A watershed is the land area that drains to a particular stream, river, or lake, with water flowing downhill from high points to low points where streams run. Small streams that flow into larger rivers are called tributaries, and scientists classify streams by how many tributaries they have, from first order streams with none to higher order streams formed by multiple lower order tributaries. The River Raisin has many tributaries, including Goose Creek, Wolf Creek, and Mason Creek, and its watershed is part of the Lake Erie watershed, which also includes parts of Michigan, Ohio, Pennsylvania, New York, and Ontario. For thousands of years before European settlement, the River Raisin watershed was an expanse of forest, swamp, and prairie with a few Native American settlements. In the mid-1800s, arriving settlers changed the watershed by building dams, clearing forests, and draining wetlands for agriculture. In the 1900s, factories were built along the Raisin and in many cases discharged pollutants directly into the stream, while its lower reaches were lined with concrete to reduce flooding. Although these changes contributed to regional growth and productivity, they also caused environmental costs, including habitat loss for fish and wildlife and reduced water quality. In recent decades, communities have increasingly recognized the importance of a healthy watershed, and regulations, programs, better farming practices, reduced wetland destruction, land preservation, and water quality monitoring have helped move the River Raisin watershed toward a better future. Formed by meltwater from glaciers that retreated from Michigan 10,000 years ago, the River Raisin and its tributaries drain approximately 1,080 square miles, including portions of five Michigan counties. The river is 150 miles long and drops 480 feet from its headwaters to its mouth at Lake Erie. Tradition holds that French explorers named the River Raisin after seeing Native Americans drying wild grapes on its banks, while its Native American name was Nummasepee, or River of Sturgeon. The watershed contains 429 lakes and ponds and more than 60 man-made dams.