Beamer, E. and Henderson R., 2012. Fisher Slough Floodgate Report for Water Year 2011. Skagit River System Cooperative, La Conner, WA. pp. 36.

This report describes floodgate monitoring efforts for the Fisher Slough floodgate, located in the South Fork Skagit River tidal delta. Monitoring results relate to measurements of floodgate door openness, water depth, water velocity, water surface elevation, and floodgate operation for Water Year (WY) 2011, the period of October 1, 2010 through September 30, 2011. The floodgate was originally installed to provide flood protection from Skagit River floods to the Fisher Slough basin. In 2009, old barn-style doors were replaced with self-regulating floodgates designed to improve fish passage through the gates while still maintaining flood protection. We compare monitoring results to floodgate operational criteria that were established based on fish passage and flood protection needs. We also discuss monitoring results and criteria in order to provide information useful to adaptively manage the Fisher Slough floodgate, if necessary, to achieve and balance its multiple objectives of fish passage, flood protection, drainage, and irrigation. Monitoring instruments used to collect water surface elevation and floodgate door openness data were shown to collect results accurately with one exception. Instrumentation on the south floodgate door did not measure door openness accurately and should be replaced or repaired. Fisher Slough floodgate operation met all Hydraulic Project Approval criteria established for all three operational time periods in WY 2011 (see table immediately below), demonstrating good management and maintenance of the floodgate structure throughout the water year and that the floodgate’s design is synchronized with Hydraulic Project Approval criteria. The newly replaced floodgate structure is capable of achieving the criteria. While all Hydraulic Project Approval criteria were met at Fisher Slough, we are concerned whether some specific values of some criteria are appropriate for a conclusion that juvenile salmon upstream passage opportunity is adequate when criteria are met. Therefore, we offer discussion and recommendations to help with future monitoring and floodgate operation at the site. More specific or different criteria related to water velocity, floodgate door openness width, and the tidal stages when doors are open are recommended. It is likely water velocity conditions on ebb tides (especially larger ebbs) at the Fisher Slough floodgates do not allow for upstream movement of fry-sized Chinook salmon even when floodgate doors are open. We recommend monitoring water velocity as part of the floodgate monitoring plan in order to better determine the conditions that allow juvenile salmon upstream migration potential. How wide a floodgate door is open likely relates to fish passage opportunity at floodgate sites. Yet, Fisher Slough criteria related to door openness only states when doors should be open (seasonally with a minimum water elevation for gate closure) and how long they should be open (percent of time), not how wide they should be open or whether variation in openness influences fish passage opportunity. We recommend calculating floodgate door openness results stratified by tidal stage and percentage of the channel width that is available for fish movement.