Cal. WAT § 4125

The owner of every reservoir which is subject to regulation by a watermaster and which is located across or upon the bed of a natural stream or requires the use of a natural stream channel shall construct and maintain such measuring devices below the reservoir in the stream and such measuring devices above the reservoir on each stream or source of supply discharging into the reservoir as may be required and approved by the department for the purpose of assisting the department or the watermaster in determining the amounts of water to which the owner is entitled and the amounts of water which the owner is diverting, storing, and applying to beneficial use.