<PAGE>
                                                                    EXHIBIT 4.02
                             SUPPLEMENTAL INDENTURE


                        (DATED AS OF SEPTEMBER 15, 2002)


                                    --------


                       PUBLIC SERVICE COMPANY OF COLORADO


                                       TO


                      U.S. BANK TRUST NATIONAL ASSOCIATION,
                                                        AS TRUSTEE


                                    --------


                   CREATING AN ISSUE OF FIRST MORTGAGE BONDS,
                               COLLATERAL SERIES I

                                    --------

      (SUPPLEMENTAL TO INDENTURE DATED AS OF DECEMBER 1, 1939, AS AMENDED)
<PAGE>
         SUPPLEMENTAL INDENTURE, dated as of September 15, 2002, between PUBLIC
         SERVICE COMPANY OF COLORADO, a corporation organized and existing under
         the laws of the State of Colorado (the "Company"), party of the first
         part, and U.S. BANK TRUST NATIONAL ASSOCIATION (FORMERLY FIRST TRUST OF
         NEW YORK, NATIONAL ASSOCIATION), a national banking association, as
         successor trustee (the "Trustee") to Morgan Guaranty Trust Company of
         New York (formerly Guaranty Trust Company of New York), party of the
         second part.

         WHEREAS, the Company heretofore executed and delivered to the Trustee
         its Indenture, dated as of December 1, 1939 (the "Principal
         Indenture"), to secure its First Mortgage Bonds from time to time
         issued thereunder; and

         WHEREAS, the Company has heretofore executed and delivered to the
         Trustee the Supplemental Indentures referred to in Schedule A hereto
         for certain purposes, including the creation of series of bonds, the
         subjection to the lien of the Principal Indenture of property acquired
         after the execution and delivery thereof, the amendment of certain
         provisions of the Principal Indenture and the appointment of the
         successor Trustee; and

         WHEREAS, the Principal Indenture as supplemented and amended by all
         Supplemental Indentures heretofore executed by the Company and the
         Trustee is hereinafter referred to as the "Indenture", and, unless the
         context requires otherwise, references herein to Articles and Sections
         of the Indenture shall be to Articles and Sections of the Principal
         Indenture as so amended; and

         WHEREAS, the Company proposes to create a new series of First Mortgage
         Bonds to be designated as First Mortgage Bonds, Collateral Series I
         (the "Collateral Series I Bonds"), to be issued and delivered to the
         trustee under the 1993 Mortgage (as hereinafter defined) as the basis
         for the authentication and delivery under the 1993 Mortgage of a series
         of securities, all as hereinafter provided, and to vary in certain
         respects the covenants and provisions contained in Article V of the
         Indenture, to the extent that such covenants and provisions apply to
         the Collateral Series I Bonds; and

         WHEREAS, the Company, pursuant to the provisions of the Indenture, has,
         by appropriate corporate action, duly resolved and determined to
         execute this Supplemental Indenture for the purpose of providing for
         the creation of the Collateral Series I Bonds and of specifying the
         form, provisions and particulars thereof, as in the Indenture provided
         or permitted and of giving to the Collateral Series I Bonds the
         protection and security of the Indenture; and

         WHEREAS, the Company represents that all acts and proceedings required
         by law and by the charter and by-laws of the Company, including all
         action requisite on the part of its shareholders, directors and
         officers, necessary to make the Collateral Series I Bonds, when
         executed by the Company, authenticated and delivered by the Trustee and
         duly issued, the valid, binding and legal obligations of the Company,
         and to constitute the Principal Indenture and all indentures
         supplemental thereto, including this Supplemental Indenture, valid,
         binding and legal instruments for the security of the bonds of all
         series, including the Collateral Series I Bonds, in accordance with the
         terms of such bonds and
<PAGE>
         such instruments, have been done, performed and fulfilled, and the
         execution and delivery hereof have been in all respects duly
         authorized;

         NOW, THEREFORE, THIS SUPPLEMENTAL INDENTURE WITNESSETH:

         That Public Service Company of Colorado, the Company named in the
         Indenture, in consideration of the premises and of One Dollar to it
         duly paid by the Trustee at or before the ensealing and delivery of
         these presents, the receipt whereof is hereby acknowledged, and in
         pursuance of the direction and authority of the Board of Directors of
         the Company given at a meeting thereof duly called and held, and in
         order to create the Collateral Series I Bonds and to specify the form,
         terms and provisions thereof, and to secure the payment of the
         principal of and premium, if any, and interest, if any, on all bonds
         from time to time outstanding under the Indenture, including the
         Collateral Series I Bonds, according to the terms of said bonds, and to
         secure the performance and observance of all of the covenants and
         conditions contained in the Indenture, has executed and delivered this
         Supplemental Indenture and has granted, bargained, sold, warranted,
         aliened, remised, released, conveyed, assigned, transferred, mortgaged,
         pledged, set over and confirmed unto U.S. Bank Trust National
         Association, as Trustee, and its successor or successors in the trust
         and its and their assigns forever;

         TO HAVE AND TO HOLD all and singular the properties, rights, privileges
         and franchises described in the Principal Indenture and in the several
         Supplemental Indentures hereinabove referred to and owned by the
         Company on the date of the execution and delivery hereof (other than
         property of a character expressly excepted from the lien of the
         Indenture as therein set forth) unto the Trustee and its successor or
         successors and assigns forever;

         SUBJECT, HOWEVER, to permitted encumbrances as defined in the
         Indenture;

         IN TRUST, NEVERTHELESS, upon the terms and trusts set forth in the
         Indenture, for the equal and proportionate benefit and security of all
         present and future holders of the bonds and coupons issued and to be
         issued under the Indenture, including the Collateral Series I Bonds,
         without preference, priority or distinction as to lien (except as any
         sinking, amortization, improvement or other fund established in
         accordance with the provisions of the Indenture or any indenture
         supplemental thereto may afford additional security for the bonds of
         any particular series) of any of said bonds over any others thereof by
         reason of series, priority in the time of the issue or negotiation
         thereof, or otherwise howsoever, except as provided in Section 2 of
         Article IV of the Indenture.

                                   ARTICLE ONE

            CREATION AND DESCRIPTION OF THE COLLATERAL SERIES I BONDS

                  SECTION 1. A new series of bonds to be issued under and
secured by the Indenture is hereby created, the bonds of such new series to be
designated First Mortgage Bonds, Collateral Series I. The Collateral Series I
Bonds shall be limited to an aggregate principal amount of Five Hundred Thirty
Million dollars ($530,000,000), excluding any Collateral Series I


                                       2
<PAGE>
Bonds which may be authenticated and exchanged for or in lieu of or in
substitution for or on transfer of other Collateral Series I Bonds pursuant to
any provisions of the Indenture. The Collateral Series I Bonds shall mature on
June 27, 2003. The Collateral Series I Bonds shall not bear interest.

                  The principal of each Collateral Series I Bond shall be
payable, upon presentation thereof, at the office or agency of the Company in
the city in which the principal corporate trust office of the 1993 Mortgage
Trustee (as hereinafter defined) is located, in any coin or currency of the
United States of America which at the time of payment shall be legal tender for
the payment of public and private debts.

                  The Collateral Series I Bonds shall be issued and delivered by
the Company to U.S. Bank Trust National Association, as successor trustee under
the Indenture, dated as of October 1, 1993, as supplemented (the "1993
Mortgage"), of the Company to such successor trustee (the "1993 Mortgage
Trustee"), as the basis for the authentication and delivery under the 1993
Mortgage of a series of securities. As provided in the 1993 Mortgage, the
Collateral Series I Bonds will be registered in the name of the 1993 Mortgage
Trustee or its nominee and will be owned and held by the 1993 Mortgage Trustee,
subject to the provisions of the 1993 Mortgage, for the benefit of the holders
of all securities from time to time outstanding under the 1993 Mortgage, and the
Company shall have no interest therein.

                  Any payment or deemed payment by the Company under the 1993
Mortgage of the principal of the securities which shall have been authenticated
and delivered under the 1993 Mortgage on the basis of the issuance and delivery
to the 1993 Mortgage Trustee of Collateral Series I Bonds (other than by the
application of the proceeds of a payment in respect of such Collateral Series I
Bonds) shall, to the extent thereof, be deemed to satisfy and discharge the
obligation of the Company, if any, to make a payment of principal of such
Collateral Series I Bonds which is then due.

                  The Trustee may conclusively presume that the obligation of
the Company to pay the principal of the Collateral Series I Bonds as the same
shall become due and payable shall have been fully satisfied and discharged
unless and until it shall have received a written notice from the 1993 Mortgage
Trustee, signed by an authorized officer thereof, stating that the principal of
specified Collateral Series I Bonds has become due and payable and has not been
fully paid, and specifying the amount of funds required to make such payment.

                  Each Collateral Series I Bond shall be dated as of the date of
its authentication.

                  The Collateral Series I Bonds shall be issued as fully
registered bonds only, in denominations of $1,000 and multiples thereof.

                  The Collateral Series I Bonds shall be registerable and
exchangeable at the office or agency of the Company in the city in which the
principal corporate trust office of the 1993 Mortgage Trustee is located, in the
manner and upon the terms set forth in Section 5 of Article II of the Indenture;
provided, however, that the Collateral Series I Bonds shall not be transferrable
except to a successor trustee under the 1993 Mortgage. No service charge shall
be made for any exchange or transfer of any Collateral Series I Bond.




                                       3
<PAGE>
                  SECTION 2. The text of the Collateral Series I Bonds shall be
substantially in the form attached hereto as Exhibit A.

                  SECTION 3. The Collateral Series I Bonds may be executed by
the Company and delivered to the Trustee and, upon compliance with all
applicable provisions and requirements of the Indenture in respect thereof,
shall be authenticated by the Trustee and delivered (without awaiting the filing
or recording of this Supplemental Indenture) in accordance with the written
order or orders of the Company.

                                   ARTICLE TWO

                   REDEMPTION OF THE COLLATERAL SERIES I BONDS

                  SECTION 1. Each Collateral Series I Bond shall be redeemable
at the option of the Company in whole at any time, or in part from time to time,
prior to maturity, at a redemption price equal to 100% of the principal amount
thereof to be redeemed.

                  SECTION 2. The provisions of Sections 3, 4, 5, 6 and 7 of
Article V of the Indenture shall be applicable to the Collateral Series I Bonds,
except that (a) no publication of notice of redemption of the Collateral Series
I Bonds shall be required and (b) if less than all the Collateral Series I Bonds
are to be redeemed, the Collateral Series I Bonds to be redeemed shall be
selected in the principal amounts designated to the Trustee by the Company, and
except as such provisions may otherwise be inconsistent with the provisions of
this Article Two.

                  SECTION 3. The holder of each and every Collateral Series I
Bond hereby agrees to accept payment thereof prior to maturity on the terms and
conditions provided for in this Article Two.

                                  ARTICLE THREE

                         ACKNOWLEDGMENT OF RIGHT TO VOTE
                           OR CONSENT WITH RESPECT TO
                         CERTAIN AMENDMENTS TO INDENTURE

                  The Company hereby acknowledges the right of the holders of
the Collateral Series I Bonds to vote or consent with respect to any or all of
the modifications to the Indenture referred to in Article Three of the
Supplemental Indenture, dated as of March 1, 1980, irrespective of the fact that
the Bonds of the Second 1987 Series are no longer outstanding; provided,
however, that such acknowledgment shall not impair (a) the right of the Company
to make such modifications without the consent or other action of the holders of
the Bonds of the 2020 Series or the bonds of any other series subsequently
created under the Indenture with respect to which the Company has expressly
reserved such right or (b) the right of the Company to reserve the right to make
such modifications without the consent or other action of the holders of bonds
of one or more, or any or all, series created subsequent to the creation of the
Collateral Series I Bonds.




                                       4
<PAGE>
                                  ARTICLE FOUR

                                   THE TRUSTEE

                  The Trustee accepts the trusts created by this Supplemental
Indenture upon the terms and conditions set forth in the Indenture and this
Supplemental Indenture. The recitals in this Supplemental Indenture are made by
the Company only and not by the Trustee. Each and every term and condition
contained in Article XII of the Indenture shall apply to this Supplemental
Indenture with the same force and effect as if the same were herein set forth in
full, with such omissions, variations and modifications thereof as may be
appropriate to make the same conform to this Supplemental Indenture.

                                  ARTICLE FIVE

                            MISCELLANEOUS PROVISIONS

                  SECTION 1. Subject to the variations contained in Article Two
of this Supplemental Indenture, the Indenture is in all respects ratified and
confirmed and the Principal Indenture, this Supplemental Indenture and all other
indentures supplemental to the Principal Indenture shall be read, taken and
construed as one and the same instrument. Neither the execution of this
Supplemental Indenture nor anything herein contained shall be construed to
impair the lien of the Indenture on any of the properties subject thereto, and
such lien shall remain in full force and effect as security for all bonds now
outstanding or hereafter issued under the Indenture.

                  All covenants and provisions of the Indenture shall continue
in full force and effect and this Supplemental Indenture shall form part of the
Indenture.

                  SECTION 2. If the date for making any payment or the last date
for performance of any act or the exercising of any right, as provided in this
Supplemental Indenture, shall not be a Business Day (as defined in the 1993
Mortgage), such payment may be made or act performed or right exercised on the
next succeeding Business Day with the same force and effect as if done on the
nominal date provided in this Supplemental Indenture.

                  SECTION 3. The terms defined in the Indenture shall, for all
purposes of this Supplemental Indenture, have the meaning specified in the
Indenture except as set forth in Section 4 of this Article or otherwise set
forth in this Supplemental Indenture or unless the context clearly indicates
some other meaning to be intended.

                  SECTION 4. Any term defined in Section 303 of the Trust
Indenture Act of 1939, as amended, and not otherwise defined in the Indenture
shall, with respect to this Supplemental Indenture and the Collateral Series I
Bonds, have the meaning assigned to such term in Section 303 as in force on the
date of the execution of this Supplemental Indenture.

                  SECTION 5. This Supplemental Indenture may be executed in any
number of counterparts, and all of said counterparts executed and delivered,
each as an original, shall constitute but one and the same instrument.



                                       5
<PAGE>
                  IN WITNESS WHEREOF, Public Service Company of Colorado, party
hereto of the first part, has caused its corporate name to be hereunto affixed,
and this instrument to be signed by its President, an Executive Vice President,
a Senior Vice President or a Vice President, and its corporate seal to be
hereunto affixed and attested by its Secretary or an Assistant Secretary for and
in its behalf; and U.S. Bank Trust National Association, the party hereto of the
second part, in evidence of its acceptance of the trust hereby created, has
caused its corporate name to be hereunto affixed, and this instrument to be
signed and its corporate seal to be affixed by one of its Assistant Vice
Presidents and attested by one of its Trust Officers, for and in its behalf, all
as of the day and year first above written.

                                     PUBLIC SERVICE COMPANY OF
                                      COLORADO



                                     By: /s/ Paul E. Pender
                                        -------------------
                                         Name:   Paul E. Pender
                                         Title:  Vice President and Treasurer


ATTEST: /s/ Anne Ziebell
        ----------------
        Name:   Anne Ziebell
        Title:  Assistant Secretary

STATE OF MINNESOTA                )
                                  )  ss.:
CITY OF MINNEAPOLIS               )


                  On this 20th day of September, 2002, before me, Sharon M.
Quellhorst, a duly authorized Notary Public in and for said City in the State
aforesaid, personally appeared Paul E. Pender and Anne Ziebell to me known to be
a Vice President and Treasurer and the Assistant Secretary, respectively, of
PUBLIC SERVICE COMPANY OF COLORADO, a corporation organized and existing under
the laws of the State of Colorado, one of the corporations that executed the
within and foregoing instrument; and the said Vice President and Treasurer and
Assistant Secretary severally acknowledged the said instrument to be the free
and voluntary act and deed of said corporation, for the uses and purposes
therein mentioned, and on oath stated that they were authorized to execute said
instrument and that the seal affixed thereto is the corporate seal of said
corporation.

                  IN WITNESS WHEREOF, I have hereunto set my hand and affixed my
official seal the day and year first above written.

                                  /s/ Sharon M. Quellhorst
                                  ------------------------
                                  Name:  Sharon M. Quellhorst
                                  Notary Public, State of Colorado
                                  Commission Expires:  January 31, 2005



                                       6
<PAGE>
                                     U.S. BANK TRUST
                                       NATIONAL ASSOCIATION,
                                              as Trustee


                                     By: /s/ Ignazio Tamburello
                                         ----------------------
                                         Name:   Ignazio Tamburello
                                         Title:  Assistant Vice President


ATTEST:  /s/ Adam Berman
         ---------------
         Name:   Adam Berman
         Title:  Trust Officer


STATE OF NEW YORK                    )
                                     )  ss.:
CITY AND COUNTY OF NEW YORK          )


                  On this 20th day of September, 2002, before me, Doris Ware, a
duly authorized Notary Public in and for said City and County in the State
aforesaid, personally appeared Ignazio Tamburello and Adam Berman to me known to
be an Assistant Vice President and a Trust Officer, respectively, of U.S. BANK
TRUST NATIONAL Association, a national banking association, one of the
corporations that executed the within and foregoing instrument; and the said
Assistant Vice President and Trust Officer severally acknowledged the said
instrument to be the free and voluntary act and deed of said corporation, for
the uses and purposes therein mentioned, and on oath stated that they were
authorized to execute said instrument and that the seal affixed thereto is the
corporate seal of said corporation.

                  IN WITNESS WHEREOF, I have hereunto set my hand and affixed my
official seal the day and year first above written.


                                     /s/ Doris Ware
                                     --------------
                                     Name:  Doris Ware
                                     Notary Public, State of New York
                                     Commission Expires:  November 9, 2005




                                       7
<PAGE>
                                                                       EXHIBIT A



                        FORM OF COLLATERAL SERIES I BOND

         THIS BOND IS NOT TRANSFERABLE EXCEPT TO A SUCCESSOR TRUSTEE UNDER THE
INDENTURE, DATED AS OF OCTOBER 1, 1993, AS SUPPLEMENTED, BETWEEN PUBLIC SERVICE
COMPANY OF COLORADO AND U.S. BANK TRUST NATIONAL ASSOCIATION (FORMERLY FIRST
TRUST OF NEW YORK, NATIONAL ASSOCIATION), AS SUCCESSOR TRUSTEE THEREUNDER.

                       PUBLIC SERVICE COMPANY OF COLORADO

                              FIRST MORTGAGE BOND,

                               Collateral Series I

                                    DUE 2003

REGISTERED                                                            REGISTERED

No. 1                                                               $530,000,000


         FOR VALUE RECEIVED, PUBLIC SERVICE COMPANY OF COLORADO, a corporation
organized and existing under the laws of the State of Colorado (hereinafter
sometimes called the "Company"), promises to pay to U.S. Bank Trust National
Association (formerly known as First Trust of New York, National Association),
as successor trustee (the "1993 Mortgage Trustee") under the Indenture, dated as
of October 1, 1993 (the "1993 Mortgage"), of the Company, or registered assigns,
Five Hundred Thirty Million Dollars on June 27, 2003, at the office or agency of
the Company in the city in which the principal corporate trust office of the
1993 Mortgage Trustee is located. This bond shall not bear interest. The
principal of this bond shall be payable in any coin or currency of the United
States of America which at the time of payment shall be legal tender for the
payment of public and private debts.

         Any payment or deemed payment by the Company under the 1993 Mortgage of
the principal of securities which shall have been authenticated and delivered
under the 1993 Mortgage on the basis of the issuance and delivery to the 1993
Mortgage Trustee of this bond (the "1993 Mortgage Securities") (other than by
the application of the proceeds of a payment in respect of this bond) shall, to
the extent thereof, be deemed to satisfy and discharge the obligation of the
Company, if any, to make a payment of principal of this bond which is then due.

         This bond is one of an issue of bonds of the Company, issued and to be
issued in one or more series under and equally and ratably secured (except as
any sinking, amortization, improvement or other fund, established in accordance
with the provisions of the indenture hereinafter mentioned, may afford
additional security for the bonds of any particular series) by a certain
indenture, dated as of December 1, 1939, made by the Company to U.S. BANK TRUST



                                  EXHIBIT A-1
<PAGE>
NATIONAL ASSOCIATION (formerly First Trust of New York, National Association),
as successor trustee (hereinafter called the "Trustee") to Morgan Guaranty Trust
Company of New York (formerly Guaranty Trust Company of New York), as amended
and supplemented by several indentures supplemental thereto, including the
Supplemental Indenture dated as of September 15, 2002 (said Indenture as amended
and supplemented by said indentures supplemental thereto being hereinafter
called the "Indenture"), to which Indenture reference is hereby made for a
description of the property mortgaged, the nature and extent of the security,
the rights and limitations of rights of the Company, the Trustee, and the
holders of said bonds, under the Indenture, and the terms and conditions upon
which said bonds are secured, to all of the provisions of which Indenture and of
all indentures supplemental thereto in respect of such security, including the
provisions of the Indenture permitting the issue of bonds of any series for
property which, under the restrictions and limitations therein specified, may be
subject to liens prior to the lien of the Indenture, the holder, by accepting
this bond, assents. To the extent permitted by and as provided in the Indenture,
the rights and obligations of the Company and of the holders of said bonds
(including those pertaining to any sinking or other fund) may be changed and
modified, with the consent of the Company, by the holders of at least 75% in
aggregate principal amount of the bonds then outstanding (excluding bonds
disqualified from voting by reason of the Company's interest therein as provided
in the Indenture); provided, however, that without the consent of the holder
hereof no such modification or alteration shall be made which will extend the
time of payment of the principal of this bond or reduce the principal amount
hereof or effect any other modification of the terms of payment of such
principal or will reduce the percentage of bonds required for the aforesaid
actions under the Indenture. The Company has reserved the right to amend the
Indenture without any consent or other action by holders of any series of bonds
created after October 31, 1975 (including this series) so as to change 75% in
the foregoing sentence to 60% and to change certain procedures relating to
bondholders' meetings. This bond is one of a series of bonds designated as the
First Mortgage Bonds, Collateral Series I, of the Company.

         This bond shall be redeemable at the option of the Company in whole at
any time, or in part from time to time, prior to maturity, at a redemption price
equal to 100% of the principal amount thereof to be redeemed.

         The principal of this bond may be declared or may become due before the
maturity hereof, on the conditions, in the manner and at the times set forth in
the Indenture, upon the happening of an event of default as therein provided.

         This bond is not transferable except to a successor trustee under the
1993 Mortgage, any such transfer to be made at the office or agency of the
Company in the city in which the principal corporate trust office of the 1993
Mortgage Trustee is located, upon surrender and cancellation of this bond, and
thereupon a new bond of this series of a like principal amount will be issued to
the transferee in exchange therefor, as provided in the Indenture. The Company,
the Trustee, any paying agent and any registrar may deem and treat the person in
whose name this bond is registered as the absolute owner hereof for the purpose
of receiving payment and for all other purposes. This bond, alone or with other
bonds of this series, may in like manner be exchanged at such office or agency
for one or more new bonds of this series of the same aggregate principal amount,
all as provided in the Indenture. No service charge shall be made to any holder
of any bond of this series for any exchange or transfer of bonds.


                                  EXHIBIT A-2
<PAGE>
         No recourse under or upon any covenant or obligation of the Indenture,
or of any bonds thereby secured, or for any claim based thereon, or otherwise in
any manner in respect thereof, shall be had against any incorporator, subscriber
to the capital stock, shareholder, officer or director, as such, of the Company,
whether former, present or future, either directly, or indirectly through the
Company or the Trustee, by the enforcement of any subscription to capital stock,
assessment or otherwise, or by any legal or equitable proceeding by virtue of
any statute or otherwise (including, without limiting the generality of the
foregoing, any proceeding to enforce any claimed liability of shareholders of
the Company based upon any theory of disregarding the corporate entity of the
Company or upon any theory that the Company was acting as the agent or
instrumentality of the shareholders), any and all such liability of
incorporators, shareholders, subscribers, officers and directors, as such, being
released by the holder hereof, by the acceptance of this bond, and being
likewise waived and released by the terms of the Indenture under which this bond
is issued.

         This bond shall not be valid or become obligatory for any purpose until
the certificate of authentication endorsed hereon shall have been signed by U.S.
Bank Trust National Association, or its successor, as Trustee under the
Indenture.

         IN WITNESS WHEREOF, Public Service Company of Colorado has caused this
bond to be signed in its name by a Vice President and its corporate seal to be
affixed hereto and attested by its Secretary or an Assistant Secretary.

Dated:                                         PUBLIC SERVICE COMPANY OF
                                               COLORADO



                                               By:
                                                  ------------------------------
                                                   Vice President and Treasurer

ATTEST:
       ------------------------------
           Assistant Secretary


                          CERTIFICATE OF AUTHENTICATION

         This is one of the securities of the series designated therein referred
to in the within-mentioned Supplemental Indenture.

Dated:                                         U.S. BANK TRUST
                                               NATIONAL ASSOCIATION,
                                                     AS TRUSTEE


                                               By:
                                                  ------------------------------
                                                              Authorized Officer



                                  EXHIBIT A-3
<PAGE>
                                                                      SCHEDULE A


                             SUPPLEMENTAL INDENTURES

<TABLE>
<CAPTION>
         DATE OF                      SERIES OF BONDS                    PRINCIPAL               PRINCIPAL
      SUPPLEMENTAL                    ---------------                  AMOUNT ISSUED               AMOUNT
        INDENTURE                                                      -------------            OUTSTANDING
        ---------                                                                               -----------
<S>                          <C>                                   <C>                       <C>
March 14, 1941                              None                             --                      --
May 14, 1941                                None                             --                      --
April 28, 1942                              None                             --                      --
April 14, 1943                              None                             --                      --
April 27, 1944                              None                             --                      --
April 18, 1945                              None                             --                      --
April 23, 1946                              None                             --                      --
April 9, 1947                               None                             --                      --
June 1, 1947*                      2-7/8% Series due 1977          $    40,000,000                  None
April 1, 1948                               None                             --                      --
May 20, 1948                                None                             --                      --
October 1, 1948                    3-1/8% Series due 1978               10,000,000                  None
April 20, 1949                              None                             --                      --
April 24, 1950                              None                             --                      --
April 18, 1951                              None                             --                      --
October 1, 1951                    3-1/4% Series due 1981               15,000,000                  None
April 21, 1952                              None                             --                      --
December 1, 1952                            None                             --                      --
April 15, 1953                              None                             --                      --
April 19, 1954                              None                             --                      --
October 1, 1954*                   3-1/8% Series due 1984               20,000,000                  None
April 18, 1955                              None                             --                      --
April 24, 1956                              None                             --                      --
May 1, 1957*                       4-3/8% Series due 1987               30,000,000                  None
April 10, 1958                              None                             --                      --
May 1, 1959                        4-5/8% Series due 1989               20,000,000                  None
April 18, 1960                              None                             --                      --
April 19, 1961                              None                             --                      --
October 1, 1961                    4-1/2% Series due 1991               30,000,000                  None
March 1, 1962                      4-5/8% Series due 1992                8,800,000                  None
June 1, 1964                       4-1/2% Series due 1994               35,000,000                  None
May 1, 1966                        5-3/8% Series due 1996               35,000,000                  None
July 1, 1967*                      5-7/8% Series due 1997               35,000,000                  None
July 1, 1968*                      6-3/4% Series due 1998               25,000,000                  None
April 25, 1969                              None                             --                      --
</TABLE>



                                  SCHEDULE A-1
<PAGE>
<TABLE>
<CAPTION>
         DATE OF                      SERIES OF BONDS                    PRINCIPAL               PRINCIPAL
      SUPPLEMENTAL                    ---------------                  AMOUNT ISSUED               AMOUNT
        INDENTURE                                                      -------------            OUTSTANDING
        ---------                                                                               -----------
<S>                          <C>                                   <C>                       <C>
April 21, 1970                              None                             --                      --
September 1, 1970                  8-3/4% Series due 2000               35,000,000                  None
February 1, 1971                   7-1/4% Series due 2001               40,000,000                  None
August 1, 1972                     7-1/2% Series due 2002               50,000,000                  None
June 1, 1973                       7-5/8% Series due 2003               50,000,000                  None
March 1, 1974                    Pollution Control Series A             24,000,000                  None
December 1, 1974                 Pollution Control Series B             50,000,000                  None
October 1, 1975                    9-3/8% Series due 2005               50,000,000                  None
April 28, 1976                              None                             --                      --

April 28, 1977                              None                             --                      --
November 1, 1977*                  8-1/4% Series due 2007               50,000,000                  None
April 28, 1978                              None                             --                      --
October 1, 1978                    9-1/4% Series due 2008               50,000,000                  None
October 1, 1979*                 Pollution Control Series C             50,000,000                  None
March 1, 1980*                      15% Series due 1987                 50,000,000                  None
April 28, 1981                              None                             --                      --
November 1, 1981*                Pollution Control Series D             27,380,000                  None
December 1, 1981*                 16-1/4% Series due 2011               50,000,000                  None
April 29, 1982                              None                             --                      --
May 1, 1983*                     Pollution Control Series E             42,000,000                  None

April 30, 1984                              None                             --                      --
March 1, 1985*                      13% Series due 2015                 50,000,000                  None
November 1, 1986*                Pollution Control Series F             27,250,000                  None
May 1, 1987*                       8.95% Series due 1992                75,000,000                  None
July 1, 1990*                      9-7/8% Series due 2020               75,000,000                  None
December 1, 1990*           Secured Medium-Term Notes, Series A        191,500,000**             15,000,000
March 1, 1992*                   8-1/8% Series due 2004 and            100,000,000              100,000,000
                                   8-3/4% Series due 2022              150,000,000              146,340,000
April 1, 1993*                   Pollution Control Series G             79,500,000               79,500,000
June 1, 1993*                    Pollution Control Series H             50,000,000               50,000,000
November 1, 1993*                   Collateral Series A                134,500,000              134,500,000

January 1, 1994*              Collateral Series B due 2001 and         102,667,000                  None
                                Collateral Series B due 2024           110,000,000              110,000,000
September 2, 1994                           None                             --                      --
(Appointment of
Successor Trustee)
May 1, 1996                         Collateral Series C                125,000,000              125,000,000
</TABLE>


                                  SCHEDULE A-2
<PAGE>
<TABLE>
<CAPTION>
         DATE OF                      SERIES OF BONDS                    PRINCIPAL               PRINCIPAL
      SUPPLEMENTAL                    ---------------                  AMOUNT ISSUED               AMOUNT
        INDENTURE                                                      -------------            OUTSTANDING
        ---------                                                                               -----------
<S>                          <C>                                   <C>                       <C>
November 1, 1996                    Collateral Series D                250,000,000              175,000,000
February 1, 1997                    Collateral Series E                150,000,000                  None
April 1, 1998                       Collateral Series F                250,000,000              250,000,000
August 15, 2002                     Collateral Series G                 48,750,000               48,750,000
September 1, 2002                   Collateral Series H                600,000,000              600,000,000
</TABLE>

----------------
* Contains amendatory provisions
** $200,000,000 authorized













































                                  SCHEDULE A-3