The Salinas River begins in the central California Coast Ranges and flows 175 miles (282 km) generally northwest to Monterey Bay, approximately 12 miles (19 km) north-northeast of Monterey and 3 miles (5 km) west-southwest of Castroville, California. The river drains a watershed of 2.7 million acres (1 million hectares) and the final reach forms a lagoon protected by 367 acres (149 hectares) of the Salinas River National Wildlife Refuge. The river is named after the Indigenous Salinan people, who lived by hunting and gathering before Spanish colonization. The northern Salinas Valley is carved into granite and other crystalline rocks, such as biotite schists and limestone. Granite intruded into the schists as magma, metamorphosing the limestone. During the Pliocene and Miocene epochs, sediments filled the valley. These sediments were later covered by unconsolidated alluvial fans, marine clays, and wind-blown sand deposits from the Pleistocene to the Holocene epochs, which now hold a vast volume of groundwater. Since the early 1940s, continuous groundwater pumping along the coastal area of the Salinas Valley has lowered water pressure, allowing seawater encroachment and degrading groundwater quality. The Salinas River’s outflow to Monterey Bay is typically blocked by beach sand and dunes, except during winter high river discharge. The river’s connectivity to the ocean depends on flow discharge, which is influenced by precipitation and upstream water extraction. During prolonged droughts, low flow conditions allow sandbars to build at the river mouth, often blocking it entirely. When this occurs, the estuarine portion of the Salinas River temporarily becomes a freshwater lagoon. Estuarine conditions return when high discharge events breach the sand barrier, re-establishing ocean connectivity.

In 1769, the Portola expedition first encountered the Salinas River, describing it as watering a luxuriant plain teeming with fish. In the late 18th and early 19th centuries, the river valley served as a route for travelers, known as El Camino Real. This became the main overland path from southern to northern Alta California, used by Spanish explorers, missionaries, and early settlers. Spanish colonial missions—San Miguel Arcángel, San Antonio de Padua, and Nuestra Señora de la Soledad—were founded in the Salinas Valley in the late 18th century. After the Mexican War of Independence in 1821, mission lands were secularized. The area near the mouth of the Salinas River became part of Rancho Bolsa Nueva y Moro Cojo, a 30,901-acre (12,505-ha) estate. This rancho combined three Mexican land grants: Rancho Bolsa Nueva y Moro Cojo, given in 1844 by Governor Manuel Micheltorena to María Antonia Pico de Castro, Rancho Bolsa del Potrero y Moro Cojo, granted in 1822 by Governor Pablo Vicente de Solá to Joaquín de la Torre; and land granted in 1837 by Governor Juan Alvarado to Simeon Castro. The rancho extended from Moss Landing inland to present-day Prunedale and south to Castroville, where the land was crisscrossed by sloughs and swamps. By the mid-19th century, Chinese immigrants had established a presence in Castroville and were instrumental in clearing these wetlands to grow crops. The 1906 San Francisco earthquake caused the Salinas River mouth at Monterey Bay to be diverted 6 miles (9.7 km) south, from an area between Moss Landing and Watsonville to a new channel just north of Marina. The Old Salinas River channel, which diverts north behind the dunes along the ocean, serves as an overflow channel during the rainy season.

Irrigation in the Salinas Valley began with the gravity systems of the Spanish missions, followed by the construction of diversion ditches in the late 1870s and early 1880s. In 1882, Max J. Brandenstein made the first large water claim, constructing 6 miles (10 km) of canal through the San Bernardo and Salinas Valley Canal and Irrigation Company to irrigate alfalfa. By 1901, seventy claims to water from the Salinas River and its tributaries had been filed. Gravity irrigation soon proved unreliable for the growing demands of agriculture, prompting the construction of storage ponds and pumping stations. The historical increase in agriculture and settlement, along with the related rise in water consumption, has significantly impacted the area. The use of river water for irrigation makes the Salinas Valley one of California’s most productive agricultural regions. However, by the end of 2016, the river had become little more than a dry-bedded runoff feature for most of its length. The Salinas River is contaminated by agricultural and industrial effluent, primarily pesticides and nitrates from the surrounding farmlands. The river also suffers from eutrophication and severe algal growth. Groundwater now provides over 95% of the water used in the Salinas Valley. The increasing demand for high-quality water for agriculture, domestic, and municipal uses has made addressing seawater encroachment a critical objective for county and state water resource agencies associated with the Salinas Valley groundwater basin. Efforts to resolve these issues are vital to ensure the sustainability of the region’s water supply. Effective management strategies are needed to mitigate contamination and prevent further degradation of water quality, which is essential for maintaining the region’s agricultural productivity and supporting local communities. Read more here and here. Explore more of the Salinas River and Castroville here: