 Irrigation with water from the Merced River continued to grow substantially until most of the arable land around the river , some 120 @,@ 000 acres ( 490 km2 ) , was under cultivation . By the late 1950s and early 1960s , irrigation in the San Joaquin Valley was to such an extent that many of the rivers ran dry in sections . Upriver of the Merced River confluence with the San Joaquin , the latter river was usually dry , only regaining flow where the Merced River enters . In the mid @-@ 20th century , the flow in the Merced River diminished to such a degree that very few salmon returned to spawn in the lower section of the Merced River . In 1991 , a fish hatchery , the Merced River Hatchery , was built beside the Merced River just downstream of the Crocker @-@ Huffman Diversion Dam , the lowermost Merced River dam . Fall chinook salmon travel up a fish ladder into the hatchery 's pools , which are supplied with water diverted from the Merced River . 
