The purpose of this supplement is to update and revise certain information presented in the original report. These corrective measures became necessary when: The non-federal interest rate used for discounting and determining annual costs was changed from 3 to 4 per cent; A structure at the Laneport site was added to the plan which had originally called for just two reservoirs on the San Gabriel River; A revised estimate was made of the amount of ground water that municipal and industrial interests might totally develop in the area by the year 2010; The size and cost of the proposed Somerville project on Yegua Creek were changed. The information provided is intended to revise only those sections or topics of the original report which are related to the four items mentioned above.