Thursday, April 15, 2021

Understanding CDFW Instream Flow Recommendations

The California Department of Fish and Wildlife (CDFW) released Draft instream flow recommendations for the lower Ventura River and Coyote Creek in February 2021.   According to CDFW, "This information will be used to enhance flows in the watershed in several ways, including the development of flow criteria and identification of important flow thresholds for conservation, restoration, and protection of southern steelhead in the Ventura River watershed."

The Draft Flow Recommendations are summarized in the table from the draft document below:

Understanding CDFW Instream Flow Recommendations 

The CDFW Instream Flow Recommendations are the result of a combination of different analytical approaches developed over several years and published in multiple documents.  This is necessary in developing a standard approach that applies to the entire State of California, but the result can be difficult to unravel.  The outcome and implementation of these recommendations depend on a clear understanding of the approach and intent, not only within the scientific and regulatory communities, but also amongst the diverse stakeholders that currently manage and benefit from the resource.  

In order to better understand the origin of these flow recommendations, two graphics were developed to more clearly illustrate the relationships between the various criteria, methodologies, and recommendations.  These graphics and a description of the processes are shown below.

The Summary Chart consolidates all of the values published in the Draft instream flow recommendations document, each aligned by month so that they can be easily viewed and cross referenced.  At the top of this chart is the general Functional Flow diagram from the Watershed Criteria Report with the Ventura River specific values inserted for each functional flow component.  The other watershed criteria are shown in the boxes below, color coded for consistency with the CDFW documentation.  

But where did this information come from?  

The Summary of Watershed Criteria Methodology table illustrates how each of the watershed criteria were derived.  
  • Stream gage records are used to develop a statistical analysis of flows in the Ventura River which become the basis for understanding Flow Variation and Functional Flow.  
  • A computer model was used to develop the Natural Flows criteria, from which Ecosystem Baseflow and Salmonid Habitat Optimum Flows were derived.   
  • Field methods were applied to  determine Sensitive Period Indicators and Steelhead Passage Flows.

Note that due to obvious inaccuracies in the statewide "Natural Flows Database" computer model, the analysis for the Ventura River use the "synthetic least-impaired USGS gage Ventura R NR Ventura + Div 11118501 based on the period of record 1965–2007."  This data is included in Appendix A of the Watershed Criteria Report No. 2020-01 Version 2, updated May 2020.

As illustrated in the Summary Chart above, the Ventura River Flow Recommendations were derived from Steelhead Passage Flows, Sensitive Period Indicators, and Steelhead Optimum Flows.   (The other criteria were developed to provide context based on historical flows in the Ventura River Watershed.) 

Therefore, the CDFW Instream Flow Recommendations are primarily based upon the physical properties of the river as determined by Field Methods.  The exceptions are two cases where Steelhead Optimum Flows are applied, for Fall Pulse Flows in November (40cfs) and for Adult Migration (80cfs) in Reach 3 only.

Field Methods Overview

CDFW Staff surveyed a total of 22 riffle transects on San Antonio Creek and the lower Ventura River. Fifteen of the original riffle transects surveyed for this project were included in the final analysis. These transects were used to develop both Sensitive Period Indicators and Steelhead Passage Flows. 

The Wetted Perimeter Method is a field survey used to determine the Sensitive Period Indicator, which is "threshold flows below which the ecosystem is likely to be particularly sensitive to additional flow reductions and other stressors." 

Once wetted perimeters and associated flows for the streambed cross-sections are obtained for the range of important flows, a wetted perimeter discharge curve is developed by plotting wetted perimeter against discharge. The breakpoint and incipient asymptote, as thresholds of important habitat conditions, are then identified to determine instream flow needs necessary for maintaining ecological and riffle productivity flows.  The Sensitive Period Indicator flow must produce a wetted perimeter that covers at least 50% of the bankfull channel perimeter in streams up to 50 feet wide and 60–70% in wider streams.

An example of this method is shown below (taken from AppendixB) for a transect of the Ventura River.  In this location it was determined that 16 cfs is the breakpoint below which the river is sensitive to flow reduction and other stressors.

The Habitat Retention Method is used to determine Salmonid Passage Flows. This method seeks to identify flows required to permit salmonid passage across the shallowest part of a channel, the hydraulic control.  Passage Flows for juvenile steelhead must produce a mean depth of 0.4 feet at the hydraulic control, while also meeting additional criteria for wetted perimeter or flow to preserve connectivity between mesohabitat units.


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