n the early 1920s the United States Geological Survey explored the Green, San Juan and Colorado Rivers, designating sites for an unbroken string of dams from the mountains of Colorado and Wyoming to the last canyons of the Colorado River along the California/Arizona border. Boulder Canyon Dam, later renamed Hoover Dam, was the first one built, back in the early ‘30s. Having proved their ability to do something that monumental, and having whetted their appetites for it, the Bureau of Reclamation began planning many more dams upstream.Several were in the Upper Basin, north of the Utah/Arizona border. Two more big ones, however, were proposed for the Lower Basin, both in Grand Canyon—one at the head of Lake Mead near Bridge Canyon, and one in the high Redwall Limestone walls of Marble Canyon. The choice was narrowed down to two potential damsites by the 1940s—one just above Redwall Cavern, and another slightly taller site a ways above Buck Farm Canyon.
From the base of the lower Marble Canyon damsite to the top of the projected reservoir of the Bridge Canyon Dam, the river would drop nearly one thousand feet. Enough hydrologic head (drop in elevation) for two more good-sized dams. Only one thing stood in the way of building these additional two as well—Grand Canyon National Park. (At that time, Grand Canyon National Park began at Nankoweap Canyon on the north side, Little Colorado on the south; and ended at Tapeats Creek on the north, Havasu Creek on the south) The Bureau knew they couldn't build a dam or a major reservoir within a park without an act of congress. Instead, they developed a plan to circumvent the park while still using the hydrologic head within the park.
The Bureau planned to do this by diverting some 90% of the Colorado River's water through a 38-mile long tunnel from the base of Marble Canyon Dam to western Grand Canyon below Deer Creek, where the northern shoreline was in Kaibab National Forest. A powerhouse placed at river level could then operate with some 1,300 feet of pressure spinning the turbines.
The tunnel would pass just beneath upper Tapeats Creek, where a major tunneling operation would be based. Deregulation pools (depending on which of four different plans might be adopted) would be formed behind a large dam in upper Deer Creek, in the dry lake beds above the Deer Creek Valley, the canyon above Fishtail, or behind a 450 foot dam in Kanab Creek. The Kanab Dam would have the added benefit of stopping Kanab Creek's muddy waters from silting in both the powerhouse and the Bridge Canyon Reservoir.
A minimum of 1,000 cfs would be allowed to run through Grand Canyon National Park—“a scenic trickle”—so there would be a powerhouse at the dam as well. The 36-foot diameter tunnel would only be able to carry around 12,000 cfs, so at times there would be considerable releases at the dam.
Another tunnel was proposed by the residents of Phoenix—it would run southward to the Verde River Valley, thus supplying irrigation water to the Valley of the Sun. This concept, for one reason or another, never caught on with the Bureau.
The Marble Canyon Dam would not be one of the really big ones—the crest of the thin arch concrete dam would be just over 300 feet above the river, backing the reservoir up to the very foot of Glen Canyon Dam. It's capacity would be a meager 363,000 acre-feet (compared to 23 million acre feet at Glen Canyon Dam). With such small capacity, a silt control dam would need to be built in the Paria Canyon. A site was chosen just below Buckskin Gulch for a 380 foot dam, spectacular for its narrowness—70 feet wide at the base and 120 feet wide at the top of the inner gorge. Like the Coconino Dam on the Little Colorado, it was estimated to hold 100 years worth of silt. (That figure includes deltaic deposits above the crest of the dam!)
Even with the silt trap on the Paria, Edward P. Marsh of the Federal Power Commission reckoned that in 104 years the Marble Canyon reservoir would be filled with sediment.
The entire project carried a price tag of over a billion dollars in 1950s, back when a billion was worth something. By the time the final report was submitted in 1964, the Kanab Tunnel had been dropped, bringing costs down considerably. The power plant at the base of the dam would have a capacity of 600,000 kilowatts, producing 2.3 billion kilowatt hours per year. With a 100-year 3% interest loan from the government, the project was then estimated to have a 1.7:1 cost benefit ratio.
Glenn Rink and Brad Dimock
In our last issue we printed Neil Murdock's reminiscences of the Bridge Canyon Dam Project in the early 1940s. During the course of planning for that dam, the Bureau of Reclamation considered several other projects it considered essential to make the dam viable.
Although it would have been a high dam at over 700 feet, its narrow reservoir would only hold 3.7 million acre feet of water (or silt)—16 % of what Glen Canyon Dam now holds. Hence siltation would be a serious threat to the dam. In the 1940s, with no dams upstream, silt retention dams were planned on the silty San Juan and Little Colorado Rivers. On the San Juan a high dam at Lime Ridge would have backed up a substantial reservoir over Bluff and Montezuma Creek, capturing the heavy silt loads of the San Juan and Chinle Creek. Once the dam at Glen Canyon was built, however, the Bluff dam was dropped from the project.
On the Little Colorado River a high, narrow dam was planned nine miles below Cameron. Called the Coconino Dam, this 250 foot high structure would have backed water over Cameron and well up into the Little Colorado Valley and Wupatki National Monument. It was estimated this would hold one hundred years worth of silt.
In addition to the upstream dams, the Bureau investigated the feasibility of a tunnel from the reservoir to supply water to some point in central Arizona. The extreme cost and impracticality of this idea caused it to be dropped in favor of pumping stations downstream.
Railroad lines were planned beginning at Peach Springs and winding into the damsite with spur lines to a construction camp/permanent town on the Tonto Platform above the dam, and to a gravel quarry at the confluence of Diamond Creek and Peach Springs Wash. The rail plans were eventually dropped, as the construction of Glen Canyon Dam proved rail access was not essential. Instead, a 25 mile paved road would wind down through Hindu Canyon and Bridge Canyon, passing through a 4,100 foot tunnel on its way to the dam.
By the early 1960s the project had been pared down to a 736 foot dam at the Lower Gneiss Site, with road access and a town on the Tonto, and the 250 foot dam on the Little Colorado at the Coconino site. The cost of just over a half billion dollars would be repaid to the government at 3% interest over 100 years. Power revenue, fishing and recreation revenue would double the annual payments. With a 2:1 cost benefit ratio, the project seemed a shoe-in.
The mouth of Havasu Creek would be under 89 feet of water, and the head of the reservoir would be somewhere just below Deer Creek—right at the powerhouse fed by the 38-mile tunnel from the Marble Canyon Dam
Glenn Rink and Brad Dimock.