. Dissolved nitrite plus nitrate concentrations at selected sites in the Rio Grande Valley

study unit, water years 1993-95. Location of sites in figure 1.

DISSOLVED NITRITE PLUS NITR

ATE CONCENTRATION, IN MILLIGRAMS PER LITER

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

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

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

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

0.20

0.15

0.10

0.05

0

0.40

0.30

0.20

0.10

0

0.08

0.07

0.06

0.05

0.04

0.40

0.30

0.20

0.10

0

EXPLANATION

Minimum

reporting level

Rio Grande near

Del Norte, Colo.

(RGDelN)

Minimum

reporting level

Rio Grande above

mouth of Trinchera

Creek, near

Lasauses, Colo.

(RGTrinch)

Minimum

reporting level

Conejos River near

Lasauses, Colo.

(Conejos)

Minimum

reporting level

Rio Grande near

Lobatos, Colo.

(RGLob)

13A

13B

13C

13D

29
0.40

0.30

0.20

0.10

0

13E EXPLANATION

Minimum

reporting level

Rio Grande below

Taos Junction

Bridge, near

Taos, N. Mex.

(RGTJB)

13G

Minimum

reporting level

Rio Chama near

Chamita, N. Mex.

(Chama)

Minimum

reporting level

Rio Grande at

Otowi Bridge, near

San Ildefonso, N. Mex.

(RGOtowi)

13F

13H

Minimum

reporting level

Rio Grande at

Isleta, N. Mex.

(RGIsleta)

Figure 13. Dissolved nitrite plus nitrate concentrations at selected sites in the Rio Grande Valley

study unit, water years 1993-95--Continued. Location of sites in figure 1.

3.00

2.00

1.00

0

0.40

0.30

0.20

0.10

0

0.12

0.10

0.08

0.06

0.04

DISSOLVED NITRITE PLUS NITR

ATE, IN MILLIGRAMS PER LITE

R

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

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

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

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

30
13I

13J

13K

13L

EXPLANATION

Minimum

reporting level

Rio Grande

Conveyance Channel

at San Marcial, N. Mex.

(RGConvey)

Minimum

reporting level

Rio Grande at

El Paso, Tex.

(RGELP)

Minimum

reporting level

Rio Grande below

Leasburg Dam, near

Leasburg, N. Mex.

(RGLeasb)

Minimum

reporting level

Rio Grande

Floodway at

San Marcial, N. Mex.

(RGFldwy)

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

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

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

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

Figure 13. Dissolved nitrite plus nitrate concentrations at selected sites in the Rio Grande Valley

study unit, water years 1993-95--Concluded. Location of sites in figure 1.

DISSOLVED NITRITE PLUS NITR

ATE, IN MILLIGRAMS PER LITER

0.8

0.6

0.4

0.2

0

2.0

1.5

1.0

0.5

0

0.4

0.3

0.2

0.1

0

1.0

0.8

0.6

0.4

0.2

0

31

33

3,000

2,500

2,000

1,500

1,000

500

0

STREAMFLOW, IN CUBIC FEET PER SECOND

Annual mean streamflow

Estimated daily mean total phosphorous load

Estimated daily mean dissolved nitrite plus nitrate load

Figure 15. Annual mean streamflow and estimated daily mean dissolved nitrite plus nitrate and total phosp

loads at selected sites in the Rio Grande Valley study unit, water year 1994. Station names in 

(Sum1 = RGTrinch + Conejos; Sum2 = RGTJB + Chama; Sum3 = RGConvey + RGFldwy)

RGDelN RGTrinch Conejos Sum1 RGLob RGTJB Chama Sum2 RGOtowi RGConvey RGFldwy Sum3 RGLeasb RGELP

0.09 0.05 0.04 0.09 0.08

0.23 0.15

0.38

0.53

0.18

0.16

1.67

2.40

2.58

1.84

0.26

0.19

0.64

0.28

0.09

0.18

Table 3. Estimated daily mean dissolved nitrite plus nitrate load and standard error of prediction, water years 199

[Station names in table 1.

Sum3 = RGConvey + RGFldwy; NA, not applicable]

Dissolved nitrite plus nitrate

Site USGS Estimated daily Standard error Standard error

identification station Water mean load of prediction of prediction

(table 1) number year (tons/day) (tons/day) (percent)

RGConvey 08358300 1993 0.1975 0.0593 30.06

RGConvey 08358300 1994 0.1802 0.0227 12.62

RGConvey 08358300 1995 0.1942 0.0304 15.65

RGFldwy 08358400 1993 2.523 1.021 40.47

RGFldwy 08358400 1994 2.397 0.354 14.78

RGFldwy 08358400 1995 2.613 0.379 14.52

Sum3 NA 1993 2.720 NA NA

Sum3 NA 1994 2.577 NA NA

Sum3 NA 1995 2.807 NA NA

RGLeasb 08363500 1993 0.1061 0.0334 31.47

RGLeasb 08363500 1994 0.1860 0.0327 17.58

RGLeasb 08363500 1995 0.3516 0.0777 22.09

RGELP 08364000 1993 0.4229 0.0641 15.16

RGELP 08364000 1994 0.6359 0.0452 7.11

RGELP 08364000 1995 0.6635 0.0668 10.07
34

nitrate load increased from RGLeasb to RGELP

because, although streamflow decreased (fig. 3),

dissolved nitrate concentrations increased (fig. 12).

Healy (1996) indicated that in this area, dissolved

nitrate loads to the Rio Grande from non-agricultural

sources (such as WWTP inflow) were greater than

those from return flows.

Total Phosphorous Concentrations

Spatial Variations in Total Phosphorous

Concentrations

Concentrations of total phosphorus generally

were small (less than 0.1 mg/L) and had small

variability from RGDelN to RGOtowi (fig. 16). Total

phosphorous concentrations were largest and had the

largest variability at RGIsleta and RGFldwy.

The median total phosphorous concentration was

larger at RGTrinch (0.10 mg/L) than at RGDelN (0.04

mg/L) because of inflow between the two sites. The

median total phosphorous concentration increased

from RGOtowi (0.07 mg/L) to RGIsleta (0.32 mg/L)

most likely because of WWTP inflow. The median total

concentration increased from RGIsleta (0.32 mg/L) to

RGFldwy (0.43 mg/L) because of inflows from the Rio

Puerco, which can have large concentrations of

phosphorus (Healy, 1997). The median total

phosphorous concentration decreased from 0.43 to

0.06 mg/L from RGFldwy and RGLeasb because of

settling and nutrient uptake in Elephant Butte and

Caballo Reservoirs. The median total phosphorous

concentration increased from RGLeasb to RGELP

(0.12 mg/L) in part because of WWTP inflows (Healy,

1996).

Temporal Variations in Total Phosphorous

Concentrations

Total phosphorous concentrations at five sites

(RGDelN, RGTrinch, Conejos, RGTJB, and

RGOtowi) were smaller during the winter months

(December–February) and larger throughout the rest of

the year (March–November); total phosphorous

concentrations were largest during snowmelt (May and

June) (fig. 17A-C, 17E, 17G). The larger total

phosphorous concentrations during snowmelt are due

to increased streamflow and the associated increase in

suspended-sediment concentrations. The larger total

phosphorous concentrations in May and June could

also be related to fertilizer applications; concurrent

increases in dissolved nitrate concentrations were not

observed, however. Total phosphorous conc

at RGLob and Chama seemed to lack a cons

seasonal pattern (fig. 17D, 17F). This could in

presence of a variety of phosphorous source

basins.

Total phosphorous concentrations at R

and RGFldwy generally were smallest from 

through June (fig. 17H, 17J) due to dilution 

inflow by large streamflow. Total phosphoro

concentrations in wastewater effluent (3 to 1

Snoeyink and Jenkins, 1980, p. 301) were la

those at RGOtowi during snowmelt (as muc

mg/L, fig. 17G). For this reason, whereas la

streamflow (during snowmelt) resulted in in

total phosphorous concentrations at some si

17A-C, 17E, 17G), large streamflow caused 

phosphorous concentrations to decrease at s

affected by WWTP inflow because the conc

(upstream from WWTP inflows) were still s

relative to those in wastewater effluent.

Total phosphorous concentrations incr

during snowmelt because nonpoint sources d

the large flows. In contrast, total phosphorou

concentrations decreased during snowmelt a

because the point sources were dominated by 

runoff.

At RGConvey, total phosphorous conc

were largest at two distinct times throughou

during March and August through October. 

concentrations at RGLeasb and RGELP wer

in April, May, November, and December.

Downstream Temporal Variations in Tota

Phosphorous Concentrations

Generally, total phosphorous concentr

were larger at RGLob than at RGTJB (fig. 1

indicating that ground-water and surface-wa

between the two sites diluted the larger conc

detected at RGLob. However, during the 199

1995 snowmelt seasons, total phosphorous

concentrations were larger at the downstream 

RGTJB, which indicates a source of total ph

between the two sites.

Total phosphorous concentrations gen

increased between RGOtowi and RGIsleta (

The smallest increases in concentration occu

during the snowmelt season because of mor

due to increased streamflow. Large increase

concentration occurred in July 1993 and Au

because of less dilution of WWTP inflow or 

increased storm runoff.

Figure 17. Total phosphorous concentrations at selected sites in the Rio Grande Valley study unit,

water years 1993-95. Location of sites in figure 1.

0.20

0.15

0.10

0.05

0

0.20

0.15

0.10

0.05

0

Minimum

reporting level

Minimum

reporting level

Minimum

reporting level

Minimum

reporting level

Rio Grande near

Del Norte, Colo.

(RGDelN)

EXPLANATION

0.80

0.60

0.40

0.20

0

0.20

0.15

0.10

0.05

0

Rio Grande above

the mouth of

Trinchera Creek,

near Lasauses, Colo.

(RGTrinch)

Conejos River near

Lasauses, Colo.

(Conejos)

Rio Grande near

Lobatos, Colo.

(RGLob)

TOTAL PHOSPHOROUS CONCENTRATION, IN MILLIGRAMS PER LITER

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

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

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

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

17A

17B

17C

17D

36
Figure 17. Total phosphorous concentrations at selected sites in the Rio Grande Valley study unit,

water years 1993-95--Continued. Location of sites in figure 1.

Rio Grande below

Taos Junction

Bridge, near

Taos, N. Mex.

(RGTJB)

EXPLANATION 0.40

0.30

0.20

0.10

0

0.40

0.30

0.20

0.10

0

0.20

0.15

0.10

0.05

0

1.00

0.80

0.60

0.40

0.20

0

Minimum

reporting level

Minimum

reporting level

Minimum

reporting level

Minimum

reporting level

Rio Chama near

Chamita, N. Mex.

(Chama)

Rio Grande at

Otowi Bridge, near

San Ildefonso, N. Mex.

(RGOtowi)

Rio Grande at

Isleta, N. Mex.

(RGIsleta)

17E

17F

17G

17H

TOTAL PHOSPHOROUS CONCENTRATION, IN MILLIGRAMS PER LITER

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

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

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

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

37
Figure 17. Total phosphorous concentrations at selected sites in the Rio Grande Valley study unit,

water years 1993-95--Concluded. Location of sites in figure 1.

0.8

0.6

0.4

0.2

0

10

8

6

4

2

0

0.4

0.3

0.2

0.1

0

0.8

0.6

0.4

0.2

0

EXPLANATION

Minimum

reporting level

Minimum

reporting level

Minimum

reporting level

Minimum

reporting level

Rio Grande

Conveyance Channel

at San Marcial, N. Mex.

(RGConvey)

Rio Grande

Floodway at

San Marcial, N. Mex.

(RGFldwy)

Rio Grande below

Leasburg Dam near

Leasburg, N. Mex.

(RGLeasb)

Rio Grande at

El Paso, Tex.

(RGELP)

17L

17K

17J

17I

TOTAL PHOSPHOROUS CONCENTRATION, IN MILLIGRAMS PER LITE

R

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

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

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

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

38

40

Total phosphorous concentrations at RGIsleta

and RGFldwy generally were similar (fig. 18C). This

similarity indicates either (1) little settling or nutrient

uptake of phosphorus occurs between sites or (2)

increases in concentrations due to surface-water inflow

(from the Rio Puerco or Rio Salado) are balanced by

decreases in total phosphorous concentrations due to

settling or nutrient uptake. Occasionally, total

phosphorous concentrations at RGFldwy were much

larger (by as much as 1 mg/L) than those at RGIsleta.

These larger concentrations at RGFldwy indicate that

either (1) surface-water inflow from the Rio Puerco or

Rio Salado occurred or (2) sediment (and sorbed

phosphorus), which was previously deposited in the

Rio Grande by the Rio Puerco or Rio Salado, was

resuspended.

Estimated Total Phosphorous Loads

In WY 1994, the total phosphorous load

decreased from RGDelN (0.09 ton/d) to RGTrinch

(0.05 ton/d) as did streamflow (fig. 3), indicating a net

outflow of total phosphorus. The load at Conejos (0.04

ton/d) was approximately equivalent to that at

RGTrinch (0.05 ton/d). From Sum1 (0.09 ton/d) to

RGLob (0.08 ton/d) (table 5), the estimated daily mean

load remained fairly constant, indicating that

phosphorous inputs equaled phosphorous outputs in

this reach. The total phosphorous load increased

between RGLob (0.08 ton/d) and RGTJB (0.23 ton/d),

as did streamflow; however, concentrations decreased.

The total phosphorous load from the Rio Chama (0.15

ton/d) was smaller than that at RGTJB (0.23 ton/d).

The increase in estimated daily mean load between

Sum2 (0.38 ton/d) and RGOtowi (0.53 ton/d) indicates

a source of total phosphorus (other than surface-water

inflow from the Rio Chama to the Rio Grande) between

RGTJB and RGOtowi.

The estimated daily mean total phosphorous load

increased from RGOtowi (0.53 ton/d) to Sum3 (1.84

tons/d) despite a decrease in streamflow (fig. 15 and

table 4). The increase in load indicates an inflow. The

decrease in streamflow indicates that outflows are

greater than inflows between RGOtowi and Sum3.

Therefore, the total phosphorous concentration of the

inflow must have been greater than that at RGOtowi.

Possible sources of inflow would be WWTP inflow,

tributary inflow, and return flow. Wastewater effluent

can have total phosphorous concentrations as large as

15 mg/L (Snoeyink and Jenkins, 1980, p. 301). Both

the Rio Puerco and the Rio Salado discharge directly

into the Rio Grande. Water-quality data for 

Puerco show that the median total phosphor

concentration for WY 1993-95 was 2.7 mg/

1997), which is large relative to the concent

RGOtowi (0.07 mg/L). The estimated daily m

phosphorous load at RGConvey (0.16 ton/d) 

approximately equal to that at Chama; howe

streamflow at Chama was about 2.5 times gr

streamflow at RGConvey.

The estimated daily mean total phosph

at RGLeasb (0.26 ton/d) was smaller than tha

(1.84 tons/d) because of a combination of (1

streamflow at RGLeasb, due to changes in s

Elephant Butte and Caballo Reservoirs, and (

total phosphorous concentrations that result

nutrient uptake and the settling of phosphoru

reservoirs.

Total phosphorous loads at RGLeasb 

ton/d) and RGELP (0.28 ton/d) were about t

despite decreases in streamflow; this indicat

inflow with larger concentrations than at RG

Inflows could include WWTP inflow and ret

Healy (1996, p. 39) indicated that total phos

loads from these two sources are approxima

SUSPENDED SEDIMENT

Suspended sediment is a measure of s

clay-sized suspended material in a water sam

percentage of sand, silt, and clay varies (1) fr

site and (2) over time at a particular site. Su

sediment concentrations are related to a vari

physical factors, such as streamflow velocity

availability of erodible material in a watershe

grain size and distribution of material in a wa

the RIOG study unit, suspended-sediment

concentrations generally are positively corre

streamflow (Anderholm and others, 1995).

Ephemeral streams periodically contri

amounts of suspended sediment to the Rio G

its tributaries. Some of this sediment is depos

channel of the Rio Grande near the mouth o

stream. This sediment can then be resuspend

transported later during periods of increased

streamflow; this results in large variability o

suspended-sediment concentrations.
Table 4. Estimated daily mean total phosphorous load and standard error of prediction, water years 1993-95 

[Station names in table 1.

Sum1 = RGTrinch + Conejos; Sum2 = RGTJB + Chama; Sum3 = RGConvey + RGFldwy; NA, not applicable]

Total phosphorus

Site USGS Estimated daily Standard error Standard error

identification station Water mean load of prediction of prediction

(table 1) number year (tons/day) (tons/day) (percent)

RGDelN 08220000 1993 0.1502 0.0249 16.58

RGDelN 08220000 1994 0.0919 0.0138 14.96

RGDelN 08220000 1995 0.1954 0.0352 18.01

RGTrinch 08240000 1993 0.0475 0.0142 29.97

RGTrinch 08240000 1994 0.0475 0.0087 18.42

RGTrinch 08240000 1995 0.1522 0.0609 40.01

Conejos 08249000 1993 0.0667 0.0134 20.07

Conejos 08249000 1994 0.0411 0.0086 20.86

Conejos 08249000 1995 0.0947 0.0177 18.73

Sum1 NA 1993 0.1142 NA NA

Sum1 NA 1994 0.0886 NA NA

Sum1 NA 1995 0.2469 NA NA

RGLob 08251500 1993 0.1397 0.0264 18.87

RGLob 08251500 1994 0.0813 0.0089 10.94

RGLob 08251500 1995 0.2189 0.0428 19.56

RGTJB 08276500 1993 0.1917 0.0497 25.93

RGTJB 08276500 1994 0.2302 0.0484 21.01

RGTJB 08276500 1995 0.4179 0.1051 25.14

Chama 08290000 1993 0.0815 0.0226 27.70

Chama 08290000 1994 0.1542 0.0344 22.31

Chama 08290000 1995 0.2089 0.0519 24.83

Sum2 NA 1993 0.2732 NA NA

Sum2 NA 1994 0.3844 NA NA

Sum2 NA 1995 0.6268 NA NA

RGOtowi 08313000 1993 0.2424 0.0659 27.19

RGOtowi 08313000 1994 0.5334 0.1167 21.88

RGOtowi 08313000 1995 0.8144 0.1936 23.77

RGConvey 08358300 1993 0.0725 0.0170 23.53

RGConvey 08358300 1994 0.1609 0.0248 15.42

RGConvey 08358300 1995 0.1799 0.0313 17.42

RGFldwy 08358400 1993 0.3424 0.1551 45.29

RGFldwy 08358400 1994 1.6744 0.4363 26.06

RGFldwy 08358400 1995 2.6799 0.6861 25.60

Sum3 NA 1993 0.4149 NA NA

Sum3 NA 1994 1.8353 NA NA

Sum3 NA 1995 2.8598 NA NA

RGLeasb 08363500 1993 0.1792 0.0695 38.80

RGLeasb 08363500 1994 0.2637 0.0688 26.10

RGLeasb 08363500 1995 0.3664 0.1116 30.44

RGELP 08364000 1993 0.1832 0.0486 26.55

RGELP 08364000 1994 0.2765 0.0495 17.89

RGELP 08364000 1995 0.6001 0.1598 26.62
Table 5. Estimated daily mean suspended-sediment load and standard error of prediction, water years 1993-95 

[Station names in table 1.

Sum1 = RGTrinch + Conejos; Sum2 = RGTJB + Chama; Sum3 = RGConvey

 + RGFldwy; NA, not applicable]

Suspended sediments

Site USGS Estimated daily Standard error Standard error

identification station Water mean load of prediction of prediction

(table 1) number year (tons/day) (tons/day) (percent)

RGDelN 08220000 1993 152 57.0 37.4

RGDelN 08220000 1994 169 68.7 40.7

RGDelN 08220000 1995 241 103 42.8

RGTrinch 08240000 1993 44 13 29.3

RGTrinch 08240000 1994 41 10 24.8

RGTrinch 08240000 1995 56 16 28.4

Conejos 08249000 1993 73 25 33.8

Conejos 08249000 1994 30 9 30.1

Conejos 08249000 1995 74 20 27.1

Sum1 NA 1993 117 NA NA

Sum1 NA 1994 71 NA NA

Sum1 NA 1995 130 NA NA

RGLob 08251500 1993 111 32 29.0

RGLob 08251500 1994 52 9 16.5

RGLob 08251500 1995 158 58 37.0

RGTJB 08276500 1993 175 54 30.7

RGTJB 08276500 1994 341 93 27.2

RGTJB 08276500 1995 405 138 34.1

Chama 08290000 1993 221 123 55.6

Chama 08290000 1994 1,181 447 37.9

Chama 08290000 1995 1,846 778 42.1

Sum2 NA 1993 396 NA NA

Sum2 NA 1994 1,523 NA NA

Sum2 NA 1995 2,251 NA NA

RGOtowi 08313000 1993 484 147 30.4

RGOtowi 08313000 1994 3,035 822 27.1

RGOtowi 08313000 1995 2,653 703 26.5

RGConvey 08358300 1993 368 155 42.0

RGConvey 08358300 1994 270 61 22.7

RGConvey 08358300 1995 593 182 30.7

RGFldwy 08358400 1993 9,424 4,241 45.0

RGFldwy 08358400 1994 10,517 3,005 28.6

RGFldwy 08358400 1995 9,915 2,886 29.1

Sum3 NA 1993 9,792 NA NA

Sum3 NA 1994 10,786 NA NA

Sum3 NA 1995 10,508 NA NA

RGLeasb 08363500 1993 447 153 34.3

RGLeasb 08363500 1994 644 164 25.5

RGLeasb 08363500 1995 4,533 1,469 32.4

RGELP 08364000 1993 1,125 318 28.2

RGELP 08364000 1994 637 114 18.0

RGELP 08364000 1995 2,129 583 27.4
43

Spatial Variations in Suspended-Sediment

Concentrations

Median suspended-sediment concentrations

generally increased from RGDelN to RGFldwy

(fig. 19). The range of median suspended-sediment

concentrations throughout the study unit was three

orders of magnitude, which is greater than the range

observed in other constituents (figs. 8, 12, 16).

Concentrations increased from 12 to 52 mg/L between

RGDelN and RGTrinch. Suspended-sediment

concentrations at RGLob (42 mg/L) decreased slightly

because of dilution from Conejos (22 mg/L), which had

small concentrations relative to those at RGLob.

Suspended-sediment concentrations decreased

between RGLob (42 mg/L) and RGTJB (36 mg/L),

possibly because of a decrease in available sediment or

dilution from ground-water and surface-water inflow.

The interquartile range of suspended-sediment

concentrations at Chama was larger than that at any

other site, indicating that suspended-sediment

concentrations on the Rio Chama were highly variable.

Surface-water inflow from the Rio Chama and other

tributaries resulted in an increase in suspendedsediment concentrations from RGTJB to RGOtowi.

RGIsleta had a slightly smaller concentration (272

mg/L) than RGOtowi (332 mg/L), possibly because of

settling of suspended sediment in Cochiti Lake. From

RGIsleta to RGFldwy, the median suspended-sediment

concentration increased from 272 to 1,610 mg/L; this

was caused by the exceptionally large concentrations in

the Rio Puerco and the Rio Salado (Anderholm and

others, 1995). Downstream from RGFldwy,

concentrations decreased because of settling of

suspended sediment in Elephant Butte and Caballo

Reservoirs. Suspended-sediment concentrations

increased from RGLeasb (141 mg/L) to RGELP (219

mg/L) most likely as a result of agricultural activities,

surface-water inflow from ephemeral streams, and

resuspension and erosion of previously deposited

sediments.

Generally, suspended-sediment concentrations

at a particular site can vary by an order of magnitude or

more, whereas dissolved-solids, dissolved nitrate, and

total phosphorous concentrations at a particular site

have much smaller variability. Sites with the largest

variability in suspended-sediment concentrations

include RGDelN, Chama, and RGIsleta.

Concentrations at RGDelN were generally less than 50

mg/L, whereas suspended-sediment concentrations at

Chama and RGIsleta generally were greater than 7

mg/L.

Suspended Sediment/Streamflow

Relations

In general, suspended-sediment concentratio

are positively correlated with streamflow, but the

strength of the relation, which depends on a variety 

factors, varies from site to site. Sites that had the

strongest suspended-sediment/streamflow relation 

RGDelN, Conejos, RGTJB, RGLeasb, and RGELP

(fig. 20). These sites generally had the largest

suspended-sediment concentrations from May to

August.

Estimated Suspended-Sediment Loads

Estimated daily mean suspended-sediment lo

varied at individual sites from year to year and betw

sites (table 5 and fig. 21) because of (1) natural

variation in streamflow and delivery of sediment to

various reaches of the Rio Grande and (2) data

limitations related to the large variability in suspend

sediment concentrations at a site from year to year 

the short period of record used for model calibratio

The large standard errors of prediction associated w

estimated suspended-sediment loads (table 5) are d

in part to the larger variability in suspended-sedime

concentrations (at a particular site) and trend-relate

coefficients that are statistically different from zero 

and B4, eq. 1). The errors resulting from large

variability in suspended-sediment concentrations co

indicate that more frequent instantaneous

concentrations are needed to accurately characteriz

the variability in suspended-sediment concentration

Trend-related coefficients that are statistically differ

from zero indicate time trends; these could be due 

the short period of record used for model calibratio

Because of the larger annual variability in suspend

sediment loads, WY 1993-95 are discussed.

Estimated daily mean suspended-sediment lo

at RGDelN and RGTrinch were smallest in WY 19

and 1994 and largest in WY 1995 (table 5 and fig. 2

Between these sites, the load decreased by about a

factor of four (every year) despite increases in

suspended-sediment concentrations; the decrease in

suspended-sediment load was due to the decrease i

streamflow.


RGDelN RGTrinch Conejos Sum1 RGLob RGTJB Chama Sum2 RGOtowi RGConvey RGFldwy Sum3 RGLeasb RGELP

RGDelN RGTrinch Conejos Sum1 RGLob RGTJB Chama Sum2 RGOtowi RGConvey RGFldwy Sum3 RGLeasb RGELP

RGDelN RGTrinch Conejos Sum1 RGLob RGTJB Chama Sum2 RGOtowi RGConvey RGFldwy Sum3 RGLeasb RGELP

Annual mean streamflow

Estimated daily mean suspended-sediment load

Annual mean streamflow

Estimated daily mean suspended-sediment load

Annual mean streamflow

Estimated daily mean suspended-sediment load

E S T I M A T E D D A I L Y 

M E A N L O A

D, I

N

T O 

N S P E R D 

A

Y

A N N U A L M E A N 

S T R E A M F L 

O W, I N C U B I C F E E T P E R S E C O N 

D

2,400

2,000

1,600

1,200

800

400

0

2,400

2,000

1,600

1,200

800

400

0

2,400

2,000

1,600

1,200

800

400

0

12,000

10,000

8,000

6,000

4,000

2,000

0

12,000

10,000

8,000

6,000

4,000

2,000

0

12,000

10,000

8,000

6,000

4,000

2,000

0

Figure 21. Annual mean streamflow and estimated daily mean suspended-sediment load at selected sites in the

Rio Grande Valley study unit, water years 1993-95. Station names in table 1; location of sites in figure 1.

(Sum1 = RGTrinch + Conejos; Sum2 = RGTJB + Chama; Sum3 = RGConvey + RGFldwy)

21A Water year 1993

21B Water year 1994

21C Water year 1995

46
47

Estimated daily mean suspended-sediment loads

at Conejos were approximately equal in WY 1993 and

1995; the load in WY 1994 was less than half the load

in WY 1993 or 1995. When the incoming load from

Conejos was small (29.6 tons/d in WY 1994), there was

little change in the suspended-sediment load from

RGTrinch (41.2 tons/d) to RGLob (51.6 tons/d) (table

5). However, when larger loads entered the Rio Grande

from the Conejos River (73.2 tons/d in WY 1993 and

74.4 tons/d in WY 1995), the suspended-sediment

loads increased by factors of about two and three,

respectively (table 5).

Although the suspended-sediment

concentrations decreased from RGLob to RGTJB (fig.

19), the suspended-sediment load increased every year.

This resulted in estimated daily mean suspendedsediment loads ranging from 175 tons/d (WY 1993) to

405 tons/d (WY 1995) at RGTJB (table 5).

Suspended-sediment loads at Chama varied by

an order of magnitude, from 221 tons/d (WY 1993) to

1,846 tons/d (WY 1995). The large variation in loads at

Chama is reflected in the large variation in suspendedsediment loads at RGOtowi (table 5). By comparing

the estimated daily mean total suspended-sediment

load of Sum2 to the load at RGOtowi, it is apparent that

the load entering the Rio Grande (between RGTJB and

RGOtowi), besides the Rio Chama, varied greatly from

year to year. The smallest incoming suspendedsediment load occurred in WY 1993 with a minimum

of 88 tons/d entering the Rio Grande; the largest

incoming load occurred in WY 1994 with a minimum

of 1,512 tons/d entering the Rio Grande.

The estimated daily mean total suspendedsediment load at Sum3 was relatively consistent from

year to year, as was streamflow. Suspended-sediment

loads at RGConvey varied from 270 tons/d in WY 1994

to 593 tons/d in WY 1995. Loads at RGConvey were

approximately equal to loads at RGTJB, although the

streamflow at RGConvey was generally less than half

the streamflow at RGTJB (fig. 21).

Estimated daily mean suspended-sediment loads

at RGFldwy were larger than those at any other site in

the RIOG study unit and exhibited little variation from

year to year (table 5). Although Cochiti Lake allows for

increased settling of suspended sediment between

RGOtowi and RGFldwy, the suspended-sediment load

increased from 484 to 9,420 tons/d in WY 1993 and

tripled in WY 1994 and 1995 despite decreases in

streamflow. The increases in load at RGFldwy can be

attributed, at least partially, to the incoming suspendedsediment loads from ephemeral streams; the median

suspended-sediment load in the Rio Puerco for WY

1993-95 was 12,900 tons/d (Healy, 1997).

The estimated daily mean suspended-sedime

load decreased from Sum3 to RGLeasb because of

decreases in streamflow and settling of suspended

sediment in Elephant Butte and Caballo Reservoirs

Suspended-sediment loads at RGLeasb varied by a

order of magnitude, from 447 tons/d in WY 1993 t

4,533 tons/d in WY 1995 (table 5). Although

streamflow at RGLeasb did not vary by an order of

magnitude, the smallest and largest streamflows

correspond to the smallest and largest suspendedsediment loads.

The suspended-sediment load at RGELP var

from year to year, with the smallest load occurring 

WY 1994 (637 tons/d) and the largest load occurrin

WY 1995 (2,130 tons/d). In WY 1993, the suspend

sediment load increased from RGLeasb to RGELP

WY 1994, the load at RGELP was approximately eq

to the load at RGLeasb. In WY 1995, the suspende

sediment load decreased from RGLeasb (4,530 tons

to RGELP (2,130 tons/d). The variation may be due 

inflow of ephemeral streams, including the deposit

of suspended sediment in the Rio Grande and the

subsequent slow, downstream transport of suspend

sediment.

EFFECTS OF VARIATIONS IN

STREAMFLOW AND CONCENTRATION

ON LOADS IN THE RIO GRANDE VALLE

STUDY UNIT

From RGDelN to RGTrinch (fig. 22), streamfl

decreased from 722 to 194 ft3

/s because outflows

(primarily from agricultural diversions) are larger th

inflows. Median dissolved-solids concentrations

increased by a factor of four (from 73 to 298 mg/L

median dissolved nitrogen concentrations increased

from less than 0.05 to 0.06 mg/L, median total

phosphorous concentrations increased from 0.04 to

0.10 mg/L, and median suspended-sediment

concentrations increased by a factor of four (from 1

52 mg/L). Changes in streamflow and water quality

resulted in an increase in the estimated daily mean

dissolved-solids load (from 106 to 137 tons/d), a

decrease in total phosphorous load (from 0.092 to

0.048 ton/d), and a decrease in suspended-sedimen

load (from 169 to 41 tons/d). Therefore, the incom

dissolved-solids load was larger than that diverted, 
Figure 22. Spatial variations in streamflow, dissolved solids, dissolved nitrite plus nitrate,

total phosphorus, and suspended sediment throughout the Rio Grande

Valley study unit. Station names in table 1.

(Conc., concentration; DS, dissolved solids; Diss. N, dissolved nitrite plus nitrate; Total P,

total phosphorus; SED, suspended sediment)

Base from U.S. Geological Survey digital data, 1:100,000, 1983

Albers Equal-Area Conic projection

Standard parallels 29°30 ́ and 45°30 ́, central meridian -96°00 ́

Rio

Puerco

Rio

Jose

Chama

Rio

Rio Grande

Rio

San

Albuquerque

Saguache

Las Cruces

Alamosa

Caballo Res

Elephant Butte Res

San Ildefonso Pueblo

Taos

Conejos

River

Lobatos

Lasauses

Great Sand Dunes

Saguache Cr

San Luis Closed Basin

San Juan Mountains

Del Norte

Sangre de

Cristo Range

Sangre de Cristo Range

Chama Basin

Jemez Mountains

Cochiti

Lake

Santa Fe R

Santa Fe

San Luis Valley

Isleta

Bernardo

Bosque del

Apache NWR

San

Marcial

Leasburg

Dam

Radium

Springs

Sunland Park

Alamosa

NWR

RGDelN

Conejos

RGLob

RGTrinch

Chama RGTJB

RGOtowi

RGIsleta

RGConvey

RGFldwy

RGLeasb

RGELP El Paso

Chihuahuan

Desert

San Juan Mountains

Mesilla

Valley

Rincon

Valley

Grande

San Acacia

Rio Grande

Gorge

Creede

Red R

Trinchera

Creek

Franklin Eddy

Canal

Continental

Divide

Continental Divide

0 20 40 60

0 20 40 60

80 MILES

80 KILOMETERS

UNITED STATES OF AMERICA

REPUBLIC

OF MEXICO

Española

Chamita

COLORADO

NEW MEXICO

TEXAS

Abiquiu

Res

Rio Pueblo de Taos

Embudo

Creek

Goose

Cr

S. Fork

Heron

Res

El

Vado

Res

Conveyance

Channel

Rio Grande

Floodway

Rio Salado

34°

35°

33°

32°

36°

37°

108°

38° 107°

106°

STUDY

UNIT

BOUNDARY

Rio

Rancho

RGDelN to RGTrinch

Conc . S trea mflow Loa d

DS

Diss . N NA

To tal P

SED

RGTrinch to RGOtow i

Conc . S tream flow Load

DS

Diss. N NA

To tal P

SED

RGFldw y to R GLeasb

Conc . S tream flow Loa d

DS

Diss. N

To tal P

SED

RGOtow i to R GFld wy

Conc . S tream flow Load

DS

Diss. N NA

To tal P

SED

EX PLANAT ION

Increase

Decrease

No change

Inconsistent change

EXPLANATION

Increase

Decrease

No change

Inconsistent change

RGLeasb to RGEl P

Conc . S trea mflow Loa d

DS

Diss . N

To tal P

SED

RGLeasb to RGELP

Jemez

River

48

RGFldwy to RGLeasb

RGOtowi to RGFldwy

RGTrinch to RGOtowi

RGDelN to RGTrinch
49

the incoming total phosphorous and suspendedsediment loads were smaller than those diverted.

Furthermore, this indicates that inflow in this reach had

large dissolved-solids concentrations (relative to

RGDelN) but small total phosphorous and suspendedsediment concentrations (relative to RGDelN).

From RGTrinch to RGOtowi, streamflow

increased from 194 to 1,902 ft3

/s because of inflow

(primarily tributary inflow). Inflow from the Conejos

River accounts for most of the increases in streamflow

(184 ft3

/s) between RGTrinch and RGLob; however,

comparison of Sum1 and RGLob indicates additional

inflow (approximately 21 ft3

/s). Inflow from the Rio

Chama accounts for most of the increases in

streamflow (862 ft3

/s) between RGTJB and RGOtowi;

however, comparison of Sum2 and RGTJB indicates

additional inflow (approximately 153 ft3

/s). From

RGTrinch to RGOtowi, median dissolved-solids

concentrations decreased from 298 to 196 mg/L

(primarily because of dilution by tributary inflow),

median dissolved nitrate concentrations did not

change, median total phosphorous concentrations

decreased from 0.10 to 0.07 mg/L, and median

suspended-sediment concentrations increased from 52

to 332 mg/L. The large increase in streamflow caused

increases in dissolved-solids load (137 to 885

tons/d), total phosphorous load (0.048 to 0.53

ton/d), and suspended-sediment load (41 to 3,035

tons/d), indicating that incoming loads are greater than

outgoing loads in this reach.

From RGOtowi to RGFldwy (fig. 22),

streamflow decreased from 1,902 to 1,279 ft3/s because

outflows were larger than inflows. Median dissolvedsolids concentrations increased from 196 to 299 mg/L,

median dissolved nitrate concentrations increased from

0.06 to 0.71 mg/L, median total phosphorous

concentrations increased from 0.07 to 0.43 mg/L, and

median suspended-sediment concentrations increased

from 332 to 1,610 mg/L; these changes in

concentrations are due to inflows from WWTP’s,

tributaries, and return flows. Despite the decrease in

streamflow, all loads increased. The dissolved-solids

load increased from 885 to 943 tons/d, total

phosphorous load increased from 0.53 to 1.67 tons/d,

and suspended-sediment load increased from 3,035 to

10,517 tons/d. Because return flows and

evapotranspiration do not affect dissolved-solids loads,

the most likely cause is regional ground-water inflow

and tributary inflow. The increases in nutrient loads are

caused by WWTP inflow, return flows, and inflow from

ephemeral channels. The increase in suspendedsediment load is caused by inflow from ephemeral

streams (the Rio Puerco and Rio Salado).

From RGFldwy to RGLeasb (fig. 22), Eleph

Butte and Caballo Reservoirs have major impacts o

streamflow. During this study, the reservoirs genera

caused decreases in streamflow because of changes 

storage and evapotranspiration, increases in dissolv

solids concentrations due to evapotranspiration,

decreases in nutrient concentrations due to settling 

nutrient uptake, and decreases in suspended-sedim

concentrations due to settling. The result is an incre

in dissolved-solids load and decreases in dissolved

nitrate, total phosphorous, and suspended-sedimen

loads.

From RGLeasb to RGELP (fig. 22), streamfl

decreased from 883 to 670 ft3

/s, indicating that

outflows were greater than inflows. Median dissolv

solids concentrations increased from 448 to 652 mg

median dissolved nitrate concentrations increased fr

0.08 to 0.31 mg/L, median total phosphorous

concentrations increased from 0.06 to 0.12 mg/L, a

median suspended-sediment concentrations increas

from 141 to 219 mg/L. Despite the decrease in

streamflow, the dissolved-solids load increased fro

1,076 to 1,214 tons/d, the dissolved nitrate load

increased from 0.19 to 0.64 ton/d, the total

phosphorous load increased from 0.26 to 0.28 ton/d

and changes in suspended-sediment loads varied fr

year to year (see previous discussion). The increase 

dissolved-solids load is due to regional ground-wat

inflow. Increases in nutrient loads are due to WWT

inflow and return flows. Changes in suspendedsediment loads are caused by tributary inflow.

SUMMARY

This report describes spatial and temporal

variations in streamflow and water quality in the R

Grande from Del Norte, Colorado, to El Paso, Texa

Selected water-quality constituents presented in thi

report are dissolved solids, dissolved nitrite plus nitr

as nitrogen, total phosphorus, and suspended sedim

A multivariate linear regression model,

ESTIMATOR2000, was used to estimate loads for

selected constituents.

Streamflow decreases in the downstream

direction throughout most of the basin because

outflows (from agricultural use, leakage to ground

water, and evapotranspiration) are greater than inflo

Streamflow increases between RGTrinch and
50

RGOtowi because ground-water and tributary inflow

are greater than outflow.

Median dissolved-solids concentrations ranged

from 73 mg/L at RGDelN to 652 mg/L at RGELP. An

increase in the median dissolved-solids concentration

was fourfold between RGDelN and RGTrinch because

of ground-water discharge to the Rio Grande and

surface-water inflow from the Franklin Eddy Canal.

Dissolved-solids concentrations increased from

RGOtowi to RGFldwy as the result of surface-water

inflow and ground-water discharge to the Rio Grande.

Large dissolved-solids concentrations at RGConvey

resulted from irrigation effects, evapotranspiration in

riparian areas, and regional ground-water discharge to

agricultural drains. Dissolved-solids concentrations

increased between RGFldwy and RGELP because of

evapotranspiration, irrigation effects, and groundwater discharge to the Rio Grande. Dissolved-solids

concentrations varied seasonally at most sites;

concentrations were smaller during the spring and

summer months (April through August).

In WY 1994, dissolved-solids loads increased in

the Rio Grande from RGDelN to Sum3 (RGFldwy plus

RGConvey), decreased from Sum3 to RGLeasb, and

increased from RGLeasb to RGELP. From RGOtowi to

Sum3, the dissolved-solids load increased because of

larger concentrations and a smaller streamflow. From

RGLeasb to RGELP, the dissolved-solids load

increased because of increases in concentrations,

which result from regional ground-water inflow and

smaller streamflow.

Dissolved nitrate concentrations ranged from

less than the minimum reporting level (0.05

mg/L) to 2.0 mg/L throughout the Rio Grande. Nitrate

concentrations increased between RGDelN and

RGTrinch and again between RGLob and RGTJB

because of ground-water and surface-water inflows.

The median dissolved nitrate concentration increased

from 0.06 to 0.66 mg/L between RGOtowi and

RGIsleta because of WWTP inflow and, to a lesser

extent, return flows. The median concentration

increased from 0.08 mg/L at RGLeasb to 0.31 mg/L at

RGELP because of WWTP inflow, ground-water

discharge to the Rio Grande, and (or) return flow.

Dissolved nitrate loads were not estimated

upstream from San Marcial, New Mexico, because of

insufficient data for model calibration. Loads

decreased between Sum3 and RGLeasb because of

decreases in streamflow and concentrations. Dissolved

nitrate loads increased between RGLeasb and RGELP

because although streamflow decreased, dis

nitrate concentrations increased.

Total phosphorous concentrations gen

were small (less than 0.1 mg/L) and had sm

variability from RGDelN to RGOtowi. Tota

phosphorous concentrations were largest an

largest variability at RGIsleta and RGFldwy

From RGDelN to RGOtowi, estimate

mean phosphorous loads generally reflect in

and decreases in streamflow, indicating that 

phosphorous concentration in inflows was re

similar to that of the Rio Grande. From RGO

Sum3, the total phosphorous load increased 

to 1.8 tons/d despite a decrease in streamflow

indicates the presence of inflow(s) with a to

phosphorous concentration greater than that 

RGOtowi, most likely WWTP effluent or in

ephemeral streams.

Median suspended-sediment concentr

generally increased from RGDelN to RGFld

downstream variation in suspended-sedimen

concentrations (three orders of magnitude) w

greater than downstream variation observed 

constituents. Median suspended-sediment

concentrations increased from 12 to 52 mg/L 

RGDelN and RGTrinch. From RGIsleta to R

the median suspended-sediment concentrati

increased from 272 to 1,610 mg/L because o

from ephemeral streams. Downstream from 

suspended-sediment concentrations decrease

of settling of suspended sediment in Elepha

and Caballo Reservoirs.

Estimated daily mean suspended-sedim

varied at individual sites from year to year an

sites because of (1) natural variation in stream

delivery of sediment to various reaches of th

Grande and (2) data limitations related to th

variability in suspended-sediment concentra

site from year to year and the short period o

used for model calibration. Between RGDel

RGTrinch, the suspended-sediment load dec

about a factor of four (every year) despite in

suspended-sediment concentrations; the dec

load is due to the decrease in streamflow. Su

sediment loads at RGFldwy were larger than 

any other site in the RIOG NAWQA study u

exhibited little variation from year to year. T

decreased from Sum3 to RGLeasb because 

decreases in streamflow and settling of susp

sediment in Elephant Butte and Caballo Res
51

Several instances of decreasing streamflow and

increasing loads indicate the presence of inflows with

large constituent concentrations (relative to those of the

Rio Grande immediately upstream from that inflow);

this occurs (1) between RGDelN and RGTrinch for

dissolved solids, (2) between RGOtowi and RGFldwy

for all constituents, and (3) between RGLeasb and

RGELP for all constituents.

Streamflow increases along every reach of the

Rio Grande between RGTrinch and RGOtowi. These

increases in streamflow result in increases in dissolvedsolids loads, total phosphorous loads, and suspendedsediment loads regardless of changes in concentrations.

SELECTED REFERENCES

Anderholm, S.K., Radell, M.J., and Richey, S.F., 1995,

Water-quality assessment of the Rio Grande Valley

study unit, Colorado, New Mexico, and

Texas—Analysis of selected nutrient, suspendedsediment, and pesticide data: U.S. Geological Survey

Water-Resources Investigations Report 94-4061, 203 p.

Clark, G.M., Mueller, D.K., and Mast, M.A., 2000, Nutrient

concentrations and yields in undeveloped stream basins

of the United States: Journal of the American Water

Resources Association, v. 36, no. 4, p. 849-860.

Cohn, T.A., 2000, Unofficial Estimator2000 homepage:

http://www159.pair.com/cohns/TimCohn/

TAC_software/estimator/ (accessed November 6,

2002).

Cohn, T.A., Caulder, D.L., Gilroy, E.J., Zynjuk, L.D., and

Summers, R.M., 1992, The validity of a simple

statistical model for estimating fluvial constituent

loads—An empirical study involving nutrient loads

entering Chesapeake Bay: Water Resources Research,

v. 28, no. 9, p. 2353-2363.

Colorado Division of Water Resources, 2002, Rio Grande

Decision Support System: http://

hbserver.riverside.com/overview/rgdss/rgdss.html

(accessed May 2002).

Crowfoot, R.M., Ugland, R.C., Maura, W.S., Steger, R.D.,

and O’Neill, G.B., 1996, Water resources data,

Colorado, water year 1995: U.S. Geological Survey

Water-Data Report CO-95, 506 p.

Edelmann, P., and Buckles, D.R., 1984, Quality of the

ground water in agricultural areas of the San Luis

Valley, south-central Colorado: U.S. Geological Survey

Water-Resources Investigations Report 83-4281, 37 p.

Ellis, S.R., Levings, G.W., Carter, L.F., Richey, S.F., and

Radell, M.J., 1993, Rio Grande Valley, Colorado, New

Mexico and Texas: Water Resources Bulletin, v. 29, no.

4, p. 617-645.

Emery, P.A., Snipes, R.J., Dumeyer, J.M., and Klein, J.M

1973, Water in the San Luis Valley, south-central

Colorado: Colorado Water Resources Circular 18, 

p., 10 pls.

Healy, D.F., 1997, Water-quality assessment of the Rio

Grande Valley, Colorado, New Mexico, and

Texas—Summary and analysis of water-quality data 

the basic-fixed-site network, 1993-95: U.S. Geolog

Survey Water-Resources Investigations Report

97-4212, 82 p.

———1996, Water-quality assessment of the Rio Grand

Valley, Colorado, New Mexico, and

Texas—Occurrence and distribution of selected

pesticides and nutrients at selected surface-water si

in the Mesilla Valley, 1994-95: U.S. Geological Sur

Water-Resources Investigations Report 96-4069, 85 

Helsel, D.R., and Hirsch, R.M., 1992, Statistical method

water resources: Elsevier Science Publishers B.V.,

1992, 529 p.

Hem, J.D., 1985, Study and interpretation of the chemic

characteristics of natural water (3d ed.): U.S.

Geological Survey Water-Supply Paper 2254, 263 

Leahy, P.P., Rosenshein, J.S., and Knopman, D.S., 1990

Implementation plan for the National Water-Qualit

Assessment Program: U.S. Geological Survey Ope

File Report 90-174, 10 p.

Levings, G.W., Healy, D.F., Richey, S.F., and Carter, L.F

1998, Water quality in the Rio Grande Valley, Colora

New Mexico, and Texas, 1992-95: U.S. Geological

Survey Circular 1162, 39 p.

New Mexico Water Quality Control Commission, 2000,

State of New Mexico—Standards for interstate and

intrastate surface waters: Filed with State Records

Center, January 24, 2000, as New Mexico AC 6.1, 5

Nickerson, E.L., 1995, Selected hydrologic data for the

Mesilla ground-water basin, 1987 through 1992 wa

years, Doña Ana County, New Mexico, and El Pas

County, Texas: U.S. Geological Survey Open-File

Report 95-111, 123 p.

Ortiz, D., and Lange, K.M., 1996, Water resources data, N

Mexico, water year 1995: U.S. Geological Survey

Water-Data Report NM-95-1, 628 p.

Richey, S.F., and Ellis, S.R., 1993, National Water-Qual

Assessment Program—Water use in the Rio Grand

Valley, 1990: U.S. Geological Survey Open-File Re

93-456, 2 p.

Robertson, D.M., and Roerish, E.D., 1999, Influence of

various water quality sampling strategies on load

estimates for small streams: Water Resources Resea

v. 35, no. 12, p. 3747-3759.

Shelton, L.R., 1994, Field guide for collecting and

processing stream-water samples for the National

Water-Quality Assessment Program: U.S. Geologic

Survey Open-File Report 94-455, 42 p.
52

Snoeyink, V.L., and Jenkins, David, 1980, Water chemistry:

New York, John Wiley and Sons, 463 p.

Trench, E.C.T., 1999, Nutrient sources and loads in the

Connecticut, Housatonic, and the Thames River Basins:

U.S. Geological Survey Water-Resources

Investigations Report 99-4236, 66 p.

Wilson, C.A., White, R.R., Orr, B.R., and Roybal, G.R.,

1981, Water resources of the Rincon and Mesilla

Valleys and adjacent areas, New Mexico: New Mexico

State Engineer Technical Report 43, 514 p.

Winograd, I.J., 1959, Ground-water conditions and geology

of Sunshine Valley and western Taos County, New

Mexico: New Mexico State Engineer Technical Report

12, 70 p.