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                                                                    Exhibit 4.05

                                                                  EXECUTION COPY

                      NORTH AMERICAN SITE DEVELOPERS, INC.

                             SUPPLEMENTAL INDENTURE

                  SUPPLEMENTAL INDENTURE (this "SUPPLEMENTAL INDENTURE"), dated
as of the 24th of April, 2001, among North American Site Developers, Inc. (the
"GUARANTEEING SUBSIDIARY"), a subsidiary of Great Lakes Dredge & Dock
Corporation (or its permitted successor), a Delaware corporation (the
"COMPANY"), the Company, the other Subsidiary Guarantors (as defined in the
Indenture referred to herein) and The Bank of New York, as trustee under the
indenture referred to below (the "TRUSTEE").

                               W I T N E S S E T H

                  WHEREAS, the Company has heretofore executed and delivered to
the Trustee an indenture (the "INDENTURE"), dated as of August 19, 1998
providing for the issuance of an aggregate principal amount of up to
$165,000,000 of 11 1/4% Senior Subordinated Notes due 2008 (the "NOTES");

                  WHEREAS, the Indenture provides that under certain
circumstances the Guaranteeing Subsidiary shall execute and deliver to the
Trustee a supplemental indenture pursuant to which the Guaranteeing Subsidiary
shall unconditionally guarantee all of the Company's Obligations under the Notes
and the Indenture on the terms and conditions set forth herein (the "SUBSIDIARY
GUARANTEE"); and

                  WHEREAS, pursuant to Section 9.01 of the Indenture, the
Trustee is authorized to execute and deliver this Supplemental Indenture.

                  NOW THEREFORE, in consideration of the foregoing and for other
good and valuable consideration, the receipt of which is hereby acknowledged,
the Guaranteeing Subsidiary and the Trustee mutually covenant and agree for the
equal and ratable benefit of the Holders of the Notes as follows:

                  1.  CAPITALIZED TERMS.  Capitalized terms used herein without
definition shall have the meanings assigned to them in the Indenture.

                  2.  AGREEMENT TO GUARANTEE.  The Guaranteeing Subsidiary
hereby agrees as follows:

         (a)   Along with all Subsidiary Guarantors named in the Indenture, to
         jointly and severally Guarantee to each Holder of a Note authenticated
         and delivered by the Trustee and to the Trustee and its successors and
         assigns, the Notes or the obligations of the Company hereunder or
         thereunder, that:

               (i)   the principal of and interest on the Notes will be
          promptly paid in full when due, whether at maturity, by acceleration,
          redemption or otherwise, and interest on the overdue principal of and
          interest on the Notes, if any, if lawful, and all other obligations
          of the Company to the Holders or the Trustee hereunder or thereunder
          will

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          be promptly paid in full or performed, all in accordance with the
          terms hereof and thereof; and

               (ii)  in case of any extension of time of payment or renewal of
          any Notes or any of such other obligations, that same will be promptly
          paid in full when due or performed in accordance with the terms of the
          extension or renewal, whether at stated maturity, by acceleration or
          otherwise. Failing payment when due of any amount so guaranteed or any
          performance so guaranteed for whatever reason, the Subsidiary
          Guarantors shall be jointly and severally obligated to pay the same
          immediately.

          (b) The obligations hereunder shall be unconditional, irrespective of
          the validity, regularity or enforceability of the Notes or the
          Indenture, the absence of any action to enforce the same, any waiver
          or consent by any Holder of the Notes with respect to any provisions
          hereof or thereof, the recovery of any judgment against the Company,
          any action to enforce the same or any other circumstance which might
          otherwise constitute a legal or equitable discharge or defense of a
          guarantor.

          (c) The following is hereby waived: diligence, presentment, demand of
          payment, filing of claims with a court in the event of insolvency or
          bankruptcy of the Company, any right to require a proceeding first
          against the Company, protest, notice and all demands whatsoever.

          (d) This Subsidiary Guarantee shall not be discharged except by
          complete performance of the obligations contained in the Notes and the
          Indenture, and the Guaranteeing Subsidiary accepts all obligations of
          a Subsidiary Guarantor under the Indenture.

          (e) If any Holder or the Trustee is required by any court or otherwise
          to return to the Company, the Subsidiary Guarantors, or any Custodian,
          Trustee, liquidator or other similar official acting in relation to
          either the Company or the Subsidiary Guarantors, any amount paid by
          either to the Trustee or such Holder, this Subsidiary Guarantee, to
          the extent theretofore discharged, shall be reinstated in full force
          and effect.

          (f) The Guaranteeing Subsidiary shall not be entitled to any right of
          subrogation in relation to the Holders in respect of any obligations
          guaranteed hereby until payment in full of all obligations guaranteed
          hereby.

          (g) As between the Subsidiary Guarantors, on the one hand, and the
          Holders and the Trustee, on the other hand, (x) the maturity of the
          obligations guaranteed hereby may be accelerated as provided in
          Article 6 of the Indenture for the purposes of this Subsidiary
          Guarantee, notwithstanding any stay, injunction or other prohibition
          preventing such acceleration in respect of the obligations guaranteed
          hereby, and (y) in the event of any declaration of acceleration of
          such obligations as provided in Article 6 of the Indenture, such
          obligations (whether or not due and payable) shall forthwith become
          due and payable by the Subsidiary Guarantors for the purpose of this
          Subsidiary Guarantee.

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          (h) The Subsidiary Guarantors shall have the right to seek
          contribution from any non-paying Subsidiary Guarantor so long as the
          exercise of such right does not impair the rights of the Holders under
          the Guarantee.

          (i) Pursuant to Section 11.02 of the Indenture, after giving effect to
          any maximum amount and any other contingent and fixed liabilities that
          are relevant under any applicable Bankruptcy or fraudulent conveyance
          laws, and after giving effect to any collections from, rights to
          receive contribution from or payments made by or on behalf of any
          other Subsidiary Guarantor in respect of the obligations of such other
          Subsidiary Guarantor under Article 11 of the Indenture, this new
          Subsidiary Guarantee shall be limited to the maximum amount
          permissible such that the obligations of such Subsidiary Guarantor
          under this Subsidiary Guarantee will not constitute a fraudulent
          transfer or conveyance.

                  3.  EXECUTION AND DELIVERY.  Each Guaranteeing Subsidiary
agrees that the Subsidiary Guarantees shall remain in full force and effect
notwithstanding any failure to endorse on each Note a notation of such
Subsidiary Guarantee.

                  4.  GUARANTEEING SUBSIDIARY MAY CONSOLIDATE, ETC. ON CERTAIN
TERMS.

                  (a) The Guaranteeing Subsidiary may not consolidate with or
merge with or into (whether or not such Subsidiary Guarantor is the surviving
Person) another corporation, Person or entity whether or not affiliated with
such Subsidiary Guarantor unless:

         (i)   subject to Sections 11.05 and 11.06 of the Indenture, the Person
         formed by or surviving any such consolidation or merger (if other than
         a Subsidiary Guarantor or the Company) unconditionally assumes all the
         obligations of such Subsidiary Guarantor, pursuant to a supplemental
         indenture in form and substance reasonably satisfactory to the Trustee,
         under the Notes, the Indenture and the Subsidiary Guarantee on the
         terms set forth herein or therein; and

         (ii)  immediately after giving effect to such transaction, no Default
         or Event of Default exists;

         provided, however, that this Section shall not apply to the merger of
         Great Lakes/North American Site Developers, Inc. with and into North
         American Site Developers, Inc.

                  (b) In case of any such consolidation, merger, sale or
conveyance and upon the assumption by the successor corporation, by supplemental
indenture, executed and delivered to the Trustee and satisfactory in form to the
Trustee, of the Subsidiary Guarantee endorsed upon the Notes and the due and
punctual performance of all of the covenants and conditions of the Indenture to
be performed by the Subsidiary Guarantor, such successor corporation shall
succeed to and be substituted for the Subsidiary Guarantor with the same effect
as if it had been named herein as a Subsidiary Guarantor. Such successor
corporation thereupon may cause to be signed any or all of the Subsidiary
Guarantees to be endorsed upon all of the Notes issuable hereunder which
theretofore shall not have been signed by the Company and delivered to the
Trustee. All

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the Subsidiary Guarantees so issued shall in all respects have the same legal
rank and benefit under the Indenture as the Subsidiary Guarantees theretofore
and thereafter issued in accordance with the terms of the Indenture as though
all of such Subsidiary Guarantees had been issued at the date of the execution
hereof.

                  (c) Except as set forth in Articles 4 and 5 and Section 11.05
of Article 11 of the Indenture, and notwithstanding clauses (a) and (b) above,
nothing contained in the Indenture or in any of the Notes shall prevent any
consolidation or merger of a Subsidiary Guarantor with or into the Company or
another Subsidiary Guarantor, or shall prevent any sale or conveyance of the
property of a Subsidiary Guarantor as an entirety or substantially as an
entirety to the Company or another Subsidiary Guarantor.

                  5.   RELEASES.

                  (a)  In the event of a sale or other disposition of all of the
assets of any Subsidiary Guarantor, by way of merger, consolidation or
otherwise, or a sale or other disposition of all to the capital stock of any
Subsidiary Guarantor, in each case to a Person that is not (either before or
after giving effect to such transaction) a Restricted Subsidiary of the Company,
then such Subsidiary Guarantor (in the event of a sale or other disposition, by
way of merger, consolidation or otherwise, of all of the capital stock of such
Subsidiary Guarantor) or the corporation acquiring the property (in the event of
a sale or other disposition of all or substantially all of the assets of such
Subsidiary Guarantor) will be released and relieved of any obligations under its
Subsidiary Guarantee; PROVIDED that the Net Proceeds of such sale or other
disposition are applied in accordance with the applicable provisions of the
Indenture, including without limitation Section 4.10 of the Indenture. Upon
delivery by the Company to the Trustee of an Officers' Certificate and an
Opinion of Counsel to the effect that such sale or other disposition was made by
the Company in accordance with the provisions of the Indenture, including
without limitation Section 4.10 of the Indenture, the Trustee shall execute any
documents reasonably required in order to evidence the release of any Subsidiary
Guarantor from its obligations under its Subsidiary Guarantee.

                  (b)  Any Subsidiary Guarantor not released from its
obligations under its Subsidiary Guarantee shall remain liable for the full
amount of principal of and interest on the Notes and for the other obligations
of any Subsidiary Guarantor under the Indenture as provided in Article 11 of the
Indenture.

                  6.   NO RECOURSE AGAINST OTHERS. No past, present or future
director, officer, employee, incorporator, stockholder or agent of the
Guaranteeing Subsidiary, as such, shall have any liability for any obligations
of the Company or any Guaranteeing Subsidiary under the Notes, any Subsidiary
Guarantees, the Indenture or this Supplemental Indenture or for any claim based
on, in respect of, or by reason of, such obligations or their creation. Each
Holder of the Notes by accepting a Note waives and releases all such liability.
The waiver and release are part of the consideration for issuance of the Notes.
Such waiver may not be effective to waive liabilities under the federal
securities laws and it is the view of the SEC that such a waiver is against
public policy.

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                  7.   NEW YORK LAW TO GOVERN. THE INTERNAL LAW OF THE STATE OF
NEW YORK SHALL GOVERN AND BE USED TO CONSTRUE THIS SUPPLEMENTAL INDENTURE BUT
WITHOUT GIVING EFFECT TO APPLICABLE PRINCIPLES OF CONFLICTS OF LAW TO THE EXTENT
THAT THE APPLICATION OF THE LAWS OF ANOTHER JURISDICTION WOULD BE REQUIRED
THEREBY.

                  8.   COUNTERPARTS The parties may sign any number of copies of
this Supplemental Indenture. Each signed copy shall be an original, but all of
them together represent the same agreement.

                  9.   EFFECT OF HEADINGS. The Section headings herein are for
convenience only and shall not affect the construction hereof.

                  10.  THE TRUSTEE. The Trustee shall not be responsible in any
manner whatsoever for or in respect of the validity or sufficiency of this
Supplemental Indenture or for or in respect of the recitals contained herein,
all of which recitals are made solely by the Guaranteeing Subsidiary and the
Company.

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                  IN WITNESS WHEREOF, the parties hereto have caused this
Supplemental Indenture to be duly executed and attested, all as of the date
first above written.

Dated:  April 24, 2001


                                       NORTH AMERICAN SITE DEVELOPERS, INC.

                                       By: /s/   Deborah A. Wensel
                                          --------------------------------------
                                       Name:     Deborah A. Wensel
                                       Title:    Vice President

                                       GREAT LAKES DREDGE & DOCK CORPORATION

                                       By: /s/   Deborah A. Wensel
                                          --------------------------------------
                                       Name:     Deborah A. Wensel
                                       Title:    Vice President and CFO


                                       GREAT LAKES DREDGE & DOCK COMPANY

                                       By: /s/   Deborah A. Wensel
                                          --------------------------------------
                                       Name:     Deborah A. Wensel
                                       Title:    Vice President and CFO


                                       DAWSON MARINE SERVICES COMPANY

                                       By: /s/   Deborah A. Wensel
                                          --------------------------------------
                                       Name:     Deborah A. Wensel
                                       Title:    Vice President and CFO

                                       GREAT LAKES CARIBBEAN DREDGING, INC.

                                       By: /s/   Deborah A. Wensel
                                          --------------------------------------
                                       Name:     Deborah A. Wensel
                                       Title:    Vice President and CFO

                                       FIFTY-THREE DREDGING CORPORATION

                                       By: /s/   Paul Dinquel
                                          --------------------------------------
                                       Name:     Paul Dinquel
                                       Title:    Vice President

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                                       GREAT LAKES/NORTH AMERICAN SITE
                                       DEVELOPERS, INC.

                                       By: /s/   Leslie A. Braun
                                          --------------------------------------
                                       Name:     Leslie A. Braun
                                       Title:    Clerk

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                                       THE BANK OF NEW YORK,
                                       as Trustee

                                       By: /s/   Mary LaGunima
                                          --------------------------------------
                                       Name:     Mary LaGunima
                                       Title:    Vice President







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                                   SCHEDULE I

                        SCHEDULE OF SUBSIDIARY GUARANTORS

                  The following schedule lists each Subsidiary Guarantor under
the Indenture as of the date of the Indenture:



Great Lakes Dredge & Dock Company
Dawson Marine Services Company
Great Lakes Caribbean Dredging, Inc.
Fifty-Three Dredging Corporation
Great Lakes/North American Site Developers, Inc.
North American Site Developers, Inc.