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There is something cozy about the soft plopping of water on the roof. I noticed this again the other night when I momentarily woke at about three. Usually this restful sound prompts an easy return to slumber, but not that night.

Rather than a sense of peace, I experienced a disconcerting wave of displacement. Something was amiss, and it took a few moments to clear my head and realize what. It was the first week of March, the middle of the night; the moon was shining, the stars were out, and here I was, 6,000 feet above sea level in the mountains of Idaho, listening to the sound of rain.

In 1993, The Nature Conservancy, in association with a group of both public and private organizations, contacted the University of Idaho and Charles Brockway to commission a study of the Wood River Valley's water resource system. Initially, the study was to concentrate on the portion of the drainage below Hailey, with a special focus on Silver Creek. As data was collected, however, it became apparent that limiting the study to only part of the system would leave many important issues unexamined, so a second phase of information gathering, analysis, and model building was undertaken in 1995. The results of these efforts were finally consolidated in a summary report written by A. Lee Brown, Ph.D., and released in May 2000.

The report, using April 1993 to April 1994 as its reference year, looks at the dynamics of the entire watershed complex and explains in general terms where the Wood River Valley gets its water, how much it gets, when the water is used, and what factors affect historical patterns. Dr. Brown, unlike some professional hydrologists, takes great care to advise the reader that hydrology is "a developing physical science" and that the facts, numbers, and conclusions in the report are best interpreted in a relative, rather than absolute, context. With this in mind, one can read the report confident that, while specific numbers may occasionally be off the mark, the relationships and patterns described represent a sound basis for understanding the behavior of this Valley's dearest commodity.

The most obvious and in many ways most important component of our watershed is the Big Wood River itself. From Titus Lake to the Picabo Hills, the river travels over fifty miles—collecting tributary drainages, feeding irrigation canals, providing fish and wildlife habitat, removing treated wastewater, supplying recreational activities, increasing property values, and creating an overall sense of wellness. Commingling with the visible, surface flow on its journey south is the other main component of the watershed, the ground water. The unseen, hard-to-measure nature of ground water and its medium, the aquifer, presents challenges for this study and keeps it from being pure science. Yet it is also the very interaction of the visible with the invisible that defines hydrology as a science and allows it to be studied.

The Big Wood River watershed drains an area of about 880 square miles, including mountains and valleys, and receives on average, in various forms of precipitation, the equivalent of about 56 billion cubic feet of water a year. Forty percent of this water, both above and below ground, eventually collects in the main Valley anywhere from below Galena Summit to the Bellevue Triangle. At its start, near the headwaters, and in the higher elevations where the Valley is narrow, the river is fed by rivulets and, through its bed and banks, by seeping and percolating ground water. The pattern continues until such place and time that the topmost fully-saturated ground (the water table) falls below the riverbed. The point at which this occurs is, of course, highly variable and not particularly scientific. Moreover, there is apparently a transitional section where water table and riverbed alternate elevation supremacy. Generally, however, the river from the headwaters to Hailey pulls water from the ground. Then, from Hailey south, the aquifer takes water back from the river.

Roughly 1.3 million acre-feet (af) of water falls in the whole watershed on average each year, with 1.1 million falling in the upper (Hailey and north) Valley. Of this upper Valley precipitation, almost 700,000 af are consumed, actually used up, through evapotranspiration (ET), a combination of evaporation and transpiration (plants and animals). Landscaping and agriculture consume another 18,000 af. This represents just 1/3 of the nearly 55,000 af diverted for human activities (most of the diverted water returns by one means or another to the watershed). About 25 percent of the 55,000 af total, including all the water (8,800 af) for the municipalities of Hailey, Ketchum, and Sun Valley, comes from underground. And of the 41,000 af diverted from surface waters, almost 90 percent goes to irrigation. In most years by the time the watershed leaves Hailey, after hydrating 625 square miles and nearly 75 percent of the total Valley population, there is somewhere between 300,000 and 400,000 af still in the system, 10 percent as ground water, 90 percent in the river. >>>


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