Excerpt from Shenandoah National Park Long-Term Ecological Monitoring System: Section III., Aquatic Component User Manual

Geological formations were selected as the sec ond level of classification because the snp has a uniquely diverse geology and resulting water chemis try, particularly alkalinity. There are great differences among watersheds, and often between adjacent watersheds. Alkalinity and related water chemistry are probably significant factors affecting the distribu tion of the aquatic biota. Information on geological formations and alkalinity can be obtained from Gathright (1976) and Dise respectively. Streams in the snp seldom have their watersheds entirely within a single geological formation. An effort should be made to establish stream sites in watersheds which have at least 75% of their area within one of the geological formation categories.

Elevation affects the distribution of aquatic biota indirectly because of factors such as stream width, depth, current velocity, valley shading and tempera ture. The snp is located entirely within the northern section of the Blue Ridge physiographic province, which is the narrow (about 8 km) mountainous area between the low-lying Piedmont province to the east and the Ridge and Valley province to the west (hoffman 1969, Gathright Because of the narrow moun tainous, characteristics of the Blue Ridge, drainage basins in this province typically have a steep gradient. Drainage basins on the west slope follow -a trellis pattern, in which the small streams flowing down the mountains are parallel and join the main stream in the valley (shenandoah River) at right angles (reid and Wood Those on the east slope follow a den dritic pattern, so-called because of the resemblance to tree roots (reid and Wood The criterion of m (1800 ft) for distinguishing upper and lower sites was selected because field observations indi cated that streams in the snp tended to become very narrow and shallow (several m wide and less than knee deep) approximately above that elevation. This delimitation is somewhat arbitrary and can be varied within reason when it is desirable to have both an upper and lower site on the same stream. Elevation is best determined from the Potomac Appalachian Trail Club maps (scale The usgs min topographic maps are not as useful because they are not as accurate regarding trails, fire roads, and stream permanency.

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