Sometimes a company of miners would build a dam across a river or stream, and turn it from its course, so they could dig out and wash the rich gravel in the river-bed. A flume, or ditch, would often carry all the water to a lower part of the river, leaving the bed of the upper stream dry for miles. In this kind of mining the "pay-dirt" was shovelled into long wooden boxes called sluices, and a constant stream of water kept the gravel and earth moving on out to a dumping-place. The gold dropped down or settled into riffles, or spaces between bars placed across the bottom of the sluices, and once a week the water was turned off and a "clean-up" made of the gold. It was not long before the rivers, creeks, and gulches had all been worked over and most of the gold taken out. The miners knew that this loose gold had been washed out of the hills by the rains and storms of countless years. So some one thought of using a heavy stream of water to break down the foot-hills themselves and to carry the gold-bearing gravel to sluice boxes. This is called hydraulic mining and is the cheapest way of handling earth, as water does all the work and very little shovelling is needed. But since a strong water-power is necessary, a large reservoir and miles of ditches or wooden flumes must be built, so the first expense is large. The water usually comes from higher up in the mountains, and is forced under great pressure through iron pipes, the nozzle or "giant" being directed at the hillside, which has already been shattered by heavy blasts of powder. The water tears thousands of tons of earth and gravel apart, and the muddy stream flows through sluices, where the gold is left. In this kind of mining a great quantity of débris, or "tailings," must be disposed of. For years this débris was washed into the rivers or on farming lands, filling up and ruining both, and leading to endless quarrels between farmers and miners. But at last the courts stopped hydraulic mining except in northern counties, where débris went into the Klamath River, upon which no boats could run and near which was little farming. But all the mines in the Sacramento and San Joaquin river-basins were idle till, in 1893, Congress appointed a Débris Commission. These mining engineers issue licenses to work the mines when satisfied that the débris will be kept out of the rivers. There are in the state many hundred thousand acres of gold-bearing gravel lands yet untouched, that could be worked by hydraulic mining. In drift-mining the rich gravel is covered by hard lava rock thrown up by some old volcanic outburst. Tunnels are driven by blasting with dynamite, or by drilling under the rock to reach the gravel which usually lies in the buried channel of an old river. The long drifts, or tunnels, needed are very expensive and only mine owners with capital can work these claims.