Physical indicators of hydrologic permanence in forested headwater streams

Recent court cases have questioned whether all headwater streams are jurisdictional waters under the US Clean Water Act. Rapid field-based indicators of hydrologic permanence are needed for making jurisdictional determinations. Our study objectives were to: 1) identify physical characteristics of forested headwater streams that best distinguish perennial, intermittent, and ephemeral reaches and 2) assess the applicability of existing rapid field-based tools for classifying hydrologic permanence across a wide geographic range. We surveyed reach- and drainage-scale characteristics at 113 sites across 10 study forests in the US. Streams in 4 core forests (61 core sites) were sampled over 2 consecutive years and were used in model construction. Streams in 6 satellite forests (72 satellite sites) were used to validate the models over a broader geographic range. Discriminant function models successfully differentiated hydrologic permanence categories at core sites. Drainage area, the Ohio Environmental Protection Agency Headwater Habitat Evaluation Index (HHEI), and the North Carolina Department of Water Quality Stream Classification Method (NCSC) were strongly correlated with the discriminant function that separated ephemeral from perennial and intermittent sites. Entrenchment ratio was the most consistent variable discriminating intermittent from perennial sites across the core forests. The models had mixed results when applied to the validation data set, but did classify correctly most intermittent and ephemeral sites. Classification trees were used to assess broad regional applicability of existing rapid field-based protocols and to identify important metrics. Scores from the Rapid Bioassessment Protocol Habitat Assessment, HHEI, and NCSC all clearly distinguished ephemeral from intermittent and perennial sites, but no differences were detected between intermittent and perennial sites across all sites. However, data from core sites do indicate that a suite of physical variables can be used successfully to identify hydrologic permanence at regional scales.