HIGH-RESOLUTION MONITORING OF SUSPENDED-SEDIMENT

CONCENTRATION AND GRAIN SIZE IN THE COLORADO RIVER USING LASERDIFFRACTION INSTRUMENTS AND A THREE-FREQUENCY ACOUSTIC SYSTEM

David J. Topping, Research Hydrologist, U.S. Geological Survey, Golden, Colorado, dtopping@usgs.gov;

Scott A. Wright, Hydrologist, U.S. Geological Survey, Flagstaff, Arizona, sawright@usgs.gov; Theodore S.

Melis, Physical Scientist, U.S. Geological Survey, Flagstaff, Arizona, tmelis@usgs.gov; David M. Rubin,

Research Geologist, U.S. Geological Survey, Santa Cruz, California, drubin@usgs.gov

Abstract: In August 2002, we began testing a laser-acoustic system to monitor suspended-sediment transport on the

Colorado River in Grand Canyon. This system consists of laser-diffraction, i.e., LISST1 (Laser In-Situ Scattering

and Transmissometry), instruments connected to an automatic pump sampler and an array of three sideways-looking

acoustic-Doppler profilers at different frequencies. Stable functions have been developed relating the pump, laserdiffraction, acoustic-attenuation, and acoustic-backscatter measurements to cross-sectionally integrated

measurements of suspended-sediment concentration and grain size. We relate acoustic attenuation to the

concentration of suspended silt and clay; this approach yields accurate silt and clay concentrations over the range

from less than 10 mg/l to about 20,000 mg/l. Suspended-sand concentration can then be computed as functions of

the acoustic backscatter and acoustic attenuation; this approach yields accurate sand concentrations over the range

from about 10 mg/l to over 3,000 mg/l. LISST-100 and three-frequency-acoustic measurements of the median grain

grain size of the suspended sand are typically within 10% of the values of the median grain size measured by

conventional methods. Silt and clay loads and sand loads computed by either the LISST-100, LISST-25X, or the

three-frequency acoustic system are well within 5% of the values computed using conventional cross-section data

(that have errors for silt and clay concentration of ~8% and errors for sand concentration of ~22%). This result, in

conjunction with the fact that orders of magnitude more sediment-transport data can be collected each day by the

laser-diffraction and acoustic instruments, indicates that a much more complete, and therefore more accurate record

of suspended-sediment transport can be collected by the laser-acoustic instruments than by conventional methods

alone.

INTRODUCTION

The grain-size distribution of suspended sediment in the regulated Colorado River below Glen Canyon Dam is

bimodal, with a silt and clay mode (dominated by clay-sized particles) and a sand mode. Transport of both modes is

limited by episodic resupply from tributaries. Transport of the sand mode is regulated by both the discharge of

water and short-term changes in the grain size of sand available for transport. During tributary floods, sand on the

bed of the river fines; this causes the suspended sand to fine and the suspended-sand concentration to increase, even

when the discharge of water remains constant. Subsequently, the bed is winnowed of finer sand, the suspended sand

coarsens, and the suspended-sand concentration decreases independently of discharge. This prohibits the

computation of sand-transport rates in the Colorado River using stable relations between water discharge and sand

transport, and therefore requires a more continuous method for measuring sand transport. To monitor sediment

transport in the Colorado River in Grand Canyon, Arizona, we have designed and evaluated a laser-acoustic system

for measuring the concentration and grain size of suspended sediment every 15 minutes. Earlier tests of this system

were reported in Melis et al. (2003) and Topping et al. (2004).

TEST SITE AND EQUIPMENT

The instrument test site for this study is the Colorado River near Grand Canyon, Arizona, gaging station, hereafter

referred to as the Grand Canyon gaging station (Fig. 1). At this test site, the laser-acoustic system consists of: (1) a

Sequoia Scientific LISST-100 type C laser-diffraction instrument and a Sequoia Scientific LISST-25X type C laserdiffraction instrument, (2) an ISCO 6712 automatic pump sampler, and (3) three single-frequency Nortek acousticDoppler profilers. The ISCO pump sampler is triggered by either of the LISST instruments when laser transmission

drops below a user-defined threshold (due to higher suspended-sediment concentrations) and then samples at a user- 

 1 Use of brand and firm names in this paper does not constitute endorsement by the U.S. Geological Survey.
Figure 1 Map of the study area showing the location of the test site at the Colorado River near Grand

Canyon, Arizona gaging station, U.S. Geological Survey (USGS) station number 09402500.

defined rate. This allows samples to be collected when the suspended-sediment concentrations exceed the upper

sand limit for the LISST and the acoustic-Doppler profilers (around 2,000-3,000 mg/l). The two LISST instruments

are suspended in the river from a steel cable on pulleys attached to a vertical cliff (Fig. 2a). The sampling path

length of each LISST instrument is 1 cm. The LISST-100 averages 100 samples collected over 30 consecutive

seconds out of every 15 minutes; output is volume concentration of suspended sediment in each of 32 log-spaced

size classes from 0.0025 to 0.50 mm, water temperature, pressure, and laser transmission. The LISST-25X averages

1000 samples collected over 110 consecutive seconds out of every 15 minutes; output is volume concentration of

suspended total sediment (0.0025 to 0.50 mm), volume concentration of suspended sand (0.0625 to 0.50 mm),

Sauter mean sizes of suspended total sediment and sand, and laser transmission.

The three-frequency sideways-looking acoustic-Doppler profiler array is mounted on a cart attached to a vertical Hbeam bracket anchored to a vertical concrete wall at the base of the gaging station stilling well (Fig. 2a). The cart is

stationary at the base of the bracket during periods of data collection, and is raised to the surface (Fig. 2b) only to

perform maintenance. The three acoustic-Doppler profilers mounted on this cart are a 1 MHz Nortek EZQ, 2 MHz

Nortek EZQ, and a 600 kHz Nortek Aquadopp. These instruments were set to sample 13 out of every 15 minutes in

the early phases of our tests; to conserve power, they are now set to sample 10 out of every 15 minutes.

The ISCO 6712 automatic pump sampler (Figs. 2a & 2c) has a capacity of 24 one-liter bottles. We designed the

LISST-ISCO pump-sampler control circuit to allow data collection during periods when the river is so greatly

enriched with suspended silt and clay that no usable data can be collected by either the LISST or acoustic-Doppler

profilers. The protocol is as follows: when the LISST laser transmission is less than the user-defined threshold, the

automated program of the ISCO pump sampler is enabled and samples are pumped at pre-defined intervals until

either the laser transmission exceeds the user-defined threshold, or the supply of 24 one-liter sample bottles is

exhausted.

Data from the laser-acoustic system are downloaded over the internet via a two-way-broadband satellite system

installed at the test site. This system was designed by Glenn Bennett and Tim Andrews (USGS, Flagstaff, Arizona)

and consists of an on-site computer running the Symantec software “pcAnywhere,” a satellite modem, a second

computer that boots the system up each morning at a user-defined time for a user-defined duration, and a satellite

dish (located on the cliff about 40 m above the gage house in Fig. 2a). The power supplies for the laser-acoustic

system and the satellite system consist of 12V deep-cycle batteries (Fig. 2c) recharged by solar panels located on the

cliff above the gage house.

DATA USED TO CALIBRATE THE LASER-ACOUSTIC MEASUREMENTS

Computation of cross-sectionally averaged suspended-sediment concentration and grain size from the automatic

pump, LISST, and acoustic data was a two-step process. First, the at-a-point pump-measured values of

concentration and grain size were calibrated to the corresponding velocity-weighted cross-sectionally averaged

values measured at the cableway 200 m downstream (Fig. 2d) using the Equal-Discharge-Increment (EDI) method
(a) (b)

(c) (d)

Figure 2 (a) Locations of instrument deployments at Grand Canyon gaging station. (b) Threefrequency acoustic-Doppler profiler array (out of water). (c) Contents of pump shelter in (a) showing

automatic pump sampler (on right) and batteries. (d) Upstream view of reach showing measurement

cableway and gage house in (a); cableway is 200 m downstream from gage house.

(described in Edwards and Glysson, 1999). The following point values measured with the pump were thus

calibrated to the comparable cross-sectionally averaged values from the EDI measurements: the concentration of

silt and clay (with R2 = 0.997), the discharge-weighted concentration of total sand (with R2 = 0.971), the dischargeweighted concentration of sand in three size classes [0.0625-0.088 mm (with R2 = 0.947), 0.088-0.177 mm (with R2

= 0.967), and 0.177-1.0 mm (with R2 = 0.891)], and the median grain size of the suspended sand (with R2 = 0.826).

Calibration of the pump data for the three size classes of sand was conducted to aid in the calibration of the three

single-frequency acoustic-Doppler profilers. This first step in the calibration process used 102 paired EDI and

pump samples collected between January 2002 and November 2004 in discharges ranging from 190 to 1,210 m3/s.

Because depth-integrated samplers do not collect time-averaged samples (and therefore do not average over the

fluctuating component of the flux due to the turbulent fluctuations in velocity and concentration), the errors

associated with EDI measurements are large, i.e., about 8% for silt and clay concentration, 22% for sand

concentration, and 12% for the median grain size of the suspended sand (Topping et al., JGR, in prep.). Therefore,

to ensure that the combination of these large errors with a small sample size (i.e., if only the EDI measurements

were used) did not bias the calibrations of the LISST and acoustic measurements, we calibrated the LISST and

acoustic measurements to the much larger combined EDI and calibrated-pump dataset.

CALIBRATION OF THE LISST INSTRUMENTS

Calibration of the at-a-point LISST measurements to the “cross-sectionally averaged” (i.e., combined EDI and

calibrated-pump) measurements depended primarily on sediment concentration, and secondarily on the discharge of
water (which affects how sediment is mixed between the main flow and the LISST deployment site). The data used

for the LISST-100 calibrations were the 443 cross-sectionally averaged measurements made between January 2003

and November 2004; the data used for the LISST-25X calibrations were the 269 cross-sectionally averaged

measurements made between June 2003 and November 2004. The LISST-100 and 25X measurements of

concentration and grain size were calibrated to the cross-sectionally averaged concentration and grain-size data after

segregating the data into five discrete discharge increments: <280 m3/s, 280-420 m3/s, 420-570 m3/s, 570-850 m3/s,

and >850 m3/s. Equations were then developed to relate the calibrations of suspended-silt and clay concentration,

suspended-sand concentration, and suspended-sand median grain size among the five discharge increments. The

equations relating the at-a-point silt and clay concentration measured by the LISST-100 and 25X to the crosssectionally averaged silt and clay concentration were power laws, with exponents that increased non-linearly from

0.55 to 0.74 as a function of increasing discharge (with R2 = 0.960 for the LISST-100 and R2 = 0.823 for the LISST25X). The equations relating the at-a-point sand concentration measured by the LISST-100 and 25X to the crosssectionally averaged sand concentration were linear, with coefficients that increased linearly from 0.3 to 3.0 as a

function of increasing discharge (with R2 = 0.971 for the LISST-100 and R2 = 0.932 for the LISST-25X). Finally,

the equation relating the at-a-point median grain size of the suspended sand measured by the LISST-100 to the

cross-sectionally averaged median grain size of the suspended sand was linear, with a coefficient that increased

logarithmically from 0.2 to 0.4 as a function of increasing discharge (with R2 = 0.937). No stable relation existed

between the Sauter mean diameter of the suspended sand measured by the LISST-25X and either the median grain

size of the suspended sand measured by the LISST-100 or the cross-sectionally averaged median grain size of the

suspended sand. Therefore, although the LISST-25X accurately measures suspended-sediment concentration in two

size classes (i.e, silt and clay, and sand), it does not provide accurate suspended-sand grain-size data at our test site.

CALIBRATION OF THE MEASUREMENTS OF ACOUSTIC ATTENUATION AND

ACOUSTIC BACKSCATTER

Theory and measurements of underwater acoustics indicate that, for a given frequency of sound in water, bimodal

grain-size distributions of suspended sediment such as those in the Colorado River can be segregated into two

acoustic size classes: (1) a finer acoustic size class in which increasing concentration (or decreasing grain size at a

constant concentration) results in increased attenuation of sound due to viscous losses (Urick, 1948; Flammer, 1962;

Lohrmann, 2001; Gartner, 2004), and (2) a coarser acoustic size class in which increasing concentration (or

increasing grain size at a constant concentration) results mainly in increased backscatter of sound (Thorne and

Campbell, 1992; Thorne et al., 1993; Lohrmann, 2001; Thorne and Hanes, 2002; Gartner, 2004). At low

concentrations of this second acoustic size class (<100 mg/l), concentration is related to backscattered sound

pressure (in Pa) increased to the second power, or backscattered sound intensity (in decibels) increased to the fourth

power (Thorne and Campbell, 1992; Thorne et al., 1993; Gartner, 2004). At higher concentrations of this second

acoustic size class, however, increases in concentration result in increased attenuation of the acoustic energy due to

scattering losses (Flammer, 1962; Gartner, 2004), and, at extremely high concentrations (>>1,000 mg/l), result in

either no increase or a decrease in the backscattered acoustic energy (Thorne and Campbell, 1992; Thorne et al.,

1993). The concentration threshold at which attenuation becomes important in this coarser acoustic size class

depends on frequency, with this threshold being lower for higher frequencies of sound (Flammer, 1962). For

example, Thorne and Campbell (1992) showed, using a 3 MHz transducer, that attenuation of acoustic energy

resulted in substantial deviation from the power of two relation between backscattered sound pressure and sand

concentration when the concentration exceeded about 100 mg/l. Power-law curves fit to the theoretically derived

relations in Fig. 8 in Thorne and Campbell (1992) have exponents >10 (i.e., all much larger than two) as a result of

the increased importance of attenuation relative to backscatter. The threshold grain size between the two acoustic

size classes of sediment is negatively correlated with the frequency of sound (Flammer, 1962; Lohrmann, 2001). For

a given concentration of sediment, lower frequencies of sound will return proportionately more backscatter from

coarser sediment than finer sediment (Thorne and Hanes, 2002, Eqs. 7-10).

The approach employed builds on that in Topping et al. (2004) and takes advantage of the acoustical effects of the

two acoustic size classes of sediment to: (1) relate increases in the attenuation of acoustic energy measured by the

three single-frequency acoustic-Doppler profilers to increases in silt and clay concentration (this can be

accomplished using any one of the three frequencies), (2) for small ranges in this acoustic attenuation due to silt and

clay (expressed as a ratio of the acoustic signal strength in a far-field cell to the acoustic signal strength in a near-
field cell), relate increases in the amplitude of the acoustic signal strength to increases in the concentration of either

total sand or a specified size class of sand (Fig. 3). The relation between the acoustic attenuation (expressed as this

ratio) and silt and clay concentration is linear (Fig. 4a). At lower sand concentrations, the amplitude of the acoustic

signal strength in a cell is dominated by backscattered acoustic energy, whereas at higher sand concentrations, the

amplitude of the acoustic signal strength results from a combination of backscatter and attenuation. Because of the

combined effects of backscatter and attenuation, the relation between the amplitude of the acoustic signal strength

and sand concentration is a power law with an exponent that ranges between 10 and 20, depending on frequency and

site geometry (Fig. 4b). At a fixed frequency at a given site, this exponent is constant and does not depend on silt

and clay concentration. For small ranges in the above-described acoustic-attenuation ratio (arising from suspended

silt and clay), a unique power-law relation will exist between the amplitude of the acoustic signal strength in a given

cell and the concentration of sand. Thus, a family of power-law curves with identical exponents will exist over a

broader range of silt and clay concentration. These curves are related to one another by a coefficient k that varies as

a function of the acoustic-attenuation ratio. Next, the fact that the threshold grain size between the two acoustic

size classes decreases as a function of increasing frequency of sound is used to develop empirical relations between

0

50

100

150

200

250

0 5 10 15 20

EFFECTS OF VARYING SAND AND SILT&CLAY

CONCENTRATIONS ON THE PROFILE SHAPES

SAND CONC. = 239 mg/l SILT&CLAY CONC. = 2660 mg/l

SAND CONC. = 208 mg/l SILT&CLAY CONC. = 1140 mg/l

SAND CONC. = 194 mg/l SILT&CLAY CONC. = 370 mg/l

SAND CONC. = 13 mg/l SILT&CLAY CONC. = 445 mg/l

AMPLITUDE OF THE BACKSCATTERED 

ACOUSTIC SIGNAL STRENGTH (counts)

HORIZONTAL DISTANCE FROM INSTRUMENT (m)

CELL 3

CELL 1

CELL 2

Figure 3 Effects of varying sediment concentration on acoustic profiles collected with the 1 MHz

EZQ. Changes in silt and clay concentration mainly result in changes in the slope of the profile;

changes in sand concentration mainly result in upward or downward shifts in the profile without

changing the slope.

the amplitudes of the acoustic signal strength in a given cell at 600 kHz, 1 MHz, and 2 MHz to the concentration of

sand in three size classes. These three size classes of sand were chosen on the basis of the minima in the

attenuation-coefficient curves in Flammer (1962, Fig. 1) and are the same as those used above in the calibration of

the pump data to the EDI measurements: 0.0625-0.088 mm, 0.088-0.177 mm, and 0.177-1.0 mm. Finally,

logarithmic interpolation between the acoustically computed sand concentration in each size class is used to

compute an “acoustic median grain size” (i.e., acoustic D50) for the suspended sand.

The standard method for working with acoustic-attenuation and backscatter data is to convert the measured

amplitudes of the signal strength in each cell (in counts) to sound intensity (in decibels) using the sonar equation of

Urick (1975). This equation takes into account the two-way transmission losses due to beam spreading, the

attenuation due to absorption by the water (which depends on pressure, temperature, and salinity), and the

attenuation due to viscous or scattering losses by the suspended sediment. Because all of these terms are linear, and

the pressure, temperature, and salinity do not vary greatly at the Grand Canyon gaging station test site, one does not

gain much in the analysis by converting to sound intensity and using the sonar equation (besides determining that

the values of the attenuation due to the suspended sediment fall in the normal range of about 0 to 20 dB/m). In the

examples below, therefore, measured raw amplitudes in the acoustic signal strength (in counts) are used to compute

the acoustic attenuation in terms of a ratio of counts in a far-field cell to counts in a near-field cell (Figs. 3 & 4).
(a)

1

10

100

1,000

10,000

100,000

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

CROSS-SECTIONALLY INTEGRATED

SILT & CLAY CONCENTRATION (mg/l)

ACOUSTIC-ATTENUATION RATIO 

(CELL 2 COUNTS)/(CELL 1 COUNTS)

2MHz SILT & CLAY CALIBRATION

concentration (mg/l) = 18,700 - 21,300(cell 2 counts)/(cell 1 counts) 

R2= 0.941

 (b) 

1

10

100

1,000

10,000

20 40 60 80 100 120

0.775

0.750

0.725

0.700

0.675

0.650

0.625

0.600

0.575

0.550

0.525

0.500

0.475

0.450

0.425

0.400

0.375

0.350

0.325

0.300

CROSS-SECTIONALLY INTEGRATED SAND CONCENTRATION (mg/l)

CELL 3 AMPLITUDE OF THE BACKSCATTERED 

ACOUSTIC SIGNAL STRENGTH (counts)

concentration (mg/l) = 2 + k(CELL 3 AMP)16 R2 = 0.700+0.042

ACOUSTICATTENUATION RATIO

1 MHz TOTAL SAND CALIBRATION

Figure 4 (a) Example of the relation between acoustic attenuation at 2 MHz (expressed as a ratio in

counts between cells) and silt and clay concentration. (b) Example of the relations between the

amplitude of the acoustic signal strength measured by the 1 MHz EZQ and total sand concentration for

small ranges in the acoustic-attenuation ratio arising from silt and clay. The coefficient k relates the

sand-calibration curves as a function of the acoustic attenuation ratio, i.e., silt and clay concentration

(relation has R2 = 0.986).

RESULTS

The laser-acoustic measurements are in excellent agreement with the EDI and calibrated pump measurements. The

levels of agreement between the integrated loads (computed by summing the instantaneous loads) and integrated

grain size are shown in Table 1. For comparison, errors associated with the EDI measurements against which the

Table 1 Difference in the integrated laser-acoustic measurements relative to the integrated combined EDI, calibrated

pump measurements (using both calibrated and uncalibrated laser-acoustic data).

LISST-100 LISST-25X 1 MHz only 3-frequency

Silt and clay load +5.0% -0.5% -2% -2%

Total sand load +0.4% +1.0% -4% -3%

0.0625-0.088 mm sand load Not analyzed N/A N/A -10%

0.088-0.177 mm sand load Not analyzed N/A N/A -4%

0.177-1.0 mm sand load Not analyzed N/A N/A -1%

Suspended-sand D50 +0.5% N/A N/A +4.8%

laser-acoustic measurements were calibrated are much larger than the values in Table 1: ~8% for silt and clay

concentration, ~22% for sand concentration, and ~12% for suspended-sand median grain size. Comparison of laseracoustic, pump, and cableway measurements of silt and clay concentration, sand concentration, and suspended-sand

median grain size beyond the calibration period ending in November 2004 are shown in Fig. 5. As shown in Fig. 5,

the calibrations for concentration and grain size are stable for the pump, LISST, and acoustic measurements and the

agreement remains excellent beyond the period used for calibration. Therefore, because of the larger errors

associated with the EDI measurements, the stability of the laser-acoustic calibrations, and the fact that orders of

magnitude more sediment-transport data can be collected each day by the laser-diffraction and acoustic instruments,

a much more complete, and therefore more accurate record of suspended-sediment transport can be collected by the

laser-acoustic instruments than by conventional methods alone.
 (a)

0

500

1,000

1

10

100

1,000

10,000

100,000

1-12-2005

1-14-2005

1-16-2005

1-18-2005

1-20-2005

1-22-2005

1-24-2005

1-26-2005

1-28-2005

SUSPENDED-SILT & CLAY CONCENTRATION

WATER DISCHARGE

LISST-100

LISST-25X

3-FREQ. ACOUSTIC

CABLEWAY

PUMP

WATER DISCHARGE (m3/s)

CONCENTRATION (mg/l)

 (b)

0

500

1,000

1

10

100

1,000

10,000

100,000

1-12-20051-14-20051-16-20051-18-20051-20-20051-22-20051-24-2005

1-26-20051-28-2005

SUSPENDED-SAND CONCENTRATION

WATER DISCHARGE

LISST-100

LISST-25X

3-FREQ. ACOUSTIC

CABLEWAY

PUMP

WATER DISCHARGE (m3/s)

CONCENTRATION (mg/l)

Figure 5 Examples of the high-resolution laser-acoustic suspended-sediment data from beyond the

period of calibration. No EDI measurements were made from the cableway during this 17-day period.

Silt and clay concentrations were too high during the first 3 1/2 days of this period to get LISST or

acoustic measurements of suspended-sand concentration or median grain size. (a) Silt and clay

concentration, (b) Sand concentration, (c) Suspended-sand median grain size (D50); on next page.
 Figure 5 (c)

0

500

1,000

0.07

0.09

0.11

0.13

0.15

0.17

0.19

1-12-2005

1-14-2005

1-16-2005

1-18-2005

1-20-2005

1-22-2005

1-24-2005

1-26-2005

1-28-2005

SUSPENDED-SAND D50

WATER DISCHARGE

LISST-100

3-FREQ. ACOUSTIC

CABLEWAY

PUMP

WATER DISCHARGE (m3/s)

D

50 (mm)

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