Construction of dams in the San Joaquin watershed has led to nearly all of the good spawning streams, located higher in the mountains near the headwaters of the San Joaquin and its tributaries, being cut off to the salmon. The remaining areas left for the salmon to spawn in are undesirable. Prior to the building of the Friant Dam in 1944, the San Joaquin River was believed to have over 6,000 miles of streambed suitable and available to spawning salmon. However, in 1993 it was estimated that there was just 300 miles.