The Illinois Confederation, also referred to as the Illiniwek or Illini, were made up of 12 to 13 tribes who lived in the Mississippi River Valley. Eventually member tribes occupied an area reaching from Lake Michicigao (Michigan) to Iowa, Illinois, Missouri, and Arkansas. The five main tribes were the Cahokia, Kaskaskia, Michigamea, Peoria, and Tamaroa. The name of the confederation was derived from the transliteration by French explorers of "Illiniwek" to a simplified "Illinois", more in keeping with the sounds of their own language. The tribes are estimated to have had tens of thousands of members, before the advancement of European contact in the 17th century that inhibited their growth and resulted in a marked decline in population.