Physical Stream Types
Classifying streams by physical (non-biological) characteristics allows researchers to examine and categorize streams by the habitat type and the variety of biological assemblages the stream can support. An examination of stream habitat types and their distribution across the region should help to inform and advance aquatic conservation work in the study area.
To perform this classification, three types of data (bedrock geology, stream gradient and watershed size) were linked to individual stream reaches using GIS. The data were divided into categories based on the effect of the variable on aquatic biota; while this habitat classification is based solely on physical criteria, it was our objective that the habitat types developed be biologically meaningful. See descriptions of these three variables below, as well as a table that shows the categories used for each.
Geology - Geology classes were based on work done by The Nature Conservancy (TNC, Anderson and Olivero 2003). TNC's research combined factors that influence water chemistry and hydrologic regime into categories based on bedrock geology types. In order to create a similar classification based on watershed geology, we decided that six geology classes adequately reflected the effects of geology on chemical and hydrological stream variables in Pennsylvania.
Stream Gradient - Stream gradient was calculated as a measure of change in elevation from the start to end of an individual stream reach. Stream segments were defined by the RF3 stream reach files, published by the U.S. Environmental Protection Agency (Dewald and Olsen, 1994).
Watershed Size - watershed area was calculated for each stream reach by summing the land area that contributes to the basin of each stream reach. We delineated four categories of watershed size that reflect patterns in biological assemblages as well as patterns in the watershed size dataset.