Park Central drive and along the western edge of Medical City Dallas Hospital complex, where it widens, then flows under U.S. Highway 75 (North Central Expressway) south of Forest Lane. From Belt Line Road to White Rock Lake, the creek's lowland flood plain is a park and a popular bicycling and running venue for Dallas residents. DART rail bridges the creek east of US 75, where Johnson Creek joins White Rock Creek. The creek crosses Royal Lane where, at this point, it has entered a more urban environment, crossing under Greenville Avenue and Walnut Hill Lane east of Presbyterian Hospital-Dallas. From here, Cottonwood Creek (containing Floyd Brook) joins White Rock Creek which soon enters (or creates) White Rock Lake, a man-made reservoir no more than 20 feet (6 m) deep serving the city of Dallas. After White Rock Lake, the creek flows south past Interstate 30 and U.S. Highway 175 (the C.F. Hawn Freeway) and merges with the Trinity River. Geology and ecology The creek (and the lake it feeds) takes its name from the weathered-white Austin Chalk which lines its bed and banks, thus revealing its place geologically within the Balcones Escarpment. In some places, this chalk becomes quite conspicuous as the creek cuts deep through canyon-like walls of rock and tree roots. Considered part of the Texas Blackland Prairie (locally White Rock Prairie) and Bottomland Forest ecosystems, the creek is host to multitudes of flora and fauna. Wildlife includes river otter, mink, bobcats, coyotes, red fox, ducks, raccoons, skunks, opossum, armadillos, gray squirrels, mice, rats and eastern cottontails. Where the banks are muddy, crayfish make their conical-shaped homes. Many birds including great blue herons, great egrets and belted kingfishers make their homes along and eat from the creek, and double-crested cormorants, American white pelicans and American coots often visit the lake. Herpetofauna include lizards, water moccasins, salamanders, red-eared sliders and common snapping turtles. The most common fish are sunfish, largemouth bass, white crappie and channel catfish. The most common trees are cedar elm, pecan and green ash. Forbs are encountered in the drier areas, whereas species more adapted to flooding are found where the banks are lower, such as cottonwood, black willow, American elm, sycamore, hackberry, chinquapin oak, and Texas buckeye. Berry-producing trees along the creek include yaupon holly, cherry laurel, fruiting mulberry, rusty blackhaw, and American sweetgum. In June 2018, over one million gallons of untreated waste