1 - With respect to David Brower's concept to drain Lake Powell completely, the following can be noted:a - The spillway discharge tunnels, to which are
connected the river bypass tunnels used during construction, have a "flip bucket" elevation of 3158 feet, approximately 14 feet below the original stream bed elevation of 3172 feet at the dam, and a tunnel
bottom elevation of 3134 feet near the flip bucket. The flip buckets would likely be removed to allow free flow of both water and sediment through the tunnel. Thus with the bypass tunnels open, the lake would be
completely drained, even the dead storage which cannot be released from the hollow jet outlet tubes that were used for the spike flow test.
b - The river bypass tunnels have a diameter of 41 feet. With the concrete
plugs removed, the dam would function much like during the initial construction period when the tunnels were used to bypass the entire Colorado River flow. Under an assumption of non-pressurized, open channel flow, the
tunnels have a combined release capacity of about 100,000 cfs. Greater amounts could be discharged if Lake Powell refilled enough to pressurize the tunnels (about 10 to 20 feet of head).
c - Peak flows on the Colorado
River at Lees Ferry typically occur in May or June and have a broadly based shape. Statistical analysis revealed that the 100-year peak flows are about 155,000 cfs, the 20-year peak flows are about 125,000 cfs, and the
average peak flows are about 65,000 cfs. This means that the rive bypass tunnels probably could pass all but about 10% of the flows in all years. Only years such as 1952, 1957, 1983, and 1984 would there be more water
than the tunnels could pass. In those cases, there would be some filling (10 to 20 feet) of the reservoir, creating some head on the tunnel resulting in river flows of about 120,000 to 130,000 cfs.
d - High spring
runoff is highly sediment laden. With the tunnels open, sediment would be bypassed downstream under a natural flow and sediment regime. It is likely that the Grand Canyon would be brought back near to pre-dam conditions.
e - One would expect that under this action, the "Law of the River" would likely be revisited, however, only the 8.23 MAF minimum objective release and the obvious equalization provisions might need to be
eliminated. With an average annual natural inflow during the historic critical hydrologic period of about 12 MAF and current upper basin use of 4.2 MAF, the difference of 7.8 MAF is enough to satisfy the Colorado River
Compact obligation of 75 MAF/10 years to the lower basin without needing the storage of Lake Powell. In addition, recovered evaporation losses from Lake Powell would help to meet any potential deficiency in the Mexican
Treaty obligation.
However, future increases in upper basin depletions will depend on the ability of Lake Powell to supplement upper basin outflow during drought periods. This drawing from storage is the foundation
purpose for the construction of Glen Canyon Dam. The most serious impacts of emptying Lake Powell is that there would no longer be reservoir storage to support additional upper basin uses and that the lower basin would
receive uncontrolled flow. If flows to the lower basin were to fall below 75 MAF/10 years, then upper basin use would have to be curtailed to meet the delivery requirement of the 1922 Colorado River Compact.
f - The
loss of existing benefits from Lake Powell itself are self evident: power generation, lake recreation, downstream trout fishery, and the extended and moderated white water Grand Canyon boating season. The entire
ecosystem of the Grand Canyon would likely change with the loss of the current food base, and nonnative predator fish from Lake Mead could be a major concern to native fish species in the Grand Canyon.
g - Significant
increases could occur in lower basin consumptive uses as the result of curtailed upper basin uses, perhaps as much as 2 MAF annually during surplus conditions (equal to the foregone additional upper basin development up
to the upper basin yield). Lower basin shortages could be eliminated as an alternative to increased uses since future risks of shortages to lower basin uses are primarily due to increases in upper basin use. The risk of
flood control releases from Lake Mead would rise dramatically, meaning extra deliveries and flood flows to Mexico. As a result of these higher flows along the Parker - Davis floodway, river channel maintenance costs in
the lower basin could also rise. Since upper basin increases in consumptive use would end, projected increases in salinity impacts might also decline, decreasing the need for additional salinity control projects.
h -
In order to carry out the Brower proposal, additional legislation would be required to negate the constraints of the 1968 Colorado River Basin Project Act. Specifically, these constraints permit annual releases of water
from Glen Canyon Dam in quantities greater than 8.23 MAF only if (1) required for lower basin consumptive use, (2) to equalize storage with Lake Mead, or (3) to avoid anticipated spills. Much greater releases would have
to be made to empty Lake Powell.
I - With such a radical change in the "plumbing" of the Colorado River reservoir system, it is likely that the Colorado River Compact, the Upper Colorado River Basin Compact,
and the Mexican Water Treaty would be renegotiated.
2 - With respect to the Glen Canyon Institute's concept to drain Lake Powell to elevation 3500 feet, the following can be noted:
a - The minimum power pool
elevation at Glen Canyon Dam is elevation 3490 feet. It is possible that some power production could occur at this elevation but there would be the possibility of a vortex, since the centerline of the power plant
penstock is 3470 feet. As shown on the attached reservoir capacity allocation sheet, the lake has a total volume of 5.9 MAF at this elevation. From our area-elevation curves, the lake has an area of about 52,000 acres,
32 percent of the maximum area of 161,000 acres.
b - The release capacity of the outlet works is about 15,000 cfs with a gate opening of about 60 percent at the full pool elevation of 3700 feet. While this amount can
be released at elevation 3500 feet, a gate opening of 100 percent is required to achieve this release. This may be questionable from a maintenance point of view since at 100 percent gate, the velocities in the outlet
tubes currently causes severe cavitation. If restricted to 60 percent gate, the capacity at elevation 3500 feet is about 11,600 cfs.
Powerplant release capacity is unclear under this concept, but existing rating
curves suggest that about 30,000 cfs could be released at elevation 3500 feet. About 900 megawatts of capacity are available at this level, a reduction of about one-third from the current maximum capacity due to the
lower head. While power plant bypasses would be eliminated, energy production would also be reduced.
b - Routings of several magnitudes of inflow were performed to evaluate the amount of unintended storage that would
temporarily accumulate in Lake Powell due to the routing effect. With the uncertainties of release capacity, the combined powerplant/outlet works capacity was assumed to be about 45,000 cfs. An examination of historic
Lake Powell inflows showed that 45,000 cfs was exceeded about 70 percent of the time. This means that Lake Powell would gain storage in the spring this frequency of years in the future.
c - Routings of average (1996),
upper quartile (1993), and extreme (1983) years were made to determine the duration of inundation at various levels upstream of the dam. Starting conditions were assumed as an elevation of 3500 feet and an ability to
pass inflow up to the 45,000 cfs level. The following table shows the routing results:
