Hurdle dykes, first used on the Mississippi, were employed on the Indus in 1902 to close partially the main channel of the river. There were to be three dykes, each dyke consisting of three lines of very long piles--some were 60 feet long,--driven into the bed of the stream, which was to be protected with mattresses made of fascines and extending right across it, with their heads above flood-level. The idea was not to wholly stop the flow of the water, but to obstruct it so much that silt would deposit, the channel become choked up, and the water find a course down another channel. The work was begun in March 1902, and was in progress in May of the same year when an unusually early flood put a stop to it. The dykes had at this time advanced considerable distances from the right bank of the stream, but none had been completed. Two dykes out of the three were for the most part carried away. The river, however, took a new course, starting from a point far upstream, the western channel became a creek, and the remains of the dykes were soon embedded in silt. In any case in which the provision of a proper mattress has been omitted, or when the mattress has been destroyed, or when a breach has occurred in an embankment, whenever, in short, it is evident that the gap cannot be closed until some other escape for the water is provided, it may be possible to provide such an escape by cutting partly through the dam or embankment on the downstream side at another place, and thoroughly protecting the place and extending the protection downstream and away from the dam or embankment. The water can then be let in, and the closure of the old gap attempted. If a closure is effected, the protected gap can then be closed. Sometimes it may be desirable to make such a protected gap beforehand and with deliberation. Dams for closing streams which are dry can be made similarly to flood embankments (CHAP. XII., _Art. 7_). Sand does very well, provided it is protected by a covering of clay or by fascining. 3. =Instances of Closures of Streams.=--In 1904 the Colorado River broke into the Salton Sink--a valley covering 4000 square miles. Unsuccessful attempts were made to close the stream by two rows of piles with willows and sandbags between them, by a gate 200 feet long, supported on 500 piles, and by twelve gates each 12 feet wide. A “rock-fill” dam was then constructed on a mattress 100 feet wide and 1·5 feet thick. The river, which was 600 feet wide, broke through, but was stopped by the construction of three parallel rock-fill dams in the gap (_Min. Proc. Inst. C.E._, vol. clxxi.). [Illustration: FIG. 25.]