A stream may be made to scour its channel by opening an escape or branch. This causes a draw in the stream, and an increase in velocity for a long distance upstream of the bifurcation (_Hydraulics_, CHAP. VII., _Art. 6_). This procedure is sometimes adopted on irrigation canals. The escape is generally opened in order to reduce the quantity of water passing down, but it may be opened solely to induce scour or prevent silting. The floor of the escape head is usually higher than the bed of the canal, but this does not interfere with operations except at low supplies. It may (CHAP. IV., _Art. 2_) have some effect on the quantity of rolled material passed out of the escape. If there is a weir in the river below the off-take of the canal, and if the escape runs back to the river and thus has a good fall, the scouring action in the canal may be very powerful. If the main channel has a uniform slope throughout, the slope of its water surface is greater upstream of the escape than downstream of the escape, and there is thus an abrupt reduction of velocity and possibly a deposit of silt in the main channel below the escape. This may or may not be objectionable. In the case of an irrigation canal, it is far less objectionable than deposit in the head of the canal. The best point for the off-take of any escape or scouring channel depends on the position of the deposits in the main channel. The off-take should be downstream of the chief deposits, but as near to all of them as possible. A breach in a bank acts of course in the same way as an escape. A stream of clear water when sent down a channel will scour it if the material is sufficiently soft. In the case of the Sirhind Canal, it has already been mentioned (CHAP. IV., _Art. 3_), that when the river water became clear after the floods the proportion of coarse sand, _i.e._ sand above the (·10) class, carried by the canal water was about 1/15,000 by volume. This was in the period from 22nd September to 7th October. From 8th to 23rd October the proportion averaged 1/32,000 from 24th October to 8th November 1/44,000, and from 9th to 24th November 1/85,000. The reason of this reduction was that the comparatively clear water kept picking up the sand from the bed and moving it on, the finer kinds being moved most quickly. As the coarse sand left on the bed became less in quantity, the water took up less. It appears, however, that the water also picked up some clay which was left, and that the total suspended silt in November was 1/9000 of the water. All the observations mentioned in this paragraph appear to have been made at Garhi, 26 miles from the head of the canal. 3. =Production of Silt Deposit.=--Works or measures for causing silt deposit may be undertaken in order to cause silt deposit in specific places where it will be useful, or in order to free the water from silt. Sometimes both objects are combined.