This project was implemented to blend treated water from the Poseidon Desalination Plant through a 54 inch pipeline into a 96 inch San Diego County Water Authority potable water transmission main. The work consisted of designing and constructing a water tight, reinforced concrete weir structure around the existing 96 inch CML&C transmission main. The new 15 feet tall weir wall needed to be capable of handling flows of 450 CFS, required two 54 inch CML&C pipeline interconnects, and a 54 inch butterfly valve installation. Continuous work was necessary 24/7 for nearly 3 months to meet a strict shutdown schedule deadline. Other challenging aspects included noise monitoring and mitigation, extensive rock breaking, limited access, shoring, and earthwork.