In cooperation with the Edwards Aquifer Authority 

Evaluation of Acoustic Doppler Velocity 

Meters to Quantify Flow From Comal 

Springs and San Marcos Springs, Texas 

Scientific Investigations Report 2008–5083 

U.S. Department of the Interior 

U.S. Geological Survey 
Cover:

Top left, U.S. Geological Survey divers installing Argonaut SL on left bank of west outflow channel of 

Spring Lake, San Marcos. 

Top right, Argonaut SW installed inside steel pipe over diversion spring at San Marcos Springs. 

Bottom left, U.S. Geological Survey diver testing velocity data acquisition in one of the spring sand boils 

of San Marcos Springs using an Argonaut ADV 3–D. Photograph by Tom Wiles.

Bottom right, U.S. Geological Survey divers installing acoustic Doppler velocity meter at Comal Springs 

new channel gage station. 
Evaluation of Acoustic Doppler Velocity 

Meters to Quantify Flow From Comal 

Springs and San Marcos Springs, Texas

By Marcus O. Gary, Robin H. Gary, and William H. Asquith

In cooperation with the Edwards Aquifer Authority

Scientific Investigations Report 2008–5083

U.S. Department of the Interior

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

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2008

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Gary, M.O., Gary, R.H., and Asquith, W.H., 2008, Evaluation of acoustic Doppler velocity meters to quantify flow from 

Comal Springs and San Marcos Springs, Texas: U.S. Geological Survey Scientific Investigations Report 2008–5083, 

37 p. 
iii

Contents

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

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

Purpose and Scope .............................................................................................................................5

Previous Acoustic Doppler Velocity Meter Applications ..............................................................5

Physical Setting ....................................................................................................................................5

Comal Springs ..............................................................................................................................5

San Marcos Springs ...................................................................................................................5

Acoustic Doppler Velocity Meters ....................................................................................................6

Principles of Operation ..............................................................................................................6

Description of Models Used .....................................................................................................6

Methods of Study ..........................................................................................................................................7

Monitoring Approaches ......................................................................................................................7

Description of Monitoring Sites and Instrumentation ...................................................................7

Comal Springs—Spring Run 3 ..................................................................................................7

Comal Springs—Spring 7 ..........................................................................................................7

Comal Springs—New Channel .................................................................................................7

Comal Springs—Old Channel .................................................................................................10

San Marcos Springs—Diversion Spring ..............................................................................12

San Marcos Springs—West Outflow ....................................................................................12

San Marcos Springs—East Outflow .....................................................................................12

San Marcos Springs—San Marcos River Streamflow Gage ............................................15

Data Quality Assurance and Archival ............................................................................................18

Evaluation of Acoustic Doppler Velocity Meters to Quantify Flow .....................................................18

Comal Springs—Spring Run 3 .........................................................................................................19

Comal Springs—Spring 7 .................................................................................................................19

Comal Springs—New Channel ........................................................................................................19

Comal Springs—Old Channel ..........................................................................................................23

San Marcos Springs—Diversion Spring .......................................................................................23

San Marcos Springs—West Outflow .............................................................................................23

San Marcos Springs—East Outflow ..............................................................................................27

San Marcos Springs—San Marcos River Streamflow Gage .....................................................27

Implications of Findings Pertaining to Improving Real-Time Springflow Estimates ...............27

Comal Springs ............................................................................................................................27

San Marcos Springs .................................................................................................................30

Summary .......................................................................................................................................................30

References ...................................................................................................................................................31

Appendix 1—Acoustic Doppler Velocity Meter Data ...........................................................................33

Figure 1.1. Hydrograph showing all stream velocity data collected using acoustic 

Doppler velocity meters at monitoring sites at Comal Springs, New Braunfels, 

Texas ...................................................................................................................................35

Figure 1.2. Hydrograph showing all stream velocity magnitude data collected using 

acoustic Doppler velocity meters at monitoring sites at San Marcos Springs, 

San Marcos, Texas ...........................................................................................................35
iv

Figure 1.3. Listing of R code for computation of a mean velocity correction factor for 

bypass pipe at station 08168932 Comal River (new channel) near Landa 

Lake, New Braunfels, Texas, based on pipe geometry and settings of 

Argonaut SW .....................................................................................................................36

Figures

1–3. Maps showing: 

1. San Antonio segment of the Edwards aquifer, Texas, and locations of Comal 

and San Marcos Springs ...................................................................................................2

2. Locations of monitoring sites at Comal Springs ............................................................3

3. Locations of monitoring sites at San Marcos Springs .................................................4

4–7. Photographs showing: 

4. U.S. Geological Survey diver testing velocity data acquisition in one of the 

spring sand boils of San Marcos Springs using an Argonaut ADV 3–D ....................8

5. Argonaut SW installed on right-bank wall, midway down run of spring run 3 

at Comal Springs .................................................................................................................8

6. Argonaut ADV 2–D installed with acoustic sensors oriented to make pointvelocity measurements of water flowing from spring 7 at Comal Springs ...............9

7. U.S. Geological Survey diver installing Argonaut SW just upstream of 61-footwide weir on new channel of Comal River .....................................................................9

8. Schematic showing location of data-collection platform (DCP) and both acoustic 

Doppler velocity meters (ADVM) (one in front of weir and one in bypass pipe) at 

new channel of Comal River ....................................................................................................10

9–10. Photographs showing: 

9. View of new channel of Comal River looking upstream as bypass gate valve 

is being closed by U.S. Geological Survey staff member ..........................................11

10. Acoustic Doppler velocity meter installed in old channel of Comal River 

seen (circled in yellow) as small black area with two yellow circles 

(transducers), 7 feet upstream from concrete and stone bridge and 

upstream from culverts ....................................................................................................11

11. Schematic showing location of acoustic Doppler velocity meter (ADVM) in old 

channel of Comal River .............................................................................................................12

12–15. Photographs showing: 

12. U.S. Geological Survey diver approaching steel pipe structure over diversion 

spring during acoustic Doppler velocity meter installation .......................................13

13. View of cave orifice through 26-inch-diameter pipe at diversion spring ................13

14. Biota net installed on steel pipe structure at diversion spring at San Marcos 

Springs ................................................................................................................................14

15. Argonaut SW installed inside steel pipe over diversion spring at San Marcos 

Springs ................................................................................................................................14

16. Schematic showing diversion spring cave at San Marcos Springs, pipe assembly 

covering spring, and location of acoustic Doppler velocity meter (ADVM) inside 

pipe................................................................................................................................................15

17–20. Photographs showing: 

17. View looking downstream from west outflow channel of Spring Lake, San 

Marcos ...............................................................................................................................16
v

18. U.S. Geological Survey divers installing Argonaut SL on left bank of west 

outflow channel of Spring Lake, San Marcos ..............................................................16

19. Argonaut SW installed in east outflow channel of Spring Lake, San Marcos .......17

20. Argonaut SW installed at station 08170500 San Marcos River at San Marcos 

attached to bridge pier in primary channel of flow .....................................................17

21. Schematic showing location of acoustic Doppler velocity meter (ADVM) next to 

bridge pier at station 08170500 San Marcos River at San Marcos ...................................18

22–37. Graphs showing: 

22. Velocity magnitude from spring run 3 at Comal Springs and streamflow from 

station 08169000 Comal River at New Braunfels during acoustic Doppler 

velocity meter deployment ..............................................................................................20

23. Velocity magnitude from spring run 3 at Comal Springs and streamflow from 

station 08169000 Comal River at New Braunfels, January 2007 ...............................20

24. Velocity magnitude from spring 7 at Comal Springs and streamflow from 

station 08169000 Comal River at New Braunfels during acoustic Doppler 

velocity meter deployment ..............................................................................................21

25. Velocity magnitude from spring 7 at Comal Springs and streamflow from 

station 08169000 Comal River at New Braunfels, November 2006 ...........................21

26. Velocity magnitude from spring 7 at Comal Springs and streamflow from 

station 08169000 Comal River at New Braunfels, March–April 2007 .......................22

27. Velocity magnitude over weir and in bypass pipe from new channel of 

Comal River; gage height at weir from new channel of Comal River; and 

streamflow from station 08169000 Comal River at New Braunfels during 

acoustic Doppler velocity meter deployment ..............................................................22

28. Velocity magnitude over weir and in bypass pipe from new channel of 

Comal River, and gage height at weir from new channel of Comal River 

during bypass valve experiment ....................................................................................24

29. Velocity magnitude over weir from new channel of Comal River; gage 

height at weir from new channel of Comal River; and streamflow from 

station 08169000 Comal River at New Braunfels, June–July 2007 ...........................24

30. Velocity magnitude and gage height from old channel of Comal River during 

acoustic Doppler velocity meter deployment ..............................................................25

31. Velocity magnitude and gage height from old channel of Comal River 

showing influence of debris and anthropogenic control on streamflow ................25

32. Velocity magnitude in pipe from diversion spring at San Marcos Springs 

and streamflow from station 08170500 San Marcos River at San Marcos 

during acoustic Doppler velocity meter deployment ..................................................26

33. Velocity magnitude from west outflow of Spring Lake and streamflow from 

station 08170500 San Marcos River at San Marcos during acoustic Doppler 

velocity meter deployment ..............................................................................................26

34. Velocity magnitude from east outflow of Spring Lake and streamflow from 

station 08170500 San Marcos River at San Marcos during acoustic Doppler 

velocity meter deployment ..............................................................................................28

35. Velocity magnitude from east outflow of Spring Lake and streamflow from 

station 08170500 San Marcos River at San Marcos, March 7–21, 2007 ..................28

36. Velocity, gage height, and streamflow from station 08170500 San Marcos 

River at San Marcos during acoustic Doppler velocity meter deployment ............29

37. Streamflow from station 08170500 San Marcos River at San Marcos, August 

1–October 31, 2006 ............................................................................................................29
vi

Table

1. Acoustic Doppler velocity meter sites at Comal Springs, New Braunfels, Texas, 

and San Marcos Springs, San Marcos, Texas .....................................................................10
Abstract

Comal Springs and San Marcos Springs are the two 

largest springs in Texas, are major discharge points for the 

San Antonio segment of the Edwards aquifer, and provide 

habitat for several Federally listed endangered species that 

depend on adequate springflows for survival. It is therefore 

imperative that the Edwards Aquifer Authority have accurate 

and timely springflow data to guide resource management. 

Discharge points for Comal Springs and San Marcos Springs 

are submerged in Landa Lake and in Spring Lake, respectively. Flows from the springs currently (2008) are estimated 

by the U.S Geological Survey in real time as surface-water 

discharge from conventional stage-discharge ratings at sites 

downstream from each spring. Recent technological advances 

and availability of acoustic Doppler velocity meters (ADVMs) 

now provide tools to collect data (stream velocity) related to 

springflow that could increase accuracy of real-time estimates 

of the springflows. The U.S. Geological Survey, in cooperation 

with the Edwards Aquifer Authority, did a study during May 

2006 through September 2007 to evaluate ADVMs to quantify flow from Comal and San Marcos Springs. The evaluation was based on two monitoring approaches: (1) placement 

of ADVMs in important spring orifices—spring run 3 and 

spring 7 at Comal Springs, and diversion spring at San Marcos 

Springs; and (2) placement of ADVMs at the nearest flowing streams—Comal River new and old channels for Comal 

Springs, Spring Lake west and east outflow channels and 

current (2008) San Marcos River streamflow-gaging site for 

San Marcos Springs. For Comal Springs, ADVM application 

at spring run 3 and spring 7 was intended to indicate whether 

the flows of spring run 3 and spring 7 can be related to total 

springflow. The findings indicate that even though velocity 

data from both discharge features reflect changes in flow, the 

data do not reliably show a direct relation to measured streamflow and thus to total Comal Springs flow. ADVMs at the 

Comal River new channel and old channel sites provide data 

that potentially could yield more accurate real-time estimates 

of total Comal Springs flow than streamflow measured at the 

downstream Comal River site. For San Marcos Springs, the 

findings indicate shortcomings with ADVM installations at 

diversion spring and in the west and east outflow channels. 

However, the accuracy of streamflow measured at the San 

Marcos River gage as an estimate of real-time San Marcos 

Springs flow could potentially be increased through use of 

ADVM data from that site.

Introduction

Comal Springs in Comal County and San Marcos Springs 

in Hays County are the two largest springs in Texas and are 

major discharge points for the San Antonio segment of the 

Edwards aquifer (fig. 1). This segment of the Edwards aquifer 

is the principal source of municipal and agricultural water in 

the San Antonio region and also is the sole habitat for a number of Federally listed endangered species (Crowe and Sharp, 

1997). Springs such as Comal and San Marcos are natural 

outflow features that reflect the condition and water levels 

of the aquifer. This is especially true in karst settings such as 

the Edwards aquifer because water-level changes in multiple 

spatial and temporal scales are rapidly reflected in changes 

in flow from the springs. Therefore, accurate and continual 

quantification of springflow is a primary tool for assessing the 

condition of karst aquifers and aquatic habitats.

Springflows (discharges) of Comal and San Marcos 

Springs indicate critical aquifer conditions relative to the survival of several endangered species. Crowe and Sharp (1997) 

reported that appreciable alterations to habitats of endangered 

species in these springs will occur if springflows decrease 

to low rates. The U.S. Fish and Wildlife Service (1996) has 

defined low (jeopardy) springflows for the Federally listed 

endangered fountain darter at Comal and San Marcos Springs 

to be 150 and 100 cubic feet per second, respectively. When 

springflows decrease below the jeopardy flow thresholds, 

the Edwards Aquifer Authority (EAA) initiates restrictions 

on withdrawals (pumping) of water from the aquifer. It is 

therefore imperative that the EAA have accurate and timely 

springflow data to guide resource management.

The EAA has identified improving methods of flow 

measurement at Comal and San Marcos Springs as a priority 

(Geary Schindel, Edwards Aquifer Authority, oral commun., 

Evaluation of Acoustic Doppler Velocity 

Meters to Quantify Flow From Comal 

Springs and San Marcos Springs, Texas

By Marcus O. Gary, Robin H. Gary, and William H. Asquith
2 Evaluation of Acoustic Doppler Velocity Meters to Quantify Flow From Comal Springs and San Marcos Springs, Texas

2005). Measurement of flow directly from the spring orifices 

has been technically difficult or impossible because (1) there 

are many distributed and focused discharge points at Comal 

and San Marcos Springs, and (2) many springs are submerged 

in Landa Lake at Comal Springs and in Spring Lake at San 

Marcos Springs. 

Flow from Comal Springs currently (2008) is estimated 

by the U.S Geological Survey (USGS) in real time as streamflow computed from a conventional stage-discharge rating at 

USGS station (site) 08169000 Comal River at New Braunfels. 

This station is about 1.6 miles downstream from the major 

spring orifices at Landa Lake (fig. 2). The stage-discharge 

rating is a relation between water-surface elevation (stage 

or gage height) in the channel and discharge; the rating is 

developed and maintained by periodic streamflow measurements over a range of heights in the channel (Rantz and others, 

1982).

The real-time estimates of springflow are useful for 

timely aquifer-management decisions; however, using streamflow at station 08169000 as a direct estimate of springflow 

has several drawbacks that can result in an inaccurate record 

of true springflow: (1) Inflow of local surface-water runoff 

and base flow downstream from Comal Springs and upstream 

from the gaging station can cause estimated springflow to be 

artificially high. (2) Evaporation from the 1.6-mile stream 

reach between the springs and station 08169000 might cause 

estimated spring discharge to be artificially low. (3) Underflow of water through permeable streambed rocks might 

cause estimated spring discharge to be artificially low because 

this underflow is not measured during direct discharge 

measurements. Accordingly, the USGS estimates daily mean 

springflow of Comal Springs, although not in real time, 

as (1) streamflow at station 08169000 when runoff and 

base flow are essentially negligible, and (2) streamflow at 

Comal

Springs

San Marcos

Springs

Austin

San

Antonio

MAVERICK ZAVALA FRIO KARNES ATASCOSA DE

WITT

WILSON

KINNEY UVALDE MEDINA

BEXAR GONZALES

GUADALUPE

BANDERA COMAL

REAL

CALDWELL KENDALL

KERR EDWARDS

HAYS

GILLESPIE BLANCO

TRAVIS

SUTTON

KIMBLE

MASON LLANO BURNET WILLIAMSON

Wimberley

San Marcos

New Braunfels

100° 99° 98°

30° 29°

Location map

EXPLANATION

Recharge zone

Confined zone

0 3 15 0 6 45 0 Miles

Figure 1. San Antonio segment of the Edwards aquifer, Texas, and locations of Comal and San Marcos Springs. 
Introduction 

3Figure 2. Locations of monitoring sites at Comal Springs. 

#*

#*

#*

#*

#*

#*

#*

Spring 2

Spring 1

(Main spring)

Spring 2

Spring run 3

USGS site 294250098080900

Comal River at New Braunfels

USGS site 08169000

Old channel

USGS site 08168913

New channel

USGS site 08168932

Spring 7

USGS site 294255098080501

New channel

USGS site 08168932

98°7'30" 98°8'

29°43' 29°42'30"

0 1,000 2,000 Feet Base from National Agriculture Imagery Program

2005 Digital Orthophoto QuadrangleUniversal Transverse Mercator projection, Zone 14N

Landa

Lake

Old channel

New channel

EXPLANATION

#*#*

Monitoring site with acoustic Doppler velocity meter—Site 

identifier and U.S. Geological Survey (USGS) site number (table 1)

Site without acoustic Doppler velocity meter—Site identifier and 

USGS site number (if applicable)
4 Evaluation of Acoustic Doppler Velocity Meters to Quantify Flow From Comal Springs and San Marcos Springs, Texas

station 08169000 adjusted using computerized base-flow 

separation techniques described by Wahl and Wahl (1995) 

that depend on water levels in nearby index wells when 

runoff and base flow are not negligible. This non-real-time 

springflow record is archived under station 08168710 Comal 

Springs at New Braunfels (station number for archival purposes only), and the record comprises only daily mean 

values.

As with Comal Springs monitoring, the USGS estimates real-time flow from San Marcos Springs as streamflow 

computed from stage-discharge ratings at station 08170500 

San Marcos River at San Marcos, which is about 0.5 mile 

downstream from Spring Lake (fig. 3). Daily mean springflow 

is estimated by accounting for base flow in the stream reach 

between Spring Lake and the gage using the computerized 

base-flow separation techniques of Wahl and Wahl (1995). 

This non-real-time daily mean springflow record is archived 

under station 08170000 San Marcos Springs at San Marcos 

(station number for archival purposes only), and the record 

comprises only daily mean values.

Figure 3. Locations of monitoring sites at San Marcos Springs. 

Diversion spring (LR-67-01-825)

USGS site 295336097555201

Base from Texas State University

6-inch resolution Digital 

Orthophoto Quadrangle

Universal Transverse Mercator 

projection, Zone 14N 

EXPLANATION

East outflow

USGS site 08169944

Monitoring site with acoustic Doppler velocity meter—Site

identifier or name and U.S. Geological Survey (USGS) site 

number (table 1)

Spring

Lake

San Marcos River

San Marcos River

Spring

Lake

0 250 500 Feet

0 250 500 Feet 0 100 200 Feet

West outflow

USGS site 08169945

West outflow

USGS site 08169945

San Marcos River at San Marcos 

USGS site 08170500

97°56'

29°53'30"

Aquarena Springs Drive
Introduction 5

Recent technological advances and availability of acoustic Doppler velocity meters (ADVMs) now provide tools to 

collect stream velocity data related to springflow that could 

increase accuracy of real-time estimates of flow from Comal 

and San Marcos Springs. ADVMs use the Doppler-shift 

principle to reliably measure the velocity of water (Morlock 

and others, 2002; U.S. Geological Survey, 2007). Several 

instrument configurations are available from different manufacturers. Each instrument configuration provides measurements appropriate for specific environmental conditions. The 

velocity of water flowing through a specific cross-sectional 

area can be directly related to discharge through this area. 

ADVM velocity data for sites at Comal and San Marcos 

Springs have the potential to complement and possibly replace 

the stage-discharge ratings for the Comal River and the San 

Marcos River streamflow-gaging sites. Accordingly, the 

USGS, in cooperation with the EAA, did a study during May 

2006 through September 2007 to evaluate ADVMs to quantify 

flow from Comal and San Marcos Springs, with the overall 

objective of improving measurement of total springflow. 

Purpose and Scope

This report documents the monitoring approaches applied 

using ADVMs, describes the monitoring sites, and evaluates 

the performance of ADVMs with regard to increasing the 

accuracy of real-time records of springflow for Comal and San 

Marcos Springs. Eight sites were equipped with ADVMs for 

periods of months during 2006–07 to evaluate the performance 

of ADVMs: one ADVM each at three sites and two ADVMs at 

one site at Comal Springs; and one ADVM each at four sites 

at San Marcos Springs. The study focused on two approaches 

to monitoring springflow with ADVMs: one involved placing 

ADVMs at important spring orifices to measure flow directly 

at discrete discharge points; and the other involved placing 

ADVMs at the nearest flowing streams to measure total spring 

discharge as outflow from the impounded lakes fed by the 

springs. 

Previous Acoustic Doppler Velocity Meter 

Applications 

ADVMs commonly are applied at USGS gaging stations 

across the United States, and standard methods have been 

developed to use the velocity data to compute flow records 

(Morlock and others, 2002). The application of ADVMs in 

karst springs has been successfully tested at station 08155500 

Barton Springs at Austin (Asquith and Gary, 2005; Gary and 

Asquith, 2005, 2006) and at station 08170990 Jacob’s Well 

Spring near Wimberley (Gary and Asquith, 2006; Gary, 2007). 

At both sites, ADVMs were placed in discrete discharge 

orifices or underwater caves. Acoustic velocity data collected 

at Barton Springs were used to augment the official discharge 

record during storms when stage data alone were insufficient or unreliable to monitor springflow. The application of 

ADVMs at Barton Springs and Jacob’s Well Spring provided 

background information for this report and identified strengths 

and weaknesses of ADVM applications in karst springs.

Physical Setting

Comal Springs

Comal Springs is the largest spring in the State with an 

average long-term (December 1927–September 2007) discharge of 291 cubic feet per second (U.S. Geological Survey, 

2008, station 08168710). The springs emerge from a number 

of discrete orifices in and around Landa Lake (fig. 2). These 

individual orifices discharge directly into Landa Lake and are 

identified as two distinct groups of springs that discharge from 

separate flow systems. Spring 1 (main spring), spring 2, and 

spring 3 (spring run 3) flow into stream runs and are thought 

to represent a shallow flow system. Spring 7 and numerous 

other springs on the northern segment of Landa Lake are 

thought to emerge from a deep flow system (Kreitler and 

others, 2004; LBG Guyton Associates, 2004). 

The total spring discharge from Comal Springs is integrated into short-term storage in Landa Lake. Landa Lake 

discharges through two separate stream channels. The original 

(old channel) stream from Landa Lake exits on the southeastern edge of the lake and flows through a swimming pool 

through a series of valves and locks. Flow through the swimming pool can be diverted to a separate channel for pool 

draining and cleaning or periods of non-use. During summer 

when the pool is open for recreation, it is drained weekly 

for cleaning. A new outflow channel from Landa Lake was 

constructed to supply water to a power plant and is now 

the major discharge feature from the lake. Based on direct 

discharge measurements made on July 2, 2007, and July 

30, 2007, about 85 percent of the total outflow from Landa 

Lake is through the new channel. The old and new channels 

merge with the Comal River about 1 river mile downstream 

from Landa Lake. The station 08169000 Comal River at New 

Braunfels that is used to quantify springflow from Comal 

Springs is about 900 feet downstream from the confluence.

San Marcos Springs

San Marcos Springs is the second largest spring in the 

State with an average long-term (May 1955–September 2007) 

discharge of 175 cubic feet per second (U.S. Geological 

Survey, 2008, station 08170000). Similar to Comal Springs, 

San Marcos Springs consists of a number of focused and diffuse discharge sites. These sites include discrete orifices and 

numerous sand boils at the bottom of Spring Lake (fig. 3). 

LBG Guyton Associates (2004) estimated that about 25 percent of the total flow from San Marcos Springs emerges 

from discrete orifices and the remaining 75 percent from sand 

boils.
6 Evaluation of Acoustic Doppler Velocity Meters to Quantify Flow From Comal Springs and San Marcos Springs, Texas

The water flowing from San Marcos Springs collects in 

the impounded Spring Lake and discharges over two separate 

outflow structures as the headwaters of the San Marcos River. 

The San Marcos Spring Lake west overflow (west outflow) 

channel exits over a series of waterfalls with variable-height 

gates that can control the water-surface elevation in Spring 

Lake and thus the rate of flow out of the lake. The San Marcos 

Spring Lake east overflow (east outflow) channel flows over 

a concrete control structure and then over boulders. The two 

channels converge below the outfall structures and immediately upstream from the Aquarena Springs Drive bridge. 

The concrete dam that impounds Spring Lake is located 

between the two outflow structures and likely allows a 

substantial amount of underflow. On May 11, 2006, measurements made on both outflow structures (west overflow and 

east overflow) yielded a total flow of 107 cubic feet per second. Measurements made downstream from the confluence of 

the two channels yielded a total flow of 130 cubic feet per second. The downstream measurements indicate that, on that day, 

about 18 percent of the flow from San Marcos Springs passed 

beneath the dam. Station 08170500 San Marcos River at San 

Marcos, which is used to quantify flow from San Marcos 

Springs in real time, is at the Aquarena Springs Drive bridge. 

The streambed at this site is prone to substantial changes in 

vegetation density and gravel movement. These changes can 

influence the stage-discharge rating for the site, which makes 

the site challenging with regard to using a conventional stagedischarge rating to estimate streamflow.

Acoustic Doppler Velocity Meters

An ADVM is an instrument that operates underwater to 

measure the velocity of water as it passes the device. It uses 

the Doppler-shift principle applied to sound waves underwater 

relative to moving particles suspended in the water column.

Principles of Operation

ADVMs use one or more monostatic acoustic transducers, oriented in a known, fixed position, that transmit and 

receive underwater sound waves. The transducers produce 

pulses of sound at a specific frequency in a narrow acoustic 

beam. As particles in the water pass through the beam, the 

sound pulses bounce off the particles, scattering the sound 

pulses and reflecting some back to the transducer. The 

reflected pulses have a frequency shift that is proportional to 

the velocities of the particles from which they were reflected. 

Thus the velocity of moving particles is measured, and water 

surrounding these particles is assumed to travel at the same 

velocity (Morlock and others, 2002; SonTek Corporation, 

2000). The acoustic beam transmitted from the ADVM can be 

programmed by the user to compute velocity in a set part of 

the beam (referred to as sample volume or cell). The velocity measured in this sample volume is a mean velocity of all 

sound pulses received by the transducer.

Each transducer on an ADVM is capable of producing 

one-dimensional velocity data for moving particles in the 

water column. For instance, a single transducer can measure 

values in the x-direction (relative to a set datum orientation). 

Because water flow seldom is one-dimensional, additional 

transducers are added to an ADVM to acquire data in additional dimensions. An ADVM with two transducers yields 

a two-dimensional measurement (x and y components), and 

an ADVM with three transducers yields a three-dimensional 

measurement (x, y, and z components). The orientation of the 

coordinate system is relative to the mounting orientation of the 

ADVM. A typical two-transducer ADVM will produce acoustic velocity data with magnitudes of both the x (downstream) 

and y (lateral or cross-stream) components. All ADVMs collect velocity data for a set period defined by the user, known 

as the averaging interval. The length of the averaging interval 

is determined by the number of independent acoustic velocity measurements that are averaged to produce a single value 

for recording. In clear water, averaging intervals are longer as 

fewer particles are present to return a strong acoustic signal. 

The longer averaging interval allows for a statistically more 

precise velocity measurement.

Description of Models Used

Four models of ADVMs were tested in the study, all 

manufactured by SonTek Corporation, San Diego, Calif. 

The Argonaut SW (shallow water) was used at most evaluation sites. This ADVM is a two-dimensional instrument for 

open-channel and pipe flow monitoring with a compact, 

rugged design that enables flexible deployment. A third 

acoustic transducer on the instrument allows for water-level 

measurements to be recorded when the instrument is mounted 

on the bottom of a stream channel. The Argonaut SW has a 

resolution of 0.003 foot per second and can measure velocities 

as high as 16 feet per second (SonTek Corporation, 2007). 

The acoustic transducers transmit at a frequency of 3 megahertz and have a maximum sample-cell range of 16 feet. 

The Argonaut SW is equipped to vary the sample-cell length 

dynamically as water depths change. 

Three ADVM units were used for specified applications. 

The Argonaut SL (side looker) also operates with a resolution 

of 0.003 foot per second, transmitting frequency of 3 megahertz, and maximum sample-cell range of 16 feet (SonTek 

Corporation, 2007). The Argonaut SL uses two acoustic 

transducers (is two-dimensional) and averages water velocity 

laterally across the stream channel. It also has a third acoustic 

transducer that measures water level above the instrument. 

The Argonaut ADV 2–D has a resolution of 0.003 foot per 

second, transmits at a 5-megahertz frequency, and has a small 

sample volume of 0.015 cubic inch. It is designed to give a 

precise point-velocity measurement about 0.5 inch upstream 

from the instrument in a narrow flow structure such as a 

discrete spring orifice. The Argonaut ADV 3–D is a threedimensional, self-powered, hand-held version of the Argonaut 

ADV. 
Methods of Study 7

All units tested are capable of self-logging with internal memory or communicating directly to a data-collection 

platform (DCP) by way of SDI–12 (serial digital interface) 

communication protocol, which is the global standard for 

interfacing data-loggers with microprocessor-based sensors. 

The x-vector, y-vector, and resultant velocity magnitude data 

are collected and logged with other diagnostic information 

including data related to signal strength. Temperature measurements also are standard on all models.

Methods of Study

Monitoring Approaches

The evaluation of ADVMs at Comal Springs and San 

Marcos Springs is based on two monitoring approaches: 

(1) placement of ADVMs in important spring orifices to 

measure ground-water velocity at discrete discharge points, 

and (2) placement of ADVMs at the nearest flowing streams to 

measure total springflow as surface-water outflow from 

Landa Lake and Spring Lake. The first approach relies on 

the hypothesis that a single ground-water discharge orifice 

or feature would be representative of the total springflow of 

the system, and thus changes in the velocity and resulting 

flow from this feature would be related to changes in the total 

springflow of the system. Two sites at Comal Springs, spring 

run 3 and spring 7 (fig. 2), and one site at San Marcos Springs, 

diversion spring (fig. 3), were monitored using this approach. 

Discharge from sand boils was measured at the discharge point 

by a diver using the hand-held Argonaut ADV 3–D (fig. 4). 

Testing indicated that three-dimensional velocity vectors are 

too variable in the sand boils to produce useful time-series 

data; therefore, no continuous monitoring was done at sand 

boils.

The second approach involves more standard methods 

of stream gaging to quantify total flow from the springs. At 

Comal Springs, new streamflow gages with ADVMs were 

installed in the new channel and the old channel (fig. 2) as 

close to Landa Lake as site characteristics permitted. At San 

Marcos Springs, both outflow channels at Spring Lake (fig. 3) 

were instrumented with ADVMs, as well as existing station 

08170500. In all, eight sites were monitored (table 1), four at 

Comal Springs (two in spring orifices and two at stream sites) 

and four at San Marcos Springs (one in a spring orifice and 

three at stream sites).

Description of Monitoring Sites and 

Instrumentation

Comal Springs—Spring Run 3

Springflow emerging from the left bank and bottom 

of the channel of spring run 3 represents about 13 percent 

of the total springflow from Comal Springs (LBG Guyton 

Associates, 2004). The run is about 300 feet long, and the 

channel gains streamflow from numerous springs along the 

run. The regular geometry of the channel midway down the 

spring run provided an adequate site to mount an Argonaut 

SW to the 2-foot-high rock wall on the right bank (fig. 5). 

The ADVM was programmed to sample water velocity across 

the entire width of the channel minus a 6-inch “blanking 

distance” next to the instrument and a 1-foot section adjacent to the left bank. An averaging interval of 300 seconds 

(5 minutes) was used to produce and record data values every 

15 minutes. The x (downstream) and y (lateral) components of 

flow were measured, and a resultant velocity magnitude was 

computed. Data were recorded in the ADVM, using the internal logging capabilities of the instrument. ADVM monitoring 

at this site was intended to indicate whether flow (estimated 

from velocity and stage) of spring run 3 could be related to 

total springflow.

Comal Springs—Spring 7

About 600 feet north of spring run 3, a discrete discharge 

point referred to as spring 7 flows from a small underwater cave. The spring is in limestone bedrock and extends a 

minimum of 5 feet into the rock. An Argonaut ADV 2–D was 

anchored to the limestone above the water surface with the 

acoustic probe extending into the middle of the spring orifice 

underwater (fig. 6). The instrument measured the x (downstream) and y (lateral) components of flow velocity and 

computed a resultant velocity magnitude. A 0.015-cubic-inch 

sample volume 3.9 inches from the probe sensors provided a 

point-velocity measurement, as opposed to a large spatially 

averaged velocity measurement that would have been provided 

by the Argonaut SW. An averaging interval of 600 seconds 

(10 minutes) was used to produce and record a data value 

every 15 minutes. As with data from spring run 3, ADVM 

monitoring at this site was intended to indicate whether flow at 

spring 7 is related to changes in total springflow.

Comal Springs—New Channel

The new channel of the Comal River passes over a lowwater, concrete weir about 800 feet downstream from Landa 

Lake (fig. 7). The concrete weir is 61 feet wide. A 42-inch 

bypass pipe with a gate valve is located on the left bank of the 

weir. Two Argonaut SW instruments were installed at this site 

to monitor the flow over the weir and through the bypass pipe 

(fig. 8). The primary instrument is mounted 5 feet upstream 

from the weir, 20 feet from the left bank (fig. 7); flow characteristics at the mounting location measured by a flowmeter 

indicate this location is minimally affected by the baffling 

effect of the weir wall. The weir ADVM records the velocity 

of water before it flows over the weir, where the flow is less 

turbulent than flow downstream from the weir. The instrument 

also uses the upward-facing transducer to record the water 

level. The second ADVM is mounted on the upstream segment 

of the bypass pipe 4 feet upstream from the gate valve. 
8 Evaluation of Acoustic Doppler Velocity Meters to Quantify Flow From Comal Springs and San Marcos Springs, Texas

Figure 4. U.S. Geological Survey diver testing velocity data acquisition in one of the spring sand boils of San Marcos Springs using an 

Argonaut ADV 3–D. Inset shows close-up of sand boil being measured with hand-held acoustic Doppler velocity meter. Photograph by 

Tom Wiles. 

Figure 5. Argonaut SW installed on right-bank wall, midway down run of spring run 3 at Comal Springs. Instrument mounted 0.8–1.2 

feet below water surface, depending on water levels, and measured water velocity across 7-foot cross section. 
Methods of Study 9

Figure 6. Argonaut ADV 2–D installed with acoustic sensors oriented to make point-velocity measurements of water flowing from 

spring 7 at Comal Springs. 

Figure 7. U.S. Geological Survey diver installing Argonaut SW just upstream of 61-foot-wide weir on new channel of Comal River. 
10 Evaluation of Acoustic Doppler Velocity Meters to Quantify Flow From Comal Springs and San Marcos Springs, Texas

The two ADVMs at the new channel of the Comal River 

are intended to monitor the position of the bypass valve (open, 

partially closed, or fully closed) and measure the flow of water 

over the weir. Under normal operating conditions, the bypass 

valve (fig. 9) is fully closed, causing all water to flow over the 

weir. When the bypass valve is fully closed, the weir ADVM 

can accurately measure total flow in the new channel. Direct 

discharge measurements using a flowmeter indicate that the 

flow is distributed evenly across the entire width of the weir, 

which enables reliable correlation of velocity measurements 

from the mounted ADVM with velocities across the channel 

width and the discharge at the weir. When the bypass valve is 

partially or fully open, some percentage of water is diverted 

into the pipe and can be monitored by the second ADVM. The 

second ADVM also can be used to indicate whether the valve 

is open. Both ADVMs are programmed to collect velocity 

and stage measurements every 15 minutes using 300-second 

(5-minute) averaging intervals, and record x-component, 

y-component, and resultant velocity magnitude data. The 

instruments are connected to a DCP that logs and transmits 

data to the USGS National Water Information System (NWIS) 

(U.S. Geological Survey, 2008). The control structure at this 

site makes the site ideal for direct discharge estimation using 

an index-velocity rating.

Comal Springs—Old Channel

The old channel of the Comal River flows from Landa 

Lake through a municipal golf course after passing the swimming pool. The streambed is mostly loose gravel and cobbles 

with dense bottom vegetation. The closest adequate streamgaging site to Landa Lake is immediately upstream from a golf 

cart bridge. This concrete and rock bridge spans two 42-inch 

concrete culverts through which all water flows except during 

large storms. An Argonaut SW was installed using a concrete 

pad and steel brace about 7 feet upstream from the bridge 

culverts, directly between them (figs. 10, 11). Although the 

ADVM installed at this location might be affected by flow 

Left edge

of water

Right edge

of water

Plan view

Water flow

ADVM

ADVM

Left edge

of water

Right edge

of water

Weir

Bypass pipe

 42

inches

Gate valve

Bypass pipe

Gate valve

24 inches

Elevation view

 42 

inches

61 feet

61 feet

ADVM

ADVM

Not to scale

DCP

DCP

Not to scale

Figure 8. Schematic showing location of data-collection 

platform (DCP) and both acoustic Doppler velocity meters (ADVM) 

(one in front of weir and one in bypass pipe) at new channel of 

Comal River. 

Table 1. Acoustic Doppler velocity meter sites at Comal Springs, New Braunfels, Texas, and San Marcos Springs, San Marcos, Texas. 

[USGS, U.S. Geological Survey; DD, spring orifice; OF, stream channel]

Site identifier USGS site number USGS site (station) name Site type

Comal Springs

 Spring run 3 294250098080900 DX-68-23-321 DD

 Spring 7 294255098080501 DX-68-23-324 DD

 New channel 08168932 Comal River (new channel) near Landa Lake, New Braunfels, Texas OF

 Old channel 08168913 Comal River (old channel) near Landa Lake, New Braunfels, Texas OF

San Marcos Springs

 Diversion spring 295336097555201 LR-67-01-825 DD

 West outflow 08169945 San Marcos Spring Lake west overflow, San Marcos, Texas OF

 East outflow 08169944 San Marcos Spring Lake east overflow, San Marcos, Texas OF

 Streamflow gage 08170500 San Marcos River at San Marcos, Texas OF
Methods of Study 11

Figure 9. View of new channel of Comal River looking upstream as bypass gate valve is being closed by U.S. Geological Survey staff 

member. Data-collection platform seen next to valve. 

Figure 10. Acoustic Doppler velocity meter installed in old channel of Comal River seen (circled in yellow) as small black area with two 

yellow circles (transducers), 7 feet upstream from concrete and stone bridge and upstream from culverts. Data-collection platform seen 

on hill above gage.
12 Evaluation of Acoustic Doppler Velocity Meters to Quantify Flow From Comal Springs and San Marcos Springs, Texas

contraction caused by the bridge culverts, no other mounting 

location was practically feasible. An analysis of the velocity 

measurements suggests that the location and orientation of this 

ADVM allows for reliable monitoring of stream stage (water 

depth above instrument) and stream velocity. The ADVM was 

programmed to collect stage and velocity measurements every 

15 minutes using 300-second (5-minute) averaging intervals. 

Data are logged in an attached DCP (fig. 10) that transmits 

real-time data from the site.

San Marcos Springs—Diversion Spring

Diversion spring is the largest discrete discharge orifice 

of San Marcos Springs, with water flowing from a cave on 

the bottom of Spring Lake at a depth of 22 feet (Zara Environmental LLC, 2005). A steel pipe structure about 10 feet high 

with a broad base (fig. 12) has been placed over the 3.5- by 

1.5-foot entrance to the orifice (fig. 13), which diverts most 

of the flow from the orifice through the pipe. A large net is 

normally on the end of the 26-inch inside diameter pipe to 

catch biota that flow out with spring water (fig. 14). An Argonaut SW was installed inside the pipe near the outflow when 

the biota net had been removed for repairs (figs. 15, 16). The 

ADVM measured water velocity in the pipe as it flows either 

directly into Spring Lake or into the biota net. Stream velocity 

components in the x and y directions were measured and the 

resultant velocity magnitude was computed by the instrument. 

Data were collected in 15-minute intervals with an averaging interval of 600 seconds (10 minutes) and transmitted to a 

DCP on the surface of Spring Lake that transmitted real-time 

data from the site. The data record from diversion spring was 

hypothesized either to correlate directly with total springflow 

or to identify subtle patterns of discharge from the aquifer. 

San Marcos Springs—West Outflow

The west outflow channel of Spring Lake discharges 

water over a group of four 4-foot-wide gates (fig. 17) with 

boards that can be added or removed to adjust the water level 

of the lake as flow conditions vary. About 20 feet upstream 

from this gated dam, both stream banks are vertical concrete 

walls. The streambed is gravel and sand with moderate vegetation including the Federally endangered Texas wild rice. An 

Argonaut SL was installed on the left bank of the stream channel and attached directly to the concrete wall (fig. 18). The 

side-looking ADVM makes integrated velocity measurements 

across the stream channel, recording x and y components of 

flow and computing the resultant velocity magnitude. The 

instrument has an upward-facing transducer that measures the 

depth of water above the instrument. Data were collected in 

15-minute intervals with an averaging interval of 120 seconds 

(2 minutes) and logged internally on the ADVM. This site was 

monitored to indicate whether direct gaging of the overflow 

was feasible.

San Marcos Springs—East Outflow

The east outflow channel of Spring Lake flows over a 

low-water concrete dam that provides an excellent control 

structure for stream gaging. Water depth over the 41-foot-wide 

dam typically is less than 1 foot, and stream depth 10 feet 

upstream from the dam is about 1.5 feet. An Argonaut SW was 

installed in the east outflow channel 15 feet upstream from the 

dam and anchored to a concrete apron (fig. 19) and oriented to 

collect x and y components of flow. The instrument computed 

the resultant velocity magnitude and measured water depth 

over the instrument. Data were collected in 15-minute intervals with an averaging interval of 300 seconds (5 minutes). 

This site was monitored to indicate whether direct gaging 

here, coupled with gaging of the west outflow channel, could 

provide total flow from Spring Lake and thus an estimate of 

total springflow.

Figure 11. Schematic showing location of acoustic Doppler 

velocity meter (ADVM) in old channel of Comal River. 

Left edge of water Right edge of water

Elevation view

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

-

-

-

-

-

-

-

-

Bridge

Not to scale

ADVM

42 inches 42 inches

22.5 feet

Depth below water surface, in feet

Not to scale

42 inches

Plan view

22.5 feet

Water flow

ADVM

Left edge of water Right edge of water

42 inches

Bridge
Methods of Study 13

Figure 12. U.S. Geological Survey diver approaching steel pipe structure over diversion spring during acoustic Doppler velocity meter 

installation. 

Figure 13. View of cave orifice through 26-inch-diameter pipe at diversion spring.
14 Evaluation of Acoustic Doppler Velocity Meters to Quantify Flow From Comal Springs and San Marcos Springs, Texas

Figure 14. Biota net installed on steel pipe structure at diversion spring at San Marcos Springs. 

Figure 15. Argonaut SW installed inside steel pipe over diversion spring at San Marcos Springs. 
Methods of Study 15

San Marcos Springs—San Marcos River 

Streamflow Gage

The streamflow-gaging station 08170500 San Marcos 

River at San Marcos has operated continually since 1995 

and is the primary source of real-time flow data from San 

Marcos Springs. Station 08170500 is on Aquarena Springs 

Drive bridge, and a standard stage-discharge rating is used to 

compute continuous discharge. The streambed is gravel and 

cobbles with periodically dense vegetation including Texas 

wild rice. Channel width at the gage is 86 feet. An Argonaut 

SW was installed in the primary flow channel of the river 

and attached to a bridge pier using a custom clamp-on bracket 

(no bolts drilled into pier). The ADVM bracket extends laterally 4 feet from the pier so that the ADVM is a sufficient 

distance from the pier to avoid excessive turbulent flow 

around the pier (figs. 20, 21). Placement of the ADVM was 

limited to the immediate vicinity of the pier to address the 

safety of swimmers, as the gage location is in a public park. 

The x and y velocity components are measured and resultant 

velocity magnitude is computed. The instrument collects 

data in 15-minute intervals using a 300-second (5-minute) 

averaging interval. The data are logged in the station DCP that 

transmits real-time data from the site.

Figure 16. Schematic showing diversion spring cave at San Marcos Springs, pipe assembly covering spring, and location of acoustic 

Doppler velocity meter (ADVM) inside pipe

Elevation view

Water flow

Water flow from

sand boil

Water flow from

bedding plane

Sandbag

Leakage around

sandbag

ADVM

Sandbag

30 feet

Not to scale
16 Evaluation of Acoustic Doppler Velocity Meters to Quantify Flow From Comal Springs and San Marcos Springs, Texas

Figure 17. View looking downstream from west outflow channel of Spring Lake, San Marcos. Four gates seen on dam between 

concrete columns, and Federally listed endangered species, Texas wild rice, seen in foreground. 

Figure 18. U.S. Geological Survey divers installing Argonaut SL on left bank of west outflow channel of Spring Lake, San Marcos. 
Methods of Study 17

Figure 19. Argonaut SW installed in east outflow channel of Spring Lake, San Marcos. 

Figure 20. Argonaut SW installed at station 08170500 San Marcos River at San Marcos attached to bridge pier in primary channel of 

flow. Inset shows close-up of instrument. 
18 Evaluation of Acoustic Doppler Velocity Meters to Quantify Flow From Comal Springs and San Marcos Springs, Texas

Data Quality Assurance and Archival

Data collected from ADVMs were checked for outliers 

and anomalous and missing data on a monthly or more 

frequent basis during the study. ADVMs connected to a 

DCP transmitted measurements to the USGS NWIS database 

(U.S. Geological Survey, 2008). The ADVM data in NWIS 

were checked each week to ensure that accurate velocity data 

were being recorded. ADVMs programmed to record measurements internally were serviced and data were downloaded 

every few weeks and manually uploaded into NWIS. All data 

for each of the eight ADVM monitoring stations were quality 

checked, analyzed, and archived in NWIS. For convenience, 

select ADVM data from this study also are included in appendix 1 (figs. 1.1, 1.2) of this report.

Evaluation of Acoustic Doppler 

Velocity Meters to Quantify Flow 

Data were collected for eight ADVM monitoring sites 

during May 2006 through September 2007, although all 

eight sites were not operational concurrently throughout 

the entire 17-month period. As data were collected and 

Elevation view

0

1

2

3

4

5

6

-

-

-

-

-

-

-

86 feet

86 feet

Not to scale

Bridge

ADVM

Left edge of water Right edge of water

Depth below water surface, in feet

Left edge of water Right edge of water

Plan view

Water flow

Bridge

Existing stage

recorder

ADVM

Figure 21. Schematic showing location of acoustic Doppler velocity meter (ADVM) next to bridge pier at station 08170500 San Marcos 

River at San Marcos. 
Evaluation of Acoustic Doppler Velocity Meters to Quantify Flow 19

reviewed, the data also were evaluated with regard to 

several criteria to indicate whether results for each site met 

the overall objectives of the study to improve methods of 

flow measurement and thus estimates of total springflow for 

Comal and San Marcos Springs. The evaluation criteria were: 

(1) The ADVM was capable of collecting accurate data, even 

in clear spring water with few particles to generate an acoustic 

signal return. (2) The environmental conditions (vegetation) at 

the site had little or no effect on velocity measurements. 

(3) The site provided consistent velocity data directly reflective of the environment with little or no influence from anthropogenic changes to the physical flow system. If anthropogenic 

effects exist, were they directly quantifiable? (4) Velocity 

data from a specific site had sufficient resolution relative to 

total springflow fluctuations to provide a reasonable index 

to quantify total instantaneous springflow. (5) The datacollection techniques can be associated with standard, published methods to generate an official discharge record for the 

site.

Comal Springs—Spring Run 3

ADVM application at this site was intended to indicate 

whether the flow of spring run 3 could be related to total 

springflow. LBG Guyton Associates (2004) reported that 

flow measured in spring run 3 represented 13 percent of total 

Comal Springs flow as indicated by streamflow recorded at 

station 08169000 Comal River River at New Braunfels in each 

of three measurements made. Data were collected from the 

ADVM from June 2006 through early March 2007 (fig. 22). 

The velocity record shows the highest shift in the data resulted 

from a change in the stream geometry in July 2006. A rock 

dam immediately downstream from the ADVM was created 

by unknown persons, raising the water level 0.2 foot and thus 

dropping the velocity by about 30 percent; however, no similar 

decrease in total springflow (as measured at station 08169000) 

occurred then. The data show subtle patterns of variation in 

velocity that can be distinguished from the pattern of streamflow at station 08169000—for example, in November 2006 

when two decreases in stream velocity of about 10 percent 

occurred, although no change in streamflow at the gage was 

observed. 

The velocity data collected by the ADVM during 

January 2007 do not correlate with the hydrograph of streamflow from station 08169000 (fig. 23). The ADVM measured 

about a 20-percent increase in stream velocity in spring run 3 

on January 10 with no subsequent recession; no increase 

in streamflow was recorded at station 08169000 on that 

day. An increase in flow of about 300 cubic feet per second 

was recorded 3 days later at station 08169000, but no corresponding response in velocity is indicated by the ADVM 

data at spring run 3. Two subsequent smaller peaks in flow 

were recorded at the gage on January 15 and 24, but again 

no corresponding response in velocity is indicated by the 

ADVM data at spring run 3. Velocity data from spring run 3 

indicate changes in flow in this discharge feature, but the 

data do not reliably show a direct relation to total Comal 

Springs flow, assuming that total springflow is indicated by 

streamflow at station 08169000. 

Comal Springs—Spring 7

As at spring run 3, the ADVM application at spring 7 

was intended to indicate whether the flow computed from 

velocity data recorded at this discrete discharge point could 

be related to total springflow for Comal Springs. The velocity data record from spring 7 extends from July 2006 through 

April 2007 (fig. 24). An initial concern regarding collecting ADVM data at this site was that water flowing from this 

orifice is exceptionally clear, and the acoustic signal strength 

might be too low to measure accurate velocities. Although 

the data noise (variation) is about 25 percent of measured 

velocities, distinct temporal patterns in the record are observable. The velocity data generally track with streamflow 

data recorded at station 08169000, particularly during 

summer 2006, a period of negligible base flow when streamflow represented essentially all springflow. However, an 

unexplained sharp increase in velocity (about 38 percent) 

followed closely by a sharp decrease (about 92 percent) 

occurred in mid-November 2006 (fig. 25). No corresponding 

change in streamflow was recorded at station 08169000. During the later part of the record (2007), the orifice velocity data 

and streamflow data likely show the influence of more storms 

than in the early part of the record, and the relation between 

the two time series becomes difficult to resolve. The velocity 

data in spring 2007 show intense fluctuations and the streamflow data show several large spikes in flow, but the changes 

in the two time series do not appear to be closely related 

(fig. 26). As with data from spring run 3, data from spring 7 

do not reliably show a direct relation to streamflow at station 

08169000, and thus to total Comal Springs flow, assuming 

that total springflow is indicated by streamflow at station 

08169000.

Comal Springs—New Channel

The ADVM installed in the new channel of the Comal 

River operated from June through September 2007 (fig. 27). 

The new channel site has several advantages with regard 

to ADVM use: (1) The weir provides a stable control that 

can be rated easily, particularly when an ADVM is deployed. 

Standard discharge rating using velocity-stage-index 

methods described by Morlock and others (2002) can be 

applied to quantify flow over the weir. (2) The bypass 

valve can be closed, allowing for all water to flow over the 

weir. When the bypass valve is open, flow in the bypass 

pipe can be quantified by the second ADVM in the pipe. 

(3) The new channel gaging site is considerably closer to 

Landa Lake than station 08169000, which reduces the 
20 Evaluation of Acoustic Doppler Velocity Meters to Quantify Flow From Comal Springs and San Marcos Springs, Texas

Figure 22. Velocity magnitude from spring run 3 at Comal Springs and streamflow from station 08169000 Comal River at New Braunfels 

during acoustic Doppler velocity meter deployment. 

Figure 23. Velocity magnitude from spring run 3 at Comal Springs and streamflow from station 08169000 Comal River at New Braunfels, 

January 2007. 

2006

JUNE

21 28

JULY

7 14 21 28

AUG

7 14 21 28

SEPT

7 14 21 28

OCT

7 14 21 28

NOV

7 14 21 28

DEC

7 14 21 28

JAN

7 14 21 28

FEB

7 14 21 28 7

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

100

150

200

250

300

350

400

450

500

550

600

STREAM VELOCITY, IN FEET PER SECOND

DISCHARGE, IN CUBIC FEET PER SECOND

2007

Equipment

malfunction

Rock dam placed in streambed 

16 feet downstream from velocity meter

MAR

Instantaneous stream velocity from spring run 3, site 294250098080900 (DX-68-23-321)

Instantaneous daily mean discharge from Comal River at New Braunfels, site 08169000 

EXPLANATION

JAN 2007

56789 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

0.250

0.275

0.300

0.325

0.350

0.375

0.400

0.425

0.450

0.475

0.500

0.525

150

200

250

300

350

400

450

500

550

600

650

STREAM VELOCITY, IN FEET PER SECOND

DISCHARGE, IN CUBIC FEET PER SECOND

Instantaneous stream velocity from spring run 3, site 294250098080900 (DX-68-23-321)

Instantaneous discharge from Comal River at New Braunfels, site 08169000 

EXPLANATION
Evaluation of Acoustic Doppler Velocity Meters to Quantify Flow 21

Figure 24. Velocity magnitude from spring 7 at Comal Springs and streamflow from station 08169000 Comal River at New Braunfels 

during acoustic Doppler velocity meter deployment. 

Figure 25. Velocity magnitude from spring 7 at Comal Springs and streamflow from station 08169000 Comal River at New Braunfels, 

November 2006. 

JULY

14 21 28

AUG

7 14 21 28

SEPT

7 14 21 28

OCT

7 14 21 28

NOV

7 14 21 28

DEC

7 14 21 28

JAN

7 14 21 28

FEB

7 14 21 28

MAR

7 14 21 28

APR

7 14

2006 2007

-0.25

0

0.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

2.25

100

150

200

250

300

350

400

450

500

550

STREAM VELOCITY, IN FEET PER SECOND

DISCHARGE, IN CUBIC FEET PER SECOND

Instantaneous stream velocity from spring 7, site 294255098080501 (DX-68-23-324)

Instantaneous daily mean discharge from Comal River at New Braunfels, site 08169000 

EXPLANATION

NOV 2006

123456789 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

200

220

240

260

280

300

320

340

-0.2

0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

STREAM VELOCITY, IN FEET PER SECOND

DISCHARGE, IN CUBIC FEET PER SECOND

Instantaneous stream velocity from spring 7, site 294255098080501 (DX-68-23-324)

Instantaneous discharge from Comal River at New Braunfels, site 08169000 

EXPLANATION
22 Evaluation of Acoustic Doppler Velocity Meters to Quantify Flow From Comal Springs and San Marcos Springs, Texas

Figure 26. Velocity magnitude from spring 7 at Comal Springs and streamflow from station 08169000 Comal River at New Braunfels, 

March–April 2007. 

Figure 27. Velocity magnitude over weir and in bypass pipe from new channel of Comal River; gage height at weir from new channel of 

Comal River; and streamflow from station 08169000 Comal River at New Braunfels during acoustic Doppler velocity meter deployment. 

MAR

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

APR

12345678 9 10 11 12 13 14 15

2007

-0.25

0

0.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

2.25

100

200

300

400

500

600

700

800

900

1,000

STREAM VELOCITY, IN FEET PER SECOND

DISCHARGE, IN CUBIC FEET PER SECOND

Instantaneous stream velocity from spring 7, site 294255098080501 (DX-68-23-324)

Instantaneous discharge from Comal River at New Braunfels, site 08169000 

EXPLANATION

2007

JUNE

15 18 21 24 27 30

JULY

3 6 9 12 15 18 21 24 27 30

AUG

3 6 9 12 15 18 21 24 27 30

SEPT

3 6 9 12 15 18

-1

0

1

2

3

4

5

6

7

8

9

0

1,000

2,000

3,000

4,000

5,000

6,000

GAGE HEIGHT, IN FEET

Bypass valve 

partially 

opened Bypass valve

closed

Bypass valve experiment

Bypass valve

closed

Velocity at weir

Velocity in

bypass pipe

Q = 276

(over weir only)

Q = 327 Q = 359 Q = 389 Q = 402

STREAM VELOCITY, IN FEET PER SECOND

DISCHARGE, IN CUBIC FEET PER SECOND

Instantaneous stream velocity over weir from new channel, site 08168932

Instantaneous discharge from Comal River at New Braunfels, site 08169000

Instantaneous stream velocity in bypass pipe from new channel, site 08168932

Instantaneous gage height at weir from new channel, site 08168932

EXPLANATION Note: Q is discharge in

 cubic feet per second

Bypass valve partially open

Bypass valve closed

Bypass valve open
Evaluation of Acoustic Doppler Velocity Meters to Quantify Flow 23

length of stream reach subject to streamflow gains and 

losses that lessen the reliability of gaged streamflow at 

station 08169000 as the indicator of real-time total 

springflow. (4) The site is secure in that it is not easily 

accessed by the public, which lessens the potential for 

vandalism. 

Velocity data from the two ADVMs, when used in 

tandem, provide excellent stage and velocity data that can 

be used to produce an accurate discharge rating at this site. 

With both instruments deployed on June 26, 2007, the bypass 

valve was completely closed, and velocity data values from 

the ADVM in the bypass pipe fluctuated ±0.3 foot per second 

(fig. 27), which indicates only turbulent, circular water movement. With the valve partially open, water velocity in the 

pipe was stable and averaged 0.4 foot per second. The 

observable difference in recorded velocity data between 

the valve-open and valve-closed periods indicates that the 

ADVM can be used to reliably determine whether the valve 

is open or closed, and if open, the flow can be directly 

quantified. 

A flow experiment was conducted September 12–14, 

2007, to evaluate the variation in flow over the weir and 

through the bypass pipe (fig. 28). With the valve completely closed and all water flowing over the weir before 

September 12, the measured flow over the weir was 389 cubic 

feet per second. After complete opening of the bypass valve, 

water velocity in the pipe increased from about 0 to about 

7.6 feet per second, the gage height (stage) at the weir 

decreased 0.23 foot, and the velocity of flow over the weir 

decreased about 0.3 foot per second. Flow conditions were 

allowed to stabilize for 2 days, and then discharge over the 

weir was directly measured at 334 cubic feet per second. 

Flow in the pipe was computed to be 68 cubic feet per second 

using pipe-flow hydraulics (appendix 1, fig. 1.3). The bypass 

valve was then completely closed, and flow conditions 

returned to essentially those before the experiment. A slight 

decrease in gage height was observed, which could be a 

result of loss of storage in Landa Lake that occurred because 

the bypass valve was open for 2 days. The data indicate flow 

at the weir during the experiment increased from 389 to 

402 cubic feet per second (334 cubic feet per second over the 

weir plus [estimated] 68 cubic feet per second through the 

pipe); however, the difference of 13 cubic feet per second is 

within the acceptable error of USGS flow measurement and 

estimation.

The velocity and gage-height records from the new 

channel site provided a good indication that springflow could 

be discriminated from surface-water inflows downstream 

from this site and upstream from station 08169000. Streamflow response during June 28–29, 2007, to a local rain event 

was recorded by the Comal River gage, but no corresponding response in velocity or gage height was recorded by the 

ADVM at the new channel site (fig. 29). The findings for the 

new channel site indicate that ADVM use there could potentially contribute to increased accuracy of real-time springflow 

estimates.

Comal Springs—Old Channel

To quantify total springflow from Comal Springs, both 

outflow channels (new channel and old channel) must be 

gaged. The gaging site on the old channel is not as well suited 

for measuring flow as the site on the new channel, but data 

of acceptable accuracy can be collected at the old channel site. Data were collected from June through September 

2007 (fig. 30). Recorded data at this site varied with frequent 

(generally weekly) changes in flow associated with upstream 

pool maintenance in summer. Sharp peaks in velocity and gage 

height were recorded as the pool was rapidly drained, then 

decreases in velocity and gage height were recorded as the 

pool was refilled.

Data recorded at the site during July 20–August 20, 

2007 (fig. 31) reflect a series of rises that covered the 

ADVM with debris and obstructed flow through the culverts 

immediately downstream. Velocities in general were low 

during this period, as outflow from the swimming pool was 

reduced. 

With velocity and stage data collected and applied to 

a velocity-stage discharge rating for the old channel site, it 

is possible to continuously gage the flow at the site. Without velocity data, stage data alone would not be adequate to 

quantify flow when debris obstructs the downstream culverts. 

However, when clear, the culverts maintain stable water levels 

and provide a control structure that produces uniform flow 

over the ADVM. The findings for the old channel site indicate 

that ADVM use there, assuming concurrent ADVM use at the 

new channel site, could potentially contribute to increased 

accuracy of real-time springflow estimates.

San Marcos Springs—Diversion Spring 

Initially, diversion spring with its 26-inch diameter pipe 

appeared to be a promising site to index the discharge of 

San Marcos Springs. An ADVM was installed and data were 

collected during May through September 2006. Data show 

that when the biota net is installed on the pipe, water velocity in the pipe decreases 25–30 percent (fig. 32). The net was 

installed on May 16 and removed on May 19; then on June 

1, the net was replaced for the remainder of the study. The 

authors believe that the decrease in velocity is a result of 

back pressure exerted by the net, and the flow lost out of the 

pipe is diverted to leakage around the base structure (fig. 16). 

Although the general trend of velocity correlates well with 

flow measured at station 08170500, the uncertainty associated 

with the biota net installation and its influence on velocity 

measurements in the pipe make this site less desirable for 

accurately indexing the total flow of San Marcos Springs. 

San Marcos Springs—West Outflow

Of the two outflow channels at Spring Lake, discharge at 

the west site is twice the volume on average of discharge at the 
24 Evaluation of Acoustic Doppler Velocity Meters to Quantify Flow From Comal Springs and San Marcos Springs, Texas

Figure 28. Velocity magnitude over weir and in bypass pipe from new channel of Comal River, and gage height at weir from new 

channel of Comal River during bypass valve experiment. 

Figure 29. Velocity magnitude over weir from new channel of Comal River; gage height at weir from new channel of Comal River; and 

streamflow from station 08169000 Comal River at New Braunfels, June–July 2007. 

SEPT 2007

11

0000 0600 1200 1800

12

0000 0600 1200 1800

13

0000 0600 1200 1800

14

0000 0600 1200 1800

15

0000 0600 1200 1800

-1

0

1

2

3

4

5

6

7

8

9

1.325

1.350

1.375

1.400

1.425

1.450

1.475

1.500

1.525

1.550

1.575

1.600

1.625

Velocity in pipe

Velocity at weir

Q = 389

over weir

Q = 334

over weir

Q = 68

in pipe

Q = 0

in pipe

GAGE HEIGHT AT WEIR, IN FEET

STREAM VELOCITY, IN FEET PER SECOND

Instantaneous stream velocity in bypass pipe from new channel, site 08168932

Instantaneous stream velocity over weir from new channel, site 08168932

Instantaneous gage height at weir from new channel, site 08168932

EXPLANATION Note: Q is discharge in

 cubic feet per second

Bypass valve open Bypass valve closed

27

0000 1200

28

0000 1200

29

0000 1200

30

0000 1200

1

0000 1200

2

0000 1200

3

0000 1200

4

0000 1200

5

0000 1200

6

0000 1200

JUNE JULY

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

0

200

400

600

800

1,000

1,200

1,400

1,600

2007

GAGE HEIGHT, IN FEET

STREAM VELOCITY, IN FEET PER SECOND

DISCHARGE, IN CUBIC FEET PER SECOND

Instantaneous stream velocity over weir from new channel, site 08168932

Instantaneous discharge from Comal River at New Braunfels, site 08169000

EXPLANATION

Instantaneous gage height at weir from new channel, site 08168932
Evaluation of Acoustic Doppler Velocity Meters to Quantify Flow 25

Figure 30. Velocity magnitude and gage height from old channel of Comal River during acoustic Doppler velocity meter deployment. 

Figure 31. Velocity magnitude and gage height from old channel of Comal River showing influence of debris and anthropogenic control 

on streamflow. 

18 21 24 27 30

JULY

3 6 9 12 15 18 21 24 27 30

AUG

3 6 9 12 15 18 21 24 27 30

SEPT

369 12 15 18

2007

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

-3

-2

-1

0

1

2

3

4

5

6

7

Swimming pool 

drained and filled

Q = 52 Q = 46 Q = 59 Q = 42

JUNE

GAGE HEIGHT, IN FEET

STREAM VELOCITY, IN FEET PER SECOND

Instantaneous stream velocity from old channel, site 08168913

Instantaneous gage height from old channel, site 08168913

EXPLANATION

Note: Q is discharge in

 cubic feet per second

19 20 21 22 23 24 25 26 27 28 29 30 31

AUG

123456789 10 11 12 13 14 15 16 17 18 19 20 21 22

2007

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

-3

-2

-1

0

1

2

3

4

5

6

7

JULY

Debris 

blocking culverts

Debris blocking

instrument

Debris 

blocking culverts

Equipment

malfunction

GAGE HEIGHT, IN FEET

STREAM VELOCITY, IN FEET PER SECOND

Instantaneous stream velocity from old channel, site 08168913

Instantaneous gage height from old channel, site 08168913

EXPLANATION
26 Evaluation of Acoustic Doppler Velocity Meters to Quantify Flow From Comal Springs and San Marcos Springs, Texas

Figure 32. Velocity magnitude in pipe from diversion spring at San Marcos Springs and streamflow from station 08170500 San Marcos 

River at San Marcos during acoustic Doppler velocity meter deployment. 

Figure 33. Velocity magnitude from west outflow of Spring Lake and streamflow from station 08170500 San Marcos River at San 

Marcos during acoustic Doppler velocity meter deployment. 

2006

MAY

12 16 20 24 28

JUNE

4 8 12 16 20 24 28

JULY

4 8 12 16 20 24 28

AUG

4 8 12 16 20 24 28

SEPT

4 8 12 16 20

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

40

60

80

100

120

140

160

180

200

220

240

Biota net installed

STREAM VELOCITY, IN FEET PER SECOND

DISCHARGE, IN CUBIC FEET PER SECOND

Instantaneous stream velocity in pipe from diversion spring, site 295336097555201 (LR-67-01-825)

Instantaneous discharge from San Marcos River at San Marcos, site 08170500 

EXPLANATION

2006

MAY

12 14 16 18 20 22 24 26 28 30

JUNE

2468 10 12 14 16 18 20 22 24 26 28 30

JULY

2468 10 12 14 16 18 20 22 24

0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

60

80

100

120

140

160

180

200

220

240

Gates altered Vegetation

fouling

STREAM VELOCITY, IN FEET PER SECOND

DISCHARGE, IN CUBIC FEET PER SECOND

Instantaneous stream velocity from west outflow of Spring Lake, site 08169945

Instantaneous discharge from San Marcos River at San Marcos, site 08170500

EXPLANATION
Evaluation of Acoustic Doppler Velocity Meters to Quantify Flow 27

east site. The Argonaut SL installed at the west site collected 

velocity data from May through July 2006. Two conditions 

that adversely affected the velocity record (fig. 33) precluded 

obtaining an accurate record at this site: (1) the gates at the 

dam were altered multiple times, which created substantial 

shifts in the data, and (2) Texas wild rice in the path of the 

acoustic beams of the ADVM intermittently reduced the 

recorded values by about 50 percent. Because Texas wild rice 

is an endangered species, it could not be removed or trimmed 

to clear a path for the acoustic beams. ADVM application at 

this site would not be particularly effective for obtaining data 

with the potential to increase accuracy of real-time springflow 

estimates.

San Marcos Springs—East Outflow

The east outflow channel from Spring Lake is free of 

structures that can be easily altered (like the gates at the west 

outflow), and no Texas wild rice is in this section of the lake; 

thus the east outflow channel site is more favorable for gaging 

than the west site. Data were collected at the east site from 

June 2006 through March 2007, and the velocity magnitude 

tracks reasonably well with streamflow from the San Marcos 

River gage 500 feet downstream (fig. 34). During many periods, the velocity data shifted downward because of vegetation 

fouling the sensor guard. Another problem at this site is high 

data noise of about 0.5 foot per second on average, the cause 

of which is unknown. The noise range is about 50 percent of 

the total measured velocity (fig. 35), which makes a potential 

relation between ADVM-recorded velocity at this site and total 

San Marcos Springs flow problematic. 

San Marcos Springs—San Marcos River 

Streamflow Gage

Shifts in the streambed and density of vegetation at 

this site cause problems (for example, changes in stage for a 

given discharge) in obtaining an accurate record of streamflow, which currently (2008) is used to estimate real-time 

San Marcos Springs flow. The measurement of velocity by 

the ADVM at this site allows for two variables (stage and 

velocity) to be incorporated into the flow computation, thus 

potentially improving accuracy. For example, if the stream 

channel becomes obstructed with vegetation that causes a 

rise in stage, then the flow computation using stage only (the 

stage-discharge rating) would result erroneously in an increase 

in flow. However, ADVM velocity would decrease under the 

channel condition described, indicating correctly that flow had 

decreased. 

Velocity from the ADVM at station 08170500 was 

recorded from July 2006 through September 2007 (fig. 36). A 

comparison of the computed discharge record during low-flow 

conditions in late summer 2006 (fig. 37) shows variation in 

the stage-discharge and the index-velocity ratings. The stagedischarge values have been corrected by applying a shift in 

stage; no shifts have been applied to the index-velocity values. 

The uncorrected index-velocity values show variability relative to the corrected stage-discharge values: During August 

through mid-September the index-velocity values are close to 

discharge computed using the corrected stage-discharge with 

some periods of higher computed discharge and some periods lower computed discharge; from mid-September through 

October the uncorrected index-velocity values are consistently 

greater than the corrected stage discharge. This might indicate 

discrepancies in the current (2008) shifting-control method 

used in the stage-discharge rating and that shifts are required 

to correct the index-velocity rating at this site, as with the 

stage-discharge rating. 

The index-velocity discharge values also show more 

variability between each instantaneous value than the stagedischarge values. This is because two variables are used in the 

discharge equation (index-velocity discharge) rather than one 

(stage-discharge). 

Implications of Findings Pertaining to Improving 

Real-Time Springflow Estimates

Comal Springs

The findings indicate that ADVMs at the Comal River 

new channel site (08168932 Comal River [new channel] near 

Landa Lake, New Braunfels,) and old channel site (08168913 

Comal River [old channel] near Landa Lake, New Braunfels) provide data that potentially could yield more accurate 

real-time estimates of total Comal Springs flow than currently 

(2008) are available from streamflow measured at station 

08169000. The sum of the flow from these two channel stations, which were newly established for this study, provides 

an independent check on the springflow recorded at station 

08169000 farther downstream. The measurement of velocity 

at the two sites, in conjunction with stage and a discharge/

velocity-stage-index rating at each, would allow accurate 

estimation of flow in the channels. 

The combination of two velocity measurements at the 

new channel site, one at the weir and one in the bypass 

pipe, provide direct observation of the status of the valve 

controlling flow in the bypass pipe; and stage at this site 

provides constraints on the hydraulic computations over the 

weir and through the pipe. Hydraulic computations of flow 

over weirs and in pipes provide straightforward analytical 

structure for the incorporation of velocity and stage data for 

collection of springflow data. The measurement of velocity 

at the old channel site, together with stage, provides direct 

observation of the condition of the flow through the culverts 

immediately downstream from the site. A combination of conventional stage-discharge and discharge/velocity-stage-index 

ratings at both new channel and old channel sites can provide 

accurate springflow values. The channel sites are considered 
28 Evaluation of Acoustic Doppler Velocity Meters to Quantify Flow From Comal Springs and San Marcos Springs, Texas

Figure 34. Velocity magnitude from east outflow of Spring Lake and streamflow from station 08170500 San Marcos River at San 

Marcos during acoustic Doppler velocity meter deployment. 

Figure 35. Velocity magnitude from east outflow of Spring Lake and streamflow from station 08170500 San Marcos River at San 

Marcos, March 7–21, 2007. 

2006

JUNE

7 14 21 28

JULY

7 14 21 28

AUG

7 14 21 28

SEPT

7 14 21 28

OCT

7 14 21 28

NOV

7 14 21 28

DEC

7 14 21 28

JAN

7 14 21 28

FEB

7 14 21 28

MAR

7 14 21 28

0

100

200

300

400

500

600

700

800

900

-0.5

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

STREAM VELOCITY, IN FEET PER SECOND

DISCHARGE, IN CUBIC FEET PER SECOND

2007

Instantaneous stream velocity from east outflow of Spring Lake, site 08169944

Instantaneous discharge from San Marcos River at San Marcos, site 08170500

EXPLANATION

Vegetation

fouling

MAR 2007

7

1200

8

1200

9

1200

10

1200

11

1200

12

1200

13

1200

14

1200

15

1200

16

1200

17

1200

18

1200

19

1200

20

1200

21

1200

0

100

200

300

400

500

600

700

800

900

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

STREAM VELOCITY, IN FEET PER SECOND

DISCHARGE, IN CUBIC FEET PER SECOND

Instantaneous stream velocity from east outflow of Spring Lake, site 08169944

Instantaneous discharge from San Marcos River at San Marcos, site 08170500 

EXPLANATION
Evaluation of Acoustic Doppler Velocity Meters to Quantify Flow 29

Figure 36. Velocity, gage height, and streamflow from station 08170500 San Marcos River at San Marcos during acoustic Doppler 

velocity meter deployment. 

Figure 37. Streamflow from station 08170500 San Marcos River at San Marcos, August 1–October 31, 2006. 

JULY AUG SEPT OCT NOV DEC

2007

JAN FEB MAR APR MAY JUN JULY AUG SEPT

-1

0

1

2

3

4

5

6

7

8

9

0

200

400

600

800

1,000

1,200

GAGE HEIGHT, IN FEET

STREAM VELOCITY, IN FEET PER SECOND

DISCHARGE, IN CUBIC FEET PER SECOND

2006

EXPLANATION

Instantaneous stream velocity (x-direction only) from San Marcos River at San Marcos, site 08170500

Instantaneous discharge from San Marcos River at San Marcos, site 08170500

Instantaneous gage height from San Marcos River at San Marcos, site 08170500

AUGUST SEPTEMBER OCTOBER

2006

0

50

100

150

200

250

300

350

400

DISCHARGE, IN CUBIC FEET PER SECOND

Instantaneous discharge from San Marcos River at San Marcos, site 08170500, computed from stage-discharge rating

Instantaneous discharge from San Marcos River at San Marcos, site 08170500, computed from index-velocity rating

Instantaneous discharge from San Marcos River at San Marcos, site 08170500, direct discharge measurements

EXPLANATION

3 6 9 12 1 15 8 21 24 27 30 369 12 18 21 24 27 15 30 3 6 9 12 1 15 8 21 24 27 30

X

X X X X X X
30 Evaluation of Acoustic Doppler Velocity Meters to Quantify Flow From Comal Springs and San Marcos Springs, Texas

excellent (new channel) and good (old channel) for monitoring 

flow, and each site is safely and readily maintainable by 

divers. The sites also are reasonably isolated from public 

intrusion.

The simultaneous operation of the new channel, old 

channel, and river stations can enhance the quantification 

of total flow from Comal Springs and would provide data 

redundancy that could prove important during critical (lowflow) flow conditions. The sum of flows for the new channel and old channel stations provides a real-time estimate 

of springflow that is likely closer to the true flow of Comal 

Springs than streamflow measured at the river gage downstream.

San Marcos Springs

The findings indicate shortcomings with ADVM installations at diversion spring (uncertainty associated with the 

biota net installation and its influence on velocity measurements), in the west outflow channel from Spring Lake (data 

shifts associated with downstream gate alterations and vegetation issue), and in the east outflow channel (vegetation issue 

and unexplained noise in recorded velocity data). However, 

the accuracy of streamflow from station 08170500 as an 

estimate of real-time San Marcos Springs flow could potentially be increased through use of ADVM data from that site. 

Furthermore, flow computed from ADVM velocity data could 

provide an independent check on flow computed by conventional methods for station 08170500, the computation of 

which can be challenging because changes in vegetation 

density and gravel movement can affect the stage-discharge 

rating. 

Summary 

Comal Springs in Comal County and San Marcos Springs 

in Hays County are the two largest springs in Texas and are 

major discharge points for the San Antonio segment of the 

Edwards aquifer in Central Texas. Springflows (discharges) 

of Comal and San Marcos Springs indicate critical aquifer 

conditions relative to the survival of several Federally listed 

endangered species. When springflows decrease below specific thresholds, the Edwards Aquifer Authority (EAA) initiates restrictions on withdrawals (pumping) of water from the 

aquifer to protect endangered species. It is therefore imperative that the EAA have accurate and timely springflow data to 

guide resource management.

Flows from Comal Springs and San Marcos Springs 

emanate from many distributed and focused discharge points 

submerged in Landa Lake at Comal Springs and in Spring 

Lake at San Marcos Springs. Flows from the springs currently 

(2008) are estimated by the U.S Geological Survey (USGS) 

in real time as streamflow computed from stage-discharge 

ratings at USGS station 08169000 about 1.6 miles downstream 

from the major spring orifices at Landa Lake and station 

08170500 about 0.5 mile downstream from the major spring 

orifices at Spring Lake. Streamflow gains and losses that periodically occur in the stream reaches between the springs and 

the gaging stations cause streamflows measured at the stations 

to not always reflect true springflows. 

Recent technological advances and availability of 

acoustic Doppler velocity meters (ADVMs) now provide 

tools to collect data (stream velocity) related to springflow 

that could increase accuracy of real-time estimates of flows 

from Comal and San Marcos Springs. The USGS, in cooperation with the EAA, did a study during May 2006 through 

September 2007 to evaluate ADVMs to quantify flow from 

Comal and San Marcos Springs, with the overall objective 

of improving methods of flow measurement and thus estimates 

of total springflow for each spring. Eight sites were equipped 

with ADVMs for periods of months during 2006–07 to evaluate the performance of ADVMs: one ADVM each at three 

sites and two ADVMs at one site at Comal Springs; and one 

ADVM each at four sites at San Marcos Springs.

The evaluation was based on two monitoring approaches: 

(1) Placement of ADVMs in important spring orifices to 

measure flow directly at discrete discharge points. Two sites 

at Comal Springs, spring run 3, and spring 7, and one site 

at San Marcos Springs, diversion spring, were monitored 

using this approach. (2) Placement of ADVMs at the nearest 

flowing streams to measure total springflow as outflow from 

Landa Lake and Spring Lake. At Comal Springs, ADVMs 

were installed in the new channel and the old channel of the 

Comal River, which originates at Landa Lake. At San Marcos 

Springs, both west and east outflow channels at Spring Lake 

were instrumented with ADVMs, as well as existing station 

08170500 on the San Marcos River. 

For Comal Springs, ADVM application at spring run 3 

and spring 7 was intended to indicate whether the flows of 

spring run 3 and spring 7 can be related to total springflow. 

The findings indicate that even though velocity data from both 

discharge features reflect changes in flow, the data do not reliably show a direct relation to streamflow at station 08169000 

Comal River at New Braunfels, and thus to total Comal 

Springs flow, assuming that total springflow is indicated by 

streamflow at station 08169000. 

ADVMs at the Comal River new channel site and old 

channel site provide data that potentially could yield more 

accurate real-time estimates of total Comal Springs flow than 

currently (2008) are available from streamflow measured at 

station 08169000. The sum of flows for the new channel and 

old channel stations provides a real-time estimate of springflow that is likely closer to the true flow of Comal Springs 

than streamflow measured at station 08169000 downstream. 

This potential increase in accuracy is in large part because 

distance between the springs and ADVM flow-measuring sites 

is less than that between the springs and station 08169000, 

which in turn reduces the effects of streamflow gains (primarily) and losses on estimates of springflow. The sum of the flow 

from these two channel stations can provide an independent 
References 31

check on the springflow recorded at station 08169000 farther 

downstream, and simultaneous operation of all three stations 

would provide data redundancy that could prove important 

during critical (low-flow) flow conditions. 

For San Marcos Springs, the findings indicate shortcomings with ADVM installations at diversion spring (uncertainty 

associated with the biota net installation and its influence on 

velocity measurements), in the west outflow channel from 

Spring Lake (data shifts associated with downstream gate 

alterations and vegetation issue), and in the east outflow 

channel (vegetation issue and unexplained noise in recorded 

velocity data). However, the accuracy of streamflow from station 08170500 as an estimate of real-time San Marcos Springs 

flow could potentially be increased through use of ADVM 

data from that site. And, flow computed from ADVM velocity 

data could provide an independent check on flow computed by 

conventional methods for station 08170500, the computation 

of which can be challenging because changes in vegetation 

density and gravel movement can affect the stage-discharge 

rating.

References 

Asquith, W.H., and Gary, M.O., 2005, Acoustic Doppler 

velocity monitoring within Main Spring, Barton Springs, 

Austin, Texas, April–September 2004—Enhancing the 

accuracy of springflow data: U.S. Geological Survey Fact 

Sheet 2005–3044, 4 p.

Crowe, J.C., and Sharp, J.M., Jr., 1997, Hydrogeologic 

delineation of habitats for endangered species—The Comal 

Springs/River system: Springer-Verlag, Environmental 

Geology, v. 30, no. 1–2, p. 17–28.

Gary, M.O., 2007, Establishing temporal trends—The first 

two years of continuous monitoring at Jacob’s Well Spring, 

Woodcreek, Texas [abs.], in 15th National Nonpoint Source 

Monitoring Workshop—Monitoring for decision making, 

Austin, Texas, August 26–30, 2007, Proceedings: U.S. 

Environmental Protection Agency, Texas Commission on 

Environmental Quality, River Systems Institute, 1 p. 

Gary, M.O., and Asquith, W.A., 2005, Monitoring springs 

in karst aquifers [abs.], in The Abstract Book of the 2005 

Ground Water Summit Program, San Antonio, Texas, 

April 17–20, 2005: National Ground Water Association, 

p. 241.

Gary, M.O., and Asquith, W.H., 2006, Karst spring monitoring 

using acoustic Doppler velocity data, in Geological Society 

of America, Annual Meeting & Exposition, Philadelphia, 

Pennsylvania, October 22–25, 2006: Topical Session T74, 

paper 74–3.

Kreitler, C.W., Davidson, Tyler, Hamilton, Mark, Schindel, 

Geary, and Johnson, Steve, 2004, Hydrogeologic characterization of Comal Springs during low flow periods, 

Edwards aquifer, Texas—Implications to sustaining spring 

flow through aquifer management and spring augmentation [abs.], in Geological Society of America, 2004 Annual 

Meeting, Denver, Colorado, November 7–10, 2004: 

Abstracts with Programs, v. 36, no. 5, p. 391. 

LBG Guyton Associates, 2004, Evaluation of augmentation 

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San Marcos Springs, Texas: San Antonio, report prepared 

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of acoustic Doppler velocity meters for the production of 

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Investigations Report 01–4157, 56 p.

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of streamflow—Volume 1, Measurement of stage and discharge. Volume 2, computation of discharge: U.S. Geological Survey Water-Supply Paper 2175, 630 p. 

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accessed January 23, 2008, at http://www.sontek.com/

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Springs and associated aquatic ecosystems (revised) recovery plan: U.S. Fish and Wildlife Service report, 121 p.

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(OSW) hydroacoustics—Acoustic Doppler velocity meters: 

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12 p.
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Appendix 1

Appendix 1—Acoustic Doppler Velocity Meter Data
Blank Page
Appendix 1 35

Figure 1.1. Hydrograph showing all stream velocity data collected using acoustic Doppler velocity meters at monitoring sites at Comal 

Springs, New Braunfels, Texas. 

Figure 1.2. Hydrograph showing all stream velocity magnitude data collected using acoustic Doppler velocity meters at monitoring 

sites at San Marcos Springs, San Marcos, Texas. COMAL RIVER STREAM VELOCITY, IN FEET PER SECOND

JULY AUG SEPT OCT NOV DEC

2007

JAN FEB MAR APR MAY JUNE JULY AUG SEPT

-1

0

1

2

3

4

5

6

7

8

9

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

Stream velocity magnitude in bypass pipe from new channel, site 08168932

Stream velocity magnitude over weir from new channel, site 08168932

Stream velocity magnitude from old channel, site 08168913

Stream velocity magnitude from spring run 3, site 294250098080900 (DX-68-23-321)

Stream velocity (x-direction only) from spring 7, site 294255098080501 (DX-68-23-324)

EXPLANATION

2006

STREAM VELOCITY, IN FEET PER SECOND

MAY JUNE JULY AUG SEPT OCT NOV DEC

2006 2007

JAN FEB MAR APR MAY JUNE JULY AUG SEPT

0

1

2

3

4

5

-2

-1

0

1

2

Stream velocity magnitude from east outflow of Spring Lake, site 08169944

Stream velocity magnitude from west outflow of Spring Lake, site 08169945

Stream velocity magnitude from San Marcos River at San Marcos, site 08170500 

Stream velocity magnitude in pipe from diversion spring, site 295336097555201 (LR-67-01-825)

EXPLANATION

STREAM VELOCITY, IN FEET PER SECOND

DIVERSION SPRING VELOCITY IN PIPE, IN FEET PER SECOND
36 Evaluation of Acoustic Doppler Velocity Meters to Quantify Flow From Comal Springs and San Marcos Springs, Texas

Figure 1.3. Listing of R code for computation of a mean velocity correction factor for bypass pipe at station 08168932 Comal River (new 

channel) near Landa Lake, New Braunfels, Texas, based on pipe geometry and settings of Argonaut SW. 
Appendix 1 37

Appendix 1 files (Online only at http://pubs.usgs.gov/sir/2008/5083)

\readme.txt

\database 

\NWIS_advm_data.mdb (Microsoft Access 2000–2003 format)

\FGDC

\readme_fgdc.txt

\ASCII_text (text files)

 \data_daily.txt

 \data_instantaneous.txt

 \lut_dailystat.txt

 \lut_parameters.txt

 \sites.txt

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Information regarding water resources in Texas is available at 

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