The sustainable governance of freshwater resources requires that policymakers adapt to changing environmental and socioeconomic contexts. Yet scant literature has offered a systematic understanding of whether or how the issues that water policies address and the tools they use change over time. This paper explores both an original dataset of over 200 water laws passed by the state legislature of Colorado, USA throughout the 20th century and the secondary literature on the history of Colorado water policy. We show how policymakers have developed new policy tools over time in response to Colorado's changing issues, such as shifting and expanding water demands and values toward the environment. At the same time our analysis shows how Colorado's prior appropriation system of water rights places constraints on this adaptive capacity. Finally, the analysis suggests that as new issues have emerged on the agenda, old issues remain, thus challenging the ability of policymakers to respond to the full array of issues in coming years.