The main cause for the development of Saginaw was the lumber needs of the growing American nation. A virgin growth forest principally consisting of white pine trees covered most of Michigan. The convenient access to water transportation provided by the Saginaw River and its numerous tributaries fueled a massive expansion in population and economic activity. As the trees were being felled in the region, logs were floated down the rivers to sawmills located in Saginaw, then to be loaded onto ships and later railroad cars.