A watershed is the land that water flows across or under on its way to a stream, river, or lake. Its boundaries are defined geographically by ridges or lines of highest elevation above lower areas such as streams or lakes, where surface and subsurface water flows and collects. Watersheds can be small or large, and most are interconnected as parts of a larger system that eventually drains into the oceans. Watersheds vary widely in shape, size, land use, and natural features. They may include hills or mountains or be nearly flat. Within a watershed there may be farmland, rangeland, small towns, or big cities, as well as rocky or marshy areas that are better suited for certain trees, plants, and wildlife. Everyone lives in a watershed and is part of its community, along with animals, birds, and fish. How people treat soil, water, air, plants, and animals influences what happens in the watershed, for good or bad, and conditions in a smaller watershed also affect the larger watershed downstream.