<PAGE>

                                                                    EXHIBIT 4.99

                             SUPPLEMENTAL INDENTURE

                        (DATED AS OF SEPTEMBER 15, 2003)

                                   ---------

                       PUBLIC SERVICE COMPANY OF COLORADO

                                       TO

                      U.S. BANK TRUST NATIONAL ASSOCIATION,
                                                        AS TRUSTEE

                                   ---------

                   CREATING AN ISSUE OF FIRST MORTGAGE BONDS,
                               COLLATERAL SERIES L

                                   ---------

      (SUPPLEMENTAL TO INDENTURE DATED AS OF DECEMBER 1, 1939, AS AMENDED)

<PAGE>

                  SUPPLEMENTAL INDENTURE, dated as of September 15, 2003,
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 L
(the "Collateral Series L 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 L 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 L Bonds and of specifying the form, provisions
and particulars thereof, as in the Indenture provided or permitted and of giving
to the Collateral Series L 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 L 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 L Bonds, in accordance with the terms of
such bonds and such instruments, have been done, performed and fulfilled, and
the execution and delivery hereof have been in all respects duly authorized;

                                       1
<PAGE>

                  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 L
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 L 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 L 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 L 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 L. The Collateral Series L
Bonds shall be limited to an aggregate principal amount of Two Hundred Fifty
Million dollars ($250,000,000) (less the aggregate principal amount of First
Mortgage Bonds, Collateral Series K which remains outstanding), excluding any
Collateral Series L Bonds which may be authenticated and exchanged for or in
lieu of or in substitution for or on transfer of other Collateral Series L Bonds
pursuant to any provisions of the Indenture. The Collateral Series L Bonds shall
mature on March 1, 2013. The Collateral Series L Bonds shall not bear interest.

                                       2
<PAGE>

                  The principal of each Collateral Series L 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 L 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 L 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 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 L Bonds (other than by the application of
the proceeds of a payment in respect of such Collateral Series L 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 L
Bonds which is then due.

                  The Trustee may conclusively presume that the obligation of
the Company to pay the principal of the Collateral Series L 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 L 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 L Bond shall be dated as of the date of
its authentication.

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

                  The Collateral Series L 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 L 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 L Bond.

                  SECTION 2. The text of the Collateral Series L Bonds shall be
substantially in the form attached hereto as Exhibit A.

                  SECTION 3. The Collateral Series L Bonds may be executed by
the Company and delivered to the Trustee and, upon compliance with all
applicable provisions and

                                       3
<PAGE>

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 L BONDS

                  SECTION 1. Each Collateral Series L 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 L Bonds,
except that (a) no publication of notice of redemption of the Collateral Series
L Bonds shall be required and (b) if less than all the Collateral Series L Bonds
are to be redeemed, the Collateral Series L 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 L
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 L 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 L Bonds.

                                  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

                                       4
<PAGE>

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 L
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/ Benjamin G. S. Fowke III
                                            ------------------------------------
                                            Name:  Benjamin G. S. Fowke III
                                            Title: Vice President and Treasurer

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

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

                  On this 19th day of September, 2003, before me, Sharon M.
Quellhorst, a duly authorized Notary Public in and for said City in the State
aforesaid, personally appeared Benjamin G. S. Fowke III and Anne M. 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 Minnesota

                                           Commission Expires:  January 31, 2005

                                       6
<PAGE>

                                           U.S. BANK TRUST
                                              NATIONAL ASSOCIATION,
                                                 as Trustee

                                           By: /s/ Ingnazio 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 19th day of September, 2003, before me, Rouba Fakih, 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/ Rouba Fakih
                                        ----------------------------------
                                        Name: Rouba Fakih
                                        Notary Public, State of New York

                                        Commission Expires:  February 20, 2007

                                       7
<PAGE>

                                                                       EXHIBIT A

                        FORM OF COLLATERAL SERIES L 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 L

                                    DUE 2013

REGISTERED                                                           REGISTERED

No.__________                                                          $________

         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,

                                                                         Dollars
on March 1, 2013, 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 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

                                   Exhibit A-1
<PAGE>

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 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, 2003 (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 L, 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

                                  Exhibit A-2
<PAGE>

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.

         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]

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                                                                                      PRINCIPAL
  SUPPLEMENTAL                                                        PRINCIPAL                   AMOUNT
   INDENTURE                           SERIES OF BONDS              AMOUNT ISSUED               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                                                                                      PRINCIPAL
  SUPPLEMENTAL                                                        PRINCIPAL                   AMOUNT
   INDENTURE                           SERIES OF BONDS              AMOUNT ISSUED               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**                  None
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                    None
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                                                                                      PRINCIPAL
  SUPPLEMENTAL                                                        PRINCIPAL                   AMOUNT
   INDENTURE                           SERIES OF BONDS              AMOUNT ISSUED               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                    None
August 15, 2002                     Collateral Series G               48,750,000                 48,750,000
September 1, 2002                   Collateral Series H              600,000,000                    None
September 15, 2002                  Collateral Series I              530,000,000                    None
April 1, 2003                       Collateral Series J              600,000,000                600,000,000
March 1, 2003                       Collateral Series K              250,000,000                250,000,000
May 1, 2003                         Collateral Series M              350,000,000                350,000,000
September 1, 2003                   Collateral Series N              300,000,000                300,000,000
September 1, 2003                   Collateral Series O              275,000,000                275,000,000
</TABLE>

------------------
* Contains amendatory provisions

** $200,000,000 authorized

                                  Schedule A-3