How do we know that the "Hopewell" were a tradition or culture, rather than simply a trade network?

We don't know because we made up the notions of tradition, culture, trade network, and interaction sphere to try to capture and make sense of the patterns we see in the artifact assemblages that prehistoric people left behind. What we can say is that in the period from approximately 100 BCE to 300 CE there existed groups of horticultural hunter-gatherers in areas of the eastern woodlands that built very large, centralized burial mounds and geometric earthwork enclosures, and deposited in these sites both local and exotic artifacts of spectacular beauty and richness. These sites typically had burial mounds and geometric earthworks; linear mounds and round and square enclosures that covered tens to hundreds of acres. Occupants of Hopewell sites engaged in trade systems that extended from the Northeast to the Gulf of Mexico and west to the Rocky Mountains.

The highest density and finest expression of Hopewell appears to be in the Scioto River Valley of Ohio, but similar assemblages and features can be found during the same period from the far Southeastern US to north of the Great Lakes. The set of traits that are collectively considered Hopewell originated in New York and moved west into Ohio, Indiana and Illinois and then spread throughout the eastern woodlands. Many aspects of Hopewell are thought to have originated in the Adena mortuary tradition. In fact, recent research has shown that Hopewell burials from mounds in Hopewell National Historical Park are direct descendants of people buried in Adena site mounds. The number, nature and origin of Hopewell artifacts and features would seem to suggest that they were made during a later period by a fully sedentary people more akin to those of the Mississippian Tradition, but that is not the case. People during the Hopewell period were early agriculturalists (pre-maize) and lived across the landscape in farmsteads of one or two houses or small pithouse villages. They periodically came together at very large, centralized burial mounds and geometric earthwork enclosures to conduct religious/social activities and bury their people.

People during the Hopewell period (note that I am using the term as an archaeological time unit because it makes more sense that way) appear to have dedicated an inordinate amount of effort to constructing sacred spaces and interring their dead in them. The use of such sites has suggested to archaeologists that the users likely had complex religious beliefs and sufficient social organization to provide for the construction of these major public works and the rich artifact assemblages that they contain.

The most intellectually conservative position an archaeologist could take is that Hopewell is a set of cultural traits that existed in a specific time and space. Certainly more expansive anthropological descriptions of "interaction spheres", "exchange networks", "cultures" and even "peoples" abound. However, these terms are seldom rigorously defined and often very sloppily used. Terminology is a difficult thing in archaeological circles. There have been many reams of paper dedicated to systematic classification of artifacts, features and cultures. There remains a lot of confusion and baggage surrounding terms like "archaeological cultures, spheres and networks". And ultimately, the terms are really getting at the same thing. What we have are archaeological assemblages that in discrete terms (artifact numbers) are very similar, and inferentially, that reflect aspects of social and cultural belief systems of a people. The safest, and in my opinion the rational position, is to consider it in terms of archaeological time-space attributes - an archaeological tradition (although it is a short tradition in comparison to others).

Ohio Hopewell Community Organization 2002 William Dancey and Paul J Pacheco Eds.

Use of tobacco pipes by Native groups tells story of regional diversity

Community organizations in the Scioto, Mann, and Havana Hopewellian regions BJ Ruby, C Carr, DK Charles Gathering Hopewell, 119-176

Footprints-In the Footprints of Squier and Davis: Archeological Fieldwork in Ross County, Ohio MJ Lynott, J Pederson-Weinberger, OC Shane III… - 2009

Systematics in Prehistory. 1971 Robert C. Dunnell Blackburn Press

Americanist Culture History: Fundamentals of Time, Space, and Form 2013 R. Lee Lyman, Michael J. O'Brien, Robert C. Dunnell Springer Science & Business Media