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                                                                     EXHIBIT 4.8

                                 TRUST AGREEMENT
                                       OF
                              SEMCO CAPITAL TRUST I


         THIS TRUST AGREEMENT is made as of November 23, 1999 (this
"Agreement"), by and among SEMCO Energy, Inc., a Michigan corporation, as
depositor (the "Depositor"), Bank One Delaware, Inc., a Delaware corporation, as
trustee (the "Delaware Trustee"), Edric R. Mason, Jr., as trustee, and Sebastian
Coppola, as trustee (Edric R. Mason, Jr. and Sebastian Coppola being
collectively the "Administrative Trustees") (the Administrative Trustees and the
Delaware Trustee being collectively the "Trustees"). The Depositor and the
Trustees hereby agree as follows:

         1. The trust created hereby shall be known as "SEMCO Capital Trust I"
(the "Trust"), in which name the Trustees or the Depositor, to the extent
provided herein, may conduct the business of the Trust, make and execute
contracts, and sue and be sued.

         2. The Depositor hereby assigns, transfers, conveys and sets over to
the Trust the sum of $10. Such amount shall constitute the initial trust estate.
It is the intention of the parties hereto that the Trust created hereby
constitute a business trust under Chapter 38 of Title 12 of the Delaware Code,
12 Del. C. Section 3801, et seq. (the "Business Trust Act"), and that this
document constitute the governing instrument of the Trust. The Trustees are
hereby authorized and directed to execute and file a certificate of trust with
the Delaware Secretary of State in such form as the Trustees may approve.

         3. The Depositor and the Trustees will enter into an amended and
restated Trust Agreement satisfactory to each such party to provide for the
contemplated operation of the Trust created hereby and the issuance of the Trust
Preferred Securities and Trust Common Securities referred to therein. Prior to
the execution and delivery of such amended and restated Trust Agreement, the
Trustees shall not have any duty or obligation hereunder or with respect of the
trust estate, except as otherwise required by applicable law or as may be
necessary to obtain prior to such execution and delivery any licenses, consents
or approvals required by applicable law or otherwise. Notwithstanding the
foregoing, the Trustees may take all actions deemed proper as are necessary to
effect the transactions contemplated herein.

         4. The Depositor is hereby authorized, in its sole discretion, (i) to
file with the Securities and Exchange Commission (the "Commission") and to
execute, in the case of the 1933 Act Registration Statement and 1934 Act
Registration Statement, on behalf of the Trust, (a) the 1933 Act Registration
Statement, including pre-effective or post-effective amendments to such
Registration Statement, relating to the registration under the Securities Act of
1933, as amended (the "1933 Act"), of the Trust Preferred Securities of the
Trust, (b) any preliminary prospectus or prospectus or supplement thereto
relating to the Trust Preferred Securities required to be filed under the 1933
Act, and (c) if required, a Registration Statement on Form 8-A or other
appropriate form (the "Registration Statement") (including all pre-effective and
post-effective amendments thereto) relating to the registration of the Trust
Preferred Securities of the Trust

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under the Securities Exchange Act of 1934, as amended; (ii) to file with the
NASDAQ National Market or any national exchange and execute on behalf of the
Trust a listing application and all other applications, statements,
certificates, agreements and other instruments as shall be necessary or
desirable to cause the Trust Preferred Securities to be listed with the NASDAQ
National Market or any national exchange; (iii) to file and execute on behalf of
the Trust such applications, reports, surety bonds, irrevocable consents,
appointments of attorney for service of process and other papers and documents
as shall be necessary or desirable to register the Trust Preferred Securities
under the securities or "Blue Sky" laws of such jurisdictions as the Depositor,
on behalf of the Trust, may deem necessary or desirable; (iv) to execute,
deliver and perform on behalf of the Trust, an underwriting agreement with the
Depositor and the underwriter or underwriters of the Trust Preferred Securities
of the Trust; (v) to execute, deliver and perform a depository agreement with
the initial clearing agency, relating to the Trust Preferred Securities; and
(vi) to apply for and obtain a tax identification number for the Trust.

         In the event that any filing referred to in this Section 4 is required
by the rules and regulations of the Commission, PORTAL or state securities or
Blue Sky laws to be executed on behalf of the Trust by the Trustees, the
Trustees, in their capacity as trustees of the Trust, are hereby authorized and
directed to join in any such filing and to execute on behalf of the Trust any
and all of the foregoing, it being understood that Bank One Delaware, Inc., in
its capacity as trustee of the Trust, shall not be required to join in any such
filing or execute on behalf of the Trust any such document unless required by
the rules and regulations of the Commission, PORTAL or state securities or Blue
Sky laws. In connection with all of the foregoing, the Depositor hereby
constitutes and appoints Sebastian Coppola, as its true and lawful
attorney-in-fact and agent, with full power of substitution and resubstitution
for the Depositor or in the Depositor's name, place and stead, in any and all
capacities, to sign any and all amendments (including all pre-effective and
post-effective amendments) to the 1933 Act Registration Statement and any 1934
Act Registration Statement and to file the same, with all exhibits thereto, and
any other documents in connection therewith, with the Commission, granting unto
said attorney-in-fact and agent full power and authority to do and perform each
and every act and thing requisite and necessary to be done in connection
therewith, as fully to all intents and purposes as the Depositor might or could
do in person, hereby ratifying and confirming all that said attorney-in-fact and
agent or his respective substitute or substitutes, shall do or cause to be done
by virtue hereof.

         5. This Agreement may be executed in one or more counterparts.

         6. The number of trustees of the Trust initially shall be three and
thereafter the number of trustees of the Trust shall be such number as shall be
fixed from time to time by a written instrument signed by the Depositor which
may increase or decrease the number of trustees of the Trust; provided, however,
that to the extent required by the Business Trust Act, one trustee of the Trust
shall either be a natural person who is a resident of the State of Delaware or,
if not a natural person, any entity which has its principal place of business in
the State of Delaware. Subject to the foregoing, the Depositor is entitled to
appoint or remove without cause any trustee of the Trust at any time. Any
trustee of the Trust may resign upon thirty days' prior notice to the Depositor.


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         7. This Agreement shall be governed by, and construed in accordance
with, the laws of the State of Delaware (with regard to conflict of laws
principles).

         8. The Delaware Trustee shall not have any of the powers or duties of
the Trustees set forth herein, except as required under the Business Trust Act.
The Delaware Trustee shall be a Trustee hereunder for the sole and limited
purpose of fulfilling the requirements of ss. 3807(a) of the Business Trust Act.

         IN WITNESS WHEREOF, the parties hereto have caused this Agreement to be
duly executed as of the day and year first above written.


                                  SEMCO Energy, Inc., as Depositor

                                  By:      /s/ William L. Johnson
                                     ----------------------------------
                                  Name:    William L. Johnson
                                  Title:   Chairman and Chief Executive Officer


                                  Bank One Delaware, Inc.

                                  By:      /s/ J. Michael Banas
                                     ----------------------------------
                                  Name:    J. Michael Banas
                                  Title:   Authorized Officer

                                  /s/ Edric R. Mason, Jr.
                                  -------------------------
                                  Edric R. Mason, Jr.,
                                  as Administrative Trustee

                                  /s/ Sebastian Coppola
                                  -------------------------
                                  Sebastian Coppola,
                                  as Administrative Trustee