Overall Habitat Suitability
Four factors (canopy density, pool depth, pool shelter complexity, and substrate embeddedness) were used in the EMDS-based analysis to determine overall habitat suitability using habitat typing data collected from two separate time periods: 1990 to 1999, and 2000 to 2010. Suitability scores were calculated by assessing how measured values compared to target values for each factor. Overall habitat suitability and suitability of each factor used in the analysis were calculated based on a weighted (by reach or stream length surveyed) average for each subbasin in each time period, and the change in suitability values between time periods was compared for streams and reaches in each of the three subbasins, and for the entire SF Eel River Basin.
In the EMDS-based model, the lowest, most limiting value of each of the four factors was used to indicate the potential of the stream reach to sustain salmonid populations. In that sense, the model mimics nature. For example, canopy density (riparian vegetation score) is evaluated with an “in channel score” (a combination of pool depth, pool complexity, and substrate embeddedness factors), at the final decision node. The lower of the two scores was used, and in SF Eel River streams, in channel scores were almost always lower than canopy density scores. Therefore, canopy density scores were often not used as the final indicator of a stream’s potential to support salmonids.
Suitability scores calculated from factor values ranged between +1 and -1, and were divided into four categories:
1.00 - 0.50 (high suitability)
0.49 - 0
-0.01 - -0.49
-0.50 - -1.00 (low suitability)
For a detailed discussion of the analysis framework and calculation of suitability scores, see the Analysis Appendix.
Overall suitability scores improved between the 1990s and early 2000s in the Northern and Western Subbasins, and in the entire SF Eel River Basin (Table 1). Increases were due primarily to improved embeddedness scores. Eastern Subbasin overall suitability scores were lower in 2000-2010 than in 1990-1999, and were in the lowest suitability category (-0.5 - -1.0) during both sampling periods (Figures 1, 2). Reduced suitability in the Eastern Subbasin is primarily due to a decrease in pool shelter complexity scores between the two sampling periods, which resulted in low pool quality scores. Although most factor suitability scores improved over time, overall suitability was low (negative) in all subbasins and in the Basin as a whole during both sampling periods.
The watershed assessment report presents information on overall suitability, and the suitability of each individual factor (canopy density, pool depth, pool shelter complexity, and substrate embeddedness used in the analysis for the entire SF Eel River Basin, and for each subbasin.