SUPPLEMENTAL INDENTURE

Exhibit 4.6

When Recorded Mail to:

Illinois Power Company

Craig W. Stensland

One Ameren Plaza (MC 1310)

1901 Chouteau Avenue

St. Louis, MO 63103

ILLINOIS POWER COMPANY

TO

BNY MIDWEST TRUST COMPANY,

AS SUCCESSOR TRUSTEE TO

HARRIS TRUST AND SAVINGS BANK

SUPPLEMENTAL INDENTURE

DATED AS OF   MARCH 1, 2007

TO

GENERAL MORTGAGE INDENTURE AND DEED OF TRUST

DATED AS OF NOVEMBER 1, 1992

This instrument was prepared by Steven R. Sullivan, Senior Vice President, General Counsel and Secretary of Illinois Power Company c/o Ameren Corporation, One Ameren Plaza, 1901 Chouteau Avenue, St. Louis, Missouri 63103.

SUPPLEMENTAL INDENTURE dated as of March 1, 2007 (“Supplemental Indenture”), made by and between ILLINOIS POWER COMPANY, a corporation organized and existing under the laws of the State of Illinois (the “Company”), party of the first part, and BNY MIDWEST TRUST COMPANY, a corporation organized and existing under the laws of the State of Illinois, as successor trustee to Harris Trust and Savings Bank, a corporation organized and existing under the laws of the State of Illinois (the “Trustee”), as Trustee under the General Mortgage Indenture and Deed of Trust dated as of November 1, 1992, hereinafter mentioned, party of the second part;

WHEREAS, the Company has heretofore executed and delivered its General Mortgage Indenture and Deed of Trust dated as of November 1, 1992 as from time to time amended (the “Indenture”), to the Trustee, for the security of the Bonds of the Company issued and to be issued thereunder (the “Bonds”); and

WHEREAS, pursuant to the terms and provisions of the Indenture there were created and authorized by supplemental indentures thereto bearing the following dates, respectively, the Mortgage Bonds of the series issued thereunder and respectively identified opposite such dates:

and

WHEREAS, a supplemental indenture with respect to the Bonds of the 2007 Series and the Bonds of the 2012 Series listed above was executed and filed but such Bonds of the 2007 Series and Bonds of the 2012 Series were never issued and a release with respect to such supplemental indenture was subsequently executed and filed; and

WHEREAS, the Company has entered into a Credit Agreement, dated as of February 9, 2007 (as amended or otherwise modified from time to time, the “Credit Agreement”) by and among the Company, Central Illinois Light Company, Central Illinois Public Service Company, AmerenEnergy Resources Generating Company and CILCORP Inc., as borrowers, the lenders from time to time party thereto (the “Lenders”) and JPMorgan Chase Bank, N.A., as agent (in such capacity, the “Agent”) for the Lenders, providing for the making of certain financial accommodations thereunder to the Company, and pursuant to such Credit Agreement, the Company has agreed to issue to the Agent, as evidence of and security for the Obligations (as such term is defined in the Credit Agreement) of the Company (the “Company Obligations”), a new series of Bonds under the Indenture; and

WHEREAS, for such purposes, the Company desires to create a new series of Bonds to be issued under the Indenture to be known as Mortgage Bonds, 2007 Credit Agreement Series (the “2007 Credit Agreement Series Bonds”); and

WHEREAS, the 2007 Credit Agreement Series Bonds shall be issued to the Agent as evidence of and security for the Company Obligations under the Credit Agreement; and

WHEREAS, the Company, in the exercise of the powers and authority conferred upon and reserved to it under the provisions of the Indenture, and pursuant to appropriate resolutions of the Board of Directors, has duly resolved and determined to make, execute and deliver to the Trustee this Supplemental Indenture in the form hereof for the purposes herein provided; and

WHEREAS, all conditions and requirements necessary to make this Supplemental Indenture a valid, binding and legal instrument 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 Illinois Power Company, in consideration of the purchase and ownership from time to time of the Bonds and the service by the Trustee, and its successors, under the

Indenture 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, hereby covenants and agrees to and with the Trustee and its successors in the trust under the Indenture, for the benefit of those who shall hold the Bonds as follows:

ARTICLE I   

DESCRIPTION OF 2007 CREDIT AGREEMENT SERIES BONDS.

The 2007 Credit Agreement Series Bonds shall be dated as of the Interest Payment Date (as defined below) thereof to which interest was paid next preceding the date of issue, unless (a) issued on an Interest Payment Date thereof to which interest was paid, in which event it shall be dated as of such issue date, or (b) issued prior to the occurrence of the first Interest Payment Date thereof to which interest was paid, in which event it shall be dated the date of original issuance.

The 2007 Credit Agreement Series Bonds shall be issued in the aggregate principal amount of $200,000,000 and shall mature on the Maturity Date (having at any time the meaning such term has at such time under the Credit Agreement) applicable to the Company.

The 2007 Credit Agreement Series Bonds shall bear interest from their date as set forth in the form thereof hereinafter recited. Interest on the 2007 Credit Agreement Series Bonds shall be payable on each Interest Payment Date (defined below), commencing on the first Interest Payment Date next succeeding the date of the 2007 Credit Agreement Series Bonds. Payment of principal on the 2007 Credit Agreement Series Bonds shall be due on the Maturity Date. If the Maturity Date falls on a day which is not a Business Day, as defined below, principal and any interest and/or fees payable by the Company with respect to the Maturity Date will be paid on the next succeeding Business Day.

Both the principal of and the interest on the 2007 Credit Agreement Series Bonds shall be payable at the times and in the manner set forth in the form of bond set out herein and in immediately available funds at the office or agency of the Trustee, in any coin or currency of the United States of America which at the time of payment is legal tender for public and private debts.

The obligation of the Company to make payments with respect to principal under the Credit Agreement shall not give rise to an obligation to pay principal of the 2007 Credit Agreement Series Bonds except on the Maturity Date of the Company or upon redemption as provided in this Supplemental Indenture. If at any time any permanent reduction of the

Borrower Sublimit (as defined in the Credit Agreement) of the Company or the Borrower Credit Exposure (as defined in the Credit Agreement) of the Company shall result in the principal of the 2007 Credit Agreement Series Bonds being greater than the greater of the Borrower Sublimit and the Borrower Credit Exposure, a payment obligation with respect to the principal of the 2007 Credit Agreement Series Bonds in the amount of such excess shall be deemed discharged upon the effectiveness of such permanent reduction. No payment of principal under the Credit Agreement shall reduce the principal amount of the   2007 Credit Agreement Series Bonds to an amount less than the greater of the Borrower Sublimit and the Borrower Credit Exposure.

The obligation of the Company to make payments with respect to the interest on the 2007 Credit Agreement Series Bonds shall be fully or partially, as the case may be, satisfied and discharged to the extent that, at the time that any such payment shall be due, the then due interest and/or fees of the Company under the Credit Agreement shall have been fully or partially paid. Satisfaction of any obligation to the extent that payment is made with respect to the interest and/or fees of the Company under the Credit Agreement means that if any payment is made on the interest and/or fees of the Company under the Credit Agreement, a corresponding payment obligation with respect to the interest on the 2007 Credit Agreement Series Bonds shall be deemed discharged in the same amount as such payment made on the interest and/or fees of the Company under the Credit Agreement.

The Trustee may at any time and all times conclusively assume that the obligation of the Company to make payments with respect to the principal of and interest on the 2007 Credit Agreement Series Bonds, so far as such payments at the time have become due, has been fully satisfied and discharged pursuant to the foregoing paragraphs unless and until the Trustee shall have received a written notice from the Agent stating (i) that timely payment of principal of or interest on the 2007 Credit Agreement Series Bonds has not been made, (ii) that the Company is in arrears as to the payments required to be made by it to the Agent in connection with the Company Obligations pursuant to the Credit Agreement, and (iii) the amount of the arrearage.

As used herein, (A) “Business Day” shall have the meaning assigned thereto in the Credit Agreement; (B) “Interest Payment Date” shall mean each date on which Company Obligations constituting interest and/or fees are due and payable from time to time pursuant to the Credit Agreement; (C) “Interest Rate” shall mean a rate of interest per annum, adjusted as necessary, to result in an interest payment equal to the aggregate amount of Company Obligations constituting interest and fees of the Company due under the Credit Agreement on the applicable Interest Payment Date; and (D) “Record Dat e ” with respect to any Interes