The dimensions of a bank should depend to some extent on the head of water against it and on the volume of the stream whose water it holds up. A breach is obviously more serious the greater the volume of the escaping water. This volume depends on the size of the stream and on its velocity. In navigation canals in England the bank on the side opposite the towing-path is usually 4 to 6 feet wide and 1½ feet above the water. In irrigation canals in India the bank of a very large canal is 2 feet above the water and 20 feet wide, while that of a small canal with 6 feet of water is 8 or 10 feet wide and 1½ feet above the water, and that of a small distributary channel with 3 feet of water is 4 feet wide and 1 foot above the water. The soil is often poor. Further remarks, which apply to banks of special height or special importance, are given under Embankments (CHAP. XII., _Art. 6_). 2. =Navigation Canals.=--A navigation canal is sometimes all on one level, but generally different reaches are at different levels, the change being made by means of locks. A “lateral” canal--the most common kind--runs along a river valley more or less parallel to the river. It is frequently cheaper to construct such a canal than to canalise the river. A “summit” canal crosses over a ridge and connects two valleys. A navigation canal requires a supply of water to make good the losses which occur by lockage, leakage, or absorption and evaporation. A canal may be of any size, according to the size of the boats which are to be used. There is always room, except in short reaches where the expense of construction has to be kept down, for two boats to pass one another. A lateral canal obtains water from the river or from the small affluents which it crosses. For a summit canal it may be necessary to provide storage reservoirs. The canal crosses the ridge where it is low, and the reservoirs are made on higher ground. Reservoirs may be required also for other canals to hold water for use in dry seasons or in order to fill the canal quickly when laid dry for repairs. In tropical countries weeds grow profusely in canals which have still or nearly still water. Traffic tends to keep them down, but they have to be cleared periodically.