A small unnamed creek drains across the southern portion of The Gardens and flows into nearby Watkins Brook. The creek, the brook, and all of the Birmingham Botanical Gardens are within the upper portions of the Shades Creek watershed. Shades Creek is one of the major tributaries to the upper Cahaba River, joining it where the river crosses from Shelby into Bibb County 26 miles to the southwest. The Cahaba watershed, the only free-flowing river of its size in the state, is famous as one of the most biologically rich rivers in all of North America, hosting over 130 fish species and dozens of rare and endangered mussel, snail, and crayfish species. The Cahaba is also the source for about 25% of Alabama’s drinking water, with much of that going to the Birmingham metropolitan area. The river itself ends when it joins the Alabama River near Selma, AL. The Alabama and many other major watersheds of the state are part of the Mobile River Basin. Thus, rainwater falling on The Gardens can wind up in the Gulf of Mexico.