3. =Works in Tidal Rivers.=--If any works are required in the tidal portion of a river, the principles to be followed in designing them are the same as if the river was non-tidal. All that has been said in CHAP. VIII., _Arts. 1_ to _3_, applies to them. The river may be straightened or trained or dredged. Generally training and dredging are combined. Any dredging in the portion of the river nearest the sea will not, of course, alter the water levels near the mouth, but it will alter them further up. The tide will come up in greater volume and will rise higher and extend further up. The ebb will be facilitated, and the low-water level will be lowered. If any narrowing of the channel near its mouth is effected by training walls for the purpose of lowering the bed, the effect on the volume of tidal water entering the river must be taken into consideration. If the narrowing is confined to a reach near the mouth, and if the resulting deepening is not sufficient to counteract the effect of the narrowing, the volume of tidal water reaching the unnarrowed portions of the channel will be reduced, and this may be injurious. Its scouring action may be insufficient. The proper course may be to continue the narrowing upstream. If this is done, then it is obvious that the width of channel in which deep water is to be maintained at high water, or which is to be kept free from deposit, is reduced in about the same proportion as the volume of tidal water is reduced, and no harm is likely to result. Any weir or similar structure which abruptly stops the flow of the tide up a river checks it of course for a long distance back, perhaps to the mouth. Old London Bridge used to obstruct the tide, and its removal increased the range of the tide, and was beneficial. Tidal rivers generally widen out to some extent near their mouths, and are thus rather estuaries than rivers. The works in such rivers are more fully discussed in _Art. 5_.