Low dams forming service reservoirs for domestic water supplies and for irrigation comprise by far the most numerous class. They are not designed to impound a large volume of water and therefore may be built across a small ravine or depression, or even upon the summit of a hill, by excavating the reservoir basin and using the material excavated to form the embankment. These reservoirs may be used in connection with surface or gravity systems, artesian wells, or underground supplies obtained by pumping. The dams forming these reservoirs being of moderate size and height may vary greatly in shape and dimensions. The form may be made to suit the configuration of the dam site. When the earthwork requires it, they may be lined with various materials to secure water-tightness. Often such dams are made composite in character, partly of earth and partly of masonry or some other material. They are also frequently accompanied by numerous accessories, such as settling-basins, aerating devices and covers, which present a diversity in form and appearance. A presentation of the different types of dams thus employed, with a discussion of the questions pertaining to utility in design and economy in construction, would be exceedingly valuable and of general interest. Service reservoirs will receive only a passing notice, with the hope expressed that some competent authority will discuss them in the future. CHAPTER II. _Preliminary Studies and Investigations._ The preliminary studies and investigations which should be made prior to the construction of any dam for the storage of water have to do with (1) the Catchment Area, (2) the Reservoir basin, and (3) the Dam site. Catchment Area. It is thought desirable to define a number of terms as we proceed, for the purpose of correcting erroneous usage and for a clearer understanding of the subject. The catchment area of a reservoir is that portion of the country naturally draining into it. The watershed is the boundary of the catchment area and may be correctly defined as the divide between adjacent drainage systems. In regard to the catchment area it is necessary to determine: 1. Its extent and area in square miles. 2. Its topography or the character of its surface. 3. Its hydrography or precipitation and run-off. 4. Its geology, or the character of its soils and subsoils, and the nature and dip of its rock strata. 5. Its flora, or the extent to which it is clothed with forest trees or other vegetation. All of these elements affect the volumes of maximum run-off, which is the one important factor in the construction of earth dams that must not be underestimated.