In planning generating resources for an electric system, the engineer has continuously in mind a minimum of investment in plant to carry a given load. Resources are assigned to the load curve of the area by system operators with this thought in mind.

On an all-hydro system, cyclical storage is drafted in an adverse water year to supplement system energy. On a system with substantial amounts of both steam- and hydroelectric resources, steam-electric units are run around the clock to supplement system energy and hydroprojects are operated only during the maximum load-hours, so as to conserve water and maintain the maximum dependable capacity of the projects. Daily pondage at each project is used to loadfactor the available water. (By load-factoring of water is meant the controlling of daily flows so as to make streamflow energy available during the hours of maximum system load.)

Another way of stating the same thing is that hydroprojects operating in con junction with steam-electric resources are operated for maximum energy and dependable capacity, drafting reservoirs only when it is necessary to do so to preclude spilling of flood flows.

SECTION IV. DEVELOPMENT OF THE MIDDLE AND LOWER SNAKE RIVER BASIN PROJECTS AS REASONABLE COMPONENTS OF A SYSTEM WITH BOTH HYDROELECTRIC AND THERMALELECTRIC GENERATING RESOURCES

General discussion

Complete development of the Snake River Basin must give consideration to the multiple uses of water and the protection of other natural resources of the area. In planning for irrigation, navigation, power, recreation and flood control, preference must be given to beneficial consumptive use of water such as irrigation and domestic and farm requirements.

The flows of the main stem of the Snake River and its two principal tributaries, the Salmon and Clearwater Rivers, have extremely diverse seasonal flow characteristics. The Salmon and the Clearwater Rivers have their sources in drainage basins just over a single mountain from the headwaters of the Bitterroot, Clark Fork, and Missouri Rivers. The spring snowmelt flood flows of the Salmon and Clearwater are very similar to the flows of the Coeur d'Alene, Clark Fork, Kootenai, and the main stem of the Columbia.

The main stem of the Snake River rises in Yellowstone National Park, 800 miles south of the principal drainage basin area of the main stem of the Columbia River. The snowmelt floods of the main stem of the Snake River almost always occur in February, March, and April. From a power standpoint, this represents a splendid resource diversity when compared with the flow characteristics of the main stem of the Columbia River which has its lowest flows in the months of January, February and March. These early flood flows of the main stem of the Snake should not be stored for they are needed at the time of their occurence for electric power generation.

The 1948 flood on the Snake River

In planning flood control on the Snake River and its principal tributaries, consideration must be given to the effect of diversion for irrigation upon the flood characteristics of the main stem of the Snake River. In the flood of 1894, the flow on the main stem of the Snake River at the Oxbow gage was 130,000 cubic feet per second. The flow of the Spaulding gage on the Clearwater was 158,000 cubic feet per second and at the White Bird gage on the Salmon River was 120,000 cubic feet per second. There was a marked change in the flow characteristics of the main stem of the Snake during the flood of 1948. While the maximum flow at the Spaulding gage on the Clearwater River increased to 177,000 cubic feet per second and the White Bird gage recorded a flow of 103,000 cubic feet per second, the peak flow at the Oxbow gage dropped to only 55,000 cubic feet per second.

Report chart 1 has been copied directly from the review report of the Middle Snake River Basin, dated December 22, 1953, and shows graphically the relative flows of the Snake River at the Oxbow gage, the Spaulding gage on the Clearwater River and the White Bird gage on the Salmon River.

The reason for this can be found largely in the diversion of water for irrigation purposes on the upper reaches of the Snake River in southern Idaho. In an irrigation season, approximately 6 feet of water is diverted for each acre of land irrigated. About half of this amount is diverted in April, May, and June. With about 234 million acres of land under irrigation in southern Idaho, it is evident that the diversion of approximately 8 million acre-feet of water at a time when the river is tending to flood would materially decrease the crest of the stream flow. Figures XIV, XV, and XVI show the daily average readings of the Spaulding, White Bird and Oxbow Gages during the period of the 1948 spring flood. These charts have been prepared to show the relative amounts of floodwater that could be stored in these 3 streams, in each instance 100 percent effective in reducing the flood flow in the lower Columbia River Basin. Storage on the upper reaches of the Clearwater and Salmon Rivers would have the FIGURE XIV

additional benefit of controlling local floods on these two streams. There is little or no local benefit to be gained by flood control of the flows of the Oxbow gage by flood-control storage at Hells Canyon.

Storage in the Snake River Basin for power

In describing the operations of hydroelectric projects as components of, a system with both thermal-electric and hydroelectric generating resources, it was pointed out that storage reservoirs were not drafted except to preclude spilling of flood flows of the river. It was further stated that all hydroelectric projects must be operated for maximum energy production. The flows of the Snake River are discussed here with these fundamentals in mind.

Figure XVII is a daily hydrograph from October 1928 through September 1944 of the lower Snake River Basin as indicated by the Clarkston gage flows. Appendix B, plate 5 of the Army engineers' study, entitled "Selection of Sites, Lower Snake River," dated March 14, 1947, was used as the basis of figure XVII. To the original hydrograph was added the heavily dotted line at a flow of 70,000 cubic feet per second. The present plans for the Ice Harbor project called for the installation of five units with a substructure for a sixth unit. The hydraulic capacity of each of the Kaplan runners is approximately 15,000 cubic feet per second. A total capacity of five runners would be 75,000 cubic feet per second. Referring to figure XVII, it will be noted that in the years shown on this chart. there is only 1 year with a river discharge above 70,000 cubic feet per second of