Document:

<PAGE>   1

                                                                 Exhibit 10.6(a)

                           THE MONTANA POWER COMPANY

                                      and

                       PUGET SOUND POWER & LIGHT COMPANY

                                      and

                       THE WASHINGTON WATER POWER COMPANY

                                      and

                       PORTLAND GENERAL ELECTRIC COMPANY

                                      and

                         PACIFIC POWER & LIGHT COMPANY

                          COMMON FACILITIES AGREEMENT

                        COLSTRIP UNITS #1, #2, #3 and #4
<PAGE>   2

                          COMMON FACILITIES AGREEMENT

                        COLSTRIP UNITS #1, #2, #3 AND #4

                               TABLE OF CONTENTS

                                     Index

                                                                            Page
                                                                            ----

 1.   Definitions ..........................................................  1
 2.   Term .................................................................  2
 3.   Allocation of Common Facilities ......................................  2
 4.   Addition or Deletion of Common Facilities.............................  2
 5.   Cost Allocations .....................................................  2
 6.   Operation of Common Facilities .......................................  3
 7.   Budget ...............................................................  3
 8.   Provisions for Additional Facilities .................................  4
 9.   Arbitration ..........................................................  5
10.   Waiver of Right to Partition .........................................  5
11.   Conveyance of Common Facilities ......................................  6
12.   Mutual Support .......................................................  6
13.   Miscellaneous ........................................................  7
      Exhibit A - Common Facilities
      Exhibit B - Common Facilities Lands
      Exhibit C - Colstrip Plants #l-#4 Buffer Zone
<PAGE>   3

                          COMMON FACILITIES AGREEMENT

      This Agreement is made as of the 6th day of May, 1981, by and between the
following parties: THE MONTANA POWER COMPANY, a Montana corporation ("Montana"),
PUGET SOUND POWER & LIGHT COMPANY, a Washington corporation and PUGET COLSTRIP
CONSTRUCTION COMPANY, a Washington Corporation (collectively, "Puget"), THE
WASHINGTON WATER POWER COMPANY, a Washington corporation ("Water Power"),
PORTLAND GENERAL ELECTRIC COMPANY, an Oregon corporation ("Portland"), PACIFIC
POWER & LIGHT COMPANY, a Maine corporation ("Pacific"), and BASIN ELECTRIC POWER
COOPERATIVE, a North Dakota cooperative corporation ("Basin Electric").

1. DEFINITIONS.

      (a) The "#1 & #2 Owners Agreement" means the Construction and Ownership
Agreement for Colstrip Units #1 and #2 entered into July 30, 1971.

      (b) The "#3 & #4 Owners Agreement" means the Ownership and Operation
Agreement for Colstrip Units #3 and #4 entered into May 6, 1981.

      (c) "Committees" means the Owner's Committee provided for in the #1 & #2
Owners Agreement and the Committee provided for in the #3 & #4 Owners Agreement.

      (d) "Common Facilities" means all personal property listed on Exhibit A
hereto and all real property described in Exhibit B hereto, either as modified,
added to, or deleted from, from time to time in the manner provided in Section
4.

      (e) "Common Facilities Operator" means the Operator appointed under
Section 6(a) hereof.

      (f) "Owners" means Montana, Puget, Water Power, Portland, Pacific, and
Basin Electric, and their successors and assigns.

      (g) "Prudent Utility Practice" at any particular time means either any of
the practices, methods and acts engaged in or approved by a significant portion
of the electrical utility industry prior thereto or any of the practices,
methods or acts, which, in the exercise of reasonable judgment in the light of
the facts known at the time the decision was made, could have been expected to
accomplish the desired result at the lowest reasonable cost consistent with
reliability, safety and expedition. Prudent Utility Practice shall apply not
only to functional parts of the Common Facilities, but also to appropriate
structures, landscaping, painting, signs, lighting, other facilities and public
relations programs, including recreational facilities, and any other programs or
facilities, reasonably designed to promote public enjoyment, understanding and
acceptance of the Projects. Prudent Utility Practice is not intended to be
limited to the
<PAGE>   4

optimum practice, method or act, to the exclusion of all others, but rather to
be a spectrum of possible practices, methods or acts. Prudent Utility Practice
shall also include those practices, methods and acts that are required by
applicable laws and final orders or regulations of regulatory agencies having
jurisdiction.

2. TERM.

      This agreement shall be effective and binding when executed by Montana,
Puget, Water Power, Portland and Pacific and shall be effective and binding as
to Basin Electric only when executed by Basin Electric. This agreement shall
continue until the first to end of (a) the term of the #1 & #2 Owners Agreement
or (b) the term of the #3 & #4 Owners Agreement; provided, however, that section
10 hereof shall survive until the end of the term of both the #1 & #2 Owners
Agreement and the #3 & #4 Owners Agreement.

3. ALLOCATION OF COMMON FACILITIES.

      Exhibit A to this agreement allocates the Common Facilities between
Colstrip Units #1 and #2 and Colstrip Units #3 and #4. That portion of the
Common Facilities allocated to Colstrip Units #1 and #2, is part of the Project,
as that term is defined in the #1 & #2 Owners Agreement. That portion of the
Common Facilities allocated to Colstrip Units #3 and #4 is part of the Project,
as that term is defined in the #3 & #4 Owners Agreement.

4. ADDITION OR DELETION OF COMMON FACILITIES.

      Common Facilities may be modified, added to, or deleted from this
agreement only with the approval of both Committees; provided, however, that no
such action by the Committees shall be effective to divest any Owner of any
interest in the Common Facilities or reduce such interest without the written
consent of the Owner or Owners so affected.

5. COST ALLOCATIONS.

      The costs of construction, operation and maintenance of the Common
Facilities shall be allocated between the Colstrip Units #1 and #2 and Colstrip
Units #3 and #4 in the portion provided in Exhibit A to this agreement, as
modified from time to time. A Committee may at any time propose to the
Committees a revised method of allocating costs which it believes is more
equitable than the initial allocation herein provided. Each Committee shall use
every effort in good faith to agree on such revised method of allocating costs
and shall proceed under Section 9 if such effort does not result in agreement.

                                      -2-
<PAGE>   5

6. OPERATION OF COMMON FACILITIES.

      (a) The Owners hereby appoint Montana, and Montana hereby accepts
appointment, as the Common Facilities Operator. The Common Facilities Operator,
as agent for and on behalf of the Owners, shall construct, operate and maintain
the Common Facilities, hire all necessary personnel, and pay all construction,
operation and maintenance expenses (including, but not limited to, labor
payroll, materials and supplies), all in accordance with the #1 & #2 Owners
Agreement and the #3 & #4 Owners Agreement, guidelines established from time to
time by the Committees, and any applicable laws, regulations, orders, permits
and licenses, now or hereafter in effect, of any governmental authority.

      (b) The Common Facilities Operator shall not assign, transfer or delegate,
voluntarily or by operation of law, its responsibilities to any person without
the written consent of the Committees. If the Common Facilities Operator is
replaced as operator under the #3 & #4 Owners Agreement, it shall be removed as
Common Facilities Operator hereunder and in such event the replacement operator
under the #3 and #4 Owners Agreement shall be deemed to be the Common Facilities
Operator under this Agreement, provided that no such replacement of Common
Facilities Operator shall become effective earlier than the effective date of
its substitution or replacement as operator under the #3 and #4 Owners Agreement
unless:

      (i)   the existing Common Facilities Operator consents thereto; or

      (ii)  an arbitrator shall find that the Common Facilities Operator is in
            material breach of its obligations as Common Facilities Operator.

The Common Facilities Operator removed shall be entitled to recover from the
Owners an equitable amount to cover the cost impacts of such removal.

7. BUDGET

      (a) On or before September 1 of each year, the Common Facilities Operator
shall submit to the Committees a budget of its estimate of costs of
construction, operation and maintenance of the Common Facilities by calendar
months for the operating year beginning January 1, next following. Such budget
shall be subject to approval by the Committees which approval shall not
unreasonably be withheld. The Committees shall approve such budget or a revised
budget on or before November 1 in any such year. The budget will list the work
force and expense therefor, materials, supplies, and other expenses associated
with the normal maintenance program. Extraordinary items of maintenance

                                      -3-
<PAGE>   6

will be detailed to set forth the cost of labor required beyond that available
from the regular force and other expense which will be incurred. The Common
Facilities Operator will submit budget revisions as may become necessary from
time to time during any operating year which the Committees shall promptly
consider and which shall similarly be subject to approval by the Committees. The
budget will guide expenditures for construction, operation and maintenance
purposes through the ensuing year, except as may be required in an emergency.

      (b) In the event of emergency or instances of unforeseen maintenance
restricting use of the Common Facilities below that required by the Owners when
repairs could be effected more rapidly by expenditure of overtime and other
expediting costs, the Owners will be individually notified. Unless authorized by
the Committees, the Owners desiring accelerated repairs will share pro rata,
according to their respective ownerships, as determined from Exhibit A to this
Agreement and the #1 & #2 Owners Agreement and the #3 & #4 Owners Agreement, the
expediting costs expended to return the Common Facilities to the required
operating level at an earlier date.

      (c) The Owners recognize that it will be necessary for continued operation
of the Common Facilities, or to maintain them in operable condition, that the
Common Facilities Operator be in a position to meet commitments for labor,
repairs and replacements, materials and supplies, services and other expenses of
a continuing nature in order that it may fulfill its obligations to the Owners
as Common Facilities Operator under this Agreement. Accordingly, notwithstanding
any of the provisions of this Section 7, the Common Facilities Operator, on
behalf of the Owners, may make all expenditures in the normal course of business
or in an emergency, all as the same are necessary for the proper and safe
operation and maintenance of the Common Facilities. As soon as practicable after
the making of any such expenditures, the Common Facilities Operator shall make a
full report thereof to the Committees. The Common Facilities Operator shall take
any action required by a final and binding order of any public authority having
jurisdiction or in any emergency for the safety of the Common Facilities.

8. PROVISIONS FOR ADDITIONAL FACILITIES.

      (a) Each Owner shall have the right at its expense to install and operate
on the Common Facilities land, facilities for its own system; provided, however,
that the facilities of any Owner shall be so installed and operated as not to
burden or unreasonably interfere with the ultimate full utilization of the land
for Colstrip Units #1, #2, #3 and #4, or with the facilities off the other
Owners or with the construction of additional generating units. In the event
that an Owner proposes to install or operate facilities which would require the
relocation of previously installed facilities of another Owner, or of the Common
Facilities, but would otherwise meet the requirements of

                                      -4-
<PAGE>   7

the preceding sentence, the Owner desiring to install or operate such facilities
shall have the right to call for such relocation if it bears all direct and
indirect costs of such relocation.

      (b) Each Owner releases all other Owners and their agents and employees
from claims to profits, charges, rents, or benefits that may arise from use by
any Owner of Common Facilities land pursuant to Section 8(a).

9. ARBITRATION.

      (a) Any controversies arising out of or relating to this Agreement, except
those arising out of relating to Sections 3 and 4 hereof, which cannot be
resolved through negotations between the Committees within thirty (30) days
after inception of the matter in dispute shall, upon demand of either Committee,
be submitted to an Arbitrator having demonstrated expertise in the matter
submitted. If the Committees cannot mutually agree upon such Arbitrator, then
upon petition of any Committee, such Arbitrator shall be appointed by the
Superior Court of the State of Washington, in and for the County of Spokane. The
arbitration shall be conducted in Spokane, Washington, pursuant to the
Washington Arbitration Act, RCW Chapter 7.04 as the same may be amended from
time to time. The Arbitrator shall render his decision in writing not later than
thirty (30) days after the matter has been submitted to him, and such decision
shall be conclusive and binding upon the Owners. The costs incurred by any
arbitration proceedings shall be charged to costs of construction or cost of
operation, whichever may be appropriate, all in accordance with the #1 & #2
Owners Agreement and the #3 & #4 Owners Agreement, provided that each Committee
shall pay its own attorney's fees and costs of witnesses.

      (b) An Owner who disagrees with the Committees' resolution of any
controversy arising out of Section 5 hereof within thirty (30) days after such
Committee resolution of the matter in dispute may submit such matter to an
Arbitrator pursuant to Section 9(a). All references to the "Committee" in
Section 9(a) shall be changed to "Owner" for purposes of application to this
subsection.

10. WAIVER OF RIGHT TO PARTITION

      So long as the Common Facilities or any part thereof as originally
constructed, reconstructed or added to are used or useful for the generation of
electric power and energy, or to the end of the period permitted by applicable
law, whichever first occurs, the Owners waive their right to partition of the
Common Facilities whether by partition in kind or sale and division of the
proceeds thereof, and agree that they will not resort to any action at law or in
equity to partition and further waive the benefit of all laws that may now or
hereafter authorize such partition of the properties comprising the Common
Facilities. It is agreed this covenant shall be deemed to run with the land.

                                      -5-
<PAGE>   8

All instruments of conveyance which effect, evidence or vest each Owner's
respective ownership interest in the Common Facilities shall contain this waiver
of right to partition.

11. CONVEYANCE OF COMMON FACILITIES.

      Each Owner shall promptly take all action (including, but not limited to,
obtaining all requisite authorizations) necessary for participation by such
Owner in the ownership, construction, operation and maintenance of the Common
Facilities. Each Owner shall promptly take such action (including, but not
limited to, the execution, acknowledgment, delivery and recordation of
instruments of conveyance) as may reasonably be requested by any other Owner to
effect, evidence or vest each Owner's respective interests in the Common
Facilities; provided, however, that Montana and Puget shall not be obligated to
convey an interest in the Common Facilities prior to January 1, 1984.

12. MUTUAL SUPPORT.

      (a) Montana and Puget now own and may hereafter acquire certain land near
Colstrip, Montana, within the boundaries shown on Exhibit C to this Agreement,
for use in connection with Colstrip Units #1 and #2 (the "1 & 2 Lands"). Montana
and Puget shall grant to the Owners such easements, licenses and other rights in
the 1 & 2 Lands as may be reasonably necessary for the effective and efficient
construction, operation and maintenance of Colstrip Units #3 and #4 and related
improvements (including, but not limited to, substations, transmission lines,
ponds and utilities, but excluding residential housing); provided that such
rights and the exercise thereof shall not materially interfere with the
construction, operation and maintenance of Colstrip Units #1 and #2 and related
improvements (including, but not limited to, substations, transmission lines,
ponds, utilities and existing residential housing). Any such grant may be for
such compensation (not in excess of Montana's and Puget's costs which are
equitably allocable to the rights granted) and subject to such reservations,
restrictions, conditions and other provisions as may reasonably be required by
Montana and Puget.

      (b) The Owners now own and may hereafter acquire certain land near
Colstrip, Montana, within the boundaries shown on Exhibit C to this Agreement,
for use in connection with Colstrip Units #3 and #4 (the "3 & 4 Lands"). The
Owners shall grant to Puget and Montana such easements, licenses and other
rights in the 3 & 4 Lands as may be reasonably necessary for the effective and
efficient construction, operation and maintenance of Colstrip Units #1 and #2
and related improvements (including, but not limited to, substations,
transmission lines, ponds and utilities, but excluding residential housing);
provided that such rights and the exercise thereof shall not materially
interfere with the construction, operation and maintenance of Colstrip Units #3
and #4 and related improvements (including, but not limited to,

                                      -6-
<PAGE>   9

substations, transmission lines, ponds, utilities and existing residential
housing). Any such grant may be for such compensation (not in excess of the
Owners' costs which are equitably allocable to the rights granted) and subject
to such reservations, restrictions, conditions and other provisions as may
reasonably be required by the Owners.

13. MISCELLANEOUS.

      (a) The headings of the clauses of this Agreement are inserted for
convenience of reference only and shall not affect the meaning or construction
thereof.

      (b) The singular of any term in this Agreement shall encompass the plural
and the plural the singular, unless the context otherwise indicates.

      (c) This Agreement shall be construed in accordance with the laws of the
State of Montana, except that Section 9 shall be construed in accordance with
the laws of the State of Washington.

      (d) This Agreement shall not be amended except by written instrument
executed, acknowledged and delivered by all of the Owners.

      IN WITNESS WHEREOF, the parties hereto have executed this Agreement in
several counterparts.

                                  THE MONTANA POWER COMPANY

                                  By  /s/ Melvyn M. Ryan
                                      ------------------------------------------
                                  Its Executive Vice President
                                      ------------------------------------------
                                        for Administration
                                      ------------------------------------------

Attest:

/s/ T. O. McElwain
----------------------------------
   Asst. Secretary

                                  PUGET SOUND POWER & LIGHT COMPANY

                                  By  /s/ D. H. Knight
                                      ------------------------------------------
                                  Its Senior Vice President
                                      ------------------------------------------

Attest:

/s/ W. E. Watson
----------------------------------
         Secretary

                                      -7-
<PAGE>   10

                                  PUGET COLSTRIP CONSTRUCTION COMPANY

                                  By  /s/ D. H. Knight
                                      ------------------------------------------
                                  Its Senior Vice President
                                      ------------------------------------------

Attest:

/s/ W. E. Watson
----------------------------------
         Secretary

                                  THE WASHINGTON WATER POWER COMPANY

                                  By  /s/ H. W. Harding
                                      ------------------------------------------

Attest:

/s/ L. O. Falk
----------------------------------
         Secretary

                                  PORTLAND GENERAL ELECTRIC COMPANY

                                  By  /s/ Glen E. Bredemier
                                      ------------------------------------------

Attest:

/s/ Warren Hastings
----------------------------------
    Asst. Secretary

                                  PACIFIC POWER & LIGHT COMPANY

                                  By  /s/ R. B. Lisbakken
                                      ------------------------------------------

Attest:

/s/ Sally A. Nofziger
----------------------------------
    Asst. Secretary

                                  BASIN ELECTRIC POWER COOPERATIVE

                                  By
                                      ------------------------------------------

Attest:

----------------------------------
         Secretary

                                      -8-
<PAGE>   11

STATE OF MONTANA       )
                       )    ss.
COUNTY OF SILVER BOW   )

      On this 6th day of May, 1981, before me, the undersigned, a Notary Public
in and for the State of Montana, personally appeared Melvyn M. Ryan, known to me
to be the Executive Vice President of THE MONTANA POWER COMPANY and acknowledged
to me that he executed the within instrument on behalf of that corporation.

      IN WITNESS WHEREOF, I have hereunto set my hand and affixed my Notarial
Seal the day and year in this certificate first above written.

                                      /s/ James Walsh
                                      ------------------------------------------
                                      Notary Public in and for the State of
                                      Montana, residing at Butte
                                      My Commission expires 6/26/82

STATE OF WASHINGTON   )
                      )    ss.
COUNTY OF KING        )

      On this 6th day of May, 1981, before me, the undersigned, a Notary Public
in and for the State of Washington, personally appeared D. H. Knight, known to
me to be the Sr. Vice President of PUGET SOUND POWER & LIGHT COMPANY and
acknowledged to me that he executed the within instrument on behalf of that
corporation.

      IN WITNESS WHEREOF, I have hereunto set my hand and affixed my Notarial
Seal the day and year in this certificate first above written.

                                      /s/ Jeanette Ragsdale
                                      ------------------------------------------
                                      Notary Public in and for the State of
                                      Washington, residing at Seattle
                                      My Commission expires August 1, 1981

                                      -9-
<PAGE>   12

STATE OF WASHINGTON   )
                      )    ss.
COUNTY OF KING        )

      On this 6th day of May, 1981, before me, the undersigned, a Notary Public
in and for the State of Washington, personally appeared D. H. Knight, known to
me to be the Sr. Vice President of PUGET COLSTRIP CONSTRUCTION COMPANY and
acknowledged to me that he executed the within instrument on behalf of that
corporation.

      IN WITNESS WHEREOF, I have hereunto set my hand and affixed my Notarial
Seal the day and year in this certificate first above written.

                                      /s/ Jeanette Ragsdale
                                      ------------------------------------------
                                      Notary Public in and for the State of
                                      Washington, residing at Seattle
                                      My Commission expires August 1, 1981

STATE OF WASHINGTON   )
                      )    ss.
COUNTY OF             )

      On this 6th day of May, 1981, before me, the undersigned, a Notary Public
in and for the State of Washington, personally appeared H. W. Harding, known to
me to be the Vice President of THE WASHINGTON WATER POWER COMPANY and
acknowledged to me that he executed the within instrument on behalf of that
corporation.

      IN WITNESS WHEREOF, I have hereunto set my hand and affixed my Notarial
Seal the day and year in this certificate first above written.

                                      /s/ Lois L. Loveridge
                                      ------------------------------------------
                                      Notary Public in and for the State of
                                      Washington, residing at Spokane
                                      My Commission expires October 17, 1982

                                      -10-
<PAGE>   13

STATE OF OREGON        )
                       )  ss.
COUNTY OF              )

      On this 6th day of May, 1981, before me, the undersigned, a Notary Public
in and for the State of Oregon, personally appeared Glen E. Bredemeier, known to
me to be the Vice President of PORTLAND GENERAL ELECTRIC COMPANY and
acknowledged to me that he executed the within instrument on behalf of that
corporation.

      IN WITNESS WHEREOF, I have hereunto set my hand and affixed my Notarial
Seal the day and year in this certificate first above written.

                                      /s/ Shirley A. Kushner
                                      ------------------------------------------
                                      Notary Public in and for the State of
                                      Oregon, residing at Portland, OR
                                      My Commission expires 9/27/84

STATE OF OREGON        )
                       )  ss.
COUNTY OF              )

      On this 6th day of May, 1981, before me, the undersigned, a Notary Public
in and for the State of Oregon, personally appeared R. B. Lisbakken, known to me
to be the Vice President of PACIFIC POWER AND LIGHT COMPANY and acknowledged to
me that he executed the within instrument on behalf of that corporation.

      IN WITNESS WHEREOF, I have hereunto set my hand and affixed my Notarial
Seal the day and year in this certificate first above written.

                                      /s/ Hilda V. Hambach
                                      ------------------------------------------
                                      Notary Public in and for the State of
                                      Oregon, residing at Portland, Oregon
                                      My Commission expires September 28, 1982

                                      -11-
<PAGE>   14

STATE OF NORTH DAKOTA   )
                        ) ss.
COUNTY OF               )

      On this ______ day of ____________, ___, before me, the undersigned, a
Notary Public in and for the State of North Dakota, personally appeared
__________________________, known to me to be the _______________________ of
BASIN ELECTRIC POWER COOPERATIVE and acknowledged to me that he executed the
within instrument on behalf of that corporation.

      IN WITNESS WHEREOF, I have hereunto set my hand and affixed my Notarial
Seal the day and year in this certificate first above written.

                                        -------------------------------------
                                        Notary Public in and for the State of
                                        North Dakota, residing at ______________
                                        My Commission expires __________________

                                      -12-
<PAGE>   15

                                                     Page 1 of 1
                                                     Exhibit A
                                                     Common Facilities Agreement
                                                     Colstrip Units #1, #2, #3
                                                            and #4

                               COMMON FACILITIES

<TABLE>
<CAPTION>
                                           Portion of Cost   Portion of Cost
                                            Allocated to      Allocated to
Facility Description                        Units #1 & #2     Units #3 & #4
--------------------                       ---------------   ---------------
<S>                                             <C>               <C>
A.  Miscellaneous Facilities
    Sewer System                                .3186             .6814
    Fire Protection Connections                 .95               .05
    Garage/Warehouse                            .3186             .6814
    Meteorological Structure                    .3186             .6814
    Air Quality Monitoring Programs
      (Excluding Indian Reservation
      Monitoring)                               .3186             .6814
    Coal Handling Crew Facilities               .3186             .6814
    Marine Equipment                            .3186             .6814
    Diesel Fuel Storage                         .3186             .6814
    Gasoline Storage                            .3186             .6814
    115 kV Start-up Transmission Line           .3186             .6814

B.  Drainage and Runoff Retention
    Secondary Sediments Retention               .3186             .6814

C.  Intake Water System
    Intake Canal & Structure                    .3186             .6814
    Pumps (Excluding Labor)                     .6372             .3628
    Other Mechanical (Excluding Pumps
      but Including Labor)
      & Electrical Equipment                    .4779             .5221

D.  Coal Handling Facilities
    Concrete Tunnel                             .500              .500
    Receiving Hoppers                           .500              .500
    Conveyor No. 6                              .714              .286

E.   Surge Pond
     Entire Pond & Structure                    .3186             .6814
     Mechanical & Electrical Equipment          .6372             .3628
     Cutoff Wall No. 1                          .3186             .6814
     Cutoff Wall No. 2                          .3186             .6814

F.   All Land Described
     in Exhibit B Hereto                        .3186             .6814
</TABLE>
<PAGE>   16

                                 [MAP OMITTED]

<PAGE>   17

                                 [MAP OMITTED]
<PAGE>   18

                                 [MAP OMITTED]
<PAGE>   19

STATE OF OREGON         )
                        ) ss.
County of Multnomah     )

      On this 30 day of June 1998, before me, the undersigned Notary Public in
and for the State of Oregon personally appeared Walter E. Pollock, known to me
to be the Senior Vice President of PORTLAND GENERAL ELECTRIC COMPANY and
acknowledged to me that he executed the within instrument on behalf of that
corporation.

      IN WITNESS WHEREOF, I have hereunto set my hand and affixed my Notarial
Seal the day and year in this certificate first above written.

-----------------------------------        /s/ Karen J. Lewis
[SEAL]    OFFICIAL SEAL                    -------------------------------------
          KAREN J. LEWIS                   Notary Public for the State of Oregon
      NOTARY PUBLIC - OREGON               Residing at Portland Oregon
       COMMISSION NO.038820                My Commission expires 10/20/98
MY COMMISSION EXPIRES OCT 20, 1998
-----------------------------------

STATE OF OREGON         )
                        ) ss.
County of Multnomah     )

      On this __ day of _____________, 1998, before me. the undersigned Notary
Public in and for the State of Montana, personally appeared
___________________________ known to me to be the
_____________________________ of PACIFIC POWER & LIGHT COMPANY, now PACIFICORP
and acknowledged to me that he executed the within instrument on behalf of that
corporation.

      IN WITNESS WHEREOF, I have hereunto set my hand and affixed my Notarial
Seal the day and year in this certificate first above written.

                                           -------------------------------------
                                           Notary Public for the State of Oregon
                                           Residing at _________________________
                                           My Commission expires _______________<PAGE>   1

                                                                 Exhibit 10.6(b)

                             AMENDMENT NO. 1 TO THE

                           COMMON FACILITIES AGREEMENT

This Amendment No. 1, dated as of January 21, 1992, is made to that certain
Common Facilities Agreement entered into as of May 6, 1981, by THE MONTANA POWER
COMPANY, PUGET SOUND POWER AND LIGHT COMPANY, PUGET COLSTRIP CONSTRUCTION
COMPANY, THE WASHINGTON WATER POWER COMPANY, PORTLAND GENERAL ELECTRIC COMPANY,
and PACIFIC POWER AND LIGHT COMPANY.

1.    All terms defined in the Common Facilities Agreement shall have the same
      meanings when used in this Amendment No. 1.

2.    The Common Facilities Agreement is hereby amended, effective as of May 6,
      1981, as follows:

      a)    The first paragraph of Page 1 of the Common Facilities Agreement is
            deleted and the following paragraph is inserted in lieu thereof:

            "This Agreement is made as of the 6th day of May, 1981, by and
            between the following parties: THE MONTANA POWER COMPANY, a Montana
            corporation ("Montana"); PUGET SOUND POWER AND LIGHT COMPANY, a
            Washington corporation ("Puget"); THE WASHINGTON WATER POWER
            COMPANY, a Washington corporation ("Water Power"); PORTLAND GENERAL
            ELECTRIC COMPANY, an Oregon corporation ("Portland"); and PACIFIC
            POWER AND LIGHT COMPANY, now PACIFICORP, an Oregon corporation
            ("Pacific"):"

      b)    Subsection 1(f) is deleted and the following Subsection is inserted
            in lieu thereof:

            "(f)" "Owners" means Montana, Puget, Water Power, Portland, and
            Pacific, and shall include their successors and assigns of an
            ownership interest in the Project or any part thereof."

      c)    Section 2 is deleted and the following Section is inserted in lieu
            thereof:

            "2. TERM

                  This agreement shall be effective and binding when executed by
            Montana, Puget, Water Power, Portland and Pacific. This agreement
            shall continue until the end of (a) the term of the #1 and #2 Owners
            Agreement or (b) the term of the #3 and #4 Owners Agreement;
            whichever occurs first, provided, however, that Section 10 hereof
            shall survive until the end of the term of
<PAGE>   2

            both the #1 and #2 Owners Agreement and the #3 and #4 Owners
            Agreement."

      d)    Section 5 is deleted and the following section is inserted in lieu
            thereof:

            "5. COST ALLOCATION

                  The costs of construction, operation and maintenance of the
            Common Facilities shall be allocated between the Colstrip Units #1
            and #2 and Colstrip Units #3 and #4 in the portion provided in
            Exhibit A to this amendment, as modified from time to time.

            The costs of construction of these agreed Common Facilities were
            negotiated and settled between Colstrip Units #1 and #2 and Colstrip
            Units #3 and #4 and resulted in a payment of $5,340,213 which
            includes carrying charges to May 1, 1989, to the Colstrip #1 and #2
            Owners by Portland, Water Power, and Pacific. This settlement also
            recognizes Puget's and Montana's share of Units #3 and #4.

            A Committee may at any time propose to the Committees a revised
            method of allocating operation and maintenance costs which it
            believes is more equitable than the initial allocation herein
            provided. Each Committee shall use every effort in good faith to
            negotiate such revised method of allocating operation and
            maintenance costs and shall proceed under Section 9 if such effort
            does not result in agreement."

      e)    To Section 13, add the following:

            "(e) Exhibit A, attached to this Amendment No. 1, is substituted for
            Exhibit A referenced in Section 3, Section 5 and Section 7(b).

3.    The Common Facilities Agreement, as amended by this Amendment No. 1,
      remains in full force and effect.

IN WITNESS WHEREOF, the Parties hereto have executed this Amendment No. 1 is
several counterparts.

                                        THE MONTANA POWER COMPANY

                                        By /s/ G. H. Van Hoy
                                           -------------------------------------
                                        Its ____________________________________

ATTEST:

/s/ [ILLEGIBLE]
------------------------
    Asst. Secretary
<PAGE>   3

                                        PUGET SOUND POWER AND LIGHT
                                           COMPANY

                                        By /s/ [ILLEGIBLE]
                                           -------------------------------------
                                        Its Vice President Power Systems
                                            ------------------------------------

ATTEST:

/s/ [ILLEGIBLE]
------------------------
    Asst. Secretary

                                                THE WASHINGTON WATER
                                                    POWER COMPANY

                                        By
                                           -------------------------------------
                                        Its
                                            ------------------------------------

ATTEST:

------------------------
    Asst. Secretary

                                        PORTLAND GENERAL ELECTRIC
                                           COMPANY

                                        By
                                           -------------------------------------
                                        Its
                                            ------------------------------------

ATTEST:

------------------------
    Asst. Secretary

                                        PACIFICORP, doing business as
                                        PACIFIC POWER AND LIGHT
                                           COMPANY

                                        By
                                           -------------------------------------
                                        Its
                                            ------------------------------------

ATTEST:

------------------------
  Corporate Secretary
<PAGE>   4

                                        PUGET SOUND POWER AND LIGHT
                                           COMPANY

                                        By
                                           -------------------------------------
                                        Its
                                            ------------------------------------

ATTEST:

------------------------
    Asst. Secretary

                                                THE WASHINGTON WATER
                                                    POWER COMPANY

                                        By /s/ [ILLEGIBLE]
                                           -------------------------------------
                                        Its VICE PRESIDENT POWER SUPPLY
                                            ------------------------------------

ATTEST:

------------------------
    Asst. Secretary

                                        PORTLAND GENERAL ELECTRIC
                                           COMPANY

                                        By
                                           -------------------------------------
                                        Its
                                            ------------------------------------

ATTEST:

------------------------
    Asst. Secretary

                                        PACIFICORP, doing business as
                                        PACIFIC POWER AND LIGHT
                                           COMPANY

                                        By
                                           -------------------------------------
                                        Its
                                            ------------------------------------

ATTEST:

------------------------
  Corporate Secretary
<PAGE>   5

                                        PUGET SOUND POWER AND LIGHT
                                           COMPANY

                                        By
                                           -------------------------------------
                                        Its
                                            ------------------------------------

ATTEST:

------------------------
    Asst. Secretary

                                                THE WASHINGTON WATER
                                                    POWER COMPANY

                                        By
                                           -------------------------------------
                                        Its
                                            ------------------------------------

ATTEST:

------------------------
    Asst. Secretary

                                        PORTLAND GENERAL ELECTRIC
                                           COMPANY

                                        By /s/ Peggy Y. Fowler
                                           -------------------------------------
                                        Its Vice President of Power Products
                                            ------------------------------------

ATTEST:

------------------------
    Asst. Secretary

                                        PACIFICORP, doing business as
                                        PACIFIC POWER AND LIGHT
                                           COMPANY

                                        By
                                           -------------------------------------
                                        Its
                                            ------------------------------------

ATTEST:

------------------------
  Corporate Secretary
<PAGE>   6

                                        PUGET SOUND POWER AND LIGHT
                                           COMPANY

                                        By
                                           -------------------------------------
                                        Its
                                            ------------------------------------

ATTEST:

------------------------
    Asst. Secretary

                                                THE WASHINGTON WATER
                                                    POWER COMPANY

                                        By
                                           -------------------------------------
                                        Its
                                            ------------------------------------

ATTEST:

------------------------
    Asst. Secretary

                                        PORTLAND GENERAL ELECTRIC
                                           COMPANY

                                        By
                                           -------------------------------------
                                        Its
                                            ------------------------------------

ATTEST:

------------------------
    Asst. Secretary

                                        PACIFICORP, doing business as
                                        PACIFIC POWER AND LIGHT
                                           COMPANY

                                        By /s/ William C. Brauer
                                           -------------------------------------
                                        Its Vice President
                                            ------------------------------------

ATTEST:

/s/ Sally A. Nofziger
------------------------
   Corporate Secretary
<PAGE>   7

                                                                 REVISED 6/25/90

                   EXHIBIT "A" TO COMMON FACILITIES AGREEMENT

<TABLE>
<CAPTION>
                                                            ---------------------------------------
                                                               Cost Allocations Per Article 5
--------------------------------------------------------------------------------------------------
                                                            Original Costs To     Future Costs Of
                                             Facility        Be Reallocated      Replacement, Own-
                             Facility        Ownership        As Per Columns     ership, & O&M To
                            Originally      Allocation         To Left) By       Shared On Follow-
                              Funded      (Per Article 3)        Payment          ing Fractional
                            By Product:      To Project:       To Project:       Basis By Project:
                            -----------------------------------------------------------------------
FACILITY DESCRIPTION        1&2     3&4     1&2     3&4        1&2     3&4         1&2        3&4
--------------------        -----------------------------------------------------------------------
<S>                          <C>    <C>    <C>     <C>         <C>      <C>      <C>         <C>
 1.  Sewer System             x     --     .3186   .6814        x       --        .3868      .6132

 2.  Fire Protection                                                             Common Areas and
     System Installed by                                                         Fire Pumps Shared
     Units #1 & #2            x     --     .9500   .0500        x       --        .3186      .6814
     (Allowance for #3 &                                                           No Sharing For
     #4 Connection)                                                                 Other Areas

 3.  Fire Pump & Fire                                                            Common Areas And
     Protection System                                                           Fire Pumps Shared
     Installed by Units      --      x      --    1.0000       --       --        .3186      .6814
     #3 & #4                                                                       No Sharing For
                                                                                     Other Areas

 4.  Garage & Warehouse
     (Units #1 & #2)          x     --     .5000   .5000        x       --        .4208      .5792

 5.  Meteorological
     Structures               x     --     .5000   .5000        x       --        .5000      .5000

 6.  Air Quality Monitor-
     ing Structures
     (Excluding Indian        x     --     .5000   .5000        x       --        .5000      .5000
     Reservation)

 7.  Coal Handling Crew
     Facilities
     (Units #1 & #2)          x     --     .5000   .5000        x       --        .3243      .6757

 8.  Portable Dredge          x     --     .5000   .5000        x       --        .5000      .5000

 9.  Diesel Fuel Storage      x      x     .5000   .5000       --       --        .4208      .5792

10.  Gasoline Storage         x     --     .5000   .5000        x       --        .4208      .5792

11.  Waste Oil Storage       --      x     .5000   .5000       --        x        .4208      .5792

12.  115 kV Start-up
     Transmission Line
     (Switchyard to           x     --     .5000   .5000        x       --        .4208      .5792
     Common Connection)

13.  Truck Scale              x     --     .5000   .5000        x       --        .5000      .5000
</TABLE>

                                      -1-
<PAGE>   8

                                                                 REVISED 6/25/90

                   EXHIBIT "A" TO COMMON FACILITIES AGREEMENT

<TABLE>
<CAPTION>
                                                            ---------------------------------------
                                                               Cost Allocations Per Article 5
--------------------------------------------------------------------------------------------------
                                                            Original Costs To     Future Costs Of
                                             Facility        Be Reallocated      Replacement, Own-
                             Facility        Ownership        As Per Columns     ership, & O&M To
                            Originally      Allocation         To Left) By       Shared On Follow-
                              Funded      (Per Article 3)        Payment          ing Fractional
                            By Product:      To Project:       To Project:       Basis By Project:
                            -----------------------------------------------------------------------
FACILITY DESCRIPTION        1&2     3&4     1&2     3&4        1&2     3&4         1&2        3&4
--------------------        -----------------------------------------------------------------------
<S>                          <C>    <C>    <C>    <C>           <C>    <C>       <C>        <C>
14.  Fly Ash Pond Pump
     House (Excluding         x     --     .6000   .4000        x      --        1.0000     .0000
     Process Equipment

15.  Drainage and Runoff      x     --     .3200   .6800        x      --         .3200     .6800
     Retention

16.  River Pumping
     Station & Facility
     (Including):

     a. Intake Canal &        x     --     .3186   .6814        x      --         .3186     .6814
        Structure

     b. Pumps (Excluding
        Labor) Furnished      x     --     .6593   .3407        x      --         .3186     .6814
        by Units #1 & #2

     c. Other Mechanical
        (Excluding Pumps
        but Including
        Labor & Electrical
        Equipment Furnished
        by Units #1 & #2      x     --     .5000   .5000        x      --         .3186     .6814

     d. Cyclone & Auxil.
        Pump at River        --      x     .3816   .6814                x         .3816     .6814

     e. River Pumps, Other
        Mechanical & Elec.
        Equip. Furnished             x     .0000  1.0000               --         .3816     .6814
        by Units #3 & #4

17.  Coal Handling Fac.
     Including:

     a. Concrete Tunnel       x     --     .5000   .5000        x      --         (      )   (     )
     b. Receiving Hoppers     x     --     .5000   .5000        x      --         (l.0000)   (     )
     c. Conveyor #6           x     --     .5000   .5000        x      --         (  of  )   (Billed)
     d. Sampler               x     --     .7140   .2860        x      --         (normal)   (  as  )
     e. Dust Collectors                                                           (costs)    ( used )
        #2 & #3               x     --     .9000   .1000        x      --         (     )    (      )
</TABLE>

                                      -2-
<PAGE>   9

                                                                 REVISED 6/25/90

                   EXHIBIT "A" TO COMMON FACILITIES AGREEMENT

<TABLE>
<CAPTION>
                                                            ---------------------------------------
                                                               Cost Allocations Per Article 5
--------------------------------------------------------------------------------------------------
                                                            Original Costs To     Future Costs Of
                                             Facility        Be Reallocated      Replacement, Own-
                             Facility        Ownership        As Per Columns     ership, & O&M To
                            Originally      Allocation         To Left) By       Shared On Follow-
                              Funded      (Per Article 3)        Payment          ing Fractional
                            By Product:      To Project:       To Project:       Basis By Project:
                            -----------------------------------------------------------------------
FACILITY DESCRIPTION        1&2     3&4     1&2     3&4        1&2      3&4        1&2        3&4
--------------------        -----------------------------------------------------------------------
<S>                          <C>    <C>    <C>     <C>         <C>     <C>        <C>        <C>
18.  Surge Fond Structure
     and Equipment
     Including:

     a. Pond                  x     --     .3186   .6814        x      --         .3186      .6814

     b. Structures            x     --     .3186   .6814        x      --         .3186      .6814

     c. Mechanical and
        Electrical Equip.
        Furnished by          x     --     .6593   .3407        x      --         .3186      .6814
        Units #1 & #2

     d. Mechanical and
        Electrical Equip.
        Furnished by         --      x     .0000  1.0000       --      --         .3186      .6814
        Units #3 & #4

     e. Cutoff Wall #1        x     --     .3186   .6814        x      --         .3186      .6814

     f. Cutoff Wall #2        x     --     .3186   .6814        x      --         .3186      .6814

     g. Surge Pond
        Control Cable        --      x     .3186   .6814        x      --         .3186      .6814
        Replacement

19.  Environmental Lab
     in Scrubber Area        --      x     .5000   .5000       --       x         .4208      .5792

20.  Converted
     Construction            --      x     .2000   .8000       --       x         .2000      .5000
     Buildings

21.  #1 & #2 Adam. Bldg.      x     --     .5000   .5000        x      --         .5000      .5000

22.  #3 & #4 Adam. Bldg.     --      x     .3000   .7000       --       x         .3000      .7000

     Includes:
     a. Repair Shop
     b. Telephone Room
     c. Electric Shop
     d. Chem Lab
</TABLE>

                                      -3-
<PAGE>   10

                                                                 REVISED 6/25/90

                   EXHIBIT "A" TO COMMON FACILITIES AGREEMENT

<TABLE>
<CAPTION>
                                                            ---------------------------------------
                                                               Cost Allocations Per Article 5
--------------------------------------------------------------------------------------------------
                                                            Original Costs To     Future Costs Of
                                             Facility        Be Reallocated      Replacement, Own-
                             Facility        Ownership        As Per Columns     ership, & O&M To
                            Originally      Allocation         To Left) By       Shared On Follow-
                              Funded      (Per Article 3)        Payment          ing Fractional
                            By Product:      To Project:       To Project:       Basis By Project:
                            -----------------------------------------------------------------------
FACILITY DESCRIPTION        1&2     3&4     1&2     3&4        1&2      3&4        1&2        3&4
--------------------        -----------------------------------------------------------------------
<S>                          <C>   <C>     <C>     <C>         <C>     <C>        <C>        <C>
     Excludes:
     a. Aux Boiler
     b. Heating Boiler
     c. Water Treatment
        Area

23.  Machine Shop, I&C
     Shop. & Chem Lab         x     --     .5000   .5000        x      --         .4208      .5792
     (Units #1 & #2)

24.  New Warehouse and        x      x     .4000   .6000        x       x         .4000      .6000
     Offices

25.  Environmental Bldg.      x      x     .4000   .6000        x       x         .4000      .6000

26.  Paving Parking           x      x     .3186   .6814       --      --         .4208      .5792
     Areas & Roads

27.  Plant Security &         x      x     .3186   .6814       --      --         .4208      .5792
     Fencing

28.  Yard Lighting            x      x     .3186   .6814       --      --         .4208      .5792

29.  Telephone Equipment      x      x     .3168   .6814       --      --         .4208      .5792

30.  Plant Landscaping        x      x     .3186   .6814       --      --         .4208      .5792

31.  Cathodic Protection
     for River Pipeline       x      x     .3156   .6814       --      --         .3186      .6814

32.  ATR                     --      x     .3186   .6814       --      --         .3186      .6814

33.  River Water Pipeline
     to Surge Pond
     Furnished by             x     --     .8632   .1368        x      --         .3186      .6814
     Units #1 & #2

34.  River Water Pipeline
     to Surge Pond
     Furnished by            --      x     .0000  1.0000       --      --         .3186      .6814
     Units #3 and #4

35.  North Plant             --      x     .3186   .6814       --       x         .3186      .6814
     Sediment Pond
</TABLE>

                                      -4-
<PAGE>   11

                                                                 REVISED 6/25/90

                   EXHIBIT "A" TO COMMON FACILITIES AGREEMENT

<TABLE>
<CAPTION>
                                                            ---------------------------------------
                                                               Cost Allocations Per Article 5
--------------------------------------------------------------------------------------------------
                                                            Original Costs To     Future Costs Of
                                             Facility        Be Reallocated      Replacement, Own-
                             Facility        Ownership        As Per Columns     ership, & O&M To
                            Originally      Allocation         To Left) By       Shared On Follow-
                              Funded      (Per Article 3)        Payment          ing Fractional
                            By Product:      To Project:       To Project:       Basis By Project:
                            ----------------------------------------------------------------------
FACILITY DESCRIPTION        1&2     3&4     1&2     3&4        1&2      3&4       1&2        3&4
--------------------        ----------------------------------------------------------------------
<S>                          <C>    <C>    <C>     <C>         <C>     <C>       <C>        <C>
36.  Computer Capitalized     x      x     .3186   .6814       --      --        .4208      .5792
     Facilities

37.  Gas Bottle Storage       x      x     .3186   .6814       --      --        .4208      .5792
     Rack

38.  All Vehicles                                                                Costs of Owner-
     Identified on the                                                           ship & 0&M Based
     Colstrip Project                                                            on Actual Use.
     Division Vehicle                                                            Replacements
     Roster with an                                                              Based on:
     Assigned Number          x      x     .5000   .5000       --      --        .3868      .6132
     from 3700 to 3999

39.  Major Mobile Equip-
     ment as Identified                                                          Costs of Owner-
     on the Colstrip                                                             ship & O&M Based
     Project Division                                                            on Actual Use.
     Equipment Roster                                                            Replacements
     with an Assigned                                                            Based on:
     Number from 4000         x      x     .5000   .5000       --      --        .4208      .5792
     4099

40.  Miscellaneous
     portable & mobile                                                           Costs of Owner-
     equipment on the                                                            ship & O&M Based
     Colstrip Project                                                            on Actual Use.
     Division Equipment                                                          Replacements
     Roster with an                                                              Based on:
     Assigned Number          x      x     .5000   .5000       --      --        .4208      .5792
     from 4200 to 4399

41.   Machine Shop Equip.     x      x     .5000   .5000        x       x        .4208      .5792

42.   Scaffolding             x      x     .5000   .5000       --      --        .4208      .5792

43.   Warehouse Equipment
      as Identified on the
      Colstrip Project
      Division Equipment
      Roster with an
      Assigned Number         x      x     .5000   .5000       --      --        .4208      .5792
      from 4400 to 4499
</TABLE>

                                      -5-
<PAGE>   12

                                                                 REVISED 6/25/90

                   EXHIBIT "A" TO COMMON FACILITIES AGREEMENT

<TABLE>
<CAPTION>
                                                            ---------------------------------------
                                                               Cost Allocations Per Article 5
--------------------------------------------------------------------------------------------------
                                                            Original Costs To     Future Costs Of
                                             Facility        Be Reallocated      Replacement, Own-
                             Facility        Ownership        As Per Columns     ership, & O&M To
                            Originally      Allocation         To Left) By       Shared On Follow-
                              Funded      (Per Article 3)        Payment          ing Fractional
                            By Product:      To Project:       To Project:       Basis By Project:
                            ----------------------------------------------------------------------
FACILITY DESCRIPTION        1&2     3&4     1&2     3&4        1&2      3&4       1&2        3&4
--------------------        ----------------------------------------------------------------------
<S>                           <C>   <C>    <C>     <C>         <C>     <C>       <C>        <C>

44.  Laboratory and           x      x     .5000   .5000       --      --        .4208      .5792
     Special Test Equip.

45.  Furniture                x      x     .3868   .6132       --      --        .3868      .6132

46.  Land as Described
     in Exhibit B             x     --     .3186   .6814        x      --        .3186      .6814
     attached hereto
</TABLE>

                                      -6-

Source: [{"source": "alea-institute/alea-institute/kl3m-data-edgar-agreements/train-00018-of-00352.parquet"}, [{"source": "alea-institute/alea-institute/kl3m-data-edgar-agreements/train-00018-of-00352.parquet"}]]