Exhibit 10.1

SUPPLEMENTAL AGREEMENT NO. 4
BIG STONE PLANT
Agreement for Sharing Ownership
Of Generating Plant

     This Supplemental Agreement No. 4 is made as of April 24, 2003, by and
between Otter Tail Power Company, a Division of Otter Tail Corporation, a
Minnesota corporation (“Otter Tail”), Montana-Dakota Utilities Co., a Division
of MDU Resources Group, Inc. (“Montana-Dakota”) and Northwestern Public Service,
a Division of Northwestern Corporation (“Northwestern”).

RECITALS

     A.     Otter Tail, Montana-Dakota and Northwestern have entered into the
Big Stone Plant Agreement for Sharing Ownership of Generating Plant, dated as of
January 7, 1970, as amended and supplemented by Supplemental Agreement No. 1,
dated as of July 1, 1983, and by Supplemental Agreement No. 2, dated as of
March 1, 1985, and by Supplemental Agreement No. 3, dated as of March 31, 1986,
(The Big Stone Plant Agreement), to provide for the construction and sharing of
ownership by Otter Tail, Montana-Dakota and Northwestern of a lignite-fueled
electric generating plant located in the vicinity of Big Stone, South Dakota,
(the “Big Stone Plant”).

     B.     Otter Tail serves as Operating Agent pursuant to the Big Stone Plant
Agreement, and in such capacity periodically requisitions funds from the plant
owners pursuant to Article 15 of the Big Stone Plant Agreement.

     C.     Also, Otter Tail as Operating Agent uses the requisitioned funds to
pay the costs and expenditures for the operation, maintenance and repair of the
plant and other plant property, pursuant to Article 15 of the Big Stone Plant
Agreement.

     D.     Otter Tail has customarily scheduled such requisitions so that the
receipt of funds corresponds close in time to the payment date of the costs or
expenditures to be made from such funds.

     E.     Otter Tail, Montana-Dakota and Northwestern have determined that
certain provisions of the Big Stone Plant Agreement should be changed to reflect
Otter Tail’s scheduling of such requisitions and to ensure that the funds
received by Otter Tail as the Operating Agent pursuant to such requisitions will
be sufficient to pay the expenses of the plant.

     F.     Otter Tail, Montana-Dakota and Northwestern support amendment of the
Big Stone Plant Agreement and have agreed to change and amend such agreement as
hereinafter provided.

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AGREEMENTS

     In consideration of the premises and the mutual covenants herein contained,
the Parties hereby agree that the Big Stone Plant Agreement shall be, and is
hereby modified, altered, amended, and changed in the following respects only
(shown in strike-underline format for convenience):

     1.     Article 19, Defaults and Remedies, Paragraph 19.4 is amended to read
as follows:

       19.4 If an Event of Default under this Agreement shall continue for a
period of [formerly thirty] five days, or, in the event that the question of
whether an Event of Default exists is made the subject of an arbitration
pursuant to this Agreement and such Event of Default continues for a period of
[formerly thirty] five days following a final determination by the arbitrators
(or a court of competent jurisdiction as provided in section 21.9) that an Event
of Default exists, then, at any time thereafter while such Event of Default is
continuing, the non-defaulting Owners, by giving written notice to the
defaulting Owner, may suspend the right of the defaulting Owner to schedule or
receive Energy or spinning reserve from or at the Plant, in which case the
suspension shall be treated, for purposes of this Agreement, the same as if the
defaulting Owner had failed to schedule Energy or spinning reserve hereunder
during such suspension, except that the proceeds from the sale by the Agent of
such Energy and/or spinning reserve pursuant to section 18.1 shall be applied
first to the payment of the defaulting Owner’s share of current costs under this
Agreement and then to the payment of any sum owed under this Agreement to any of
the other Owners by the defaulting Owner, and any balances thereafter remaining
shall be shared by the non-defaulting Owners in the proportion which their
respective Ownership Shares bear to the total of their Ownership Shares. Such
suspension shall not relieve the defaulting Owner from any of its obligations
under this Agreement, including without limitation, its obligations under
sections 12.1, 13.2 and 13.4.

     2.     Other than as expressly provided herein, Otter Tail, as Operating
Agent, shall make advance requisitions and maintain advanced funds as it has
customarily done for the past several years.

     3.     In the event of any conflict, inconsistency, or incongruity between
the provision of this Supplemental Agreement No. 4 and any of the provisions of
the Big Stone Plant Agreement, provisions of this Supplemental Agreement No. 4
shall in all respects govern and control.

     4.     Otter Tail, Montana-Dakota and Northwestern understand and agree
that this Supplemental Agreement No. 4 shall in no way act as a waiver of any of
the conditions and obligations imposed upon the parties by the Big Stone Plant
Agreement, and any rights which any of the Parties may have by virtue of such
Big Stone Plant Agreement are to be considered as of full force and effect.

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     5.     Otter Tail, Montana-Dakota and Northwestern agree that this
Supplemental Agreement No. 4 shall be filed in the Grant County Register of
Deeds office and indexed against the property of the property described in
Exhibit A to this Supplemental Agreement No. 4, pursuant to the provisions of
Article 3, Ownership, Paragraph 3.4 of the Big Stone Plant Agreement.

     IN WITNESS WHEREOF, the parties have executed this Supplemental Agreement
No. 4 the day and year first-above written.

                                  OTTER TAIL POWER COMPANY
A DIVISION OF OTTER TAIL CORPORATION                                   By   /s/
Charles MacFarlane                

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                                  Name   CHARLES MAC FARLANE                

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                                  MONTANA-DAKOTA UTILITIES CO.
A DIVISION OF MDU RESOURCES GROUP, INC.                                   By  
/s/ C. Wayne Fox                

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                                  Name   C. Wayne Fox                

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                                  NORTHWESTERN PUBLIC SERVICE
A DIVISION OF NORTHWESTERN CORPORATION                                   By  
/s/ Michael J. Hanson                

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                                  Name   MICHAEL J. HANSON                

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