NAVIGABILITY

A body of water, such as a river, canal or lake, is navigable if it is deep and wide enough for a vessel to pass and there are no obstructions, like rocks, trees and low bridges. Shallow rivers may be made navigable by the installation of locks that increase & regulate water depth. A very high water speed may also make a channel unnavigable and high-latitude waters may be unnavigable in winter because of freezing. Navigability also depends on the size of the vessel: A small river may not be navigable by a freighter, but it might be navigable by a smaller craft, like a motor boat or kayak. Therefore, whether a water body is considered navigable or unnavigable depends on the context.
In addition, navigable waters of the United States, as defined in 33 CFR 329, are those waters that are subject to the ebb and flow of the tide and/or are presently used, or have been used in the past, or may be susceptible for use to transport interstate or foreign commerce. Section 10 of the Rivers and harbors Act approved March 3, 1899 (33 U.S.C. 403)prohibits the unauthorized obstruction or a a navigable water of the U.S. This statute also requires a permit from the U.S. Army Corps of Engineers for any construction in or over any navigable water, or the excavation or discharge of material into such water, or the accomplishment of any other work affecting the course, location, condition, or capacity of such waters.
Also, the Clean Water Act uses the term "navigable waters," "navigable waters of the United States" and "navigablity." These terms are dependent on judicial interpretation and are somewhat more flexible currently, in light of a U.S. Supreme Court decision on two joined cases: "Carabell vs United States" and "Rapanos vs United States.
[ "http://www.usace.army.mil/cw/cecwo/reg/cwa_guide/cwa_guide.htm]