The quality of water will obviously be affected by the kind of material through which it percolates or flows. Surface-waters, flowing over the surface of the ground or percolating but a short distance below the surface, naturally have contact with vegetable matter, unless they are collected in a country where the soil is sandy and where the vegetation is scarce. If such waters flow through swamps or over beds of peat or other similar vegetable mould or soil, they may become so impregnated with organic matter or so deeply colored by it that they are not available for potable purposes. Ground-waters, on the other hand, possess the advantage of having flowed through comparatively great depths of sand or other earthy material essentially free of organic matter. They may, however, in some locations, carry prejudicial amounts of objectionable salts in solution, rendering them unfit for use. As a rule, ground-waters are apt to be of better quality than surface-waters, but they do not generally stand storage in reservoirs as well as surface potable waters. It is advisable to store them in covered reservoirs from which the light is excluded, rather than in open reservoirs. They are sometimes impregnated with salts of iron to such an extent as to make it necessary to resort to suitable processes for their removal, and they are also occasionally found so hard as to require the employment of methods of softening them. Both sources of supply are much used in the United States. Table IV shows the percentages of the various classes of supplies as found in this country during the year 1897; the total number of supplies having been at that time nearly 4000. TABLE IV. WATER-SUPPLIES OF THE UNITED STATES. Source. Per Cent of Total. {Rivers 25 Surface-waters: {Lakes 7 {Impounding reservoirs 6 {Combinations .5 ----- 38.5 {Shallow wells 26 {Artesian wells 10 Ground-waters: {Spring 15 {Galleries and tunnels 1 {Combinations 2 ----- 54 Surface- and {Rivers and ground-waters 6 ground-waters: {Lakes and ground-waters 1 {Impounding reservoirs and ground-waters .5 ---- 7.5 ------- Total 100.0