<PAGE>

                                EXHIBIT 10.13D
                              FOURTH AMENDMENT TO
                              -------------------
                     AMENDED AND RESTATED CREDIT AGREEMENT
                     -------------------------------------

     THIS FOURTH AMENDMENT TO AMENDED AND RESTATED CREDIT AGREEMENT (the
"Amendment"), dated as of September 29, 2000, is entered into between MELLON
 ---------
BANK, N.A. a national association ("Bank"), and SOUTHWEST WATER COMPANY, a
                                    ----
Delaware corporation ("Borrower").
                       --------

                                   RECITALS
                                   --------

     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 and that certain Third Amendment
to Amended and Restated Credit Agreement dated as of July 19, 2000
(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.   Bank and Borrower wish 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 the 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 "Maturity Date" as set forth in Section 1.01 of
                                 -------------
the Credit Agreement is hereby amended in its entirety to read as follows:

                    "Maturity Date":  July 31, 2002
                     --------------

          (b) The definition of "Consolidated Tangible Net Worth" set forth in
                                 -------------------------------
Section 1.01 of the Credit Agreement is hereby amended and restated in its
entirety to read as follow:

               "Consolidated Tangible Net Worth": At any date of determination,
                -------------------------------
the sum of the capital stock and additional paid-in capital plus retained
earnings (or minus accumulated deficit) of the Borrower and its consolidated
Subsidiaries minus (i) treasury stock, (ii) intangible assets (including,
             ------
without limitation, franchises, patents, patent applications, trademarks, brand
names, goodwill, purchased contracts, deferred charges (including unamortized
debt discount and expense and organization costs) and research and development
expenses provided, however, that
<PAGE>

water rights shall not be considered an intangible asset and (iii) receivables,
advances, loans and all other amounts due from employees, officers,
shareholders, and/or affiliates (excluding Borrower's wholly-owned
Subsidiaries), on a consolidated basis determined in conformity with GAAP."

     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)  Renewal Fee. Receipt by Bank of the renewal fee set forth in
               -----------
     Paragraph 3 below.

          (c)  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)Bank of America, N.A. and Borrower Amendment.  Execution and
             ---------------------------------------------
     delivery to Bank of the Bank of America, N.A. and Borrower Amendment, which
     includes the same amendments contained in this Amendment.

          (e)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.  Renewal Fee.  Borrower shall pay to Bank a renewal fee in the amount of
         -----------
Six Thousand Dollars ($6,000) for the renewal of the term of the Credit
Agreement, which fee will be fully earned on the date of this Amendment.

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

         (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.

                                       2
<PAGE>

          (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.

          (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

                                       3
<PAGE>

     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.

     8.   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.

     9.   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.

     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/ PETER J. MOERBEEK
                                 -----------------------
                              Name: Peter J. Moerbeek
                              Title: Chief Financial Officer

                                       4
<PAGE>

                              "BANK"

                              MELLON BANK, N.A.,
                              A National Association


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

                                       5