Illustration of the seal of the Saint Lawrence Seaway Development Corporation medallion from s:Executive Order 10581, which defined the seal.  The design is described there as:

 On a sea green disk, Neptune's spear palewise in white, between an American bald eagle displayed and a Canadian maple leaf all gold, partially encircled by the inscription "SAINT LAWRENCE SEAWAY DEVELOPMENT CORPORATION" and in an inner arc the words "UNITED STATES OF AMERICA" in white letters.
Scanned from Title 3 of the Code of Federal Regulations, 1954-1958 Comp., p 230.
U.S. Government