<PAGE>

                                                                  EXHIBIT 10.13C

                              THIRD AMENDMENT TO
                              ------------------
                     AMENDED AND RESTATED CREDIT AGREEMENT
                     -------------------------------------

     THIS THIRD AMENDMENT TO AMENDED AND RESTATED CREDIT AGREEMENT (the
"Amendment"), dated as of July 19, 2000, is entered into between MELLON BANK,
 ---------
N.A. a national association ("Bank"), and SOUTHWEST WATER COMPANY, a Delaware
                              ----
corporation ("Borrower").
              --------

                                    RECITAL
                                    -------

     A.   Borrower and Bank have previously entered into that certain Amended
and Restated Credit Agreement dated as of December 23, 1997, as amended by that
certain First Amendment to Amended and Restated Credit Agreement dated as of
September 1, 1998 and that certain Second Amendment to Amended and Restated
Credit Agreement dated as of September 29, 1999 (collectively, the "Credit
                                                                    ------
Agreement"), pursuant to which Bank has made certain loans and financial
---------
accommodations available to Borrower. Terms used herein without definition shall
have the meanings ascribed to them in the Credit Agreement.

     B.   Borrower and Bank wish to amend the Credit Agreement (i) to increase
the Revolving Commitment from $4,000,000 to $7,000,000, and (ii) to increase the
amount of Debt that Borrower is permitted to exist or incur.

     C.   Bank is willing to amend the Credit Agreement under the terms and
conditions set forth in this Amendment. Borrower is entering into this Amendment
with the understanding and agreement that, except as specifically provided
herein, none of Bank's rights or remedies as set forth in the Credit Agreement
is being waived or modified by the terms of this Amendment.

     NOW, THEREFORE, in consideration of the foregoing and the mutual covenants
herein contained, and for other good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, the parties hereby agree as
follows:

     1.   Amendments to Credit Agreement.
          ------------------------------

          (a)   The definition of "Revolving Commitment" as set forth in Section
     1.01 of the Credit Agreement is hereby amended to read in its entirety as
     follows:

                "'Revolving Commitment': The amount of $7,000,000 as such amount
                  --------------------
          may be reduced pursuant to Section 2.01(c)."

          (b)   Clause (iii) of Section 6.02(f) of the Credit Agreement is
     hereby amended and restated in its entirety to read as follows:

          (iii) Debts, revolving lines of credit and lease obligations of
          Borrower existing as of, and disclosed to Bank prior to, the date of
          this Agreement (including $7,000,000 of unsecured debt of Borrower to
          Bank of America, $4,000,000 of unsecured debt of Suburban to Bank of
          America and $4,000,000 of unsecured debt of NMUI to First Security
          Bank),"
<PAGE>

          "(c) Clause (v) of Section 6.02(f) of the Credit Agreement is
        hereby amended and restated in its entirety to read as follow:

               "(v)" unsecured funded bank debt not to exceed an aggregate of
               $26,000,000 at any time (including, without limitation, unsecured
               funded bank debt incurred pursuant to the Loan Documents and the
               Suburban Loan Documents and unsecured funded bank debt to Bank of
               America as described in clause (iii) above)."

               "(d)  Exhibit A to the Credit Agreement, the form of "Revolving
                     ---------
        Note" is hereby replaced in its entirety with Exhibit A attached hereto
                                                      ---------
        and any and all references to the "Revolving Note" in the Credit
        Agreement shall be a reference to the "Revolving Note" in the Credit
        Agreement shall be a reference to the Revolving Note as amended hereby.

        2.     Effectiveness of this Amendment.  Bank must have received the
               -------------------------------
following items, in form and content acceptable to Bank, before this Amendment
is effective and before Bank is required to extend any credit to Borrower as
provided for by this Amendment.

               (a)  Amendment. This Amendment fully executed in a sufficient
                    ---------
        number of counterparts for distribution to Bank and Borrower.

               (b)  Authorizations.  Evidence that the execution, delivery and
                    --------------
        performance by Borrower and each guarantor or subordinating creditor of
        this Amendment and any instrument or agreement required under this
        Amendment have been duly authorized.

               (c)  Representations and Warranties.  The representations and
                    ------------------------------
        warranties set forth in the Credit Agreement must be true and correct.

               (d)  Third Amended and Restated Promissory Note.  The Third
                    ------------------------------------------
        Amended and Restated Line of Credit Note substantially in the form of
        Exhibit A, attached hereto, with any appropriate insertions and duly
        ---------
        executed by Borrower.


               (e)  Bank of America, N.A. and Borrower Amendment.  Execution and
                    --------------------------------------------
delivery of the Bank of America, N. A. and Borrower Amendment, which includes
the same increases in the Revolving Commitment an the amount of Debt the
Borrower is permitted to exist or incur as stated in this Amendment.

               (f)  Other Required Documentation.  All other documents and legal
                    ----------------------------
matters in connection with the transactions contemplated by this Amendment shall
have been delivered or executed or recorded and shall be in form and substance
satisfactory to Bank.

        3.     Amendment Fee.  Borrower shall pay to Bank an amendment fee in
               -------------
the amount of Seven Thousand Five Hundred Dollars ($7,500) for the processing
and approval of this Amendment, which fee will be fully earned on the date of
this Amendment.

        4.     Representations and Warranties.  The Borrower represents and
               ------------------------------
warrants as follows:

                                       2
<PAGE>

          (a)  Authority.  The Borrower has the requisite corporate power and
               ---------
     authority to execute and deliver this Amendment and to perform its
     obligations hereunder and under the Loan Documents (as amended or modified
     hereby) to which it is a party.  The execution, delivery and performance by
     the Borrower of this Amendment and the performance by Borrower of each Loan
     Document (as amended or modified hereby) to which it is a party have been
     duly approved by all necessary corporate action of Borrower and no other
     corporate proceedings on the part of Borrower are necessary to consummate
     such transactions.

          (b)  Enforceability.  This Amendment has been duly executed and
               --------------
     delivered by the Borrower. This Amendment and each Loan Document (as
     amended or modified hereby) is the legal, valid and binding obligation of
     Borrower, enforceable against Borrower in accordance with its terms, and is
     in full force and effect.

          (c)  Representations and Warranties.  The representations and
               ------------------------------
     warranties contained in each Loan Document (other than any such
     representations or warranties that, by their terms, are specifically made
     as of a date other than the date hereof) are correct on and as of the date
     hereof as though made on and as of the date hereof.

          (d)  No Default.  No event has occurred and is continuing that
               ----------
     constitutes an Event of Default.

     5.   Choice of Law.  The validity of this Amendment, its construction,
          -------------
interpretation and enforcement, the rights of the parties hereunder, shall be
determined under, governed by, and construed in accordance with the internal
laws of the State of California governing contracts only to be performed in that
State.

     6.   Counterparts.  This Amendment may be executed in any number of
          ------------
counterparts and by different parties and separate counterparts, each of which
when so executed and delivered, shall be deemed an original, and all of which,
when taken together, shall constitute one and the same instrument.  Delivery of
an executed counterpart of a signature page to this Amendment or the Consent by
telefacsimile shall be effective as delivery of a manually executed counterpart
of this Amendment or such Consent.

     7.   Due Execution.  The execution, delivery and performance of this
          -------------
Amendment are within the power of Borrower, have been duly authorized by all
necessary corporate action, have received all necessary governmental approval,
if any, and do not contravene any law or any contractual restrictions binding on
Borrower.

     8.   Reference to and Effect on the Loan Documents.
          ---------------------------------------------

          (a) Upon and after the effectiveness of this Amendment, each reference
     in the Credit Agreement to "this Agreement", "hereunder", "hereof" or words
     of like import referring to the Credit Agreement, and each reference in the
     other Loan Documents to "the Credit Agreement", "thereof" or words of like
     import referring to the Credit Agreement, shall mean and be a reference to
     the Credit Agreement as modified and amended hereby.

                                       3
<PAGE>

          (b)  Except as specifically amended above, the Credit Agreement and
     all other Loan Documents, are and shall continue to be in full force and
     effect and are hereby in all respects ratified and confirmed and shall
     constitute the legal, valid, binding and enforceable obligations of
     Borrower to Bank.

          (c)  The execution, delivery and effectiveness of this Amendment shall
     not, except as expressly provided herein, operate as a waiver of any right,
     power or remedy of Bank under any of the Loan Documents, nor constitute a
     waiver of any provision of any of the Loan Documents.

          (d)  To the extent that any terms and conditions in any of the Loan
     Documents shall contradict or be in conflict with any terms or conditions
     of the Credit Agreement, after giving effect to this Amendment, such terms
     and conditions are hereby deemed modified or amended accordingly to reflect
     the terms and conditions of the Credit Agreement as modified or amended
     hereby.

     9.   Ratification.  Borrower hereby restates, ratifies and reaffirms each
          ------------
and every term and condition set forth in the Credit Agreement, as amended
hereby, and the Loan Documents effective as of the date hereof.

     10.  Estoppel.  To induce Bank to enter into this Amendment and to continue
          --------
to make advances to Borrower under the Credit Agreement, Borrower hereby
acknowledges and agrees that, after giving effect to this Amendment, as of the
date hereof, there exists no Event of Default and no right of offset, defense,
counterclaim or objection in favor of Borrower as against Bank with respect to
the Obligations.

     11.  Acknowledgment and Agreement pertaining to Section 6.02(f)(v) of the
          --------------------------------------------------------------------
Southwest Water Company Amended and Restated Credit Agreement Dated December 23,
--------------------------------------------------------------------------------
1997, as amended from time to time (collectively the "Southwest Credit
----------------------------------------------------------------------
Agreement") and Section 6.02(i)(iv) of the Suburban Water Systems Credit
------------------------------------------------------------------------
Agreement dated December 23, 1997, as amended from time to time, (collectively
------------------------------------------------------------------------------
the "Suburban Credit Agreement".  Pursuant to sections 6.02(f)(v) and
------------------------------
6.02(i)(iv) of the Southwest Credit Agreement and the Suburban Credit Agreement,
respectively, the Bank hereby consents to the increase in Debt which the
Borrower is permitted to exist or incur, as described herein.

                      [Signatures set forth on next page]

                                       4
<PAGE>

     IN WITNESS WHEREOF, the parties have entered into this Amendment as of the
date first above written.

                              "Borrower"

                              SOUTHWEST WATER COMPANY,
                              a Delaware corporation

                              By: /s/ Thomas C. Tekulve
                                  ------------------------
                              Name:  Thomas C. Tekulve
                              Title: Vice President - Finance



                              By: /s/ Anton C. Garnier
                                 -------------------------
                              Name:  Anton C. Garnier
                              Title: President

                              "BANK"

                              MELLON BANK, N.A.,
                              a national association

                              By: /s/ Garry Handelman
                                  ------------------------
                              Name:  Garry Handelman
                              Title: Vice President

                                       5

<PAGE>

                                   EXHIBIT A
                                   ---------

                            Form of Revolving Note

                                [See Attached.]


                                       6
<PAGE>

                                   EXHIBIT A
                                   ---------

                            SOUTHWEST WATER COMPANY

                          SECOND AMENDED AND RESTATED
                                PROMISSORY NOTE

$7,000,000.00                                                      July 19, 2000
                                                         West Covina, California

          FOR VALUE RECEIVED, SOUTHWEST WATER COMPANY, a Delaware corporation
(the "Borrower"), promises to pay to the order of MELLON BANK, N.A. (the "Bank")
      --------                                                            ----
on the Maturity Date (as defined in the Credit Agreement referred to below) the
principal amount of Seven Million Dollars ($7,000,000.00), or, if less, the
aggregate amount of Revolving Loans (as defined in the Credit Agreement referred
to below) made by the Bank to the Borrower pursuant to the Credit Agreement
referred to below outstanding on the Maturity Date.

          The Borrower also promises to pay interest on the unpaid principal
amount hereof from the date hereof until paid at the rates and at the times
which shall be determined in accordance with the provisions of the Credit
Agreement.

          All unpaid amounts of principal and interest shall be due and payable
in full on the Maturity Date.

          All payments of principal and interest in respect of this Note shall
be made in lawful money of the United States of America in same day funds at the
office of the Bank located at Loan Administration - 199-5220, P.O. Box 7777-
W7715, Philadelphia, Pennsylvania 19175-7715 or at such other place as shall be
designated in writing for such purpose in accordance with the terms of the
Credit Agreement.  Until notified of the transfer of this Note, the Borrower
shall be entitled to deem the Bank or such person who has been so identified by
the transferor in writing to the Borrower as the holder of this Note, as the
owner and holder of this Note.  Each of the Bank and any subsequent holder of
this Note agrees that before disposing of this Note or any part hereof, it will
make a notation hereon of all principal payments previously made hereunder and
of the date to which interest hereon has been paid on the schedule attached
hereto, if any; provided, however, that the failure to make notation of any
                --------  -------
payment made on this Note shall not limit or otherwise affect the obligation of
the Borrower hereunder with respect to payments of principal or interest on this
Note.

          This Note is referred to in, and is entitled to the benefits of, the
Amended and Restated Credit Agreement dated as of December 23, 1997, as amended
from time to time (the "Credit Agreement") between the Borrower and the Bank.
                        ----------------
The Credit Agreement, among other things, (i) provides for the making of
advances (the "Loans") by the Bank to the Borrower from time to time in an
               -----
aggregate amount not to exceed at any time outstanding the U.S. dollar amount
first above mentioned, the indebtedness of the Borrower resulting from each such
Loan being evidenced by this Note, and (ii) contains provisions for acceleration
of the maturity hereof upon
<PAGE>

the happening of certain stated events and also for prepayments on account of
principal hereof prior to the maturity hereof upon the terms and conditions
therein specified.

          The terms of this Note are subject to amendment only in the manner
provided in the Credit Agreement.

          No reference herein to the Credit Agreement and no provision of this
Note or the Credit Agreement shall alter or impair the obligation of the
Borrower, which is absolute and unconditional, to pay the principal of and
interest on this Note at the place, at the respective times, and in the currency
herein prescribed.

          The Borrower promises to pay all costs and expenses, including
reasonable attorneys' fees, incurred in the collection and enforcement of this
Note.  The Borrower hereby consents to renewals and extensions of time at or
after the maturity hereof, without notice, and hereby waives diligence,
presentment, protest, demand and notice of every kind and, to the full extent
permitted by law, the right to plead any statute of limitations as a defense to
any demand hereunder.

          This Note amends and restates in its entirety that certain Amended and
Restated Promissory Note dated as of September 29, 1999 in the amount of Four
Million Dollars ($4,000,000) made by Borrower payable to the order of Bank
pursuant to the Credit Agreement.

          IN WITNESS WHEREOF, the Borrower has caused this Notice to be executed
and delivered by its duly authorized officers, as of the date and the place
first above-written.


                                    SOUTHWEST WATER COMPANY


                                    By:______________________________
                                    Name:  Thomas C. Tekulve
                                    Title: Vice President - Finance



                                    By:______________________________
                                    Name:  Anton C. Garnier
                                    Title: President
<PAGE>

                                 TRANSACTIONS
                                      ON
                                     NOTE


<TABLE>
<CAPTION>
Amount of        Amount of                            Interest Paid   Principal    Notation
Loan Made        Principal Paid      Interest Paid    Through         Balance      Made By
--------         --------------      ------------     -------         -------      ------
<S>             <C>                <C>                <C>             <C>          <C>
  </TABLE>