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The substrate of fluvial systems is regularly characterized as part of a larger physical habitat assessment. These measures are instrumental in meeting the regulatory responsibilities of bioassessment and monitoring programs, and essential to monitoring restoration and rehabilitation success. We describe and validate a commonly used technique for broadly categorizing, and thus characterizing, the substrate in non-wadeable streams and rivers called the sounding rod method. In brief, a rod, often hollow, is used to probe the substrate of non-wadeable systems to characterize the substrate. We tested the viability of this method on three different systems by comparing estimated particle class and direct particle measurements. Our results indicate that substrates can adequately be defined into six broad classes (fine-particle sediment, sand, gravel, cobble, boulder and bedrock) based on size using the sounding rod. Estimated classes were significantly positively correlated to measured classes (tau = 0.83, p< 0.001), and estimates of size class and direct measurements of size were not from significantly different distributions (chi(2)(0.05,9) = 569.51, p< 0.001). Further, there were significant differences between each category (H= 243.5, 3 d.f., p< 0.001). Although our results affirm that actual substrate class size can be directly inferred from estimated data, it should be noted that soft sediments <200mm were not always detected. This finding should be carefully considered with individual study objective. Overall, the sounding rod method can be learned quickly, and it is a low-cost and time-efficient method for substrate classification. Published 2013. This article is a U.S. Government work and is in the public domain in the USA.