The main cause for the founding and subsequent development of Saginaw was the large demand for lumber as the United States expanded westward. A virgin growth forest principally consisting of white pine trees covered most of Michigan. The convenient access to transportation provided by the Saginaw River and its numerous tributaries fueled a massive expansion in population and economic activity. As the trees were being cut down in the region, logs were floated down the rivers to sawmills located in Saginaw, destined to be loaded onto ships and later railroad cars.