This is coolbert:
From that prior blog entry:
"Is this [Kerch] an amphibious operation as that term generally, commonly, and ordinarily understood?"
"When exactly [if so] does a river crossing end and an amphibious operation start?"
And thanks to Colonel Craig USMC and the U.S. Army FM as dated 1961 we have the answer as best can be discerned.
"FM NO. 31-12 ARMY FORCES IN AMPHIBIOUS OPERATIONS 28 MARCH 1961"
"CHAPTER 1 INTRODUCTION"
"PART ONE THE AMPHIBIOUS OPERATION"
"Section II. NATURE AND SCOPE OF THE AMPHIBIOUS OPERATION"
"6. Definition. An amphibious operation is":
"a. An attack launched from the sea by naval and landing forces involving a
landing on a hostile shore."
"b. A tactical withdrawal of land forces from a hostile shore, effected by
"7. Distinguishing characteristics."
"a. By definition, the amphibious operation, as an attack, has two
"1. Major elements of the landing force are embarked in naval ships or
craft for movement to the objective area."
. . . .
"3. It does not encompass other operations which possess some like
characteristics and involve some of the same techniques of an amphibious
operation. combat operations involving shore-to-shore movement across sea
areas and inland waters in means other than naval ships are not amphibious
There in somewhat greater than the proverbial nutshell that amphibious operation defined  as being an assault FROM THE SEA and involving NAVAL VESSELS AND WARSHIPS.
Anything other than that is NOT an amphibious operation.
By that definition the crossing of the Rhine  or the Irrawaddy , the evacuation of German troops across the Strait of Messina or the Scheldt Estuary WOULD NOT be amphibious operations.
NOT amphibious regardless of scale, the widespread use of landing craft and boats immaterial, NOT amphibious operations.