[7] It flows north-northwest and drains the Salinas Valley that slices through the central California Coast Ranges south of Monterey Bay.

The river is a wildlife corridor, and provides the principal source of water from its reservoirs and tributaries for the farms and vineyards of the valley.

[9] Nonetheless, with sufficiently heavy rains, and on rare occasions, this now normally dry runoff feature is still capable of quickly transforming itself back into a fast-flowing river.

[10] The atypical drought-breaking rains of the winter of 2016–2017 restored the river's flow to its lower northern reaches in January 2017.

The current most typical dry or zero flow state of the majority of the river may be more the result of human activity than of any recent changes in weather patterns.

[11][12] Recent increases in water use, primarily in the agricultural sector, and the damming of the river and its tributaries may be contributing factors causing the now mostly-dry condition of the riverbed.

[16] The previous ecosystem of the Salinas River, which once included steelhead trout, and numerous other species throughout the full length of a once year-round flowing river, has clearly been drastically impacted in recent years by the expanding heavy demands of agricultural water use in the Salinas Valley, and the resulting most typical dry-river conditions.

The Portolá expedition included Franciscan priests, who soon thereafter established two missions along the banks of the Salinas river (then referred to as el Rio de Monterey.)

The Mission San Antonio de Padua was established during this same time period in the Salinas Valley, but not on the river itself.

Possibly because of flooding and human activity sometime between 1908 and 1910, the river mouth changed by 5.5 miles (8.8 km) to a new channel by Mulligan Hill.

[24] The river begins in southern San Luis Obispo County, approximately 2.5 miles (4.0 km) east of the summit point of Pine Ridge,[25] at a point just off of Agua Escondido Road, coming down off of the slopes of the Los Machos Hills of the Los Padres National Forest.

It flows past many small towns in the valley, including King City, Greenfield, and Soledad, where it combines with the flash-flood prone Arroyo Seco, its fourth major tributary (in wet years).

The old stream bed went from the Old Salinas River, joining Elkhorn Slough on Monterey Bay near Moss Landing, to the present course where the main channel's mouth is directly on the Pacific Ocean.

Before the arrival of Hispanic and American settlers in the area, the Salinas River was once home to abundant fish and beaver populations.

Regarding historical fish populations, the Arroyo Seco is the only major Salinas River tributary which has remained undammed and as of 2015, still supported a small remnant population of the threatened Central Coast Steelhead trout (Oncorhynchus mykiss) that once spawned throughout in the Salinas River watershed.

At one time it was also an important middle link for salmon migrating from the Salinas River to Tassajara Creek and other tributaries.

It once took over ten days for the steelhead from the upper part of the watershed to migrate to the Pacific Ocean near the City of Marina on Monterey Bay.

From there, the steelhead would migrate to the area west of the Aleutian Islands before returning to the spawning grounds in the tributaries of the Salinas River.

The underground flow results from numerous aquifers, which are recharged by water from the Salinas, especially from the Nacimiento and San Antonio lakes during the dry months.