Prior to the construction of Nacimiento Dam, the lower Nacimiento River (and the Salinas River) only flowed during the wet season between December and May. Without enough time to be absorbed into the Salinas Valley
aquifer, much of the stormwater flowed directly into the Pacific Ocean. Because the Nacimiento River is the biggest tributary of the Salinas River, the dam would capture up to half the annual peak flows in the entire Salinas River Basin. Under the county plan, water would only be stored when the flow rate at the confluence of the Nacimiento and Salinas Rivers exceeded , which was estimated as the highest volume the riverbed can naturally absorb. When natural runoff exceeded this amount, no water would be released. The
California Department of Fish and Game (DFG) opposed the project due to the detrimental impacts this would have on winter run
steelhead trout. The DFG sought a minimum flow of in the Nacimiento River, which the county believed would place too great a demand on the new reservoir. Ultimately, the county won and was granted the water rights for the reservoir in 1955 without any provisions for fishery flows. After further petitioning from the DFG, the MCWRA now maintains a minimum flow of in the Nacimiento River except in years of severe drought. Monterey County is entitled to most of the water supply from Lake Nacimiento. The dam releases an average of of water per year for groundwater recharge and instream flows in the Nacimiento River (also including occasional releases for flood control).{{cite web|url=https://wdr.water.usgs.gov/wy2013/pdfs/11149400.2013.pdf|title=USGS Gage #11149400 on the Nacimiento River below Nacimiento Dam, near Bradley, CA: Water-Data Report 2013|publisher=U.S. Geological Survey ==Specifications==