4. =Tidal Estuaries.=--If, instead of a river of uniform width, there is an estuary whose width increases steadily towards the sea so that it is funnel-shaped, the conditions described in _Art. 2_ are modified. An estuary is formed first by the waves of the sea, which wear away the angles at the mouth of the river and allow the tide to enter in greater volume, and then by the flow and ebb of the tides. The slope of the bed of the estuary is usually much flatter than that of the river, and the water surface is as shown in fig. 67. The tidal movements extend further upstream than in the case of a river, not only because of the greater difficulty experienced by the upland water in filling up the wide channel of the estuary, but because of the momentum of the tidal water driving its way up the funnel-shaped channel (_Art. 1_). The capacity of the estuary is of course much greater than is required for the discharge of the upland water alone. If the sea-level remained always at one height and if the upland water contained silt, it would tend to deposit in the estuary and would certainly deposit in it to some extent. The action of the sea water is the same as described in _Art. 2_, scouring if it is clear when entering, of less account if it is not clear. Owing to the funnel shape of the estuary, the tide rises higher at its upper end than if the estuary were replaced by a river channel, and the tide also extends further up. This may partly or wholly compensate for the greater tendency of silt to deposit in an estuary as compared with a river channel. The ebb tide in an estuary does not always follow exactly the same course as the flood tide. Of course the lowest parts of the estuary are filled first and emptied last, but the channels are not all continuous. A channel open at its lower end may have a dead end at its upper termination, and _vice versa_. Also, at sharp bends in the channels, the momentum of the water may cause differences in the paths traversed by the flowing and ebbing currents. Wherever there is a deep channel the water from the adjacent sandbanks tends, towards the close of the ebb, to flow cross-wise into the channel, and in doing this it to some extent washes down the banks into the channel.