Drainages

Aquatic ecosystems are part of a larger earth process, the hydrologic cycle. The interaction of climate with geology through erosion has created large topographic features in the Appalachian Highlands. Most areas of high elevation and relief have been formed on resistant rocks such as those of the Blue Ridge, and the major drainage systems of the region have become closely adjusted to rock type (Wallace and others 1992). The process of infiltration and runoff of precipitation drives streamflow, and interaction with geology and landforms create drainages.

Water draining from the western side of the Appalachians eventually reaches the Gulf of Mexico, while drainages on the eastern side end up in the Atlantic Ocean. Major Gulf drainages of the region are the Ohio River (New-Kanawha, Tennessee, and Cumberland River drainages) or the Alabama-Apalachicola Rivers (Coosa-Tallapoosa, Chattahoochee River drainages). Atlantic slope drainages include the Chesapeake Bay (Potomac, Rapidan-Rappahannock, and James River drainages) and those in the Carolinas (Roanoke, PeeDee, Santee-Cooper, and Savannah River drainages). The U.S. Geological Survey has delineated drainages nationwide into hydrologic units that are commonly used to summarize information about watersheds.

High-gradient streams of the crystalline Appalachians are typically dendritic. Stream density is high in this region.  Reported densities of 12 to 16 feet per acre (1.5 to 1.9 miles per square mile) (SAMAB 1996), were determined from the EPA Reach File version 3.0 (RF3), which is based on a map scale of 1:100,000.  This map does not include many first- to third-order streams (SAMAB 1996) and stream length is underestimated at this scale (Hansen 2001).  In the Ridge and Valley, or sedimentary Appalachians, streams may be dendritic, but they tend to follow a trellised pattern downstream (Wallace and others 1992). Streams are often classified by their stream order.