The United States Clean Water Act (CWA) is one of the key legal means in the USA to ‘restore and maintain the chemical, physical and biological integrity of the nation's waters’. Given the pervasive influence of human development and associated climate change in increasing water temperatures in streams of the USA, salmonids are particularly susceptible to reduction in productivity and geographic distribution. Native and introduced, self-sustaining salmonid populations can be found in most of the 50 States of the US. Despite this commonly shared resource, the highly similar temperature sensitivity among salmonids, and the legal imperative under the CWA to provide full protection to the most sensitive uses, the States supporting these thermally sensitive species have adopted a wide range of standards. As these standards are so divergent, even though the protection goal under the CWA applies uniformly to all States, it is clear that water temperature standards have been developed under conflicting interpretations of the best science available or there is a misunderstanding of the level of protection needed. The current EPA Gold Book guidance for development of protective standards, dating from 1973, still recommends the use of MWAT (maximum weekly average temperature) as a means of assigning protective chronic temperature standards to coldwater fisheries. MWAT, applied according to EPA guidance, is typically used in conjunction with an acute upper limit. From its inception, evidence was available to show that MWAT was inadequate to protect against chronic thermal impairment. This review of temperature standards, applied across the 50 States, collectively reveals a set of ecologically based principles that can be extracted from available standards and would provide a better measure of protection. It is deduced that standards might better apply to optimum temperatures for each life-history stage to protect against chronic thermal effects. These should include: geographic identification of core spawning and rearing areas; recognising cumulative warming from multiple sources; a limit on rate of warming or cooling; special standards for salmonids with exceptionally lower specific temperature requirements, requiring natural thermal patterns on a daily, seasonal, and annual cycle; and regulating the frequency of exceedence of standards on a multi-year basis. The diverse temperature standards found in the statutes of individual States to protect fish species with highly similar biological requirements are indicative of the failure of States to provide consistently high levels of protection and of the EPA to ensure State application of the best science through its standards approval process. In addition to appropriate standards, monitoring, listing of streams as water quality impaired, and development of restoration plans are essential to the success in protecting the coldwater fish resource.