Historical Changes in Streamflows, Channel Morphology, and Riparian 

Vegetation of the Rio Grande Downstream of Brownsville, Texas 

The Rio GrandefRio Bravo drains an area of more than 

440,300 square kilometers of Mexico and southwestern 

United States (Bartlett. 1984). The Rio Grande flows for 

3,000 kilometers from its headwaters in the San Juan Mountains of southern Colorado to the Gulf of Mexico downstream of Brownsville, Texas. The "Rio," as it is often 

called, drains the southern Rocky Mountains of Colorado 

f'\FW MEXfC() 

MEXICO 

COLORADO 

LOCATION MAP 

and northern New Mexico; the vast Chihuahuan Desert of 

southern New Mexico, northern Mexico, and southwestern 

Texas; ami the subtropical lower valley of southern Texas 

(fig. I). 

llJ3' 

Mean annual flow in the Rio Grande ranges from 6.0 million cubic meters in the snowmelt-dominated segments of 

EXPLANATION 

... Streamflow-golging station 

1 Rio Crande al Fori Quitman 

2 Rio Grande 31 Johnson Ranch 

3 Rio Crande n('i1 r Brownsville 

" 

I If, \ , \/lJ,L 

50 100 KilOMETERS , 

Figure 1. Rio GrandefRio Bravo Basin and location of selected streamflow-gaging stations and segment downstream of Brownsville, 

Texas. 

u.s. Department of the Interior 

U.S. Geological Survey 

USGS Fact Sheet 074-00 

July 2000 
2,000 

0 Rio Grande at Fort Quitman 1,800 

1,600 Z 1,400 0 

U 1,200 

w 1,()()() 

(f) 800 

a: 600 

w 4()() 

a. 200 

(f) 0 a: ·200 W 

t- 2,()()() w 

::; 1,800 

U 1.600 

1,400 iii 1,200 => 

U 1,()()() 

;::: BOO 

600 

W 400 

(!J 200 

a: 0 

Rio Grande at Johnson Ranch near Castolon 

I J_Jl.L I J ,I, ,I.IL ~ •. 

I 

'-~ /I • l J. , .,d, J ~." A I I }(I, 11 

« -200 

I 

U 2,000 (f) 1,800 is 1,600 

Rio Grande near Brownsville 

Z 1,400 

« 1,200 W 1,000 ::; BOO 

>- 600 II -' 400 1 ' III, 

, II Completion of Falcon International Darn/Reservoir 

« 200 0 0 

I ' I (\ , l I' 1,- _!J~ A-.. ..c 

·200 

'" '" 0> 

0 "' 0 "' 0 "' 0 "' 0 "' 0 "' '" " " "' "' '" '" ... ... '" '" 0> 0> 0> 0> 0> 0> ~ 0> ~ 0> 0> '" '" '" '" '" 

Figure 2. Hydrographs showing daily mean streamflow for three gaging stations on the Rio Grande, 

the river in northern New Mexico to 22 million cubic meters 

near its mouth at the Gulf of Mexico, The Rio Grande is 

sometimes thought of as a river that has been disconnected 

in the middle. Downstream of EI Paso, between Fort 

Quitman and Presidio, Texas, the river can be totally dry, 

Just upstream of Presidio, the RIO Conchos, which drains 

68,400 square kilometers of the Sierra Madre Occidental 

of Mexico. gives new life to Ihe river. As much as 75 perce11l 

of the flow of the Rio Grande downstream of Presidio 

comes from the Rio Conchos, 

The Rio Grande downstream of Albuquerque. New 

Mexico, is a regulated river. Four major reservoirs impound 

the Rio Grande; Elephant BUlle and Caballo Reservoirs in 

southern New Mexico and Amistad and Falcon International 

Reservoirs along the Texas border with Mexico. These reservoirs, which were construe ,ed for water storage and flood 

control, have provided a continuous source of water for irrigation, allowing year-round farming in the Rio Grande Valley of Mexico and the United States. Irrigation water for 

agriculture accounts for 80 percent of surface-water use in 

the U.S. part of the river basin (Texas Natural Resource 

Conservation Commission, 1996). The 1906 Allocation 

Treaty ensures that the United States will deliver 74 cubic 

hectometers of irrigation water annually to Mexico by way 

of International Dam between EI Paso and Ciudad Juarez 

(Meuller, 1975). The J11lernational Dam is the major flowdiversion structure on the Rio Grande. However, many 

smaller diversion dams carry water to crops in the flood2 

plain of the river. and numerous drainage ditches return this 

water, often more saline, to the main channel of the river. 

The International Boundary and Water Commission 

(mWC) has undertaken channelmodificalion of the Rio 

Grande over much of its length between Elephant Butte 

Reservoir and Fort Quitman. Between EI Paso and Fort 

Quitman, the river has been straightened from 249 to 142 

kilometers in lenglh. 

About 10 million people live in the Rio Grande Basin; the 

fastest growing areas are the EI Paso and Ciudad Juarez 

metropolitan area and the McAllen to Brownsville corridor 

in the lower Rio Grande Valley of South Texas (Texas Natural Resource Conservation Commission, 1996). In 1956, 

Mexico initiated the Border Induslrialization Program, often 

referred to as the maquiladora program. This program, 

which provides incentives for foreign companies to construct factories in Mexico, has fostered the establishment of 

over 900 maquiladoras along the inlernalional border 

between Texas, New Mexico, and Mexico. 

Population growth and agricultural and industrial development along the main stem of the Rio Grande, particul arly 

the segment of the river that forms the international boundary between Texas and Mexico, has caused changes in 

waler-quality conditions and in historical flows of the river. 

Changes to a river's flow patterns, and more specifically, 

changes in the timing and magnitude of a river's peak or 

higher flows have important consequences for channel 
1,000 1.000 I 

0 1936-50 (pre-Falcon Reservoir) 1956-95 (post-Falcon Reservoir) Z 900 900 ' 0 

U 

w 

VJ 

a: 

w 800 800 a. 

VJ 

a: 

UJ 

fUJ 700 I 700 , " u 

iii 

::J 

U 600 ~ 600 ~ 

ui 

'" a: 

'" 500 I 500 I 

U 

VJ 

0 

Z 400 f 400 '" UJ 

" >- .J 

<' 300 ' 300 0 

200 200 ' 

100 100 

0 0 

0 0.1 0.2 0.3 0.4 0 .5 0.6 0.7 0.8 0.9 1.0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 

PROPORTION OF DISCHARGE MEASUREMENTS 

Figure 3. Distribution of daily mean discharge at the Rio Grande near Brownsville before and after construction of Falcon International 

Reservoir. 

features and riparian zone vegetation (Leopold. 1994). 

These changes in channel and riparian zone features will in 

tum influence the makeup and status of riparian and aquatic 

biological communities. This report presems the results of 

an assessment of changes in general streamflow pallems at 

selected locations on the Rio Grande between EI Paso and 

Brownsville and of changes in channel and riparian features 

downstream of Brownsville that are thought 10 have been 

caused by changes in streamflows. 

Historical Changes in Streamflows in the Rio 

Grande 

Three locations (fi g. I) on the Rio Grande between EI 

Paso and Brownsville were selected to investigate changes 

in historical streamflows. The streamflow-gaging stations 

operated by the I B WC at each of these locations have been 

monitoring daily mean flows since the early 1920s or 1930s. 

These stations are Rio Grande at Fort Quitman, 2.4 river 

kilometers downstream of old Fort Quitman; Rio Grande at 

Johnson Ranch near CaslOlon, 2.2 river kilometers upstream 

of the old Johnson Ranch headquarters in Big Bend 

National Park; and Rio Grande near Brownsville, 10.9 river 

3 

kil ometers downstream of the International Bridge between 

Brownsville and Matamoros, Mexico. An equivalent period 

of streamflow record, 1936-98, was selected to characterize 

streamflow at each station. 

Daily mean streamflow at the FOlt Quitman station did 

not exceed 200 cubic meters per second during 1936-98 

(fi g. 2). This is partially a result of upstream reservoirs in 

New Mexico regu lating flows. In contrast, daily mean 

streamflow is as great as 1,800 cubic meters per second at 

the Johnson Ranch station. which is downstream of the confluence with the Rio Conch os (fi g. I). Flows at the Johnson 

Ranch station are more seasonal than flows at the Fort Quitman station. with increased flows in latc summcr corresponding to the onset of monsoonal rains in the Chihuahuan 

Desert. These seasonal flows refl ect the flow of the Rio 

Conchos, which is less regulated than the Rio Grande above 

EI Paso. The streamflow record for the station just downstream from Brownsville (fi g. 2) shows that the variability in 

flows decreased substantially after the completion of Falcon 

International Reservoir in 1958. In particular, the proportion 

of flows greater than 100 cubic meters per second decreased 
1950 1995 

Figure 4. 1950 and 1995 aerial photographs of part of segment of Ihe Rio Grande downstream of Brownsville, Texas. 

from a prop0l1ion of 0.3 or 30 percent during 1936-50 to a 

proportion of 0.1 or 10 percent during 1958- 95 (fig. 3). 

The segment of the Rio Grande from downstream of 

Falcon Reservoir to the mouth of the Rio Grande at the 

Gulf of Mexico is bordered by tracts of undeveloped riparian and floodplain lands that are part of the U.S. Fish and 

Wildlife Service (USFW) complex of wildlife refuges and 

the National Audubon Society Sabal Palm Preserve. These 

lands contain some of the last stands of native riverside vegetation such as the sabal palm. However, these native stands 

are being displaced in many areas by more aggressive exotic 

species such as African bermuda and salt cedar (Larry Ditto, 

U.S. Fish and Wildlife Service. ora l commun .. 1999). 

Changes in streamflows in the Rio Grande, specifically the 

lack of higher out-of-bank flows. can be an important cause 

of the encroachment of these exotic species. 

Historical Changes in Channel and Riparian 

Features Downstream of Brownsville, Texas 

A segment of the Rio Grande downstream of Brownsville 

(fig. I) was selected to characterize historical changes in 

channel and riparian features that might have been caused 

by changes in stream flows. The segment extends about 38 

kilometers from just upstream of the IBWC gaging station 

near Brownsville to just downstream of the Palmito Hill 

Civil War historical site (inset, fi g. I). Aerial photographs 

4 

(scale I :20,000) taken in 1950 by the Soil Conservation Service (now the Natural Resources Conservation Service) and 

U.S. Geological Survey digital orthophoto quadrangles 

(DOQs) derived from 1995 National Aerial Photography 

Program aerial photographs (scale I :40,000) were used to 

compare channel and riparian features (fig. 4). The 1950 

aerial photographs were rectified to match the scale of the 

1995 DOQs for comparison. The difference between daily 

mean discharge when the 1950 and 1995 aerial photographs 

were taken was only 0.5 cubic meter per second: the effect 

of this difference on channel features in the photographs, at 

this scale, is thought to be negligible. 

Various channel and vegelalive-cover measurements 

were made to assess changes along the Rio Grande during 

1950-95 (table I). The vegetation score for point bars was 

based on assigning values from 1.0 to 5.0 according to the 

area of the bar covered by vegetation. For example, a vegetation score of 1.0 indicates vegetation coverage of 0 to 25 

percent of the bar, and a score of 5.0 indicates coverage of 

75 to 100 percent. These measures were determined using 

geographic information system (GIS) software. 

The largest changes in channel and lipari an features in 

the segment are in liparian vegetation, point-bar area and 

vegetation, and wetted channel width (fig. 5. table I). 

Between 1950 and 1995, the frequency of breaks in riparian 

vegetation increased by an average of 35 percent; however, 
the length of these breaks increased by an average of more 

than 3UO percent. The average width of riparian vegetation 

decreased by about 57 percent on the U.S. bank and about 

47 percent on the Mex ican bank. An inc rease in the number 

and extent of breaks in riparian vegetation might have been 

caused by land-use practices including the clearing of vegetation to increase land available for cultivation. However, a 

decrease in the intensity and frequency of hi gher flows 

might have caused a reduction in recharge to the alluvial 

aquifer, thus lowe ring the alluvial water table, and similarly 

reduCing the density and extent of riparian vegetation. 

The average point-bar area increased by II I percent. and 

the average score of vegetation coverage of the same point 

bars increased by 230 pe rcent. The average wetted chan nel 

width decreased by 32 percent. A decrease in the frequency 

of higher river flows (such as out-of-bank flows) and the 

resulting increase in rooted vegetation a re probably the 

cause of channel narrowing and the stabilization of point 

bars. 

These changes in channel and riparian features in this 

segment of the Rio Grande downstream of Brownsville are 

further illustrated by comparing the 1950 aerial photograph 

to the 1995 aerial photograph for a part of the segment studied (fig. 4). Point bars in the 1950 photograph are primarily 

unvegetated sand. The same point bars in the 1995 photograph have little or no sand visible and are largely covered 

with vegetation. The channel in the 1995 photograph is visibly narrowe r and is more simplified Ulan the channel in the 

1950 photograph. 

Many native species of riparian vegetation require seasonal flooding to genninate, and the pe riodic flooding of the 

riparian zone affects soil chemistry through the import and 

removal of organic matte r, in the reple nishment of mineral 

resources, and in maintaining a large flux of energy and 

nutrients in the riparian zone (Mitsch and Gosselink, 1993). 

Riparian zone vegetation provides important nesting and 

feed ing habitat for resident and mi gratory birds and often 

provides the only substantial cover for other wildlife, particularly in more arid regions. This report provides evidence of 

a decreasing trend in higher flow events (which mi ght have 

caused the narrowing of the channel) and increased vegetative coverage leading to more stable channel bars and a less 

continuous riparian buffer zone paralleling the rive r in this 

segment of the Rio Grande downstream of Brownsville, 

Texas. 

Table 1. Channel and riparian features for the segment of the Rio Grande downstream of Brownsville, Texas 

1m, meters, m2

• squ<.tre meters I 

Feature 1950 1995 Percent change 

Linear segment length ( 111 ) 19,193 19,18 1 -0.063 

Curvi linear segment length (m) 59,323 60.847 2.57 

Sinuosity of segment 3.09 3. 17 2.59 

Left bank IU.S. llength (m) 57,770 57,922 .263 

Right bank IMex ico llength (m) 57.625 58,615 1.72 

Length of hreaks in riparian vegetation, left bank I U.S .I (m) 6,536 26.70t 309 

Length of breaks in riparian vegetation, right bank I Mexico l (m) 4,340 17,834 3 t I 

Frequency of breaks in riparian vegetation. left bank I U.S.] 43 56 30.2 

Frequency of breaks in riparian vegetation, right bank IMexico) 62 87 40.3 

Average width of riparian vegetation, left bank [U.S.I (m) 32.4 13.9 - 57. 1 

Average width of riparian vegetation, right bank IMexico l (m) 22.5 12.0 -46.7 

Average wetted channel width (m) 55.9 38.0 - 32.0 

Ave rage area of point bars (m2) 10,153 2 1.44 1 I II 

Ave rage score of vegetation coverage of point hars 1.0 3.3 230 

5 
36.000 75 

3.5 

_ Area of point bar _ Wetted channel width _ Point-bar vegetation score 70 _ Frequency of vegetalion breaks 

30.000 3.0 IZ 

W 

::; 

65 '" w <I) <I) 

a: a: 2.5 w w w a: I- > 24,000 0 w 0 u '" <I) a: <I) <: 60 '" '" Z '" OJ 0 :£ w I- 2.0 ;= I- a: z '" 0 OJ 

0 I- ~ Z <L 18,000 w ~ 

55 1 

0 .... '" w ;= w 0 > z ;0 

'" z a: '" w w 1.5 '" '" a: OJ I W '" e- u > 0 50 z w .... 12,000 0 1= 0 <L >- W u 1.0 ~ z 45 w 

=> 

0 

w 

a: 6,000 .... 0.5 40 

0 1950 1995 0 35 

1950 1995 

Figure 5. Channel and riparian zone features for segment of the Rio Grande downstream of Brownsville, Texas. 

References 

Banlett, R.A., 1984, Rolling rivers-An encyclopedia of 

America's rivers: New York, McGraw-Hili, 398 p. 

Leopold. L.B., 1994, A view of the river: Cambridge, Mass., 

Harvard University Press. 298 p. 

Meuller, J.E., 1975, Restless river: EI Paso. Texas Western 

Press, 153 p. 

Mitsch, W,J., and Gosselink, J.G., 1993. Wetlands (2<1 ed.): 

New York, Van Nostrom Reinhold, 722 p. 

Texas Natural Resource Conservation Commission, 1996, 

1996 Regional assessment of water quality in the Rio 

Grande Basin: Border Environmental Assessment 

Team, SFR- 041 , 74 p. 

-(;, u.s. GOVERNMENT PRINTING omCE: 2000 - 513-089 / 52011 Region No.8 

6 

-J. Bruce Moring and Rita Setser 

Any use vf trade, product, or finn names is lor descriptil1e 

purposes ollly alld does 1101 imply endorsement by the 

U.S. Government. 

For more information, please contact: 

Districi Chief 

U.S. Geological Survey 

8027 Exchange Dr. 

Auslin. TX 78754-4733 

E-mail: dc_tx @usgs.gov 

Phone: (512) 927- 3500 

FAX: (5 12) 927- 3590 

World Wide Web: http://tx.usgs.gov/