Houston's topography is further defined by a large number of creeks and ditches. Overall, this intricate system of waterways is essential to flood control; Houston is well known as one of the most flood-prone cities in the United States. Since the mid-20th century, the United States Army Corps of Engineers, in cooperation with the city and Harris County Flood Control District, has channelized, paved, widened, and deepened extensive sections of all of the five major bayous specified above, with the notable exception of some parts of the Buffalo near Memorial. The Buffalo Bayou watershed also features two flood control reservoirs, Addicks Reservoir and Barker Reservoir, which retain large amounts of water after extreme rainfall events.