Houston is located in the Gulf Coastal Plain biome, and its vegetation is classified as temperate grassland. Much of the city was built on marshes, forested land, swamp, or prairie, all of which can still be seen in surrounding areas.
The city's topography is very flat, making flooding a recurring problem for its residents. The city stands about 50 feet (15 m) above sea level—the highest area within city limits being 90 feet (27 m). However, subsidence, caused by extensive groundwater pumping and resource extraction, has caused the elevation to drop 10 feet (3.0 m) or more in certain areas. As a result, the city turned to surface water sources for its municipal supply, creating reservoirs such as Lake Houston and Lake Conroe (of which Houston owns two-thirds interest).
Houston has four major bayous passing through the city: Buffalo Bayou, which runs into downtown and the Houston Ship Channel; and three of its tributaries: Brays Bayou, which runs along the Texas Medical Center; White Oak Bayou, which runs through the Heights and near the northwest area; and Sims Bayou, which runs through the south of Houston and downtown Houston. The ship channel goes past Galveston and into the Gulf of Mexico.
Houston is largely located within the Northern Humid Gulf Coast Prairies level IV ecoregion (34a), a subset of the Western Gulf coastal grasslands level III region as defined by the United States Environmental Protection Agency. This region is generally underlaid by Quaternary-aged sand, silts, and clays (clay, clay loam, or sandy clay loam) and covered by grasslands with occasional clusters of oak trees.