An open crest weir discharges water driven only by the shallow depth of water flowing over the crest (Q ∝ H^1.5). A siphon spillway, once primed, acts as a closed pressurized conduit driven by the full head difference between reservoir surface and downstream tailwater (Q ∝ Hnet^0.5). A siphon can pass 3 to 5 times the discharge of an open weir of identical crest length.

As water accelerates over the crest apex, potential energy and kinetic velocity heads subtract from atmospheric pressure. If the resulting sub-atmospheric vacuum approaches water vapor pressure (typically when vacuum exceeds 24 to 26 ft of water / ~8 m), the water boils at ambient temperature. Vapor bubbles collapse violently downstream, pitting and spalling concrete and de-priming the siphon.

The air breaker is a pipe connecting the siphon crown to the reservoir water surface at normal pool level. During a flood, rising water submerges the vent, allowing the siphon to exhaust air and prime. When floodwaters recede below the vent lip, atmospheric air rushes into the crest crown, breaking the vacuum and instantly shutting off the siphon before it drains the reservoir pool.

Atmospheric pressure decreases with altitude (e.g. from 34.0 ft of water at sea level to 29.5 ft at 4,000 ft elevation). Because the maximum suction lift is limited by atmospheric pressure minus vapor pressure, high-altitude dams have a lower vacuum threshold before cavitation occurs, requiring lower crest lifts above reservoir water level.

For a siphon to prime automatically, the downstream outlet must be sealed by a tailwater basin or a deflective sealing weir. This water seal prevents air from being drawn backward up the barrel from the bottom, allowing water sweeping over the crest to entrain and eject all trapped air until full-bore pipe flow is established.