Community November 24, 2008

Topics of Conservation


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. >>>



Below Hailey the Valley widens out substantially, becoming a drier, more agricultural landscape. This warmer lower section of the watershed collects only about 220,000 af of precipitation from 255 square miles. In the research year, which long-term (80-year) river flow averages would suggest was unrepresentatively wet, precipitation plus upper valley contribution totaled 612,000 af, of which 161,000 was consumed by evapotranspiration, 107,000 by crop irrigation and less than 1,000 by residential use. Of the remaining 343,000 af that flowed out of the valley that year, only two-thirds left by way of the Big Wood River. The balance, representing underflow and seepage from irrigation canals and the river, first gathered in the aquifer and then either continued underground or surfaced through springs, before exiting the watershed at Picabo.

I did some math. Projecting a valley population of 50,000 (a possible total build-out figure, although the latest tally is not even 20,000) and a per capita water usage rate of 1,000 gallons per day (twice as much as current local averages and almost 6 times the national average), the human demand on the valley’s watershed would be 56,000 af, but as much as 85 percent of this amount would be returned to the system. A corresponding increase in landscape irrigation could account for another 77,000 af, and only half of this would actually be consumed.

So, even assuming a bias in the research year and an explosion in population and water usage, the numbers by themselves seem to make a case that the Big Wood River watershed is a healthy, well-endowed, and accommodating water resource system. But as Dr. Brown reminds us, "Science tends to be weakest at extremes. It’s much better at measuring and understanding the mid-range, the close-at-hand rather than the distant."

One such "extreme" occurred in June 1992 and was the catalyst that initiated the seven-year study. After several years of dry weather conditions and below-normal river flows, Silver Creek started suffering from reduced levels of dissolved oxygen. In one day, fifty large trout died. The same low dissolved-oxygen levels recurred last summer and caused stream managers to request a voluntary curtailment of fishing. These episodes can be considered "extreme;" or they can be taken as warning signs that the watershed is not as robust as the numbers suggest.

Perhaps more alarming is an indication that the delicate nature of the system extends beyond mere volume fluctuations. Recent reported instances of contaminated wells have exposed a potential problem in water quality, particularly in certain parts of the unincorporated county. Water quality issues will not only catch the eye of local health officials and county commissioners; they will attract the attention of the state.

In 1981, the state of Idaho designated the whole of the Big Wood River, from Titus Lake to Magic Reservoir, a Special Resource River. This means it is recognized by the state as "needing intensive protection," and it falls to the state Department of Environmental Quality to monitor its condition and implement policy. Part of this policy is to disallow any "new point source" that by reason of its discharge "can or will result in a reduction of the ambient water quality" as measured immediately below the discharge zone. The practical implication of the policy is that no new wastewater treatment facility, except where it replaces existing capacity, can be built. Nor can existing capacity be expanded. Consequently, any increased population growth will require alternative wastewater management applications, and these will probably interact, as septic systems do now, with the ground water.

Analyzing any dynamic system from a fixed perspective in time invites problems. This is particularly true when the system, such as a watershed, contains components that can neither be seen nor definitively measured, such as ground water and the aquifer. It seems important, therefore, to take great care before drawing conclusions from the Big Wood study. (For instance, in the above projection of a build-out scenario, the net consumption of water seems comfortably within the resource’s tolerances, but in fact, unless recent practices change, almost all of the increased demand would be for ground water, of which there most likely would not be enough.)

The challenge facing County planners and citizens alike is to assess the impact of each new, expanded, or changed water use with the awareness that the next development or septic system or water right transfer or land use change or riverside encroachment has the potential to permanently impair a fragile system. Normally, the temptation is to embrace generalities and dismiss anomalies. But it is the anomalies, the extremes, that may have the greatest implications for the future of the Big Wood River watershed. Just like the dripping of melting snow in the middle of a winter night, they command one’s attention.


Brought up and educated in New England, and professionalized in New York City, Bill Lowe came here in 1982 to raise a family and spend more time with his sense of humor. Currently, he’s promoting a new circuit-board technology based on vector logic.



This article appears in the Winter 2012-2013 Issue of Sun Valley Magazine.