Field methods for flowing waters are determined by whether or not the stream is wadeable. Bioassessment sampling methods are now fairly well established for wadeable streams, which constitute the majority of the length of lotic ecosystems. A better understanding of the characteristics of non-wadeable streams and rivers should advance the development of methods for those ecosystems. We use data collected by the Environmental Monitoring and Assessment Program’s Western Pilot Survey to characterize non-wadeable streams and rivers in the western U.S. Exclusive of the Colorado, Columbia, and Missouri Rivers, non-wadeable streams comprise 11% of the length of lentic systems in the West (9%, 14%, and 22% in Mountains, Xeric, and Plains ecoregions, respectively). Non-wadeable streams were widest in the Xeric ecoregion (median = 54 m) and narrowest in the Plains (median = 32 m) with 25% of Plains non-wadeable streams being < 20 m wide. Mean thalweg depths did not vary by ecoregion and were fairly shallow (median = 1.4 m), with 25% of streams having mean thalweg depths of <1 m. Thus, a substantial number of relatively small streams require the use of boat sampling methods. There was no clear break in these size parameters between wadeable and non-wadeable sites.