EXECUTIVE SUMMARY. Every State receives title to the beds of its navigable rivers when it joins the Union. The streambeds of navigable rivers become sovereign lands to be held for the public trust and to preserve corridors of trade and travel on the rivers. States own the beds of only the navigable waterways. Therefore, before a State can establish a claim to a specific river, it must first demonstrate that river was, or could have been, navigated by boats or used to float logs as of the time of Statehood. Furthermore, federal law requires that each river’s navigability be determined at a time when it was in an “ordinary and natural” condition, rather the condition of the river at the time of the navigability determination, or at Statehood, if the river’s navigability had already been altered by humans. While federal courts require consideration of a river’s navigability in its ordinary and natural condition, the courts have provided only minimal direction regarding the scientific definition of the terms “ordinary” and “natural” with respect to navigability. This paper recommends technical definitions for both terms relative to a river’s flow rate, as well as to the physical condition of the river’s boating channel. Ordinary flow is defined as the range of flows between base flow and the flow rate that creates the river’s ordinary high-water mark. The ordinary physical condition of the river is defined as the portion of the floodplain that includes the low flow channel up to the ordinary high-water mark. Natural is defined as the conditions (for both flow rate and channel characteristics) absent the impact of humans. Recommended procedures are provided that can be used to determine if a river is still in its natural condition, or if not, how to obtain information that describes the river’s boating characteristics prior to human impacts on navigability. In addition, five case histories are briefly summarized to illustrate the types of technical issues that have been raised in recent navigability decisions in Arizona and Alaska regarding the rivers’ ordinary and natural conditions.