Printed on recycled paper

Fact Sheet 2016–3027

April 2016

U.S. Department of the Interior

U.S. Geological Survey

in Texas, with 5 of these cities exceeding 

their previous record for the month of 

May (Texas A&M University, 2015). 

The onset of abundant precipitation in May 2015 resulted in the NWS 

flood stage being exceeded at USGS 

streamgages on numerous rivers. The 

widespread and prolonged nature of 

the flooding was unusual; most flood 

events in Texas are localized, typically 

affecting only one or two river basins 

and generally lasting only a few days 

(Liscum and East, 1994; Raines and 

others, 1998; Slade and Persky, 1999; 

Winters, 2012). With the exception of 

the Rio Grande, flooding was widespread 

in all of the major rivers in Texas during 

May–June 2015 (fig. 1). 

3.37 inches, with the previous record 

average monthly precipitation reported as 

6.66 inches during June 2004 (National 

Oceanic and Atmospheric Administration, 

2016). The Office of the Texas State 

Climatologist compiled monthly precipitation amounts for 19 selected cities 

throughout Texas and for 1 city in 

Louisiana; the total monthly precipitation 

amounts exceeded the monthly normal 

precipitation for 18 of the 19 selected cities 

Figure 1. Major rivers in Texas and U.S. Geological Survey streamflow-gaging stations where 

direct measurements of discharge were made during the period from May 1 to June 15, 2015.

Pecos River

Sulphur River

Red River

Sabine Rive

N

r

eches River Cypress Creek

Colorado River

Trinity River

San 

Jacinto 

River

Brazos River Canadian River

Nueces River

San Antonio River

Guadalupe River

Lavaca River

Blanco River

GULF OF MEXICO

Rio Grande

95°00' 97°30'

102°30' 100°00'

105°00'

35°00'

32°30'

30°00'

27°30'

Base modified from Texas Department 

of Transportation digital data

Lambert Conformal Conic projection

North American Datum of 1983

EXPLANATION

U.S. Geological Survey

 streamflow-gaging station

0 100 200 KILOMETERS

0 100 200 MILES

As a Federal science agency within 

the Department of the Interior, the 

U.S. Geological Survey (USGS) collects 

and disseminates streamflow stage and 

discharge information (http://www.usgs.

gov/water/) along with other types of 

water information as a major part of its 

Water mission area (http://water.usgs.gov/

about_WRD.html). Data collected at 

USGS streamflow-gaging stations (hereinafter referred to as “streamgages”) are used 

for a variety of purposes including flood 

warning, engineering design, management 

of water resources, and scientific research. 

During flood events, the need for 

timely, accurate, and complete streamflow 

data is underscored because these data 

are relied on by local, State, and Federal 

emergency management personnel for 

flood response purposes. For example, the 

National Weather Service (NWS) uses the 

data from USGS streamgages to develop 

flood forecasts for specific locations 

on a river (National Weather Service, 

2016). Tasks that the USGS performs in 

response to floods include monitoring the 

operation of gages and responding to any 

interruptions in data collection, calibrating 

and verifying stage-discharge ratings, and 

documenting extreme events including 

peak stage and peak discharge. 

Frequent, severe storms during 

May and June 2015 caused widespread 

flooding in Texas. By various measures, 

the storms that caused the flooding were 

extreme and persistent. May 2015 was 

the wettest month on record for Texas, 

with a statewide average precipitation 

of 9.06 inches (National Oceanic and 

Atmospheric Administration, 2016). In 

comparison, the long-term statewide 

average monthly precipitation is 

U.S. Geological Survey Response to Flooding in Texas, 

May–June 2015

Stage is the height of the surface of a stream measured in reference to the 

known elevation of an arbitrary datum. The stage is used to compute streamflow 

(discharge)—the total volume of water that flows past a specific point on a 

stream in a given period of time (commonly expressed as cubic feet per second). 
Figure 3. U.S. Geological Survey streamflow-gaging station 08171000, Blanco River at Wimberley, Texas, before and after the flood of 

May 23–24, 2015. A, The streamflow-gaging station in 2011. B, The streamflow-gaging station after being damaged by the flood (the new, 

temporary gage is visible in the upper right).

Figure 2. Recorded stage from May 23 to May 27, 2015, at U.S. Geological Survey 

streamflow-gaging station 08171000, Blanco River at Wimberley, Texas.

0 4 8 12 16 20 0 4 8 12 16 20 0 4 8 12 16 20 0 4 8 12 16 20 0 4 8 12 16 20 0

May 23, 2015 May 24, 2015 May 25, 2015 May 26, 2015 May 27, 2015

Time, in hours

0

5

10

15

20

25

30

35

40

45

50

Stage, in feet

Measured stage, in feet

EXPLANATION

National Weather Service flood stage = 13.0 feet

an increase of approximately 35 ft from 

the 5-ft base-flow stage before the river 

began to rise (fig. 2). When the stage 

reached 40 ft, inundation and flood 

damage caused data collection by the 

gage to cease; the gap in stage data in 

figure 2 corresponds to the period when 

the gage was inundated. Independent 

high-water marks, surveyed after 

the event, showed that water rose an 

additional 5 ft (after gage inundation) 

and reached a peak stage of 44.90 ft; 

for reference, the NWS flood stage is 

13.0 ft (fig. 2). In response to the damage, 

on the following day (May 25, 2015), 

a new, temporary gage was installed 

at the location and began transmitting 

data (fig. 3).

The missing 2 percent of the 

instantaneous unit values were caused 

by a variety of issues. The most common 

issues were equipment malfunction and 

the damage or destruction of streamgages 

caused by flooding. On the Blanco River, 

which underwent some of the most severe 

flooding, two streamgages were destroyed 

by high flows. The gage shelters that 

were destroyed on the Blanco River had 

been installed at elevations appreciably 

higher than the stage elevation associated 

with previous flood peaks but, because 

of the extreme nature of this event, were 

inundated by the rapid rise in stage. At 

USGS streamgage 08171000, Blanco 

River at Wimberley, Tex., in just 3 hours 

the river rose to a stage of 40 feet (ft)—

Operation of Gages 

During the May–June 2015 flood 

event, the USGS provided real-time 

stage and stream discharge data from 

approximately 490 USGS streamgages 

in Texas. In addition to streamgages, 

the USGS operates gages to collect 

water-quality, reservoir, and groundwater 

data throughout Texas. Although this 

fact sheet focuses on severe storms and 

resulting streamflow, the data collected 

by all of these gages are useful for evaluating changes in hydrologic conditions 

resulting from flood events and must be 

reliable, accurate, and timely.

Most USGS gages are programmed 

to record data every 15 minutes and 

to transmit data via the Geostationary 

Operational Environmental Satellite 

(GOES) system every 60 minutes. One 

metric of the reliability of the USGS 

network of different types of gages 

during events such as the May–June 2015 

flood event can be calculated as the 

number of instantaneous unit values 

available for each gage during the event 

divided by the total number of possible 

instantaneous unit values for that gage. 

From May 1 through June 30, 2015, 

this metric of reliability was determined 

for a combined 634 USGS streamflow, 

water-quality, reservoir, and groundwater 

gages operated by the USGS in Texas. 

Approximately 98 percent of the possible 

instantaneous unit values from May 1 

through June 30, 2015, were recorded, 

transmitted, and entered into the USGS 

National Water Information System 

(http://waterdata.usgs.gov/tx/nwis/nwis). 

A B
Figure 5. Example of a shift-adjusted stage-discharge rating curve.

Stage, in feet

10

11

12

13

23

33

43

Discharge, in cubic feet per second

10,000 20,000 50,000 100,000 200,000

Rating 11.0

294 295 296 297

295

Existing stage-discharge rating

Shift-adjusted stage-discharge rating

Measurement made during period 

 (May 1, 2015, to June 25, 2015)

EXPLANATION

Highest 10 measurements made

 during last 10 years

Measurement made during 

 last 2 years

Figure 4. Time-series graph of discharge and discrete discharge measurements at 

U.S. Geological Survey streamflow-gaging station 07312500, Wichita River at Wichita 

Falls, Texas, for May 5–June 10, 2015.

including the quality of the discharge 

measurement, the difference between the 

discharge measurement and the predicted 

value from the existing stage-discharge 

rating, and physical examination of 

control conditions during the measurement (Turnipseed and Sauer, 2010). 

Extending Stage-Discharge 

Rating Curves

Discharge measurements made 

during flood events are commonly used 

to extend stage-discharge rating curves 

(fig. 6). An extension of the stagedischarge rating curve is required when 

the existing rating curve does not include 

a stage-discharge combination that occurs 

during a large flood event. In this situation, 

it becomes important to make discharge 

measurements as close to the peak stage 

as possible so that the extension of the 

stage-discharge rating curve is based on 

measured stage and discharge values.

of the same material during a period 

of high flow. Discharge measurements 

are used to verify if the existing stagedischarge rating accurately reflects 

current conditions or if a change to the 

control conditions, as described above, 

has occurred. In those instances when 

the discharge measurement sufficiently 

verifies the existing stage-discharge 

rating, the rating remains in effect and 

is used directly to compute discharge. In 

those instances when a significant change 

to the control conditions is identified, 

however, the stage-discharge rating must 

be adjusted. The USGS applies temporary 

“shift adjustments” (Rantz and others, 

1982), which are based on recent discrete 

discharge measurements, to the stagedischarge rating to represent the changes 

in control conditions (fig. 5). 

To determine whether a change to the 

control conditions is significant and a shift 

adjustment to the stage discharge rating is 

required, various factors are considered, 

Verifying Stage-Discharge Ratings

At most streamgages, stage data are 

collected and used to estimate streamflow 

on a continuous basis (a few gages are 

designed to collect only specific types of 

flow information, such as peak stage). For 

the continuous streamflow gages, a stagedischarge rating is developed, which 

establishes the relation between the full 

range of stage and streamflow at a given 

gage. To develop and verify the rating, 

discrete measurements of discharge 

are made, with corresponding stream 

stage measurements, over the range of 

expected flow (Rantz and others, 1982). 

Discrete discharge measurements involve 

making direct measurements of velocity, 

depth, and width and applying standard 

computation methods as described in 

Turnipseed and Sauer (2010).

Because the relation between stage 

and streamflow can change gradually or 

rapidly as changes to the channel occur, the 

stage-discharge rating is regularly updated. 

It is important to make discharge measurements during periods of high flow to 

maintain accurate stage-discharge ratings. 

From May 1 to June 15, 2015, which 

is when most of the flooding occurred, 

the USGS made 739 discrete discharge 

measurements at 420 streamgages (fig. 1). 

For comparison, during the same period

in 2014, when high flow related to 

flooding was rare, 590 measurements 

were made at 388 streamgages. 

Because most flood events in 

Texas are typically of short duration 

(1–2 days), the opportunity to make 

discharge measurements during high flow 

is brief. The extended duration of the 

May–June 2015 flood event allowed for 

the collection of more discharge measurements at many streamgages where additional verification of the stage-discharge 

rating was desired. Figure 4 depicts an 

example of the collection of several 

discrete measurements of discharge during 

high flow at USGS streamgage 07312500, 

Wichita River at Wichita Falls, Tex.

Shifting Stage-Discharge 

Rating Curves

Over time, the conditions that 

control the stage-discharge relation at a 

streamgage can change. An example of 

this would be the deposition of sediment 

or debris on the low-water control (bed 

of the stream channel) during a period 

of low flow followed by the removal 

Discrete discharge measurement

EXPLANATION

10

20

50

100

200

500

1,000

2,000

5,000

10,000

Discharge, in cubic feet per second

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 1 2 3 4 5 6 7 8 9 10

2015 May June
ISSN 2327-6916 (print)

ISSN 2327-6932 (online)

http://dx.doi.org/10.3133/fs20163027

Publishing support provided by 

Lafayette Publishing Service Center

By Jeffery W. East

Texas A&M University, 2015, Climatic bulletins—May 2015: Texas A&M University, 

Office of the State Climatologist, accessed 

July 12, 2015, at http://climatexas.tamu.

edu/climatic-bulletins/may-2015/.

Turnipseed, D.P., and Sauer, V.B., 2010, 

Discharge measurements at gaging stations: 

U.S. Geological Survey Techniques and 

Methods, book 3, chap. A8, 87 p. [Also 

available at http://pubs.usgs.gov/tm/tm3-a8/.]

 Winters, K.E., 2012, Floods in central 

Texas, September 7–14, 2010: Texas 

Water Journal, v. 3, no. 1. [Also available

at https://journals.tdl.org/twj/index.php/

twj/article/view/3292.]

National Weather Service, 2016, National 

Weather Service reference guide: 

Accessed February 16, 2016, at 

http://www.crh.noaa.gov/Image/

mkx/pdf/NWS_Reference_Guide.pdf.

Raines, T.H., Asquith, W.H., and Brown, D.S.,

1998, Floods in south-central Texas, 

June 1997: U.S. Geological Survey Fact 

Sheet 053–98. [Also available at https://

pubs.er.usgs.gov/publication/fs05398.]

Rantz, S.E., and others, 1982, Measurement 

and computation of streamflow: U.S. Geological Survey Water-Supply Paper 2175, 

v. 2, 631 p. [Also available at http://pubs.

usgs.gov/wsp/wsp2175/html/wsp2175_

vol2.html.] 

Slade, R.M., and Persky, Kristie, 1999, 

Floods in the Guadalupe and San Antonio 

River Basins in Texas, October 1998: 

U.S. Geological Survey Fact Sheet 147–99, 

4 p. [Also available at http://pubs.usgs.gov/

fs/FS-147-99/.] 

97°30' 95°00'

102°30' 100°00'

105°00'

35°00'

32°30'

30°00'

27°30'

EXPLANATION

Period of record greater

 than 10 years

Period of record 

 5 to 10 years

Period of record less

 than 5 years

0 100 200 KILOMETERS

0 100 200 MILES

An example of a rating curve that was 

extended on the basis of three discharge 

measurements (21, 22, and 23) made 

during the May–June 2015 flood event is 

depicted in figure 6. Measurements 21–23 

were used to extend the rating from 

50,000 cubic feet per second to about 

90,000 cubic feet per second (fig. 6).

Peak Stage and Discharge

The magnitude of a flood event 

can be put into historical context by 

comparing the peak stage and discharge 

determined for that flood for a location 

to the previously determined peak stage 

and discharge for that same location. The 

USGS identified streamgages at which the 

peak discharge during the May–June 2015 

flood event exceeded the previously 

determined peak discharge for the entire 

period of record at that gage (fig. 7). The 

USGS WaterWatch Web page (http://

waterwatch.usgs.gov/?m=flood&r=tx) 

summarizes information, such as previously determined peak discharge and 

current event peak discharge, for USGS 

streamgages and is a resource for data 

that may be used to provide historical 

context for current flooding.

References

Liscum, Fred, and East, J.W., 1994, Floods

in southeast Texas, October 1994: U.S. 

Geological Survey Fact Sheet 94– 073, 2 p. 

[Also available at http://pubs.usgs.gov/fs/

fs-073-94/.]

National Oceanic and Atmospheric Administration, 2016, National overview—

May 2015: National Oceanic and Atmospheric Administration, National Centers 

for Environmental Information, accessed 

February 16, 2016, at https://www.ncdc.

noaa.gov/sotc/national/201505.

24

26

21

22

23

16

20

27

24

26

21

22

23

16

20

27

23

26

EXPLANATION

Existing stage-discharge rating—

 Based on measurements made

 prior to May 2015

Discrete discharge measurement 

 above 50,000 ft3

/s and identifier—

 Measurements made during rise

 of May and June 2015 

Discrete discharge measurement since 

 August 21, 2014, and identifier

Extension of stage-discharge rating—

 Based on measurements made

 during rise of May and June 2015

Stage, in feet (ft)

92.1

92.5

93

97

102

142

192

Discharge, in cubic feet per second (ft3/s)

1 5 10 50 100 500 1,000 5,000 10,000 50,000 100,000

Figure 7. U.S. Geological 

Survey streamflow-gaging stations at which 

peak discharges during the May–June 2015 

flood event in Texas exceeded previous 

record peak discharges, grouped by years 

of available record: less than 5 years, 5 years 

to 10 years, and greater than 10 years. 

Figure 6. Example of extending a stage-discharge rating curve. 

For more information, please contact:

Director, Texas Water Science Center

U.S. Geological Survey

1505 Ferguson Lane

Austin, TX 78754– 4501

http://tx.usgs.gov/