A large canal is provided, so far as is practicable, with “escapes” by means of which surplus water may be let out. Surplus water occurs chiefly after rain. At such times the demand for water may suddenly be reduced and if there were no escapes there would probably be serious breaches of the banks before there was time for the reduction of water, effected at the head of the canal, to take effect lower down. There is usually an escape at some point in the main line, preferably at a point where it divides into branches, and this escape runs back to the river. There may also be escapes near the tails of the longest branches. These escapes may discharge into drainages or into reservoirs formed by running a low embankment round a large area of waste land. The drainage of the whole tract irrigated by a canal must be carefully seen to. The subsoil water level of a tract of country is nearly always raised by an irrigation canal. The rise near to a canal or distributary is due to percolation from the channel and is inevitable.[3] The rise at places further away, if it occurs, is due to over-watering or to neglect of drainage. Immense damage has been done by “water-logging” of the soil when irrigation water has been supplied to a tract of flat country and the clearance and improvement of the natural drainages has not been attended to. Any drainages crossed by the banks of the irrigation channels should be provided with syphons or aqueducts or else the drainage diverted into another channel. Very frequently the main line of a canal--whether great or small--in the upper reaches near the hills, has to cross heavy drainage channels or torrents and large and expensive works are required for this. [3] But see Chap. V. as to reduction of percolation. Near the head of a canal and of every branch and distributary, there is an ordinary gauge which shows the depth of water and is read daily. The gauge near the head of a main canal is generally self-registering. The principles sketched out in this article are those generally followed in the designs of modern canals. They have by no means been followed in all cases. In some of the older Indian canals both the canal and the distributaries ran in low ground. Water-courses took off direct from the canals, and the irrigation did not generally extend far from the canal. In fact long distributaries were impracticable because they would have run into high ground. The banks of the channels obstructed drainages and caused pestilential swamps. Most canals of this type have been abolished since the advent of British rule and replaced by others properly designed. Some badly designed canals however, mostly of the inundation class, still exist but in very dry tracts where drainages are of little consequence.