Fluvial sediment and sorbed materials are the most widespread pollutants affecting United States (U.S.) rivers and streams (http://iaspub.epa.gov/waters/national_rept.control#TOP_IMP). The need for reliable, comparable, cost-effective, spatially and temporally consistent data to quantify the clarity and sediment content of waters of the U.S. has never been greater. The number of sites at which the U.S. Geological Survey (USGS) collected nationally consistent daily sediment data in 2006 was about a quarter of the number operated in 1981 (D. W. Stewart, personal communication, 2008). This precipitous decrease in sediment monitoring over a quarter century by the USGS, the federal agency responsible for collecting, archiving, and disseminating the nation's water data, including fluvial sediment [Glysson and Gray, 1997], is due to a number of factors, principally cost [Gray, 2003a, 2003b]. The decrease in monitoring is of particular concern, given that the physical, chemical, and biological damages attributable to fluvial sediment in North America alone are estimated to range between $20 billion and $50 billion annually [Pimentel et al., 1995; Osterkamp et al., 1998, 2004]. Given this dearth in adequate, consistent, and reliable data describing fluvial sediment fluxes, decision makers responsible for mitigating its deleterious effects are at best hard-pressed to develop technically supportable management and remedial plans.
 Historically, riverine suspended sediment data in the U.S. have been produced by gravimetric analyses performed on water sediment samples collected manually or by automatic samplers [Edwards and Glysson, 1999; Bent et al., 2003; Nolan et al., 2005; Davis, 2005; Gray et al., 2008]. These data collection methods tend to be expensive, difficult, labor intensive, and hazardous under some conditions. Specialized equipment and considerable training are prerequisites for obtaining reliable samples and results. The characteristic paucity of the derived data may be inadequate for defining the variability in suspended sediment concentrations (SSC) and particle size distributions (PSD), particularly for periods of storm runoff. Consequently, temporal interpolations and calibrations along with spatial corrections to the data are commonly required to develop the requisite SSC time series used with an associated time series of water discharge data to produce subdaily and daily records of suspended sediment discharges [Porterfield, 1972; Koltun et al., 2006].
 Existing and emerging sediment surrogate technologies may provide the types and density of fluvial sediment data needed to improve sediment discharge computations in a range of river types and sedimentological conditions [Gray and Gartner, 2004]. Potentially useful instruments and methods for inferring selected physical characteristics of fluvial sediments [Gartner et al., 2003; Bogen et al., 2003; Gray, 2005; Gray et al., 2003b, 2003c] are being developed and tested around the world. Through the informal Sediment Monitoring Instrument and Analysis Research Program [Gray, 2003a, 2003b], the USGS is testing instruments operating on bulk optic (turbidity), laser optic, pressure difference, and acoustic backscatter principles in U.S. rivers and in laboratory settings for measuring selected characteristics of suspended sediment, bed load, and bed material. To make the transition from research to operational applications, these new technologies must be rigorously tested with respect to accuracy and reliability in different physiographic settings, and their performances must be compared to the aforementioned traditional techniques. The performance comparisons should include concurrent collection of data by traditional and new techniques for a sufficient “shake-out” period, probably years, to identify and minimize changes in bias and precision between the old and new technologies.
 Even after the “shake-out” period, each of the four in situ technologies will require periodic calibration in field applications to define the relation of the surrogate measurement to the mean value in the cross section [Porterfield, 1972]. However, the need for routine calibration is expected to diminish over time.
 None of the technologies examined herein represents a panacea for sediment monitoring at all rivers under all flow and sediment transport conditions. However, with careful matching of proven monitoring technologies to the physical and sedimentological characteristics of selected river reaches, it may be possible in the coming years to remotely and continuously monitor suspended sediment discharges, in some cases by particle size class, with sufficient reliability to store the information as public-releasable data in the USGS National Water Information System (http://waterdata.usgs.gov/nwis). Calculation and publication of some uncertainties associated with variables used in computations of SSC and suspended sediment discharge records in a variety of river types over a large range of flow and sedimentary regimes may also be possible (http://ks.water.usgs.gov/Kansas/rtqw/sites/06892350/htmls/2005/p63680_2005_all_uv.shtml).
 The prospect of large-scale application of proven suspended sediment surrogate technologies is a revolutionary concept in fluvial sedimentology when considered from a worldwide or even national perspective. The benefits of such applied capability could be enormous, providing for safer, more frequent and consistent, arguably more accurate, and ultimately less expensive fluvial sediment data collection for use in managing the world's sedimentary resources.
 This paper describes four commercially available surrogate technologies that operate on bulk optic (turbidity), laser optic, pressure difference, or acoustic backscatter principles for monitoring SSC and in some cases PSD. These technologies are being evaluated in field settings by the USGS with varying degrees of promise toward providing continuous, largely automated subdaily time series of SSC data in rivers. The paper begins with a description of traditional techniques for suspended sediment sampling, against which the surrogate technologies are evaluated. Descriptions of the theory, applications, evaluations, and some advantages and limitations of each technology are presented and compared.
1.1. Background: Traditional Suspended Sediment Sampling Techniques in the United States
 Instruments and methods for collecting fluvial-sediment data in the U.S. have evolved considerably since 1838 when Captain Andrew Talcott of the U.S. Army Corps of Engineers first sampled the Mississippi River [Federal Interagency Sedimentation Project, 1940]. The earliest suspended sediment samples were collected by use of instantaneous samplers, such as open containers or pails. By 1939, at least nine different types of sediment samplers were being used by U.S. agencies. Most of the samplers had been developed by independent investigators, lacked calibrations, and were deployed using a variety of methods. A 1930s survey of sediment sampling equipment used in the U.S. indicated that the 30 instantaneous samplers studied had limited usefulness either because of poor intake velocity characteristics or because of the short filament of water-sediment mixture sampled [Federal Interagency Sedimentation Project, 1940; Nelson and Benedict, 1950; Glysson, 1989].
 In 1939, six federal agencies and the Iowa Institute of Hydraulic Research organized a committee to consider the development of sediment samplers, sampling techniques, and laboratory procedures and to coordinate such work among the federal agencies “actively concerned with the sedimentation problem” [U.S. Department of Agriculture, 1965]. This committee has evolved into the present-day Subcommittee on Sedimentation, Technical Committee, and Federal Interagency Sedimentation Project (FISP) [Skinner, 1989; Glysson and Gray, 1997; http://acwi.gov/sos/]. The purpose of the FISP is to study methods and equipment used in measuring the sediment discharge of streams and to improve and standardize equipment and methods where practicable.
 The bulk of suspended-sediment data obtained by federal agencies using traditional sampling techniques are collected by isokinetic samplers and methods developed by the Federal Interagency Sedimentation Project  and described by Edwards and Glysson , Davis , Nolan et al. , and Gray et al. . These include samplers with rigid sample bottles (bottle samplers) and flexible bags (bag samplers) that fill at a rate determined by the product of the ambient stream velocity at the sampler nozzle and the nozzle's area. These samplers, ranging in mass from 2 to 125 kg, are designed to collect a representative velocity-weighted sample of the water-sediment mixture at the deployment location within the sampler's flow velocity and transit rate limits. FISP isokinetic samplers are designed to ensure that the water velocity entering the nozzle is within about 10 percent of the stream velocity incident on the sampler throughout the samplers′ operable velocity range to minimize bias in SSC and PSD measurements. Figure 1 shows the effect of sampling rate on measured SSCs for four PSDs [Gray et al., 2008].
 A list of FISP samplers and selected attributes is provided by Davis  and Gray et al. . Examples of a rigid-bottle sampler, the U.S. D-74, and a bag sampler, the U.S. D-96, are shown in Figure 2.
 When deployed using either the equal discharge increment (EDI) or equal width increment (EWI) sampling method [Edwards and Glysson, 1999; Nolan et al., 2005], an isokinetic sampler integrates a sample proportionally by velocity and area, resulting in a discharge-weighted sample that contains a concentration and size distribution representative of the suspended material in transport throughout the cross section at the time that the sample is collected.
 Although manual isokinetic samplers have considerable benefits, most notably the acquisition of demonstrably reliable suspended-sediment data from rivers, they have consequential drawbacks. For example, total noncapital costs associated with manual deployment of isokinetic samplers (about a half-person day for consecutive EDI or EWI samples per site excluding transportation) and subsequent analytical costs (typically tens to hundreds of dollars depending on types of analyses) can be substantial with respect to available resources. Safety issues are paramount whenever a hydrographer works in, over, or near a stream. The time and effort required to collect manual samples by traditional methods precludes their use to resolve high-frequency sediment transport dynamics. The sparse temporal distribution of the derivative data, often a single observation or less per day, requires that daily load computations be based on estimated concentration values and (or) stochastically indexed to another more plentiful if imperfect predictive data source such as river discharge through sediment transport curves [Glysson, 1987; Gray and Simões, 2008].
1.2. Performance Criteria for Data Produced by Sediment Surrogate Technologies
 A number of advances in surrogate technologies used to compute SSCs and in some cases PSDs have been made in recent decades. However, verification data, particularly certifiably reliable verification data covering the broad range of flow and sedimentological conditions, are often lacking.
 Validation of a sediment surrogate technology requires evaluation criteria and a well-conceived and well-administered testing program [Gray et al., 2002; Gray and Glysson, 2005]. Following are some qualitative criteria for selecting and deploying a surrogate technology:
 1. Capital, operating, and analytical costs should be affordable with respect to the objectives of the program in which the monitoring instrument is deployed.
 2. The technology should be able to measure SSC, and in some cases PSD, throughout the range of interest.
 3. The instrument should be robust and reliable; that is, prone to neither failure nor signal drift.
 4. The technology should be sufficiently simple to deploy and operate by a field technician with a reasonable amount of appropriate training.
 5. The derived data should be relatively simple and straightforward to use in subsequent computations and (or) accompanied by standard analytical procedures as computational routines for processing the derived data.
 Quantitative criteria for acceptable accuracies of the derived data are difficult to develop for all potential applications, in part because of significant differences in river sedimentary and flow regimes. For example, accuracy criteria for rivers transporting mostly silt and clay in suspension should be set more stringently (intolerant of larger-magnitude uncertainties) than those for rivers that transport comparatively large fractions of sand. However, there is a clear need for consistency in PSD and SSC criteria on the part of instrument developers, marketers, and users.
 To this end, acceptance criteria developed for PSD and SSC data produced by a laser diffraction instrument [Gray et al., 2002] have been generalized for evaluating data from any suspended sediment surrogate instrument. At least 90% of PSD values between 0.002 and 0.5 mm median diameter are required to be ±25% of true median diameters. Absent a more rigorous evaluation, this criterion has been applied to all particle sizes in suspension.
|Suspended-Sediment Concentration Minimum, g/L||Suspended-Sediment Concentration Maximum, g/L||Acceptable Uncertainty, %|
|0.01||<0.1||50–25 computed linearly|
|0.1||<1.0||25–15 computed linearly|
 These criteria pertain solely to the performance of a surrogate technology within its physical realm of measurement. Routine calibrations to correlate instrument signals to mean cross-sectional SSC values are required for all of the in situ instruments presented herein.
 Because of the spatial and temporal variability in river sedimentological regimes, only generalities regarding the expected range of SSCs and PSDs in rivers can be made in the absence of site-specific data. Rainwater  produced an empirically derived map of the 48 conterminous U.S showing mean SSCs for rivers (generalized for the entire land area) over seven logarithmically based SSC ranges. The SSC ranges were computed from measurements of the annual suspended sediment load divided by the annual streamflow. Computed SSC values in the largest range exceeded about 48 g/L. Using a similar computational scheme, Meade and Parker  and the U. S. Geological Survey (http://co.water.usgs.gov/sediment/conc.frame.html) simplified the Rainwater  map to show areas characterized by SSC in the following ranges: less than 0.3 g/L; 0.3–2 g/L; 2–6 g/L; and more than 6 g/L. These maps can serve as initial, general indicators of the suitability of a selected sediment surrogate technology in a river reach of interest.
 Additional information on the range of SSCs in U.S. rivers is available from Smith et al. , who computed percentile values for SSC data collected at 267 USGS streamgages in medium and large river basins as part of the original USGS National Stream Quality Accounting Network (NASQAN) (http://water.usgs.gov/nasqan/). The 25th, 50th, and 75th SSC percentiles were 0.02, 0.07, and 0.19 g/L, respectively. In 1995, the NASQAN network was redesigned to focus on the nation's largest rivers basins, the Mississippi (including the Missouri and Ohio), Columbia, and Colorado rivers and the Rio Grande. A. Horowitz (personal communication, 2008) calculated the 10th, 25th, 50th, 75th, and 90th SSC percentiles for the 41 NASQAN streamgages in these large river basins for the period 1994–2006 as 0.01, 0.03, 0.12, 0.32, and 0.74 g/L, respectively.
 Many streams transport near-zero SSCs at various times. On the other extreme, SSCs measured during surface runoff from 1989 to 1991 in the Little Colorado River basin, Arizona and New Mexico, commonly exceeded 100 g/L [Graf et al., 1995]. Maximum SSC values measured at the USGS streamgage on the Paria River at Lees Ferry, Arizona, have exceeded 1000 g/L [Beverage and Culbertson, 1964].
 In general, most of a river's annual sediment budget is transported during infrequent high-flow periods concomitant with high SSCs. Any proposed suspended sediment surrogate technology deployment should take into consideration not only the statistics quoted above but also the potential maximum SSC and, where appropriate, maximum particle sizes that might be transported in the period of interest.
 After surrogate technology efficacy is resolved, cost considerations are often of penultimate interest. The cost of producing reliable, quality-assured suspended sediment data can be separated into four categories: (1) the purchase price of the instrument; (2) other capital costs associated with installation and initial operation of the instrument; (3) operational costs to maintain and calibrate the instrument; and (4) analytical costs to evaluate, reduce, compute, review, store, and publish the derivative data.
 Of these four categories, only the current purchase price is straightforward to quantify. The others are dependent on a number of factors, including site location and physical characteristics, hydrological and sedimentological regime, availability of electrical power, limitations associated with accessibility, safety considerations, and the time and complexity associated with data analysis. Additionally, any such cost information inevitably becomes obsolete due, in part, to technological advances, marketing competition, and changes in currency valuation. Hence, the relative purchase prices are proffered for the surrogate instruments described herein versus the actual (summer 2008) purchase price for the most common of the instruments, an in situ fully equipped turbidimeter. In some instances, other relevant cost information for a given technology that is considered reliable is provided. That information may be considered in light of the fact that the cost to compute, store, and provide daily sediment discharge data at a USGS streamgage in 2001 (adjusted for inflation in 2008 dollars) ranged from $24,000 to $78,000 [Gray, 2003a].