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  • Writer's pictureMark L. Johnson

Water Woes XXVII-Ebbs & Flows

Glen Canyon Dam (Lake Powell)-photo by Jim Quigley


This blog covers several topics concerning Southwest water items including; (1) Local Temperature & Precipitation, (2) Colorado River Conditions, (3) Near Term Colorado River Operations-SEIS, (4) Post 2026 Coordinated Operation of Colorado River Basin and (5) Glen Canyon Dam (Lake Powell) River Outlet Limitations


Local Temperature & Precipitation

Data from the National Weather Service (NWS)-Tucson Airport (4/13/24) indicates average temperatures and above normal precipitation for calendar year (CY) 2024. Total CY precipitation is 5.29" vs 2.39"(normal) or 2.9" above normal.

This is all good news for the drought and our aquifers. However, state and local fire officials are concerned that accelerated vegetation growth will be fuel for potential wildfires.


Colorado River Conditions


Snowpack in the Colorado River Basin (4/8/24) is 17.2" SWE which is 110% above the normal 15.5" SWE.

This bodes well for decent runoff this spring but not as robust as last year.

Reservoir Storage

Lake Mead (4/1/23) is at 37% capacity and Lake Powell is at 33% capacity. This compares to 28% and 22%, respectively on the same date in 2023. This is about 10% improvement over 2023.

Shortage Status

The Colorado System is in a Tier 1 shortage for 2024 and is projected to remain in Tier 1 for 2025.


Near Term Colorado River Operations-SEIS

In March, the United States Bureau of Reclamation (USBR) issued a final Supplemental Environmental Impact Statement (SEIS) for the operation of the Colorado River System through 2026. It essentially provides for an additional 3 million acre-feet of water conservation by the Lower Basin States (Arizona, California and Nevada) beyond the existing conservation programs (Interim Shortage Guidelines and Drought Contingency Plan). Download the joint statement by Arizona, California and Nevada in the link below.

Download PDF • 147KB


Post 2026 Coordinated Operation

of Colorado River Basin

Operating the Colorado River System through 2026 is essentially resolved. Now the USBR and Basin States are developing long-term operating guidelines for the Colorado River System post-2026.

The Lower Basin States and Upper Basin States (Colorado, Wyoming, Utah and New Mexico) have submitted differing proposals for this long-term plan. See graphics below. USBR intends to issue a draft SEIS by the end of 2024. TA will be providing more information on this topic in a detailed future blog.


Glen Canyon Dam (Lake Powell)

River Outlet Limitations

Glen Canyon Dam-Sections (USBR)

You may have seem some articles lately about issues related to the ability to release water at Glen Canyon Dam at very low levels. I have some experience with the design, operation and maintenance of large concrete gravity dams across the nation and will give the skinny on this one.

Here are some Glen Canyon Dam facts:

  • Impounds-Colorado River upstream of Lake Mead (Hoover Dam)

  • Height-710 feet

  • Full Live Storage-23.3 million acre-feet (maf)

  • Maximum Pool Elevation-3,700 feet above sea level

  • Turbine Capacity-1,320 Mega Watts (MW)

There are three different dam components that release water downstream (refer to the numbered sections in the above drawing):

  1. Spillway (2)-releases water between elevation 3,711' (maximum surface elevation) and elevation 3,648'. There are two spillways--one on each side of the dam and are used primarily to control flows during flooding conditions in order to protect the dam from over-topping.

  2. Penstocks (8)-water is withdrawn from the reservoir via 8 intake structures which are connected to hydroelectric turbines via large diameter (15') pipes or penstocks. The turbines then discharge water downstream of the dam. Water cannot be discharged via the penstocks below elevation 3,490'.

  3. River Outlets (2 w/4-96" Steel Pipes & Hollow Jet Discharge Valves)-water cannot be discharged below elevation 3,394'. The volume of water available between elevation 3,490' and elevation 3,394' is estimated at 3.7 maf. The river outlets were designed to augment the spillways for emergency flood releases. However, they have been used for high flow experiments (HFEs) to provide high river flows downstream of the dam to move sediment and increase sandbar size.

USBR has conducted inspections of the River Outlets after the HFEs in 1965 and 2023 and found that the outlet pipes had suffered cavitation damage. Cavitation occurs in hydraulic structures during high velocity situations and can cause pitting and other damage to the pipe, pump, turbine, etc. materials. Because of this issue, USBR investigated issued a Technical Decision Memorandum (3/26/24) (click on the link below to download) which recommended that, "USBR not use the river outlet works as the sole means for releasing water from Glen Canyon Dam as would be required at reservoir levels below the minimum power pool, elevation 3,490'."

USBR-Technical Decision Memo-Glen Canyon Dam Operations-Low Reservoir Levels-3-26-24
Download PDF • 895KB

In conclusion, if Lake Powell drops to elevation 3,490' there will be no hydroelectric power and no ability to discharge water to the Colorado River for transfer to Lake Mead. The water elevation in Lake Mead is now 3,559'---only 69' down to disaster compared to 141' up to maximum pool.

These are the ebbs and flows of water in the Southwest!!


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