The 70-mile (110 km) First Aqueduct consists of pipelines 1 and 2, which run from the Colorado River Aqueduct near San Jacinto, California, to San Vicente Reservoir, approximately 15 miles (24 km) northeast of the city.

The La Mesa-Sweetwater Branch originates from the First Aqueduct, flowing into Sweetwater Reservoir.

In 1957, the construction of Pipeline 3 of the Second Aqueduct was begun by the Metropolitan Water District (MWD), completing it in May 1960.

[3] The First Aqueduct, built of two parallel precast concrete pipes, ranging in diameter from 96 to 48 in (240 to 120 cm), branches from the Colorado River Aqueduct in San Jacinto, California, just north of the San Jacinto River, continuing 70 mi (110 km) south to its terminus at San Vicente Reservoir.

In the first 16 mi (26 km) from the Colorado River Aqueduct to Lake Skinner, Pipeline 3 consists of an open canal handling approximately 500 cu ft/s (14 m3/s).