If the proposed dam or reservoir is to be located upon a main drainage line; that is, upon a river or stream, it is necessary to know both the flood and low-water discharge of the stream. Frequently no reliable data on this subject are available, and the engineer must then make such a study of the whole situation as will enable him to estimate the minimum and maximum flow with considerable accuracy. There are numerous factors entering into the solution of this first problem, such as the shape and length of the catchment area, its general elevation, the character of its surface, whether mountainous, hilly or flat, barren or timbered. Good topographic maps, if available, furnish valuable data on these subjects and it is to be regretted that only a comparatively small portion of the United States has been thus mapped in detail. The results of stream measurements, if any have been made in the catchment basin, are especially important: These are usually few in the high areas, on account of their inaccessibility. The year 1902 marked a notable beginning of such measurements in California. In many parts of the arid region of the United States, the best storage-sites are situated in the upper or higher portions of the drainage systems. This is especially true of the streams on the Pacific Slope having their source in the High Sierras. As regulators of stream-flow and for power purposes such storage is peculiarly valuable, while storage for irrigation and domestic uses may be located nearer the valleys and the centers of population. Frequently the engineer is required to build his dam where no such data are available. In such instances he should endeavor to secure the establishment of rain gauges and make measurements of the flow of the main stream and its principal tributaries at various places to obtain the desired information. Even this may not suffice, owing to the limited time at his disposal, and he must resort to the use of certain empirical rules or formulas, and make such comparisons and deductions from known conditions and results as will best answer his purpose. The engineer should know, approximately at least, the normal yield of the catchment area, the duration of the minimum and maximum seasonal flow, and the floods he may have to provide against during the construction of his dam. These data are absolutely necessary to enable him to provide ample wasteways for his reservoirs. Many of the failures of earth dams have been the result of over-topping the embankment, which would have been averted by an ample wasteway. The most notable example of this kind in recent years was that of the South Fork Dam, at Conemaugh, Pennsylvania, in 1889, resulting in what is generally known as the “Johnstown Disaster.”