what was the main reason for their decline?
War, colder climate, introduction of bow and arrow, are few among them.
Hopewell tradition
War is a possible cause, as villages dating to the Late Woodland period shifted to larger communities; they built defensive fortifications of palisade walls and ditches. Colder climatic conditions could have also driven game animals north or west, as weather would have a detrimental effect on plant life, drastically cutting the subsistence base for these foods. The introduction of the bow and arrow, by improving hunts, may have caused stress on already depleted food populations. 
in which country does saint paul, minnesota fall under? U.S. how is the climate there? Hot summer humid continental climate what is the population of this place? Acc to 2010 census, there are 285,068 people. who were the first settlers in this place? Members of the Hopewell tradition who did their members include? It was not a single culture or society, but a widely dispersed set of related populations can you name some cultures that exists in this tradition? Ohio Hopewell and a number of other Middle Woodland period cultures was there a cultural decline? Yes
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Hopewell tradition [SEP] Cultural decline
Around 500 CE, the Hopewell exchange ceased, mound building stopped, and art forms were no longer produced. War is a possible cause, as villages dating to the Late Woodland period shifted to larger communities; they built defensive fortifications of palisade walls and ditches. Colder climatic conditions could have also driven game animals north or west, as weather would have a detrimental effect on plant life, drastically cutting the subsistence base for these foods. The introduction of the bow and arrow, by improving hunts, may have caused stress on already depleted food populations. It may have led people to live in larger, more permanent communities for protection as warfare became more deadly.