Methodology and Interpretation

Percent Forest within 120m of Streams
The percentage of forested land-cover within 120 meters of streams is calculated by summing the total number of forest land-cover cells underneath stream segments in the reporting unit and within a four cell buffer (120 meters) and dividing by the stream corridor's total land area (i.e., all cells within 120 meters of streams minus those classified as water). Cells inside the cell buffer zone but outside of the reporting unit boundary are ignored. Forested areas contain plants that are capable of uptaking, accumulating, and storing some of the chemical constituents in runoff (e.g., nitrogen from fertilizers) and may reduce the amount of soil erosion, thus reducing downstream sedimentation and transport of phosphorus to streams in runoff. In situ studies of these phenomena suggest that the runoff ameliorating effects of forest near streams occurs within an approximate 100-meter zone on the banks of rivers and streams.

Quantile: Each class contains an approximately equal number (count) of features. A quantile classification is well-suited to linearly distributed data. Because features are grouped by the number within each class, the resulting map can be misleading, in that similar features can be separated into adjacent classes, or features with widely different values can be lumped into the same class. This distortion can be minimized by increasing the number of classes. For continuity of the browser content, and consistency among maps, legend gradients are from higher values (red) to lower values (green).

Metric input GIS data:

• 244 Subwatersheds - Metadata

• 2003 Land-Cover - Metadata

• Stream Network - Metadata