Every schoolchild's aquarium hose is a siphon: fill the tube, put one end below the other, and gravity pulls water up and over the lip forever — no pump. So why not do it at planetary scale? Point one leg into Lake Shasta, drape the other over the Tehachapis, and let physics move California's water for free.

The atmospheric ceiling. Textbook siphons are powered by atmospheric pressure pushing liquid up the ascending leg. That pressure — 101 kPa at sea level — can only support a column of water:

h_max = P_atm / (ρ · g) = 101,325 / (1000 · 9.81) ≈ 10.33 m

Above that, the pressure at the crest drops to the vapor pressure of water, the column cavitates, and the siphon breaks. So the "mountain" your tube can crest is a hilariously modest three-story hop.

The cohesion loophole. Since 2011, Ramette & Ramette and independently Boatwright showed degassed water siphoned in a smooth, sealed tube can go higher — because water's molecular cohesion (tensile strength of pure water ≈ –25 MPa in theory, ~1.5 MPa achieved in glass capillaries) means the column can be pulled like a chain, not just pushed. Careful lab work reached ~15 m. Trees do it: xylem sap in a redwood ascends 115 m under negative pressures around –1.5 MPa. But trees cheat with 20 μm capillaries; scaling up to a meter-diameter aqueduct means the slightest nucleation site — a passing cosmic ray, a weld defect — triggers a cavitation avalanche.

Pressurize the whole system. The workable trick: seal both reservoirs and hold them at pressure P. Now the crest height above the source becomes:

h_max = P / (ρ · g)

To crest the Tehachapis (~600 m over the aqueduct's source), you need:

P = 1000 · 9.81 · 600 ≈ 5.9 MPa (~59 bar, ~850 psi)

That's routine for oil pipelines. Both the intake pond and the discharge pond sit inside pressurized concrete domes filled with nitrogen at 60 bar. The 100 km, 4 m-diameter steel pipeline needs a wall thickness (thin-wall approximation, σ_allow = 200 MPa):

t = P·r / σ = 5.9e6 · 2 / 2e8 ≈ 59 mm

Call it 70 mm for safety. Steel mass: π · 4 · 0.07 · 100,000 · 7850 ≈ 690,000 tonnes — one Golden Gate Bridge worth of steel per aqueduct.

Does it pay? California's Edmonston Pumping Plant lifts 120 m³/s over 610 m, consuming ~8 TWh/year — roughly $800M in electricity, and the state's single largest power draw. A pressurized siphon replaces every joule of that pumping with a one-time investment in pressurization gas and pipe steel. Steady-state losses are just pipe friction; for a smooth 4 m pipe at 10 m/s, Darcy-Weisbach gives about 15 m of head loss per 100 km, meaning the source reservoir only needs to sit 15 m higher than the sink. If it doesn't, you're back to pumps — but only against friction, not against gravity.

What kills it. Not the physics — the failure modes. A pinhole leak drops system pressure; cavitation front races through the crest at the speed of sound in water (~1500 m/s); the entire column separates; hundreds of tonnes of vacuum-slammed water hammer the pipe with pressure spikes exceeding 100 bar (Joukowsky equation: ΔP = ρ·c·Δv). Your 690,000-tonne masterpiece bursts like a party balloon. This is why real long-distance water systems use redundant pumping stations at each pass: they fail gracefully. Siphons fail catastrophically.