California Code, Government Code - GOV § 6502

If authorized by their legislative or other governing bodies, two or more public agencies by agreement may jointly exercise any power common to the contracting parties, including, but not limited to, the authority to levy a fee, assessment, or tax, even though one or more of the contracting agencies may be located outside this state.

It shall not be necessary that any power common to the contracting parties be exercisable by each such contracting party with respect to the geographical area in which such power is to be jointly exercised.  For purposes of this section, two or more public agencies having the power to conduct agricultural, livestock, industrial, cultural, or other fairs or exhibitions shall be deemed to have common power with respect to any such fair or exhibition conducted by any one or more of such public agencies or by an entity created pursuant to a joint powers agreement entered into by such public agencies.