Fluvial transport is recognized as a common manner by which bone assemblages forming in or near moving water can be taphonomically modified. Here we study an assemblage of 38 cow carcasses, killed during a mass-mortality event and deposited in a fluvial system (the Yellowstone River of eastern Montana), over the course of three years. Seven of the 38 carcasses were observable throughout the study, allowing assessment of patterns of disarticulation and element loss with respect to time and microenvironment. The probability of loss of individual elements corresponds broadly with Voorhies' bone transport experiments, modified to account for disarticulation order. Evidence of scavenger-mediated bone loss is minimal, despite abundant evidence of the presence of scavenging taxa. Individual carcasses show different patterns of element loss, depending on the magnitude of fluvial energy that the carcass was subject to. Carcasses in very high energy regimes lose all elements immediately on disarticulation, whereas carcasses in lower energy regimes show a more typical lag-assemblage profile. Comparison of disarticulation patterns from similar modern and fossil assemblages indicates the Yellowstone cow assemblages define part of a widely expressed spectrum of bone modification in fluvial systems, strongly controlled by the magnitude of transport energy to which bone assemblages are exposed.