TIDAL DATUM PLANES eyes 
Geodetic Survey shows that at Philadelphia, about 100 miles up the Delaware, the 
mean sea level (or mean river level) is about three-quarters of a foot higher than 
mean sea level on the coast, notwithstanding the fact that the range of the tide at 
Philadelphia is somewhat larger than at the mouth of the river. 
Widening the mouth of a tidal river increases the cross-sectional area of the 
channel through which the drainage waters flow into the sea. As a consequence of the 
enlarged channel the drainage waters have freer outlet, which results in a lowering of 
the mean sea level some distance upstream. Deepening the mouth has a like effect; in 
addition, by reducing the friction per unit volume of water it brings about a further 
lowering of the mean level of the water. 
Tf as a result of widening and deepening the mouth of a tidal stream the mean sea 
level at a given point of the river is lowered D feet and the range increased A feet, the 
changes in the different datums would be: Mean sea level, lowered D feet; mean high 
water, lowered D—% A feet; mean low water, lowered D+% A feet. The slope of 
mean, sea level up a tidal stream is relatively slight, but the increase in range of tide 
consequent upon improvement is relatively large. For example, at Glasgow, on the 
Clyde, the range was increased 8 feet by river improvements. It follows, therefore, 
that as a rule in the above formulas D is less than % A. This means that D—¥% A is 
negative and that high water, instead of being lowered, is raised somewhat. Low 
water, however, is lowered by the full amount of the increase in half range plus the 
depression of mean sea level. 
These considerations are important in connection with the improvement of tidal 
rivers, since the depths in these are generally referred to mean low water. When 
improvements are contemplated the wording is generally to the effect that a certain 
depth at mean low water is to be attained. At first thought it would appear that if 
the desired depth below mean low water is a feet and the present depth is 6 feet the 
channel is to be deepened a—b feet. But as the considerations outlined above show 
this is not the case, for the datum plane of mean low water is different in the two cases. 
