The Transcontinental Aqueduct. Leg 12: San Carlos Lake to the Colorado river following the Gila river, a distance of 280 miles. (Updated)

Stage 12 is a true delivery of water on demand aqueduct. The San Carlos lake has a storage capacity of a million acre-ft, the ideal buffer from the peak power demand driven uphill stages to the major delivery stage. San Carlos lake is now mostly empty, but will be normally filled to 85% of capacity, slightly less in advance of the winter snow melt. The Lake would look like this:

San Carlos lake, about half full

The Coolidge dam is now decommissioned, the lake is too often empty and the dam suffered damage in the power plant and it was no longer economical to produce power. The retrofitted dam will have a power generation capacity of up to 19,000 cfs the top of the dam is at 2535 ft, the typical water level is at 2500 ft and the drop is 215 feet, giving a maximum power output of 325 MW.

Coolidge dam before rebuilding

From there the stream follows the Gila River all the way to the Colorado River with the following dropoffs:

Where the Arizona central project waterway crosses the Gila river it will deliver up to 500 cfs to Tucson

Where the Gila river meets the Salt river it can deliver up to 1,500 cfs to the Phoenix-Scottsdale metropolitan area.

To the Martinez lake it can deliver up to 15,155 cfs, the design capacity of the All American canal. This will of course be nearly always far less, dependent on the need for water for irrigation, but we dimension the aqueduct to accomodate maximum flow. The Martinez lake is puny, and would easily be overwhelmed by surges in the water flow. To accommodate this, the Senator Wash Reservoir will have to be upgraded to be able to pump up or down at least twice as much water as is it present capacity. Lake Martinez is at about 180 feet elevation, and Senator Wash Reservoir is at a maximum elevation of 240 feet.

The Martinez lake and the Senator Wash Reservoir.

The rest of the Transcontinental Aqueduct empties out where the Gila river joins the remainder of the Colorado river a few miles downstream. It will be able to carry up to 6, 000 cfs of water to accommodate the needs of Mexico and also provide a modest amount of water to assure the Colorado river again reaches the ocean, maybe restoring some shrimp fishing in the ocean.

The 1944 water treaty with Mexico provides Mexico with 1.5 million acre-ft per year, more or less dependent of drought or surplus. It will be increased only on condition that when the Transcontinental aqueduct is finished, the New River in Mexicali will be cut off at the border, and Mexico will have to do their own complete waste water treatment.

There will be water allocated to the Salton Sea. Proposed will be the world’s largest Lithium mine, mining the deep brine, rich in Lithium. (about 40% of the world supply according to one estimate). This requires water, and as a minimum to allow mining in the Salton Sea the water needs to be cleaned. This requires further investigation, but the area around the Salton Sea is maybe the most unhealthy in the United States.

The maximum power generating drop during this last leg will be (2500 – 190 – 2.2X 280) = 1694 feet. With an average flow of 14,000 cfs this will generate 1.9 GW of power, but the realized power output will be determined by the actual water demands.

The Transcontinental Aqueduct. Leg 11: Cove Tank Reservoir to San Carlos Lake, a distance of 70 miles.

Stage 10 was a true pumped hydro-storage peak power stage, producing up to 11.5 GW electric power for up to 5 hours a day. In stage 11 the flow will be a maximum flow of up to 25,000 cfs, but with periods of less flow during low electricity demand, all to accommodate both water needs and power demands.

The Cove Tank Reservoir dam is 1 mile wide and 250 feet high, the top of the dam is at 4,000 feet. containing up to 60,000 acre-ft of water, enough storage for more than a day’s flow.

The Cove tank Reservoir starts out nearly empty when peak power demand starts, and is rapidly filling up until peak demand ends

The power generating drop is on average (3800 – 2535 – 70×2.2) = 1,111 feet. This stage is capable of generating maximum 2.2 GW of power during peak power demand, adjusted down at low power demand to not exceed the daily water supply.

San Carlos lake is located within the 3,000-square-mile (7,800 km2) San Carlos Apache Indian Reservation, and is thus subject to tribal regulations. It has been full only three times, in 1993 it overflowed the spillway and about 35,000 cfs of water caused erosion damage to natural gas pipelines. The lake contains now (April 6,2021) less than 100 acre-ft of water. All fish is dead.

When former President Coolidge dedicated the dam in 1930, the dam had not begun to fill. Humorist Will Rogers looked at the grass in the lake bed, and said, “If this were my dam, I’d mow it.”[

When the Transcontinental aqueduct is built the lake will always be nearly filled, with flood control nearly automatic, it will never overflow, and it will look like this:

The San Carlos lake, when filled will hold 1,000,000 acre-ft of water.

The Coolidge dam will have to be retrofitted for a 25,000 cfs water flow