U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2012–5182

Revised September 2012

Prepared in cooperation with the Texas Water Development Board 

Trends in Selected Streamflow Statistics at 19 Long-Term 

Streamflow-Gaging Stations Indicative of Outflows from 

Texas to Arkansas, Louisiana, Galveston Bay, and the 

Gulf of Mexico, 1922–2009

Prepared in cooperation with the Texas Water Development Board

Trends in Selected Streamflow Statistics at 19 Long-Term 

Streamflow-Gaging Stations Indicative of Outflows from 

Texas to Arkansas, Louisiana, Galveston Bay, and the 

Gulf of Mexico, 1922–2009
Background, Looking towards the right bank of the Sabine River during a flood measurement at streamflow-gaging station 08030500 - Sabine River near Ruliff, Texas, on October 23, 2006. Photograph by Doug 

McGhee, U.S. Geological Survey. 

Front cover:

Left, Discharge measurement at streamflow-gaging station 08117500 - San Bernard River near Boling, 

Texas. Photograph by Mac Cherry, U.S. Geological Survey on April 1, 2007.

Right, Gage shelter raised above 200-year flood-plain level at 08114000 - Brazos River at Richmond, 

Texas, January 15, 2007. Photograph by Joe Stuart, U.S. Geological Survey.

Back cover:

Left, U.S. Geological Survey streamflow-gaging station 08475000 - Rio Grande near Brownsville, Texas, 

August 18, 2010. Photograph by Jaimie Ingold, U.S. Geological Survey. 

Right, Gage shelter and wire-weight gage at 08041000 - Neches River near Evadale, Texas. Photograph 

by Joe Stuart, November 17, 2006. 
Trends in Selected Streamflow Statistics at 

19 Long-Term Streamflow-Gaging Stations 

Indicative of Outflows from Texas to 

Arkansas, Louisiana, Galveston Bay, and 

the Gulf of Mexico, 1922–2009

By Dana L. Barbie and Loren L. Wehmeyer

Prepared in cooperation with the Texas Water Development Board

Scientific Investigations Report 2012–5182

Revised September 2012

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012

Revised 2012 

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

reproduce any copyrighted materials contained within this report.

Suggested citation:

Barbie, D.L., and Wehmeyer, L.L., 2012, Trends in selected streamflow statistics at 19 long-term streamflow-gaging 

stations indicative of outflows from Texas to Arkansas, Louisiana, Galveston Bay, and the Gulf of Mexico, 1922–2009: 

U.S. Geological Survey Scientific Investigations Report 2012–5182, 20 p. (Revised September 2012)
iii

Contents

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

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

Purpose and Scope .............................................................................................................................1

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

Previous Studies ..................................................................................................................................3

Methods ..........................................................................................................................................................3

Trends in Selected Streamflow Statistics .................................................................................................6

Outflows to Arkansas (Red and Sulphur River Basins) .................................................................6

Outflows to Louisiana (Little Cypress Creek Basin) .......................................................................8

Outflows to Upper Coast Gulf of Mexico (Sabine and Neches River Basins) ...........................8

Outflows to Gulf of Mexico through Galveston Bay (Trinity and San Jacinto Rivers and 

Associated Creeks and Bayous) ..........................................................................................9

Outflows to Middle Coast Gulf of Mexico (Brazos, San Bernard, and Colorado 

River Basins) .........................................................................................................................13

Outflows to Lower Coast Gulf of Mexico (Guadalupe and Nueces River Basins and 

Rio Grande Basin) ................................................................................................................16

Summary .......................................................................................................................................................19

References ...................................................................................................................................................20

Figures

1. Map showing basin boundaries for major rivers in Texas that outflow to Arkansas, 

Louisiana, Galveston Bay, and the Gulf of Mexico .................................................................2

2. Map showing statistically-significant trends in the annual mean daily discharge 

for the period of record through 2009 for streamflow-gaging stations with more 

than 50 years of record indicative of outflows from Texas ...................................................4

3. Graph showing annual mean daily discharge for the period of record through 2009 

for U.S. Geological Survey streamflow-gaging station 07337000 Red River at Index, 

Arkansas .......................................................................................................................................7

4. Graph showing annual mean daily discharge for the period of record through 

2009 for U.S. Geological Survey streamflow-gaging station 08041000 Neches River 

at Evadale, Texas .......................................................................................................................10

5. Graph showing annual mean daily discharge for the period of record through 2009 

for U.S. Geological Survey streamflow-gaging station 08066500 Trinity River at 

Romayor, Texas ..........................................................................................................................12

6. Graph showing annual mean daily discharge for the period of record through 2009 

for U.S. Geological Survey streamflow-gaging station 08073500 Buffalo Bayou near 

Addicks, Texas ............................................................................................................................14

7. Graph showing annual mean daily discharge for the period of record through 

2009 for U.S. Geological Survey streamflow-gaging station 08076000 Greens Bayou 

near Houston, Texas ..................................................................................................................15

8. Graph showing annual mean daily discharge for the period of record through 

2009 for U.S. Geological Survey streamflow-gaging station 08475000 Rio Grande at 

Brownsville, Texas .....................................................................................................................18
iv

Tables

1. Selected U.S. Geological Survey or International Boundary and Water 

Commission streamflow-gaging stations with more than 50 years of record that 

were used to assess trends in outflows from Texas to Arkansas, Louisiana, and 

the Gulf of Mexico .......................................................................................................................5

2. Monotonic trends in outflows from the Red and Sulphur River Basins in Texas, 

1937–2009 ......................................................................................................................................6

3. Monotonic trends in outflows from the Little Cypress Creek Basin in Texas, 

1947–2009 ......................................................................................................................................8

4. Monotonic trends in outflows from the Sabine and Neches River Basins in Texas, 

1925–2009 ......................................................................................................................................9

5. Monotonic trends in outflows from the Trinity and San Jacinto River Basins and 

associated creeks and bayous to Galveston Bay in Texas, 1925–2009 ............................11

6. Monotonic trends in outflows from the Brazos, San Bernard, and Colorado River 

Basins in Texas, 1923–2009 ......................................................................................................16

7. Monotonic trends in outflows from the Guadalupe and Nueces River Basins, and 

the Rio Grande Basin in Texas, 1926–2009 ............................................................................17

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2

) 2.590 square kilometer (km2) 

Flow rate

cubic foot per second (ft3

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

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

A water year is the 12-month period October 1 through September 30 designated by the 

calendar year in which it ends.
Trends in Selected Streamflow Statistics at 19 Long-Term 

Streamflow-Gaging Stations Indicative of Outflows from 

Texas to Arkansas, Louisiana, Galveston Bay, and the 

Gulf of Mexico, 1922–2009

By Dana L. Barbie and Loren L. Wehmeyer 

Abstract

Trends in selected streamflow statistics during 

1922–2009 were evaluated at 19 long-term streamflowgaging stations considered indicative of outflows from 

Texas to Arkansas, Louisiana, Galveston Bay, and the Gulf 

of Mexico. The U.S. Geological Survey, in cooperation 

with the Texas Water Development Board, evaluated 

streamflow data from streamflow-gaging stations with more 

than 50 years of record that were active as of 2009. The 

outflows into Arkansas and Louisiana were represented by 

3 streamflow-gaging stations, and outflows into the Gulf 

of Mexico, including Galveston Bay, were represented by 

16 streamflow-gaging stations. Monotonic trend analyses 

were done using the following three streamflow statistics 

generated from daily mean values of streamflow: (1) annual 

mean daily discharge, (2) annual maximum daily discharge, 

and (3) annual minimum daily discharge. The trend analyses 

were based on the nonparametric Kendall’s Tau test, which is 

useful for the detection of monotonic upward or downward 

trends with time. A total of 69 trend analyses by Kendall’s 

Tau were computed—19 periods of streamflow multiplied by 

the 3 streamflow statistics plus 12 additional trend analyses 

because the periods of record for 2 streamflow-gaging stations 

were divided into periods representing pre- and post-reservoir 

impoundment. Unless otherwise described, each trend analysis 

used the entire period of record for each streamflow-gaging 

station. The monotonic trend analysis detected 11 statistically 

significant downward trends, 37 instances of no trend, and 

21 statistically significant upward trends. One general region 

studied, which seemingly has relatively more upward trends 

for many of the streamflow statistics analyzed, includes the 

rivers and associated creeks and bayous to Galveston Bay 

in the Houston metropolitan area. Lastly, the most western 

river basins considered (the Nueces and Rio Grande) had 

statistically significant downward trends for many of the 

streamflow statistics analyzed.

Introduction

The major river basins in Texas include the Red River, 

Sabine River, Neches River, Trinity River, San Jacinto River, 

Brazos River, Colorado River, Guadalupe River, San Antonio 

River, Nueces River, and the Rio Grande (fig. 1). The Red 

River flows into Arkansas and then into Louisiana; one of 

its tributaries (Big Cypress Creek) flows directly from Texas 

into Louisiana. The Sabine, Neches, Brazos, San Bernard, 

Colorado, Guadalupe, and Nueces Rivers and the Rio Grande 

flow directly into the Gulf of Mexico. The Trinity and San 

Jacinto Rivers flow into Galveston Bay, which is part of the 

Gulf of Mexico. Fresh surface-water usage in Texas is about 

two-thirds of the total freshwater usage in Texas (Kenny 

and others, 2009, p. 6). As a result, studies of streamflow 

statistics for outflows from Texas are useful in resource 

assessments, such as water-supply studies, as well as useful to 

hydrologists, biologists, and ecologists. Accordingly, the U.S. 

Geological Survey (USGS), in cooperation with the Texas 

Water Development Board, evaluated streamflow data from 

streamflow-gaging stations with more than 50 years of record 

that were active as of 2009.

Purpose and Scope

The purpose of this report is to assess monotonic trends 

in streamflow to determine whether statistically significant 

monotonic changes in selected streamflow statistics have 

occurred over time for streamflow records likely indicative 

of outflows from Texas. Streamflow data were evaluated 

and annual streamflow statistics were computed for 18 

selected USGS stations and one International Boundary 

Water Commission (IBWC) station with more than 50 

years of record and active as of 2009. Examination of 

historical patterns, cycles, and possible reasons for the 

trends in streamflow such as trends in precipitation, changes 

in quantified water use, reservoir operation, or other 
OKLAHOMA ARKANSAS LOUISIANA COLORADONEW MEXICO TEXAS GULF OF MEXICO Galveston Bay 0 100 200 MILES 0 100 200 KILOMETERS 50 150 15050 Colorado Rio Grande Guadalupe Brazos Red Trinity Nueces Neches Sabine Jacinto San San Antonio San Bernard 9

94° 98° 102° 106° 40° 36° 32° 28° Base from U.S. Geological Survey 1:2,000,000-scale digital data Albers equal-area projection North American Datum of 1983 Watershed 

EXPLANATIO

Figure 1. Basin boundaries for major rivers in Texas that outflow to Arkansas, Louisiana, Galveston Bay, and the Gulf of Mexico.
Methods 3

human-related changes, or the effects of direct groundwater 

contributions (gains or losses) to streamflow was beyond the 

scope of this study.

Description of Study Area

Streamflow outflow from Texas occurs in the eastern 

part of the State. The locations of streamflow-gaging stations 

and selected reservoirs discussed in this report are shown in 

figure 2. 

The stations used for this study, along with ancillary 

information pertaining to each station including the period 

of record, are listed in table 1. Collectively, the stations 

discussed in this report monitor streamflow from Texas into 

Arkansas, Louisiana, Galveston Bay (and subsequently the 

Gulf of Mexico), and the Gulf of Mexico directly. Outflow 

trends to Arkansas were evaluated using two stations (table 1, 

Outflows to Arkansas). Outflow trends to Louisiana were 

evaluated using one station (table 1, Outflows to Louisiana). 

Outflow trends to the Gulf of Mexico on the upper coast 

(near Louisiana) were computed at two stations (table 1, 

Outflows to upper coast Gulf of Mexico). Outflow trends to 

the Gulf of Mexico through Galveston Bay were computed 

using eight stations (table 1, Outflows to Gulf of Mexico 

through Galveston Bay). Outflow trends to the Gulf of 

Mexico on the middle coast were computed at three stations 

(table 1, Outflows to middle-coast Gulf of Mexico). Lastly, 

outflow trends to the Gulf of Mexico on the lower coast were 

computed at three stations (table 1, Outflows to lower-coast 

Gulf of Mexico). 

Previous Studies

Several statistical and historical summaries of streamflow 

data in Texas have previously been done by the USGS 

(Asquith and others, 2007a, 2007b; Asquith and Heitmuller, 

2008). Lanning-Rush (2000) also provided regional regression 

equations to estimate mean annual discharge (the arithmetic 

mean of the annual mean discharges) as well as mean seasonal 

discharge for unmonitored stream sites throughout Texas. 

Lastly, Asquith (2001) did a statewide study on the effects 

of flood-control regulation on statistics of annual peak 

streamflow time series for hundreds of streamflow-gaging 

stations in Texas.

Methods

Streamflow data were evaluated and annual streamflow 

statistics were computed for 19 streamflow-gaging stations 

consisting of 18 USGS stations and 1 IBWC station with more 

than 50 years of record and active since 2009. Streamflow data 

used in this study are published in the USGS National Water 

Information System (U.S. Geological Survey, 2012a) with 

the exception of the streamflow data for the Rio Grande near 

Brownsville, Tex., station that is assigned an eight-digit station 

identification number consistent with USGS station numbers 

but is operated by the IBWC. Streamflow data collected at the 

Rio Grande station are published by the IBWC (International 

Boundary and Water Commission, 2012).

The daily mean discharges for each station (365 to 

366 values per year) were used in the analyses. These data 

were subsequently converted to the annual streamflow 

statistics of (1) annual mean daily discharge, (2) annual 

maximum daily discharge, and (3) annual minimum 

daily discharge. 

The Kendall’s Tau statistical test was used to detect 

monotonic trends in annual mean daily, annual maximum 

daily, and annual minimum daily streamflow over time (Helsel 

and Hirsch, 2002). Specifically, the test uses the relation 

(trend) between time and ranked discharges as opposed to 

discharge magnitude. The trend analyses reported in this report 

thus refer to the direction and not the magnitude of streamflow 

change. A statistically significant positive Kendall’s Tau value 

indicates an upward monotonic streamflow trend; conversely, 

a statistically significant negative Tau indicates a downward 

monotonic streamflow trend. The probability value or p-value 

of Kendall’s Tau is a measure of the statistical significance of 

the trend. For this study, a p-value of less than 0.05 indicates 

a statistically significant trend, whereas, a p-value of 0.05 or 

greater indicates no significant trend.

Annual mean daily discharge is computed as the average 

of all the 365 or 366 daily discharges during a water year (a 

water year is defined as the 12-month period from October 1 

to September 30). The annual minimum daily discharge is 

the discharge on the day of lowest daily mean discharge in a 

given water year. The annual maximum daily discharge is the 

discharge on the day of largest daily mean discharge in a given 

water year. The software package Statistica (StatSoft, 2009) 

was used to calculate the Kendall’s Tau test statistic. For each 

station, Kendall’s Tau statistics were computed using the three 

streamflow statistics calculated for each station (annual mean, 

maximum, and minimum daily streamflow). The period of 

records evaluated for each station is listed in table 1.

A total of 69 trend analyses by Kendall’s Tau were 

computed—19 periods of streamflow from USGS or 

IBWC stations multiplied by 3 streamflow statistics plus 12 

additional trend analyses because the periods of record for 

2 USGS stations were analyzed for trends pre- and postreservoir impoundment, resulting in 4 periods of streamflow 

multiplied by 3 streamflow statistics. The period of postreservoir impoundment was defined as when at least 10 

percent of contributing drainage area was regulated. Unless 

otherwise indicated, each trend analysis used the entire period 

of record for each station. Some additional remarks concerning 

methods and decisions made for the presentation of the trend 

analysis results are required. There are many stations with 

one or more upstream reservoirs. However, for circumstances 

in which there were no changes in streamflow trends pre- or 

post-reservoir impoundment, the entire period of record was 

used for the trend analysis. Also, there are circumstances in 

which a reservoir was impounded near the beginning of the 

streamflow record—leaving perhaps only a few years prior to 
4 Trends in Selected Streamflow Statistics at 19 Long-Term Streamflow-Gaging Stations Indicative of Outflows

LOUISIANA

TEXAS

ARKANSAS

OKLAHOMA

MEXICO

UNITED STATES

"

"

"

"

"

"

"

"

"

"

"

"

"

"

"

"

"

"

" "

"

"

"

"

"

"

Ruliff

Mathis

Houston

Evadale

Boling

Wharton

Victoria

Splendora

Addicks

Cleveland

Westfield Spring

Conroe

Romayor

Houston

Richmond

Brownsville

Index

Jefferson

Talco

Base from U.S. Geological Survey

1:100,000-scale digital data

Albers equal-area projection

North American Datum of 1983 

0 25 50 75 100 MILES

0 50 75 100 KILOMETERS 25

94°

96° 98°

34°

32°

30°

28°

26°

STATES 

ARKANSAS

LOUISIANA

NEW MEXICO

OKLAHOMA

TEXAS MEXICO

Area enlarged

UNITED

Lake

Livingston

Lake

Tawakoni

Lake

Palestine 

Lake

Halbert

Lake

Whitney

Waco

Lake

Richland-Chambers

Reservoir 

Sabine 

Lake

Cypress Creek

Spring Creek

Sabine Riv er

Sam Rayburn

Reservoir

GULF OF MEXICO

Angelina

River

Town Lake

Coleto Creek

Reservoir

Lake Texoma

Lake

Austin

Toledo Bend

Reservoir

Falcon

 Reservoir

B A Steinhagen

Lake

Lake

Travis

Possum

Kindgom

Lake

Ellison Creek Reservoir

Lake Corpus

Christi

Jim

Chapman

Lake 

Canyon 

Lake

Lake O’

the Pines 

Choke Canyon 

Reservoir

Lake

Worth

Lake

Conroe

Barker Reservoir

Caney

Creek

Bayou

Greens

GULF OF

MEXICO 

Red River

Trinity River

San Antonio River

Colorado River

Medina River

Guadalupe

River

Nueces River

Frio River

Attoyac Bayou

Neches River

Galveston

Bay

Trinity

River

Brazos River

Little

Cypress

Creek

Sabine River

Red River

Big Cypress

Creek

Sulphur River

West Fork

San Jacinto River

East Fork

San Jacinto River

Lake

 Houston

Buffalo Bayo u

Addicks Reservoir

East Fork

San Jacinto River

San Bernard

River

West Fork San Jacinto River

Brazos River

Rio Grande Rio Brav o

#

#

#

# # #

#

##

# #

#

#

#

# #

#

#

#

#

#

#

#

#

#

#

#

#

08176500

08211000

08162000

08117500

08475000

07337000

07343200

07346070

08114000

08073500

08070000

08066500

08070500

08076000

08069000

08068500

08068000

08041000

08030500

EXPLANATION

# U.S. Geological Survey

 streamflow-gaging station

 and station number

Upward trend

Downward trend 

08162000

See inset

Figure 2. Statistically-significant trends in the annual mean daily discharge for the period of record through 2009 for streamflowgaging stations with more than 50 years of record indicative of outflows from Texas.
Methods 5

Table 1. Selected U.S. Geological Survey or International Boundary and Water Commission streamflow-gaging stations with more than 

50 years of record that were used to assess trends in outflows from Texas to Arkansas, Louisiana, and the Gulf of Mexico.

[mi; square miles; --, not applicable]

Station 

number Station name Period of record1

Year of first 

substantial 

reservoir 

impoundment2

Latitude 

(decimal 

degrees)

Longitude 

(decimal 

degrees)

Drainage 

area 

(mi2

)

Contributing 

drainage 

area 

(mi2

)

Outflows to Arkansas (Red and Sulphur River Basins—table 2)

07337000 Red River at Index, Ark. 1937–2009 1943 33.55207 94.04130 48,030 42,094

07343200 Sulphur River near Talco, Tex. 1957–2009 1991 33.39067 95.06244 1,405 1,405

Outflows to Louisiana (Little Cypress Creek Basin—table 3)

07346070 Little Cypress Creek near 

Jefferson, Tex.

1947–2009 -- 32.71292 94.34603 675 675

Outflows to the upper coast of the Gulf of Mexico (Sabine and Neches River Basins—table 4)

08030500 Sabine River near Ruliff, Tex. 1925–2009 1961 30.30382 93.74378 9,329 9,329

08041000 Neches River at Evadale, Tex. 1922–2009 1951 30.35576 94.09324 7,951 7,951

Outflows to the Gulf of Mexico through Galveston Bay (Trinity and San Jacinto River Basins, and Associated Creeks and Bayous—table 5)

08066500 Trinity River at Romayor, Tex. 1925–2009 before 1925 30.42521 94.85076 17,186 17,186

08068000 West Fork San Jacinto River near 

Conroe, Tex.

1940–2009 1973 30.24466 95.45716 828 828

08068500 Spring Creek near Spring, Tex. 1940–2009 -- 30.11050 95.43633 409 409

08069000 Cypress Creek near Westfield, Tex. 1945–2009 -- 30.03578 95.42883 285 285

08070000 East Fork San Jacinto River near 

Cleveland, Tex.

1940–2009 -- 30.33660 95.10410 325 325

08070500 Caney Creek near Splendora, Tex. 1945–2009 -- 30.25966 95.30244 105 105

08073500 Buffalo Bayou near Addicks, Tex. 1946–2009 before 1946 29.76190 95.60578 277 277

08076000 Greens Bayou near Houston, Tex. 1953–2009 -- 29.91828 95.30688 69 69

Outflows to the middle coast of the Gulf of Mexico (Brazos, Sen Bernard, and Colorado River Basins—table 6)

08114000 Brazos River at Richmond, Tex. 1923–2009 1941 29.58246 95.75773 45,107 35,541

08117500 San Bernard River near Boling, Tex. 1955–2009 -- 29.31358 95.89384 727 727

08162000 Colorado River at Wharton, Tex. 1939–2009 before 1939 29.30914 96.10385 42,003 30,600

Outflows to the lower coast of the Gulf of Mexico (Guadalupe and Nueces River Basins, and Rio Grande Basin—table 7)

08176500 Guadalupe River at Victoria, Tex. 1936–2009 before 1936 28.79305 97.01304 5,198 5,198

08211000 Nueces River near Mathis, Tex. 1940–2009 before 1940 28.03835 97.86028 16,503 16,503

08475000 Rio Grande near Brownsville, Tex.3 1935–2009 1953 25.87675 97.45443 176,333 176,333

1

Period of record is shown in water years; a water year is defined as the 12-month period October 1 through September 30 designated by the calendar year in 

which it ends.

2

Year reservoir impoundment first appreciably affected streamflow (Dowell, 1964).

3

Station operated by the International Boundary and Water Commission. All other stations are operated by the U.S. Geological Survey.

impoundment—for this study, the entire period of record was 

used for the trend analysis. Lastly, time series plots of annual 

mean daily discharge for six selected stations are provided 

to illustrate this streamflow statistic. Similar plots to the 

time series figures introduced later can be readily generated 

through the USGS National Water Information System Web 

(NWISWeb) Internet portal (http://waterdata.usgs.gov). 

Table 1 lists the year of the first large reservoir 

impoundment upstream from the station. For several stations, 

this year predates the activation of the station, for example 

USGS streamflow-gaging station 082111000 Nueces River 

near Mathis, Tex. However, for two stations, where the period 

of record was sufficient and the effect of impoundment on 

streamflow characteristics was evident (USGS streamflowgaging station 08041000 Neches River at Evadale, Tex., and 

IBWC streamflow-gaging station 08475000 Rio Grande 

near Brownsville, Tex.), the streamflow record was divided 

into pre- and post-impoundment periods. Trend analysis 
6 Trends in Selected Streamflow Statistics at 19 Long-Term Streamflow-Gaging Stations Indicative of Outflows

subsequently was performed for the entire period of record, 

including the years before reservoir impoundment. 

Trends in Selected Streamflow 

Statistics

This section is divided into six subsections that 

correspond to logical and convenient groupings of stations. 

Each section provides salient but limited background, 

summarizes the stations involved, and concludes with 

reporting of the analysis to detect monotonic trends for each of 

the three streamflow statistics.

For the overall assessment in trends in selected 

streamflow statistics during 1922–2009 at 19 long-term 

streamflow-gaging stations considered indicative of outflows 

from Texas to Arkansas, Louisiana, Galveston Bay, and 

the Gulf of Mexico, monotonic trend analysis detected 

11 statistically significant downward trends, 37 instances of 

no trend, and 21 statistically significant upward trends. Annual 

mean daily discharge is especially diagnostic because this 

statistic can be used to compute mean annual discharge over 

long periods of time and represents the expectation of the 

annual volume of water measured by a station and, therefore, 

water yields from a basin. Trends in annual mean daily 

discharge can be summarized as follows; (1) annual outflow 

volumes from Texas into Arkansas and Louisiana as well 

as the Gulf of Mexico (upper Texas coast) lack statistically 

significant trends; (2) annual inflows into Galveston Bay 

(Houston metropolitan area) show upward trends for four of 

eight stations; (3) annual outflow volumes from the Brazos 

River (east of Houston metropolitan area) to the Guadalupe 

River lack statistically significant trends; and (4) annual 

outflow volumes from the two most westerly stations on the 

Nueces River and Rio Grande show downward trends.

Outflows to Arkansas (Red and Sulphur 

River Basins)

Streamflows at USGS streamflow-gaging stations 

07337000 Red River at Index, Ark. (Red River at Index 

station), and 07343200 Sulphur River near Talco, Tex. 

(Sulphur River near Talco station), are indicative of outflow 

from Texas to Arkansas. After exiting Texas and flowing 

through the southwest corner of Arkansas, the Sulphur River 

flows into the Red River in Louisiana.

The Red River at Index station is located at the TexasArkansas State boundary in far northeast Texas and is more 

than 500 miles downstream from the Red River’s headwaters 

in New Mexico. The Red River Basin upstream from the Red 

River at Index station includes parts of New Mexico, Texas, 

and Oklahoma. A particularly important reservoir in terms of 

its effect on regulating flow in the Red River is Lake Texoma 

on the Texas–Oklahoma border. Since 1944, Lake Texoma, 

which is about 241 miles upstream from Index, Ark., has 

potentially affected general streamflow characteristics at the 

Red River at Index station (Dowell, 1964; U.S. Geological 

Survey, 2012b). The time period analyzed for this study is 

1937–2009 (table 2), which includes a few years prior to 

completion of Lake Texoma. A downward trend for annual 

maximum daily discharge was detected at this station. A 

time series of the annual mean daily discharge is shown in 

figure 3.

The Sulphur River near the Talco station is 2.8 miles 

northeast of Talco, Tex. Jim Chapman Lake (fig. 2) was 

impounded in 1991 and has potentially affected streamflow 

Table 2. Monotonic trends in outflows from the Red and Sulphur River Basins in Texas, 1937–2009.

[USGS, U.S. Geological Survey; p-value, probability value; <, less than; --, no statistically significant trend was detected (p-value <0.05)]

USGS 

station 

number

USGS station 

name

Time 

period

analyzed

Streamflow statistic

(discharge)

Kendall’s 

tau p-value Trenddirection

07337000 Red River at Index, Ark. 1937–2009 Annual mean daily 0.04 0.65 --

07337000 Red River at Index, Ark. 1937–2009 Annual maximum daily -0.25 <0.05 downward

07337000 Red River at Index, Ark. 1937–2009 Annual minimum daily 0.10 0.20 --

07343200 Sulphur River near Talco, Tex. 1957–2009 Annual mean daily -0.13 0.17 --

07343200 Sulphur River near Talco, Tex. 1957–2009 Annual maximum daily -0.18 0.054 --

07343200 Sulphur River near Talco, Tex. 1957–2009 Annual minimum daily 0.33 <0.05 upward
0

5,000

10,00015,00020,00025,00030,00035,000

1937

1939

1941

1943

1945

1947

1949

1951

1953

1955

1957

1959

1961

1963

1965

1967

1969

1971

1973

1975

1977

1979

1981

1983

1985

1987

1989

1991

1993

1995

1997

Discharge, in cubic feet per second

Water year

Lake Texoma Impoundment

An

Figure 3. Annual mean daily discharge for the period of record through 2009 for U.S. Geological Survey streamflow-gaging station 07337000 Red Rive
8 Trends in Selected Streamflow Statistics at 19 Long-Term Streamflow-Gaging Stations Indicative of Outflows

characteristics of the Sulphur River since its impoundment 

(U.S. Geological Survey, 2012b). The station is about 

35 miles downstream from Jim Chapman Lake and about 

70 miles upstream from Texas-Louisiana State boundary. The 

time period analyzed for this study is 1957–2009 (table 2). 

An upward trend for annual minimum daily discharge was 

detected at this station.

Outflows to Louisiana (Little Cypress 

Creek Basin)

Streamflow at USGS streamflow-gaging station 

07346070 Little Cypress Creek near Jefferson, Tex. (Little 

Cypress Creek station), is indicative of the unregulated 

contribution to outflows from Texas to Louisiana through Big 

Cypress Creek, the only stream in Texas that flows directly 

into Louisiana. Little Cypress Creek flows into the Big 

Cypress Creek, which flows into the Red River in Louisiana 

(fig. 2). Little Cypress Creek streamflows were analyzed for 

trends because Big Cypress Creek has been highly regulated 

by Lake O’ the Pines after it was impounded in 1958 (Dowell, 

1964; U.S. Geological Survey, 2012b). The streamflow in 

Little Cypress Creek can be considered reasonably natural. 

Most streams in Texas are affected by regulation, but Little 

Cypress Creek represents one of the few opportunities to 

analyze for trends in a reasonably natural stream.

The Little Cypress Creek station is 3.5 miles south of 

Jefferson, Tex. The station is about 65 miles downstream from 

the headwaters of Little Cypress Creek and about 7 miles 

upstream from Big Cypress Creek. The time period analyzed 

for this study is 1947–2009 (table 3). No trends were detected 

in streamflow at this station.

Outflows to Upper Coast Gulf of Mexico (Sabine 

and Neches River Basins)

The Sabine and Neches Rivers flow directly into the Gulf 

of Mexico along the upper coastline of Texas. The two river 

basins drain the heavily forested east Texas region noted for its 

humid, subtropical climate (Larkin and Bomar, 1983). Three 

principle reservoirs are Toledo Bend Reservoir on the Sabine 

River (impounded in 1966), Sam Rayburn Reservoir on a 

tributary to the Neches River (impounded in 1965), and B.A. 

Steinhagen Lake on the Neches River (impounded in 1951) 

(fig. 2); other reservoirs depicted on figure 2 include Lake 

Tawakoni on the Sabine River (impounded in 1960) and Lake 

Palestine on the Neches River (impounded in 1962) (Dowell, 

1964; U.S. Geological Survey, 2012b).

USGS streamflow-gaging station 08030500 Sabine River 

near Ruliff, Tex. (Sabine River near Ruliff station), is about 

2 miles north of Ruliff and about 45 miles upstream from the 

mouth with Sabine Lake that in turn drains into the Gulf of 

Mexico. This station is about 300 miles downstream from 

the headwaters of the Sabine River. Streamflow in the Sabine 

River has been potentially affected by regulated outflows 

from Lake Tawakoni (fig. 2) since 1961 (Dowell, 1964; U.S. 

Geological Survey, 2012b). More importantly, the station is 

about 50 miles downstream from the relatively large Toledo 

Bend Reservoir. The time period analyzed was 1925–2009 

(table 4). An upward trend for annual minimum daily 

discharge was detected at this station.

USGS streamflow-gaging station 08041000 Neches 

River at Evadale, Tex. (Neches River at Evadale station), is 

about 200 miles downstream from the headwaters of the river 

and about 40 miles upstream from the mouth of the river. The 

mouth of the Neches River empties into Sabine Lake, which 

drains into the Gulf of Mexico. Since 1951, streamflow in 

the Neches River has been appreciably regulated by B.A. 

Steinhagen Lake, which is about 32 miles upstream from the 

Neches River at Evadale station. The Sam Rayburn Reservoir 

on the Angelina River, a major tributary of the Neches River, 

is about 15 miles upstream from B.A. Steinhagen Lake (fig. 

2), and has affected streamflow along the lower Neches River 

since 1965 (Dowell, 1964; U.S. Geological Survey, 2012b). 

A time series of the annual mean daily discharge is shown 

in figure 4 and includes the timing of impoundment by B.A. 

Steinhagen Lake and Sam Rayburn Reservoir. A distinction 

of pre- and post-regulation at 1950 and 1951, respectively, 

is made for this analysis. The time periods analyzed for this 

study were 1922–2009, 1922–50, and 1951–2009 (table 4). 

Table 3. Monotonic trends in outflows from the Little Cypress Creek Basin in Texas, 1947–2009.

[USGS, U.S. Geological Survey; p-value, probability value; <, less than; --, no statistically significant trend was detected (p-value <0.05)]

USGS 

station 

number

USGS station 

name

Time 

period

analyzed

Streamflow 

statistic

(discharge)

Kendall’s 

tau p-value Trenddirection

07346070 Little Cypress Creek near Jefferson, Tex. 1947–2009 Annual mean daily 0.02 0.82 --

07346070 Little Cypress Creek near Jefferson, Tex. 1947–2009 Annual maximum daily 0.01 0.88 --

07346070 Little Cypress Creek near Jefferson, Tex. 1947–2009 Annual minimum daily 0.12 0.18 --
Trends in Selected Streamflow Statistics 9

A downward trend for annual maximum daily discharge 

and conversely an upward trend for annual minimum daily 

discharge were detected for Neches River at Evadale station 

for the period 1922–2009 (period of record). No trends were 

detected for the Neches River at Evadale, Tex., station for the 

period 1922–50; however, upward trends for annual mean 

daily and annual minimum daily discharge were detected for 

the period 1951–2009.

Outflows to Gulf of Mexico through Galveston 

Bay (Trinity and San Jacinto Rivers and 

Associated Creeks and Bayous)

Galveston Bay is a large estuary on the Texas coast 

and has substantial and varied sources of inflow. The Trinity 

River, with its headwaters in north Texas adjacent to the Red 

River Basin (fig. 1), is the largest tributary to the bay. The 

Trinity River is followed in size by the San Jacinto River and 

its associated tributaries in the Houston metropolitan area, 

including West Fork San Jacinto River, Spring Creek, Cypress 

Creek, East Fork San Jacinto River, Caney Creek, Buffalo 

Bayou, and Greens Bayou. 

USGS streamflow-gaging station 08066500 Trinity 

River at Romayor, Tex. (Trinity River at Romayor station), 

is about 45 miles upstream from Galveston Bay and about 

300 miles downstream from the headwaters of this river. 

Lake Livingston, about 20 miles upstream, is the reservoir 

nearest the station (fig. 2) and was impounded in 1969 (U.S. 

Geological Survey, 2012b). The time period analyzed for 

the Trinity River at Romayor station is 1925–2009 (table 5). 

In addition to Lake Livingston, there are other reservoirs 

regulating flow on the Trinity River upstream from the 

Trinity River at Romayor station, including some that were 

impounded as early as 1916 (Lake Worth) and 1921 (Lake 

Halbert) (Texas Historical Association, 2012a, b; Dowell, 

1964). An upward trend for annual minimum daily discharge 

was detected at the Trinity River at Romayor station. A time 

series of the annual mean daily discharge is shown in figure 5.

San Jacinto River flows from Lake Houston into 

Galveston Bay. Major inflows into Lake Houston are West 

Fork San Jacinto River, Spring Creek, Cypress Creek, East 

Fork San Jacinto River, and Caney Creek. USGS streamflowgaging station 08068000 West Fork San Jacinto River near 

Conroe, Tex. (West Fork San Jacinto River near Conroe 

station), is about 4 miles south of Conroe. The station is about 

40 miles downstream from the headwaters of the river and 

about 10 miles downstream from Lake Conroe. Since 1973, 

Lake Conroe has potentially affected streamflow in the West 

Fork San Jacinto River (U.S. Geological Survey, 2012b). The 

time period analyzed for this study is 1940–2009 (table 5). 

An upward trend for annual minimum daily discharge was 

detected at this station.

USGS streamflow-gaging station 08068500 Spring 

Creek near Spring, Tex. (Spring Creek near Spring station), 

Table 4. Monotonic trends in outflows from the Sabine and Neches River Basins in Texas, 1925–2009.

[USGS, U.S. Geological Survey; p-value, probability value; <, less than; --, no statistically significant trend was detected (p-value <0.05)]

USGS 

station 

number

USGS station name Time period

analyzed

Streamflow statistic

(discharge)

Kendall’s 

tau p-value Trenddirection

08030500 Sabine River near Ruliff, Tex. 1925–2009 Annual mean daily -0.02 0.79 --

08030500 Sabine River near Ruliff, Tex. 1925–2009 Annual maximum daily -0.05 0.49 --

08030500 Sabine River near Ruliff, Tex. 1925–2009 Annual minimum daily 0.29 <0.05 upward

08041000 Neches River at Evadale, Tex. 1922–2009 Annual mean daily 0.0005 1.0 --

08041000 Neches River at Evadale, Tex. 1922–2009 Annual maximum daily -0.16 <0.05 downward

08041000 Neches River at Evadale, Tex. 1922–2009 Annual minimum daily 0.4 <0.05 upward

08041000 Neches River at Evadale, Tex. 1922–1950 Annual mean daily 0.06 0.65 --

08041000 Neches River at Evadale, Tex. 1922–1950 Annual maximum daily 0.02 0.85 --

08041000 Neches River at Evadale, Tex. 1922–1950 Annual minimum daily 0.14 0.27 --

08041000 Neches River at Evadale, Tex. 1951–2009 Annual mean daily 0.23 <0.05 upward

08041000 Neches River at Evadale, Tex. 1951–2009 Annual maximum daily 0.04 0.67 --

08041000 Neches River at Evadale, Tex. 1951–2009 Annual minimum daily 0.46 <0.05 upward
0 2,000 4,000 6,000 8,000 10,000 12,000 14,000

1922

1924

1926

1928

1930

1932

1934

1936

1938

1940

1942

1944

1946

1948

1950

1952

1954

1956

1958

1960

1962

1964

1966

1968

1970

1972

1974

1976

1978

1980

1982

1984

1986

1988

1990

1992

1994

1996

Discharge, in cubic feet per second

Water year

B.A. SteinhagenLake Impoundment Sam Rayburn Reservoir

Impoundment

Annual mean daily discharge

EXPLANATION

Figure 4. Annual mean daily discharge for the period of record through 2009 for U.S. Geological Survey streamflow-gaging station 08041000 Neches 
Trends in Selected Streamflow Statistics 11

is about 2 miles northeast of Spring, Tex., and northwest of 

Houston. The station is about 50 miles downstream from the 

headwaters of this stream and about 15 miles upstream from 

Cypress Creek. Although development has been occurring in 

the Spring Creek watershed for several years, the streamflow 

in Spring Creek was considered reasonably natural for the 

period of record analyzed because there are no documented 

impoundments on Spring Creek (U.S. Geological Survey, 

2012b). The time period analyzed for this study is 1940–2009 

(table 5). Upward trends for annual mean daily and annual 

minimum daily discharges were detected at this station.

USGS streamflow-gaging station 08069000 Cypress 

Creek near Westfield, Tex. (Cypress Creek near Westfield 

station), is about 2 miles northwest of Westfield. The station 

is about 55 miles downstream from the headwaters and about 

11 miles upstream from West Fork San Jacinto River. The 

streamflow in Cypress Creek can be considered reasonably 

natural (U.S. Geological Survey, 2012b). The time period 

analyzed for this study is 1945–2009 (table 5). Upward trends 

in annual mean, maximum, and minimum daily discharges 

were detected at this station.

USGS streamflow-gaging station 08070000 East Fork 

San Jacinto River near Cleveland, Tex. (East Fork San 

Jacinto River near Cleveland station), is about 1 mile west 

of Cleveland, Tex. The station is about 35 miles downstream 

from the headwaters of the river and 20 miles upstream from 

Table 5. Monotonic trends in outflows from the Trinity and San Jacinto River Basins and associated creeks and bayous to Galveston 

Bay in Texas, 1925–2009.

[USGS, U.S. Geological Survey; p-value, probability value; <, less than; --, no statistically significant trend was detected (p-value <0.05)]

USGS 

station 

number

USGS station name

Time 

period

analyzed

Streamflow statistic

(discharge)

Kendall’s 

tau p-value Trenddirection

08066500 Trinity River at Romayor, Tex. 1925–2009 Annual mean daily 0.09 0.25 --

08066500 Trinity River at Romayor, Tex. 1925–2009 Annual maximum daily 0.12 0.10 --

08066500 Trinity River at Romayor, Tex. 1925–2009 Annual minimum daily 0.51 <0.05 upward

08068000 West Fork San Jacinto River near Conroe, Tex. 1940–2009 Annual mean daily 0.04 0.60 --

08068000 West Fork San Jacinto River near Conroe, Tex. 1940–2009 Annual maximum daily -0.06 0.43 --

08068000 West Fork San Jacinto River near Conroe, Tex. 1940–2009 Annual minimum daily 0.20 <0.05 upward

08068500 Spring Creek near Spring, Tex. 1940–2009 Annual mean daily 0.23 <0.05 upward

08068500 Spring Creek near Spring, Tex. 1940–2009 Annual maximum daily 0.10 0.23 --

08068500 Spring Creek near Spring, Tex. 1940–2009 Annual minimum daily 0.25 <0.05 upward

08069000 Cypress Creek near Westfield, Tex. 1945–2009 Annual mean daily 0.29 <0.05 upward

08069000 Cypress Creek near Westfield, Tex. 1945–2009 Annual maximum daily 0.20 <0.05 upward

08069000 Cypress Creek near Westfield, Tex. 1945–2009 Annual minimum daily 0.74 <0.05 upward

08070000 East Fork San Jacinto River near Cleveland, Tex. 1940–2009 Annual mean daily 0.09 0.25 --

08070000 East Fork San Jacinto River near Cleveland, Tex. 1940–2009 Annual maximum daily -0.01 0.88 --

08070000 East Fork San Jacinto River near Cleveland, Tex. 1940–2009 Annual minimum daily 0.18 <0.05 upward

08070500 Caney Creek near Splendora, Tex. 1945–2009 Annual mean daily 0.16 0.053 --

08070500 Caney Creek near Splendora, Tex. 1945–2009 Annual maximum daily 0.09 0.27 --

08070500 Caney Creek near Splendora, Tex. 1945–2009 Annual minimum daily 0.18 <0.05 upward

08073500 Buffalo Bayou near Addicks, Tex. 1946–2009 Annual mean daily 0.34 <0.05 upward

08073500 Buffalo Bayou near Addicks, Tex. 1946–2009 Annual maximum daily -0.19 <0.05 downward

08073500 Buffalo Bayou near Addicks, Tex. 1946–2009 Annual minimum daily 0.69 <0.05 upward

08076000 Greens Bayou near Houston, Tex. 1953–2009 Annual mean daily 0.59 <0.05 upward

08076000 Greens Bayou near Houston, Tex. 1953–2009 Annual maximum daily 0.29 <0.05 upward

08076000 Greens Bayou near Houston, Tex. 1953–2009 Annual minimum daily 0.83 <0.05 upward
0 5,000 10,000 15,000 20,000

1925

1927

1929

1931

1933

1935

1937

1939

1941

1943

1945

1947

1949

1951

1953

1955

1957

1959

1961

1963

1965

1967

1969

1971

1973

1975

1977

1979

1981

1983

1985

1987

1989

1991

1993

1995

Discharge, in cubic feet per second

Water year

Figure 5. Annual mean daily discharge for the period of record through 2009 for U.S. Geological Survey streamflow-gaging station 08066500 Trinity R
Trends in Selected Streamflow Statistics 13

Lake Houston. The streamflow in the East Fork San Jacinto 

River can be considered reasonably natural (U.S. Geological 

Survey, 2012b). The time period analyzed for this study is 

1940–2009 (table 5). An upward trend for annual minimum 

daily discharge was detected at this station.

USGS streamflow-gaging station 08070500 Caney Creek 

near Splendora, Tex. (Caney Creek near Splendora station), 

is about 8 miles west of Splendora, Tex. The station is about 

25 miles downstream from the headwaters of the stream and 

about 20 miles upstream from East Fork San Jacinto River. 

The streamflow in Caney Creek can be considered reasonably 

natural (U.S. Geological Survey, 2012b). The time period 

analyzed for this study is 1945–2009 (table 5). An upward 

trend for annual minimum daily discharge was detected at 

this station.

USGS streamflow-gaging station 08073500 Buffalo 

Bayou near Addicks, Tex. (Buffalo Bayou near Addicks 

station), is about 3 miles southeast of Addicks, Tex. The 

station is downstream from Barker and Addicks Reservoirs in 

the western part of Houston and is about 45 miles upstream 

from Galveston Bay. Barker Reservoir has regulated 

streamflow for the station over the entire period of record. 

Barker Reservoir was impounded in 1945 and Addicks 

Reservoir was impounded in 1948. Both reservoirs generally 

are empty and function as large detention reservoirs for floodpeak attenuation for the large and densely populated greater 

Houston area. The time period analyzed for this study is 

1946–2009 (table 5). Upward trends in annual mean daily and 

minimum discharges were detected at this station, whereas, a 

downward trend for the annual maximum daily discharge was 

detected. A time series of the annual mean daily discharge is 

shown in figure 6.

USGS streamflow-gaging station 08076000 Greens 

Bayou near Houston, Tex. (Greens Bayou near Houston 

station), is about 1 mile northeast of Houston. The station 

is about 25 miles downstream from the bayou’s headwaters 

and about 25 miles upstream from Buffalo Bayou. The 

streamflow in Greens Bayou is unregulated (U.S. Geological 

Survey, 2012b), but because the watershed of Greens Bayou 

has become increasingly developed since the station was 

installed, streamflow might deviate from the characteristics 

of reasonably natural flow. Konrad and Booth (2005, p. 157) 

reported that “urban development modifies the production and 

delivery of runoff to streams and the resulting rate, volume, 

and timing of streamflow” and identified “four hydrologic 

changes resulting from urban development: increased 

frequency of high flows, redistribution of water from base 

flow to stormflow, increased daily variation in streamflow, 

and reduction in low flow.” The time period analyzed for this 

study is 1953–2009 (table 5). Upward trends for annual mean, 

maximum, and minimum daily discharges were detected for 

the Greens Bayou near Houston. Tex., station. A time series of 

the annual mean daily discharge is shown in figure 7. Channel 

rectification occurred during 1974–75 (U.S. Geological 

Survey, 2012b); channel rectification is typically done by the 

U.S. Army Corps of Engineers (USACE) to improve channel 

conveyance and mitigate flood risks (U.S. Army Corps of 

Engineers, 2012).

Outflows to Middle Coast Gulf of Mexico 

(Brazos, San Bernard, and Colorado 

River Basins)

The Brazos, San Bernard, and Colorado Rivers flow 

directly into the Gulf of Mexico along the middle coastline of 

Texas. The Brazos and Colorado River Basins extend across 

Texas into New Mexico (fig. 1). The lower parts of these 

basins narrow as the rivers flow towards the Gulf of Mexico, 

particularly near about the 98th meridian. The San Bernard 

River Basin is much smaller and is located between the Brazos 

and Colorado River Basins. 

USGS streamflow-gaging station 08114000 Brazos River 

at Richmond, Tex. (Brazos River at Richmond station), is 

more than 500 miles downstream from the headwaters of the 

Brazos River in New Mexico and about 70 miles upstream 

from the Gulf of Mexico. Although it is about 300 miles 

upstream, Possum Kingdom Lake, which was impounded in 

1941, is the main reservoir affecting streamflow at the Brazos 

River at Richmond station (Dowell, 1964; U.S. Geological 

Survey, 2012b). There are additional reservoirs on the main 

stem and major tributaries of the Brazos River, including 

Lake Whitney and Waco Lake. The time period analyzed for 

this study is 1923–2009 (table 6). No trends were detected at 

this station.

USGS streamflow-gaging station 08117500 San Bernard 

River near Boling, Tex. (San Bernard River near Boling 

station), is about 5 miles northeast of Boling, Tex. The station 

is about 90 miles downstream from the headwaters and about 

45 miles upstream from the mouth with the Gulf of Mexico. 

The streamflow for the San Bernard River can be considered 

reasonably natural (U.S. Geological Survey, 2012b). The time 

period analyzed for this study is 1955–2009 (table 6). No 

trends were detected at this station.

USGS streamflow-gaging station 08162000 Colorado 

River at Wharton, Tex. (Colorado River at Wharton station), 

is more than 500 miles downstream from the headwaters 

in north Texas and about 65 miles upstream from the Gulf 

of Mexico. Although numerous reservoirs have been 

constructed along the Colorado River and many of its principle 

tributaries, the main reservoir for flood control and volume 

storage is Lake Travis, which is about 150 miles upstream 

from the Wharton station. The streamflow recorded by this 

station has been affected by reservoir operations (mostly 

by Lake Travis) over the entire period of record. The time 

period analyzed for this study is 1939–2009 (table 6). A 

downward trend in annual minimum daily discharge was 

detected at this station.
Figure 6. Annual mean daily discharge for the period of record through 2009 for U.S. Geological Survey streamflow-gaging station 08073500 Buffalo 

0

100200300400500600700800

1946

1947

1948

1949

1950

1951

1952

1953

1954

1955

1956

1957

1958

1959

1960

1961

1962

1963

1964

1965

1966

1967

1968

1969

1970

1971

1972

1973

1974

1975

1976

1977

1978

1979

1980

1981

1982

1983

1984

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

Discharge, in cubic feet per second

Water year

Annual mean daily discharge

EXPLANATION
050

100150200250

1953

1955

1957

1959

1961

1963

1965

1967

1969

1971

1973

1975

1977

1979

1981

1983

1985

1987

1989

1991

1993

1995

1997

1999

channel rectifiedwater years 1974-75

Discharge, in cubic feet per second

Water year

Annual mean daily discharge

EXPLANATION

Figure 7. Annual mean daily discharge for the period of record through 2009 for U.S. Geological Survey streamflow-gaging station 08076000 Greens 
16 Trends in Selected Streamflow Statistics at 19 Long-Term Streamflow-Gaging Stations Indicative of Outflows

Outflows to Lower Coast Gulf of Mexico 

(Guadalupe and Nueces River Basins and Rio 

Grande Basin)

The Guadalupe River at Victoria streamflow-gaging 

station 08176500 Guadalupe River at Victoria, Tex. 

(Guadalupe River at Victoria station), is about 250 miles 

downstream from the headwaters and about 30 miles upstream 

from the Gulf of Mexico. The time period analyzed for this 

station is 1936-2009, and streamflow recorded by this station 

has been affected by reservoir operations over the entire period 

of record (table 1) (Dowell, 1964). No trends were detected at 

this station.

USGS streamflow-saging station 08211000 Nueces River 

near Mathis, Tex. (Nueces River near Mathis station), is about 

4 miles southwest of Mathis, Tex. The station is about 200 

miles downstream from the headwaters and about 30 miles 

upstream from the Gulf of Mexico. Lake Corpus Christi is less 

than 1 mile upstream and has maintained a pool in some form 

throughout the entire period of record of the station (Dowell, 

1964; U.S. Geological Survey, 2012b). 

The Frio River is a major tributary to the Nueces River. 

Additional regulation of flow at the Nueces River near 

Mathis station is provided by Choke Canyon Reservoir, 

which is upstream from this station on the Frio River near 

the confluence of the Frio and Nueces Rivers (fig. 2). The 

time period analyzed for this study is 1940–2009 (table 7). 

Downward trends for annual mean and maximum daily 

discharges were detected at this station, whereas, an upward 

trend for annual minimum daily discharge was detected.

Streamflow-gaging station 08475000 Rio Grande near 

Brownsville, Tex. (Rio Grande near Brownsville station), is 

operated by IBWC and is more than 1,000 miles downstream 

from the headwaters and about 25 miles upstream from the 

Gulf of Mexico. Discharge at the Rio Grande near Brownsville 

station has been substantially affected by Falcon Reservoir 

since 1953 (Dowell, 1964; U.S. Geological Survey, 2012b). 

Falcon Reservoir is about 120 miles upstream from the station.

A time series of the annual mean daily discharge is shown 

in figure 8 and includes the timing of the Falcon Reservoir 

impoundment. The time periods analyzed were 1935–2009, 

1935–54, and 1955–2009 (table 7). Downward trends for 

annual mean and maximum daily discharges were detected at 

this station, whereas, an upward trend for annual minimum 

daily discharge was detected for 1935–2009. Additionally, 

downward trends for annual mean daily, maximum, and 

minimum discharges were detected at this station for the 

period 1935–54. No trends were detected at this station for the 

period 1955–2009.

Table 6. Monotonic trends in outflows from the Brazos, San Bernard, and Colorado River Basins in Texas, 1923–2009.

[USGS, U.S. Geological Survey; p-value, probability value; <, less than; --, no statistically significant trend was detected (p-value <0.05)]

USGS 

station 

number

USGS station 

name

Time period

analyzed

Streamflow statistic

(discharge)

Kendall’s 

tau p-value Trenddirection

08114000 Brazos River at Richmond, Tex. 1923–2009 Annual mean daily -0.01 0.88 --

08114000 Brazos River at Richmond, Tex. 1923–2009 Annual maximum daily -0.12 0.095 --

08114000 Brazos River at Richmond, Tex. 1923–2009 Annual minimum daily 0.07 0.35 --

08117500 San Bernard River near Boling, Tex. 1955–2009 Annual mean daily 0.14 0.12 --

08117500 San Bernard River near Boling, Tex. 1955–2009 Annual maximum daily 0.15 0.11 --

08117500 San Bernard River near Boling, Tex. 1955–2009 Annual minimum daily 0.14 0.12 --

08162000 Colorado River at Wharton, Tex. 1939–2009 Annual mean daily -0.02 0.78 --

08162000 Colorado River at Wharton, Tex. 1939–2009 Annual maximum daily 0.002 0.98 --

08162000 Colorado River at Wharton, Tex. 1939–2009 Annual minimum daily -0.22 <0.05 downward
Trends in Selected Streamflow Statistics 17

Table 7. Monotonic trends in outflows from the Guadalupe and Nueces River Basins, and the Rio Grande Basin in Texas, 1926–2009.

[USGS, U.S. Geological Survey; IBWC, International Boundary and Water Commission; p-value, probability value; <, less than; --, no statistically significant 

trend was detected (p-value <0.05)]

Station 

number Station name Agency 

Time 

period

analyzed

Streamflow

statistic

(discharge)

Kendall’s 

tau p-value Trenddirection

08176500 Guadalupe River at Victoria, Tex. USGS 1936–2009 Annual mean daily 0.11 0.16 --

08176500 Guadalupe River at Victoria, Tex. USGS 1936–2009 Annual maximum daily -0.006 0.94 --

08176500 Guadalupe River at Victoria, Tex. USGS 1936–2009 Annual minimum daily 0.02 0.75 --

08211000 Nueces River near Mathis, Tex. USGS 1940–2009 Annual mean daily -0.20 <0.05 downward

08211000 Nueces River near Mathis, Tex. USGS 1940–2009 Annual maximum daily -0.25 <0.05 downward

08211000 Nueces River near Mathis, Tex. USGS 1940–2009 Annual minimum daily 0.26 <0.05 upward

08475000 Rio Grande near Brownsville, Tex.1 IBWC 1935–2009 Annual mean daily -0.38 <0.05 downward

08475000 Rio Grande near Brownsville, Tex.1 IBWC 1935–2009 Annual maximum daily -0.49 <0.05 downward

08475000 Rio Grande near Brownsville, Tex.1 IBWC 1935–2009 Annual minimum daily 0.15 <0.05 upward

08475000 Rio Grande near Brownsville, Tex.1 IBWC 1935–1954 Annual mean daily -0.62 <0.05 downward

08475000 Rio Grande near Brownsville, Tex.1 IBWC 1935–1954 Annual maximum daily -0.35 <0.05 downward

08475000 Rio Grande near Brownsville, Tex.1 IBWC 1935–1954 Annual minimum daily -0.43 <0.05 downward

08475000 Rio Grande near Brownsville, Tex.1 IBWC 1955–2009 Annual mean daily -0.08 0.39 --

08475000 Rio Grande near Brownsville, Tex.1 IBWC 1955–2009 Annual maximum daily -0.17 0.064 --

08475000 Rio Grande near Brownsville, Tex.1 IBWC 1955–2009 Annual minimum daily 0.05 0.61 --

1

Streamflow data for Rio Grande near Brownsville, Tex., are currently (2012) operated and data published by the International Boundary and Water 

Commission (2012).
0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 9,000

1935

1937

1939

1941

1943

1945

1947

1949

1951

1953

1955

1957

1959

1961

1963

1965

1967

1969

1971

1973

1975

1977

1979

1981

1983

1985

1987

1989

1991

1993

1995

1997

Falcon Reservoir

Regulation

Discharge, in cubic feet per second

Water year

Figure 8. Annual mean daily discharge for the period of record through 2009 for U.S. Geological Survey streamflow-gaging station 08475000 Rio Gran
Summary 19

Summary

The major river basins in Texas include the Red River, 

Sabine River, Neches River, Trinity River, San Jacinto River, 

Brazos River, Colorado River, Guadalupe River, San Antonio 

River, Nueces River, and the Rio Grande. The Red River 

flows into Arkansas and then into Louisiana. The Sabine, 

Neches, Brazos, San Bernard, Colorado, Guadalupe, and 

Nueces Rivers and Rio Grande all flow directly into the Gulf 

of Mexico. The Trinity River, San Jacinto River, and Buffalo 

Bayou flow into Galveston Bay, which subsequently flows 

into the Gulf of Mexico. Fresh surface-water usage in Texas 

is about two-thirds of the total freshwater usage in Texas. As 

a result, studies of streamflow statistics, likely indicative of 

outflows from Texas, are useful in resource assessments, such 

as water-supply studies, as well as useful to hydrologists, 

biologists, and ecologists. Accordingly, the U.S. Geological 

Survey, in cooperation with the Texas Water Development 

Board, evaluated streamflow data from streamflow-gaging 

stations with more than 50 years of record that were active as 

of 2009.

The streamflow data for 18 selected U.S. Geological 

Survey and one International Boundary Water Commission 

streamflow-gaging stations were used to compute selected 

streamflow statistics for trend analysis. The outflows into 

Arkansas and Louisiana were represented by streamflow 

records from 3 stations, and outflows into the Gulf of Mexico 

were represented by streamflow records from 16 stations.

Monotonic trend analyses were performed on the 

following three annual streamflow statistics: (1) annual mean 

daily discharge, (2) annual maximum daily discharge, and 

(3) annual minimum daily discharge. Trends were determined 

using the nonparametric Kendall’s Tau test, which is useful for 

the detection of monotonic upward or downward trends with 

time. A positive Kendall Tau value indicates an upward trend 

and a negative Tau value indicates a downward trend. The 

p-value for the test is a measure of the statistical significance 

of the trend. Small p-values (less than 0.05) indicate a 

statistically significant trend for purposes of this study.

A total of 69 trend analyses by Kendall’s Tau were 

computed—19 periods of streamflow from USGS stations 

multiplied by 3 streamflow statistics (annual mean, maximum, 

and minumum discharge) plus 12 additional trend analyses 

because the periods of record for 2 stations were further 

analyzed for trends pre- and post-reservoir impoundment. 

Unless otherwise described, each trend analysis used the entire 

period of record for each station. The monotonic trend analysis 

detected 11 statistically significant downward trends, 37 

instances of no trend, and 21 statistically significant upward 

trends. Statistically significant trends follow for each station 

and organized by a logical grouping according to river system.

Outflows to Arkansas (Red and Sulphur River Basins)—A 

downward trend for annual maximum daily discharge was 

detected for the Red River at Index, Ark., station. An upward 

trend for annual minimum daily discharge was detected for the 

Sulphur River near Talco, Tex., station.

Outflows to Louisiana (Little Cypress Creek Basin)—

No trends were detected for the Little Cypress Creek near 

Jefferson, Tex., station.

Outflows to the upper coast of the Gulf of Mexico 

(Sabine and Neches River Basins)—An upward trend for 

annual minimum daily discharge was detected for the Sabine 

River near Ruliff, Tex., station. A downward trend for annual 

maximum daily discharge and conversely an upward trend for 

annual minimum daily discharge were detected for Neches 

River at Evadale, Tex., station for the period 1922–2009 

(period of record). No trends were detected for the Neches 

River at Evadale, Tex., station for the period 1922–50; 

however, upward trends for annual mean and minimum daily 

discharges were detected for the period 1951–2009. 

Outflows to the Gulf of Mexico through Galveston Bay 

(Trinity and San Jacinto River Basins, and associated Creeks 

and Bayous [Galveston Bay inflows])—An upward trend for 

annual minimum daily discharge was detected for the Trinity 

River at Romayor, Tex., station. An upward trend for annual 

minimum daily discharge was detected for the West Fork San 

Jacinto River near Conroe, Tex., station. Upward trends for 

annual mean and minimum daily discharges were detected for 

the Spring Creek near Spring, Tex., station. Upward trends in 

annual mean, maximum, and minimum daily discharges were 

detected for the Cypress Creek near Westfield, Tex., station. 

An upward trend for annual minimum daily discharge was 

detected for the East Fork San Jacinto River near Cleveland, 

Tex., station. An upward trend for annual minimum daily 

discharge was detected for the Caney Creek near Splendora, 

Tex., station. Upward trends in annual mean and minimum 

daily discharges were detected for the Buffalo Bayou near 

Addicks, Tex., station, whereas, a downward trend for the 

annual maximum daily discharge was detected. Upward 

trends for annual mean, maximum, and minimum daily 

discharges were detected for the Greens Bayou near Houston, 

Tex., station. 

Outflows to the middle coast of the Gulf of Mexico 

(Brazos, San Bernard, and Colorado River Basins)—No 

trends in streamflow were detected for the Brazos River 

at Richmond, Tex., or for San Bernard River near Boling, 

Tex., stations. A downward trend in annual minimum daily 

discharge was detected for the Colorado River at Wharton, 

Tex., station.

Outflows to the lower coast of the Gulf of Mexico 

(Guadalupe and Nueces River Basins, and Rio Grande 

Basin)—No trends were detected for the Guadalupe River 

at Victoria, Tex., station. Downward trends for annual mean 

and maximum daily discharges were detected for the Nueces 

River near Mathis, Tex., station, whereas, an upward trend for 

annual minimum daily discharge was detected. Downward 

trends for annual mean and maximum daily discharges were 

detected for the Rio Grande near Brownsville, Tex., station, 

whereas, an upward trend for annual minimum daily discharge 

was detected for 1935–2009. Additionally, downward trends 

for annual mean, maximum, and minimum daily discharges 

were detected for the Rio Grande near Brownsville, Tex., 

station for the period 1935–54. No trends were detected for 
20 Trends in Selected Streamflow Statistics at 19 Long-Term Streamflow-Gaging Stations Indicative of Outflows

the Rio Grande near Brownsville, Tex., station for the period 

1955–2009. 

Annual mean daily discharge is especially diagnostic 

because this statistic can be used to compute annual mean 

discharge over long periods of time and represents the 

expectation of the annual volume of water measured by 

a station. Trends in annual mean daily discharge can be 

summarized as follows: (1) annual outflow volumes from 

Texas into Arkansas and Louisiana as well as the Gulf of 

Mexico (upper Texas coast) lack statistically significant 

trends; (2) annual inflows into Galveston Bay (Houston 

metropolitan area) show upward trends for four of eight 

stations; (3) annual outflow volumes from the Brazos River 

(east of Houston metropolitan area) to the Guadalupe River 

lack statistically significant trends; and (4) annual outflow 

volumes from the two most westerly stations on the Nueces 

River and Rio Grande show downward trends.

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Barbie and Wehmeyer—Trends in Selected Streamflow Statistics at 19 Long-Term Streamflow-Gaging Stations Indicative of Outflows—SIR 2012–5182 Barbie and Wehmeyer—Trends in Selected Streamflow Statistics at 19 Long-Term Streamflow-Gaging Stations Indicative of Outflows—SIR 2012–5182