In the United States, treatment as a state (TAS) provisions enable eligible Native American tribes to assume the same responsibilities as state governments in setting and implementing water quality standards (WQSs). Following the introduction of TAS through 1987 amendments to the US Clean Water Act (CWA), forty-four US tribes have enacted TAS tribal standards, which may be more stringent than those of neighboring states; can incorporate cultural and/or ceremonial uses; and can be used to influence pollution levels coming from upstream, off-reservation users. To evaluate TAS as a model for Indigenous water co-governance, we examine how Native American tribes are advancing tribal sovereignty and environmental sustainability through TAS, and we engage with conflicting views on whether and how Indigenous self-determination can be advanced through existing bureaucratic and colonial governance systems. We specifically analyze environmental pollutant listings in tribal water quality standards for the forty-four TAS tribes. Findings suggest that TAS tribes are creating more culturally relevant WQSs, which are typically as comprehensive as, and often more stringent than, analogous state regulations. Tribal standards are diverse, and TAS tribes can set standards independently from neighboring states and one another. Further analysis reveals the complexities of TAS policy, whereby colonial entanglements both enable and constrain enhanced Indigenous self-determination.