air value at the time of
taking.[294]

Where the Government erects dams and other obstructions across a river,
causing an overflow of water which renders the property affected unfit
for agricultural use and deprives it of all value, there is taking of
property for which the Government is under an implied contract to make
just compensation.[295] The construction of locks and for "canalizing" a
river, which cause recurrent overflows, impairing but not destroying the
value of the land amounts to a partial taking of property within the
meaning of the Fifth Amendment;--the fee remains in the owner, subject
to an easement in the United States to overflow it as often as may
necessarily result from the operation of the lock and dam for purposes
of navigation.[296] Compensation has been awarded for the erosion of
land by waters impounded by a Government dam,[297] and for the
destruction of the agricultural value of land located on a nonnavigable
tributary of the Mississippi River, which as a result of the continuous
maintenance of the river's level at high water mark, was permanently
invaded by the percolation of the waters, and its drainage
obstructed.[298] When the construction of locks and dams raised the
water in a nonnavigable creek to about one foot below the crest of an
upper milldam, thus preventing the drop in the current necessary to run
the mill, there was a taking of property in the constitutional
sense.[299] A contrary conclusion was reached with respect to the
destruction of property of the owner of a lake through the raising of
the lake level as a consequence of an irrigation project, where the
result to the lake owner's property could not have been foreseen.[300]

JUST COMPENSATION

If only a portion of a single tract is taken, the owner's compensation
includes any element of value arising out of the relation of the part
taken to the entire tract.[301] Thus, where the taking of a strip of
land across a farm closed a private right of way, an allowance was
properly made for value of the easement.[302] On the other hand, if the
taking has in fact benefited the owner, the benefit may be set off
against the value of the land condemned.[303] But there may not be taken
into account any supposed benefit which the owner may receive in common
with all from the public use to which the property is appropriated.[304]
Where Congress condemned certain lands for park purposes, setting off
resulting benefits against the valu