3. =Reduction of Width.=--If a channel which is to be narrowed is not a wide one, the reduction in width can be effected by any of the processes described under bank protection (CHAP. VI.). But in a wide channel, reduction of the width by any direct process is generally impracticable. The expense would generally be prohibitive. Earth, if filled in, is liable to be washed away unless protected all along. Reduction in the width of a large channel is nearly always effected either by groynes (fig. 26) or by training walls (fig. 27). Spurs or short groynes for bank protection have been already described (CHAP. VI., _Art. 2_). Groynes for narrowing streams are made in the same way and of the same materials, but are longer. They are at right angles to the stream or nearly so. Groynes in the river Sutlej have been mentioned in CHAP. V., _Art. 5_, and are shown in fig. 6, p. 55. Whether groynes or training walls are used, the object is to confine the stream to a definite zone and to silt up the spaces at the sides. These spaces when partly silted can be planted with osiers or with anything which will grow when partly submerged, and this will assist in completing the silting. [Illustration: FIG. 26.] [Illustration: FIG. 27.] A training wall can be made of any of the materials used for groynes. In order to silt up the spaces between each wall and the adjacent bank of the stream, other walls are run at intervals across them. Usually the training walls and cross walls are carried up only to ordinary water-level, sometimes only to low-water level. Floods can thus spread out and submerge the walls and deposit silt. If the walls are carried up too high it may be necessary, in order to give room for floods, to space them too far apart, and this, as will be seen below, is objectionable. The difference between training walls and groynes is one of degree rather than one of kind. The material most commonly used is, in either case, loose stone--with pitching, if desired, above low-water level,--but it may be wattled stakes. If the water of the stream contains silt at all stages of the supply, gaps can be left in training walls so that silt deposit may occur at all times and not only in floods. If the walls are of wattled stakes, water will pass through them, and it may not be necessary to leave any gaps. Groynes are frequently made with =T=-heads (fig. 26), and they are thus equivalent to training walls with long gaps in them. The edge of the narrowed channel usually forms somewhat as shown in the figure. If the groynes are placed so near together as to give a regular channel, the cost is not likely to be much less than that of training walls.