] What
constitutes a navigable river within the purview of the commerce clause
often involves sharply disputed issues of fact and of law. In the
leading case of The Daniel Ball[286] the Court laid down the rule that:
"Those rivers must be regarded as public navigable rivers in law which
are navigable in fact. And they are navigable in fact when they are
used, or are susceptible of being used, as highways for commerce, over
which trade and travel are or may be conducted in the customary modes of
trade and travel on water."[287] In 1940, over the dissent of two
Justices, the Court held that the phrase "natural and ordinary
condition" refers to volume of water, the gradients and the regularity
of the flow. It further held that in determining the navigable character
of a river it is proper to consider "the feasibility of interstate use
after reasonable improvements which might be made."[288] A few months
later it decided unanimously that Congress may exercise the power of
eminent domain in connection with the construction of a dam and
reservoir on the nonnavigable stretches of a river in order to preserve
or promote commerce on the navigable portions.[289]

The Government does not have to compensate a riparian owner for cutting
off his access to navigable waters by changing the course of the stream
in order to improve navigation.[290] Where submerged land under
navigable waters of a bay are planted with oysters, the action of the
Government in dredging a channel across the bay in such a way as to
destroy the oyster bed is not a "taking" of property in the
constitutional sense.[291] The determination by Congress that the whole
flow of a stream should be devoted to navigation does not take any
private property rights of a water power company which holds a revocable
permit to erect dams and dykes for the purpose of controlling the
current and using the power for commercial purposes.[292] The interest
of a riparian owner in keeping the level of a navigable stream low
enough to maintain a power head for his use was not one for which he was
entitled to be compensated when the Government raised the level by
erecting a dam to improve navigation.[293] Inasmuch as a riparian owner
has no private property in the flow of the stream, a license to maintain
a hydroelectric dam, may, without offending the Fifth Amendment, contain
a provision giving the United States an option to acquire the property
at a value assumed to be less than its f