Generally the best method to adopt in a closure is to cover the bed of the channel beforehand--unless it is already hard enough--with a mattress or floor, such that it cannot be scoured as the gap closes. A floor may consist of a number of stones or sandbags dropped in from boats or by any suitable means, and placed with care so that there shall not be gaps or mounds. Sandbags should be carefully sewn up. A mattress may be made of fascines laid side by side and tied together, floated into position, weighted and sunk. Even a carpet made of matting or cloth and suitably weighted has sufficed in some cases. If the scour is likely to be such that stones or sandbags will be carried away, the stones may be placed in nets, baskets, or crates. Sandbags may also be placed in nets. Probably the long rolls of wire-netting filled with stones, described in CHAP. VI., _Art. 3_, would be better than anything, and the diameter could be reduced somewhat. The floor or mattress need not usually extend right across the stream. It must cover a width much greater than--perhaps twice as great as--the width of the gap is likely to be when scour begins. Its length, measured parallel to the direction of the stream, must be such that severe eddies in the contracted stream will have ceased before the stream reaches its downstream edge. It need not extend to any considerable distance upstream of the line of the dam. The dams when started from the banks can generally be of simple earth or gravel, or loose stones, but before they have advanced far they will probably require protection at the ends by stones, or by staking and brushwood, or by fascines. As soon as the dams have advanced well onto the mattress and their ends have been well protected, it is best to cease contracting the stream from the sides and to contract it from the bottom by laying a number of sandbags across the gap so as to form a submerged weir. In this way the rush of water is spread over a considerable width of the stream. The weir is then raised until it comes up above water. Leakage can be stopped by throwing in earth, or gravel, or bundles of grass on the upstream side. Sometimes it is best to construct the mattress over the whole width of the stream, and to effect the closure entirely by a weir, carrying each layer right across before adding another. The banks of the stream, if not hard, can be protected by sandbags, stones, staking or fascining.