The navigable areas of oceans, rivers, bays, and tidal zones are owned by the public and generally available for public use. A common area of dispute is whether a private property owner has the right to control use of and access to lakes, streams, and rivers on private property (sometimes known as riparian rights)—and whether federal or state governments have the right to regulate use of such waterways.

Much of the question turns on whether the waterway is navigable or unnavigable (nonnavigable). Navigable waters are, as the name implies, waters that can be navigated—or easily made navigable—traditionally for purposes of transporting goods in intrastate (within one state) or interstate (between two or more states) commerce.

Thus, a waterway in its original condition might have had substantial obstructions (falls, rapids, sand bars, bridges, portages, shifting currents, or similar obstructions) that were overcome by frontier boats or portages and used as a channel of commerce—even though boats had to be removed from the water in some stretches, or logs be brought around an obstruction by means of artificial chutes. But the question is ultimately a matter of degree and there is some point beyond which navigability cannot be established.

Lakes and rivers are generally navigable waters, and smaller bodies of water such as streams may also be navigable. The U.S. Supreme Court has created four tests for determining what constitutes navigable waters under federal law: (1) whether the body of water is subject to the ebb and flow of the tide; (2) whether the body of water connects with a continuous interstate waterway; (3) whether the body of water has navigable capacity; and (4) whether the body of water is actually navigable.

Navigable waterways (as defined by federal law) are under the jurisdiction of the federal government—rather than states or municipalities—based on the Commerce Clause (Article I, Section 8) of the U.S. Constitution. But waterways that do not meet the definition of navigable waters under federal law are under the jurisdiction of states and municipalities and may be considered navigable waters under state and municipal laws. Waterways that are unnavigable under both federal and state law may be subject to the control of the streamside landowner.

Laws vary from state to state, but some state courts have ruled that where the state legislature refuses to create a public trust for recreational purposes in unnavigable streams, the courts should not alter the legislature's statement of public policy by judicial legislation. These courts have reasoned that if unnavigable waters are to be appropriated for recreational use, the legislative process is the proper method to achieve this goal.

And in some states the public trust doctrine—a legal theory that requires states to hold and preserve certain natural resources for the public’s use and enjoyment—requires that navigable waterways be available for public use (at least flotation) even though they are located on private land.

In Montana, the public generally has the right to use navigable waterways, which include oceans, rivers, bays, and tidal zones. The determination of whether a waterway is navigable or not is crucial, as navigable waters are subject to federal jurisdiction under the Commerce Clause of the U.S. Constitution, while non-navigable waters fall under state and local control. The U.S. Supreme Court has established criteria for defining navigable waters, including the influence of tides, connection to interstate waterways, navigable capacity, and actual navigability. In Montana, the Stream Access Law allows the public to use rivers and streams for recreational purposes up to the high-water mark, regardless of land ownership. This means that private property owners cannot prevent the public from using waterways that are capable of recreational use. However, the extent of public access may vary based on whether the waterway is considered navigable or non-navigable under federal and state law. Montana has a strong tradition of public access to waterways for recreational purposes, reflecting the public trust doctrine, which mandates that certain natural resources be preserved for public use and enjoyment.