Cal. Probate Code § 5305

(a) Notwithstanding Sections 5301 to 5303, inclusive, if parties to an account are married to each other, whether or not they are so described in the deposit agreement, their net contribution to the account is presumed to be and remain their community property. (b) Notwithstanding Sections 2581 and 2640 of the Family Code, the presumption established by this section is a presumption affecting the burden of proof and may be rebutted by proof of either of the following: (1) The sums on deposit that are claimed to be separate property can be traced from separate property unless it is proved that the married persons made a written agreement that expressed their clear intent that the sums be their community property. (2) The married persons made a written agreement, separate from the deposit agreement, that expressly provided that the sums on deposit, claimed not to be community property, were not to be community property. (c) Except as provided in Section 5307, a right of survivorship arising from the express terms of the account or under Section 5302, a beneficiary designation in a Totten trust account, or a P.O.D. payee designation, may not be changed by will. (d) Except as provided in subdivisions (b) and (c), a multiple-party account created with community property funds does not in any way alter community property rights.