Monday, June 28, 2010

GC dams: What the dam would do, 1942

In October 1942, Bureau of Reclamation engineers in the field had become convinced that the most economically sound Bridge Canyon dam would be one high enough that its reservoir would back several miles into the Grand Canyon National Park, even as far as Tapeats Creek, mile 134. That is, the dam would reach elevation 1970', as compared to the 1772' level marking the western boundary of the Park. They informed the Park Service of their reasoning, and a trip was arranged with Reclamation's Larson and NPS's Tillotson, Bryant, Olmsted, and others to look, at rim and river level, at the potentially affected areas, with a review of the part of the Canyon that had been impacted by a decade of Boulder Project operations. On Oct 20, a "preliminary report" was prepared by NPS trying to describe just what would be "the probable effects of alternative elevations for the full reservoir surface at Bridge Canyon upon GCNP and in general upon values for the conservation of which the NPS is responsible".

Keep in mind: No one was factoring in a dam like Glen, a regulator and silt trap. Also, they focussed on the Park, not the Canyon in its entirety, as an entity. I have not quoted, but have tried to use the words and phrasing, catch the spirit, of this interesting and principled document.

Reclamation presented their case for a higher dam, always with the qualifying statements that a decision had not been made and that detailed and laborious investigations were still to be carried out. The real context is made clear by a remark following Reclamation's summary: When the trip was over, their boss told them he had received a telegram from another Reclamation survey, which had concluded 1970' would "probably" be the optimum economic height. A telegram. What happened to the "additional and laborious investigations"? Looking over these files today, gives me the picture of the Bureau of Reclamation as a teeming anthill of highly opinionated experts rushing back and forth, making and unmaking decisions, pushing against opposition, always certain, always secretive. Ideas, notions, spring up and become dogma; assurance is the prized characteristic; keeping an outsider off-balance the preferred tactic. 

So, what Larson said: Only the upper and lower Gneiss sites are being considered. The lower site could be to 1772', but a dam to 1835' would clearly add marginal value. The upper site is so good it could be even higher (1835' to 1919' "probably") with an even better ratio of cost to power producible. More study is needed to justify the costs, but comparisons of the sites indicate a huge superiority for the highest dam at the upper site, to such a degree that "it cannot properly be dismissed … unless objections to the higher dam on other grounds of public policy are very strong". Reclamation could only proceed, however, if it got "flowage rights" upstream of Havasu Canyon, the western extremity of the Park.

Having made the squeeze, Larson gets home and finds that telegram: "1919' probably" is now 1970'. "Probably." 

The bulk of the paper is on effects. Everywhere along the river's length in the Canyon, conditions are highly dynamic & radically changeable from month to month. (Less: On the 1923 USGS expedition, the group was forced for a few days to out-wait the river as it went up to 100,000 cfs almost overnight.) At low water, the river may be beautifully translucent, is often placid, and its whole appearance emphasizes the vastness of the Canyon. At high-water stages, it is a boiling flood of liquified mud and sand and coarser detritus--buoyant debris from upstream: sticks, branches, logs, uprooted trees, sawn lumber, wreckage of structures, bloated animal bodies, a gruesome procession that accumulates in Mead's backwater. Far from wholly agreeable, this burden of mineral, vegetable, and animal debris is far from wholly "natural". From the rim, this muddy stream looks a little wider, maybe diminishing any sense of the Canyon's vast scale. The great flood-time is in two months. Then debris accumulates in the still-water, much of it going rapidly to the bottom. Driftwood may eddy slowly around a limited locality, sometimes closely packed, sometimes open enough to allow a boat through.  In May 1941, the river was so solidly packed with floating, waterworn, dead-wood, from wall to wall for about ½ mile, it was impossible to force a passing; six months later, there was a clear route throughout. (There were regular Reclamation trips going up to the Bridge site.)

A technical interlude described the heavier mix of mud and sand with colder water of the flood as pushing the slack-water, then sliding underneath it, where it accumulates in the deeps. Along the sides are more continuous deposits, while the bottom has the coarser materials building up a "delta" with a rather sharp front. The water flowing down over this front drags surface water down such that this increase in volume produces a backward-flowing surface current which holds the floating debris in place. When the floodwaters warm and lighten, quite suddenly all the floating debris disperses downstream. This takes place many miles down from the intersection of river and reservoir, in Mead 30 miles and more. If this happens behind Bridge, the accumulations will be more or less below Toroweap Point, moving downstream as the "delta" extends. That is, they will be considerably downstream from Havasu; and of course, in Mead, accumulations will end.

Consequent effects: trips on Mead will be free from obstruction. In Bridge's reservoir, the bad accumulations would seriously detract from the natural appearance, and prevent boating much beyond a day trip from the dam, which would satisfy most visitors. The floating debris would need to be controlled after the concentrated accumulations break up. 

Water fluctuations will no doubt increase, perhaps 50' before the flood, with a maximum of 100' +/-. At Havasu, if the low dam reservoir were full, the fluctuations could be 65', instead of the historic 40' or so. For the highest dam, 150' would be possible. These fluctuations would be annual regularities. The high water would, of course, not at all resemble the current flood stages, being wider, slower, less turbulent. The impact upstream, toward Tapeats, depends upon rather complex hydraulic factors, and is unknown. 

As the water receded, the silt and sand deposits would be exposed. These deposits would increase the width and height, making them more conspicuous. In Mead, there are pronounced bands, distinctly different from natural silty deposits. They are sometimes unnoticeable, other times unpleasantly conspicuous, suggestive of untidy artificial interference. Should Mead and Bridge reservoirs be drawn down for a considerable time, the delta-deposits will appear, unlike anything in the natural canyon. In 1941, the nearly flat surface of the Mead delta at mile 265 was at 1194', 210' feet above the original rock bottom. Downstream a bit, the frontal escarpment dropped nearly 200'. If the reservoir stays below 1100', the river will cascade down this steep front (150' in a mile), eroding a mud inner canyon, headcutting just as the Colorado did when it jumped its track near the Salton Sea. Over a long-enough time, it will slough off the deposits, break them down and erode them away. The writer of the paper had seen a like phenomenon in the Imperial Valley; exciting and a nasty mess. 

At high-water, Bridge's reservoir will certainly be an impressive, colossal, fjord-like artificial lake, just as certainly as at low-water, it will be an equally colossal, artificial mess of mud and sand and miscellaneous debris. While the differences between the various dams are perhaps matters of degree, definitely some unmistakably artificial modifications, scenically unpleasant, will extend to the reservoir's limits.

The effects in Havasu, whatever the dam height, will be confined to the part that lies down-creek of the beautiful falls. The mingling of the waters of the creek and river would be radically altered for the worse. Also, it would be deplorable to encourage a large influx of boating tourists into Havasu or to make it a thoroughfare to the reservoir. There should be no boat landing. The river section upstream from Havasu has recently been more regularly travelled by Nevills' enterprises. He considers its 13 miles "amongst the most beautiful" of the river stretches. There will be changes here, and no rapids. 

The Esplanade (not called that in this report) going west from Tapeats/Great Thumb to Yumtheska would doubtless provide impressive views of the inner canyon, but it is extraordinarily difficult of access across a wide, dissected, desert bench. The investigators did not get there, and could not learn of any NPS staff who ever had. It is also true, that only a minute fraction of visitors have seen the inner gorge here. However, (the Esplanade) here is notable for its distinctive character and had been completely included in the original Monument, and is worthy of Park status. Its omission seems not to have been due to a deliberate and reasoned decision to reverse its protection, and the Forest Service says that scenic protection is its prime duty there. 

The reports ends pointing out that choosing the appropriate policy is a matter of broad considerations of public purpose, not technical details. So if the protective policy of 1908 establishing the Monument is to be deliberately abandoned, then the logical procedure would be for Congress to make a marginal boundary modification, shifting it, west of Tapeats, from the north bank of the river to a south bank contour above the reservoir. 

If the policy is to remain one of protection, then Bridge's height should be limited since a higher reservoir will substantially alter natural conditions and injuriously affect the natural scenery, solely for an economic gain. This conclusion is valid even if the impairment is relatively limited and would be observed by few visitors, and the economic gain is large. If such decisions are to be a matter of degree, the hair-splitting over how much is too much will be endless.

Source: copy of NPS document in my files

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