SPATIAL DISTRIBUTION OF INTERMITTENT AND PERENNIAL FLOW IN HEADWATER STREAMS, WASHINGTON

The Perennial Stream Survey (PSS) included 218 first-order headwater streams in Washington. The results indicate that continuous perennial flow was maintained at or near the channel head during the summer dry months (August – September). The significant results are: (1) Intermittent reaches were short (median <21 m) to nonexistent with channel head springs/seeps at 20% to 100% of the sites within a study area; (2) Downstream discontinuous-water perennial reaches were longer (median <120 m); (3) Drainage basins maintaining these reaches were small with the median areas for channel heads <12 acres, the initiation of discontinuous perennial flow <12 acres, and continuous perennial flow <46 acres. The proximity of perennial water to channel heads has important forest practice implications in Washington.

The Forests and Fish Report establishes default basin areas (DBA) to define the initiation of perennial water on the non-federal forestlands in Washington. The DBAs were established from limited data at 300 acres for east of the Cascade crest, 52 acres for west of the crest, and 13 acres near the coast. The DBAs can be changed through the adaptive management process in Washington. The PSS arose from that process.

The Cascade Crest separates the drier eastern part of Washington from the wetter western portion and all data were similarly stratified. The basin areas required to maintain late-summer perennial water at Pd have a lognormal distribution. The median basin areas for maintaining perennial flow east (median=12 acres) and west (median=5 acres) of the crest are different (á=0.10) from each other and from the DBAs. These differences result in the misclassification of perennial reaches as intermittent (seasonal) by 1,100 m on the eastside and 375 m on the westside. Because buffering requirements are more stringent for perennial water than seasonal water, this discrepancy could be affecting salmonids recovery efforts in Washington. The implications of these results presently are being considered through the adaptive management process.