Reservoirs serve two purposes in connection with waters drawn from streams: they allow sedimentation, and they afford storage. If a water having a turbidity of 1.00 is allowed to remain in a sedimentation-basin for 24 hours, its turbidity may be reduced by as much as 40 per cent, or to 0.60. If it is held a second day the additional reduction is much less. If samples are taken of the water in the reservoir before and after settling and sent to the chemist for analysis, he will probably report that from 70 to 80 per cent of the suspended matters have been removed by the process. The suspended matters are removed in much larger ratio than the turbidity. This arises from the fact that there is a certain proportion of comparatively coarse material in the water as it is taken from the river. This coarse material increases the weight of the suspended matters without increasing the turbidity in a corresponding degree. In 24 hours the coarser materials are removed completely, and at the end of that time only the clayey or finer particles remain in suspension. It is these clayey particles, however, that constitute the turbidity, which are most objectionable in appearance, and which are most difficult of removal by filtration or otherwise. Sedimentation thus removes the heavier matters from the water, but it does not remove the finer matters which principally affect the appearance of the water and are otherwise most troublesome. A sedimentation of 24 hours removes practically all of the coarser matters, and the clayey material remaining at the end of that time can hardly be removed by further sedimentation. The economic limit of sedimentation is about 24 hours. Sedimentation has practically no effect upon the clearer waters between flood periods.