The number of free-flowing rivers in the continental United States (U.S.) has consistently declined as a result of dam and reservoir construction over the last 200 years. Today, more than 50,000 structures impair flow across every major watershed in the U.S. Dams and reservoirs provide important benefits to human systems: mitigating floods, producing energy, providing recreation, and supplying water. However, these anthropogenic structures also bifurcate river networks, threatening the natural exchange of water, sediment, and organisms. In this study, we explore the evolution of natural and anthropogenic fragmentation of U.S. river networks. We use the complete inventory of structures available in the National Anthropogenic Barrier Dataset to map dams to the national river network and divide the network into continuous fragments. We map fragments with and without dams to quantify the degree to which anthropogenic structures have impacted river connectivity across every major U.S. river basin. We also consider both the current state as well as how fragmentation has occurred over time, and the major drivers of fragmentation based on reservoir uses. The analysis is useful for water managers and stakeholders to evaluate the current state of river bifurcation and better inform water management practices that involve dams.