Interim instream flow criteria for the protection of fishery resources in the Scott River watershed, Siskiyou County

This document describes the methods and results of an analysis using historical flow data and regional regression relationships to develop interim instream flow criteria suitable for anadromous fish in the Scott River watershed in Siskiyou County. The Scott River watershed provides aquatic habitat for four species of anadromous fish; Chinook Salmon (Oncorhynchus tshawytscha), Coho Salmon (O. kisutch), steelhead trout (O. mykiss), and Pacific Lamprey (Lampetra tridentata). Specifically, the Scott River is one of the most important Coho Salmon spawning and rearing tributaries in the Klamath River watershed.

Instream flow requirements can be generated from flow standard setting techniques or from the results of site specific studies. The interim instream flow criteria presented for the Scott River were developed using flow standard setting techniques. Stream flow standards derived from standard setting techniques are designed to identify the environmental resource in need of flow protection, identify biologically significant criterion that can be used to measure potential flow related impacts, and specify the amount of flow required to protect the resource. Most individual standards evaluate only one or more, but not all the criterion needed to fully evaluate the flow needs of an aquatic species. This limitation can lead to prescribing a single minimum threshold or “flat-line” affect (Poff et al. 1997). The seasonal and inter-annual variability in the hydrograph must be maintained to protect stream ecology and provide an ecosystem based standard (Annear 2004).

To account for the seasonal and the inter-annual hydrologic variability of the Scott River, the Department applied a detailed hydrologic analysis along with application of three standard setting methods to evaluate the life history flow needs of salmonids in the Scott River near Fort Jones.

Adult fish passage was estimated using the equation developed by R2 Resources (R2 2008) for the State Water Resources Control Board’s (SWRCB) North Coast Instream Flow Policy (SWRCB 2014), spawning and juvenile rearing were evaluated using the Hatfield and Bruce regional equations (Hatfield and Bruce 2000), and the results were adjusted monthly based on estimates of unimpaired hydrology using Tessmann’s adaptation (Tessmann 1980) of the Tennant or Montana Method (Tennant 1975).