ON PHYSICAL AND ENGINEERING FEATURES OF THE MERSEY. 551 
North Lancashire, and which, after receiving the waters of minor streams, 
passes through the city of Manchester, and joins the Mersey at Flixton, 
eight miles lower down. Between that point and Warrington the Mersey 
receives from the adjoining marshes the waters of several small streams. 
The aggregate drainage area of the combined catchment basins above 
Warrington is about 750 square miles. From Warrington downward to 
Runcorn, a distance of about ten miles, it partakes of the form of an 
ordinary narrow tidal river, passing through the low-lying marsh lands of 
the district with little fall. 
At the town of Runcorn, where the name Runcorn Gap fitly describes 
the narrow and special configuration of its high and abrupt red sandstone 
shores, the Mersey is crossed by the high-level viaduct which carries the 
London and North-western Railway to Liverpool. From this point the 
river passes into the enlarged portion of the estuary, which at high tide 
assumes the appearance of a large inland lake. Reaching seaward to the 
south end of Liverpool, a distance of about 12} miles, by from 2 to 3 
miles wide, with an area of 30 square miles, it is filled to about. half-tide 
level with a deposit of sand, which mostly becomes dry at low tide. This 
part of the river, owing to its form and size, plays an important part in 
maintaining the deep water abreast of Liverpool, as well as the sea 
channels. 
- About 24 miles below Runcorn the river Weaver passes into the 
Mersey on its left bank, and, with its tributaries, forms its most important 
adjunct, being the chief drainage basin of mid-Cheshire, with a water- 
shed of 550 square miles. 
Below the mouth of the Weaver the adjoining part of Cheshire 
is drained by the Gowy and a few other insignificant streams, while 
on the Lancashire side minor streams of a like character drain that 
district. 
The drainage of the city of Liverpool is effected by an ordinary system 
of sewers which pass into large intercepting culverts, carried at intervals 
through the Dock Estate into the river. Owing to the large volume of 
‘tidal water which daily passes backwards and forwards, the material from 
this source is swept away, leaving little or no trace of fouling along the 
foreshores. 
The aggregate drainage area of these several rivers and streams 
is computed at 1,724 square miles. The total amount of up-river water 
discharging into the estuary in each twelve hours is estimated at from 
two to three million cubic yards, while the volume of tidal water on high 
Springs is computed at about 710 million cubic yards, and on low neaps at 
281 million cubic yards. It should not be inferred from this disparity in 
quantity between the volume of upland water and that of the tides that 
the former does not play an important part in the régime of the river— 
on the contrary, the river channel is formed primarily by the land water, 
and the wandering tendency which it displays in its downward course to 
the sea is the first step towards insuring the capacity of the estuary 
being fully maintained. This is effected by its action in grooving out the 
surface of the sandbanks, so forming minor channels to receive the 
in-flowing tide, which, running through them with great velocity, enlarges 
and extends them. This process, repeated in all the varying positions 
which the channels take, ploughs up the whole area of the estuary from 
‘shore to shore, so preventing the growth of the banks by accretion and 
the tidal displacement which would follow such accretion. 
