Indians lived in the Columbia bottomlands, using the marshy sloughs for subsistence and transportation. The Columbia Slough allowed navigation from the confluence of the Sandy River inland to the Willamette when choppy, rough waters made the Columbia River dangerous. A dense stretch of villages dotted the floodplain with Chinookan villages organized around permanent structures like the Nechacolee village-house, built with four-foot wide passageways allowing five independent houselike apartments. The size of houses and number of occupants varied from village to village. When the waters of the Columbia and Willamette Rivers rose, creating a lake from the Sandy to the Willamette, residents towed house planking behind their canoes and moved to higher ground. During the summer families dispersed, gathering necessary staples from the river's abundant plant and animal resources while living in summer homes constructed of cattail mat sides with cedar bark roofs. Harvesting fish from the Columbia, its tributaries and sloughs, and gathering berries, cattails, and wapato from the marshes, allowed lower river residents to remain close to home during the summer months.