With water resources becoming scarcer every year, studying the negotiation processes that leads to the treaties that govern the distribution of water rights across national boundaries is more important than ever. Sub-state units sometimes play an important role in the negotiation of such treaties and thus it is important to better understand how specific types of regional, transborder sub-state units operate both within the context of their respective federal systems and in relation to each other. Exploring the case study of the Columbia River Treaty—governing the area known as the upper Columbia River Basin— and the Water Policy Working Group of the Pacific NorthWest Economic Region (PNWER), this thesis uses documentary sources and original qualitative interviews to examine the roles of sub-state governments and federalism on the original Columbia River Treaty negotiations, as well as the effect the historical legacy of the Treaty negotiations has left on constituent diplomacy today. Examining constituent diplomacy through a micro rather than macro lens, the thesis also explores the role of constituent diplomacy within PNWER with respect to the upcoming Columbia River Treaty negotiations and the impact that PNWER’s governance model has on the effectiveness and functionality of constituent diplomacy. The thesis demonstrates that sub-state governments, specifically the B.C. government, played a vital role in original Treaty negotiations, and suggests that the past Treaty negotiations are relevant to constituent diplomacy today because of modern-day and future Treaty negotiations and ongoing Treaty implementation. Drawing on documentary evidence and original interview data, this thesis details current PNWER governance structures and practices and then moves to argue that the structure, functions and dynamics of the Canadian and American federal systems can impact the effectiveness and functionality of constituent diplomacy.