The estuary of the Mersey differs from most others. Towards the mouth, near Liverpool, it is narrow and it widens out further inland. The tides, running through the narrow portion, to fill up the large inland basin and to empty it again, keep the narrow part scoured to a great depth. It was proposed to train the wide portion for the Manchester Ship Canal. The training would, no doubt, have succeeded, but, owing to the silting up of the greater part of the estuary, the scouring near Liverpool would have been very greatly reduced and serious damage done to that port. CHAPTER XV RIVER BARS 1. =Deltaic Rivers.=--When a river flows into a tideless sea its silt deposits and forms a shoal or bar. This shoal may in time extend and rise up to the water-level. The current of the river makes its way through it in various directions, and in this way a delta is formed and constantly extends seawards. This flattens the slope of the lower portions of the river, and causes raising of the bed in the reaches upstream, and this again may cause the water to break out further upstream and form fresh channels to the sea. The bars at the mouths of deltaic rivers are generally formed with great rapidity, and they are apt to form a complete hindrance to navigation. They are sometimes partly scoured away by floods in the river, but in this case the scoured material may deposit on the outer slopes of the bar. If a river which carries silt has no delta, it is probably because there is a littoral current, which prevents the silt from depositing. On the other hand, if a river brings down very heavy sediment, a delta may be formed even when tidal flow is not wholly absent. This occurs in the case of the Ganges.