Our society uses water for a variety of productive purposes, including domestic, agricultural, mining, manufacturing, and energy development. Most of these uses require physical removal of water from watercourses or ground water aquifers. Water can also serve useful purposes, however, when it remains a lake or stream. Flowing water helps to maintain water quality and furthers other uses such as recreation, aesthetic values, and ecological interests—referred to as “instream uses.”

Large quantities of water must remain in place to safeguard instream uses. At the same time, the increasing demands of consumptive water users are significantly reducing streamflows and lake levels in many parts of the country. Arguably, streamflows are threatened because the existing water allocation regime does not properly recognize instream uses and often favors consumptive uses at their expenses. For this reason, some legal commentators have suggested the public trust doctrine as a basis for protecting streamflows and instream uses.

The public trust doctrine is a common law principle of constitutional dimensions. The doctrine protects the public's interest in certain critical resources by treating the public's interest as a property right which the state cannot wholly alienate. The public trust doctrine has traditionally protected navigation, fishing, and commerce. In its original form, the public trust concept limited the power of private individuals to acquire title to lands beneath tidal waters; however, courts have applied the doctrine to the beds under navigable fresh waters and have extended it to other natural resources as well.

This article begins with an overview of the public trust doctrine. Part III discusses conventional water rights doctrines and how they treat instream uses and streamflows. Part IV examines the reasoning of recent decisions which have applied the public trust principle to water rights. Part V then discusses and evaluates state regulatory efforts in this area which offer alternative methods for protecting instream users.