132” U. §. COAST AND GEODETIC SURVEY 
Whatever the changes in the rise and fall of the tide along an open ocean coast due 
to gradual subsidence, it is obvious that the mean sea level, fixed with respect to local 
bench marks, will show an apparent change of the same magnitude as the subsidence, 
but in the opposite direction. That is, if Sis the subsidence in feet, from the beginning 
of the period, the mean sea level would appear to have risen S feet. But, if because of 
the alteration of the hydrographic features consequent on the subsidence, the range of 
the tide is increased by A feet, mean high water will appear to have risen S+-}4 A feet, 
while for mean low water the apparent rise will be S—% A feet. Thus the datums of 
mean high water, mean sea level, and mean low water, with respect to local bench marks 
will be changed by different amounts. 
On a rising coast the changes that would take place in the tidal datums are similar 
but in the opposite direction to those just discussed. The first effect would be a lower- 
ing of all the tidal datum planes with respect to local bench marks by the amount of 
emergence. If the emergence becomes sufficient to alter materially the hydrographic 
features, and so bring about a change in the rise and fall of the tide, the changes ensuing 
from this latter cause would, as in the case of subsidence, be different for the different 
datums. 
Changes Due to Alteration of Hydrographic Features 
It is with regard to inland tidal waters that changes in datum planes due to altera- 
tion of hydrographic features become important. On the open coast it is reasonable to 
assume that only profound changes in the hydrographic features can bring about changes 
in the range of the tide. But in inland tidal waters, because of the relatively limited 
areas and depths involved, changes in the features of considerably lesser magnitude are 
sufficient to change the range of the tide and thus bring about changes in datums. 
While the quantitative relations subsisting between changes in the body of water and 
changes in datums are difficult to establish from general considerations, qualitatively 
we may determine the changes in the datums that will follow proposed changes in the 
hydrographic features. 
Tidal rivers are good examples of such inland bodies of water. These rivers serve 
as highways to the sea for numerous ports, some of which are situated many miles from 
the coast. With the increased draft and size of modern vessels, changes in depth or 
other alterations are frequently found necessary; and such improvements, if of sufficient 
magnitude, result in changes in the local tidal datum planes. 
The tides in rivers are due to the tides sweeping into them from the seas into 
which they open. Normally the tide travels upstream until stopped by falls or rapids. 
If the mouth of the river is widened or deepened, this makes for a freer entry of the 
tide from the open sea and thus for a greater rise and fall of the tide. As a first effect, 
therefore, of widening or deepening a tidal river at its mouth, we may expect a rise in 
the high-water datums and a fall in the low-water datums. This effect, it is reasonable 
to expect, will generally be greatest near the mouth of the river, becoming gradually 
less going upstream. 
Tidal rivers serve, however, not only as highways for the tide, but also as channels 
for carrying to the sea the drainage waters from large territories. Normally the cross- 
sectional area of a river increases seaward, due to the seaward slope of the river bed and 
the increasing width between banks. As a consequence, the mean river level in a 
tidal river becomes higher in going upstream. Thus, precise leveling by the Coast and 
