The Thames, moreover, differs entirely from any of the foregoing rivers, and must be considered on its own merits. The map (Fig. 1) shows that it is already dammed and provided with locks at thirty-four places between London and Oxford, the object of these dams being the maintenance of a uniform level of water for navigation and boating purposes, and to prevent the river running dry in the dry season and exposing the muddy foreshores.
But from Teddington Weir to its estuary the Thames is tidal, and there is no obstruction to the tidal flow except the bridges and the half-tide weir at Richmond, which merely holds up sufficient water to cover the foreshores for the advantage of the riparian owners and of boating.
To understand clearly the conditions to be dealt with, it is necessary to consider the daily movements of tide, the affluents, the dock and wharf business and the traffic of the river.
The maps (Figs. 2 and 3) show the tidal river and estuary from Teddington to the North Foreland. [Transcriber’s Note: It seems ‘Teddington’ here is an error for either ‘London’ or ‘Southwark’; that’s what the maps show, anyway.]
The river proper—that is, from Teddington to Gravesend—is forty-six miles long, and averages one-third of a mile wide. Its depth at low water varies from 6 ft. at Teddington to 10 ft. at London Bridge and 40 ft. at Gravesend, and the rise of tide at London varies from 17 ft. to 21 ft. and at Gravesend from 15 ft. to 19 ft., the current usually averaging four knots per hour. At London Bridge the Spring tides flow 5 hours and ebb 7½ hours; while at Gravesend they flow 6 hours and ebb 6½ hours.
The river winds about considerably. The straight line distance from Teddington to Gravesend being thirty-three miles, shows that thirteen miles are added to the river in its bends, some of which—as those at Grays, Erith, Blackwall and Limehouse—are short and tortuous.
The longitudinal section (Fig. 4) of the river from Teddington to Gravesend gives graphically all the data necessary for our purpose. Ordnance Datum (O.D.) is the common datum line of the Government maps. Trinity High Water (T.H.W.) is the water datum usually adopted in the river. High and low water, ordinary and Spring tides (H.W.O.T.—L.W.O.T.—H.W.S.T.—L.W.S.T.) are the levels of the respective states of tide in the river at various points. The highest and lowest known tides are also given, as well as the level of the river bottom and the levels of the principal dock entrance sills and of the crowns of the Thames tunnels, showing their depths below the river bottom.