Drainage Pattern : The land area whose runoff drains through a single stream is called that stream's DRAINAGE BASIN. The area outside the Roney Building at GSW, for instance, provides runoff to the small stream that drains College Lake, and so is in the drainage basin of that stream. That stream feeds Willet Creek a few dozen meters downstream and so we are also in the Willet Creek drainage basin. Willet Creek is tributary to Mill Creek, Mill Creek to Muckalee Creek, Muckalee to the Flint River, and the Flint to the Appalachicola, which drains into the Gulf. The Roney building therefore sits in the drainage basin of each of these streams in turn. The high ground that separates two drainage basins is called a DRAINAGE DIVIDE. The Pelham Escarpment (a hill which one climbs south of Cordele on I-75) separates the drainage basins of the Apalachicola system from that of the Suwannee system, and so is the drainage divide between those basins. How water drains out of an area depends upon the bedrock geology of that area. There are numerous patterns that drainage systems follow; we will examine only four of them as summarized in the diagram below. The next page shows examples and discusses each in more detail. ENDRITIC drainage arises on rocks that are consistently resistant to erosion and/or flat-lying. As valleys develop they erode their way headward in random directions (always uphill, of course). Because "uphill" is, in general, away from the mouth of the stream, the pattern of tributary branching tends to resemble the upward branching of a tree. The name comes from the Greek rootdendros which means "tree". The southernmost half or so of Georgia is called the Coastal Plain. This region is underlain by horizontally bedded, young, soft sediments and this is why dendritic drainage has formed here. Much of the rest of the state, excepting only the northernmost counties, is called the Piedmont. This is a stage 3 ("old") erosional surface on metamorphic and igneous rocks that are, for the most part, consistently resistant to erosion. This is why dendritic drainage has formed here. Some parts of the Piedmont, the Blue Ridge (northeastern corner) and Valley and Ridge (northwestern corner) are not geologically like this and so may have different drainage types. A different type of drainage forms on sedimentary rocks that are tilted or folded and that have different susceptibilities to erosion. In such a situation the more resistant rocks tend to stand as high ridges whereas the more easily eroded softer rocks are cut down to form valleys. Both the ridges and the valleys are long parallel topographic features, running along the bands where the tilted beds intersect the Earth's surface. Such topography is called "valley and ridge" topography. The province of that name in northwest Georgia has such a landscape. Where rocks are consistently hard but have been fractured by faulting or other processes a third type of drainage pattern forms. The fractures tend to occur in two sets that at some angle, sometimes 90°. The fractured rock is usually easier to erode that the pristine rock and so the streams in the region tend to follow along them, also meeting at consistent angles. In general this drainage type is called ANGULAR, but if the fractures and streams do meet at approximately right angles it can be calledRECTANGULAR DRAINAGE. The final drainage type occurs on circular mountains and round uplifted regions called "domes". Most truly circular mountains are of volcanic origin, and so RADIAL DRAINAGE is usually associated with volcanoes. The map below shows the northern of a pair of small volcanic cones (the Menan Buttes) in southeastern Idaho. The closed depression at the top is, of course, the volcanic crater. even though there are no permanent streams that drain this mountain the "v's" on the contour lines clearly show the locations of dry washes, and these drain away from the summit, as would be expected.