Cadillac Desert_ The American West and Its Disappearing Water - Marc Reisner [259]
Though the airy caves in the rock were a shocking discovery, no one besides Robison, the contractors, Harold Arthur, and a small circle of Bureau officialdom knew about them. Gil Stamm, the commissioner, was probably never told. The people of Rexburg and Sugar City, the two towns lying directly in the Teton River floodpath, were entirely in the dark, as were the politicians who had so assiduously promoted the dam. Of course, had they known about the voids, it probably wouldn’t have mattered to them anyway. After all, the Bureau of Reclamation had the best engineers in the world.
The dam was finished, more or less, on October 3, 1975, when the flow of the river was interrupted for the first time. Even with the biggest voids left unfilled, the job had taken 503,000 cubic feet of grout—more than twice as much as the Bureau predicted it would have to use. That winter, a series of Pacific storms bashed into the Teton Mountains, depositing a big snowpack. As spring was about to arrive, Robbie Robison had two worries: how he was going to settle with the contractors over the cost of the extra grouting, and how he was going to capture the snow that was about to melt out of the Grand Tetons without violating the Bureau’s time-honored rule about filling reservoirs behind earthfill dams.
The rule is simple: the rate of fill is to be kept at or below one foot a day measured vertically along the reservoir walls. That way, if problems develop with the dam or the abutments, or back along the reservoir itself—where rising water sometimes loosens rock and causes landslides, or causes the bedrock to shift under its weight, producing the same result—they can be dealt with. At a slow rate of fill, such problems are less likely to develop in the first place. It was a sensible rule, and, like most sensible rules, it had already been violated on a number of occasions. Why not dispense with it again, with all that precious water coming down from the Teton Range? On March 3, 1976, Robison wrote Harold Arthur formally requesting permission for a two-foot-per-day filling rate. Ironically, one of the arguments he used in support of his request was that a faster rate of fill would permit the Bureau to observe how effective its grouting program had been. It was, in a way, like arguing for a hundred-mile-per-hour speed limit on the grounds that motorists would spend less time on dangerous highways if they drove twice as fast. But on March 23, Arthur readily acceded to Robison’s request.
Actually, the whole business—formal request, formal permission granted—was a meaningless charade. The main outlet works—the tunnel and appurtenances that would carry water out of the reservoir and into the adjacent canals—were not yet finished. The auxiliary outlet works were, but they were designed to carry a maximum flow of 850 cubic feet per second. Engorged by a snowpack half again as deep as normal, the Teton River was about to peak at several thousand cfs. Without a functioning main outlet works, the reservoir would rise as fast as the Teton River felt like filling it. It was likely to rise a lot faster than two feet per day.
Harold Arthur was unconcerned about such a fast rate of fill because he had ordered a series of observation wells to be drilled around the dam, which would—in theory—inform the Bureau of any developing problems. The water table around a damsite will often show a rise as the reservoir fills, because a certain amount of seepage into outlying terrain is inevitable. If the water table rises precipitously, however, and if wells far from the reservoir are affected—especially wells downstream—it could mean that the reservoir is seeping excessively. The only other possibility is a pressure response, where the adjacent water table rises out of proportion to the actual rate of seepage because of hydrologic pressure, much as the constriction of a hose nozzle turns a placid gurgle into a sixty-foot jet.
From what Arthur had heard from Robbie Robison, the observation wells in