One special feature of the Chicago area was the now-vanished Mud Lake in the Des Plaines River watershed. During heavy periods of rain or when the Des Plaines overflowed its banks due to downstream ice dams in the early spring, the river would flow through Mud Lake to the South Branch of the Chicago River, forming a favorite portage for early traders and creating the path of the future I&M and Chicago Sanitary and Ship Canals. When the city we know today was initially founded in the 1830s, the land was swampy and most of the early building began on low dunes around the Chicago River's mouth.