Beamer, E. and Henderson R., 2013. Fisher Slough Floodgate Report for Water Year 2012. Skagit River System Cooperative, La Conner, WA. pp. 45.

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) 2012, the period of October 1, 2011 through September 30, 2012. 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 and tidal exchange 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 did not meet all the Hydraulic Project Approval criteria established for all three operational time periods in WY 2012. During the Spring Juvenile Chinook Migration Period two of the four criteria were met. The two criterion related to gate closure (gates open for 90% of the time; minimum closure setting of 9.5 ft NAVD88) were not achieved. The percent of time when at least one floodgate door was open was 85.48%. The floodgate doors closed 80 times during the Juvenile Spring Chinook Migration Period and 52 of the closures (65.0%) were at water levels less than 9.5 ft NAVD88. The minimum water depth and velocity criteria were met during this management time period. Criteria for both the Fall/Winter Flood Period and the Summer Irrigation Period were met. While the HPA’s water velocity criterion was met at Fisher Slough in WY 2012, we do not believe this criterion is appropriate for successful upstream passage opportunity for juvenile salmon. We measured water velocity at the floodgate site continuously for 15 tidal cycles and found water velocity conditions on ebb tides at the Fisher Slough floodgates do not likely allow for upstream movement of fry-sized Chinook salmon based on maximum velocity thresholds from literature. This suggests upstream passage of juvenile salmon (especially fry sized salmon) occurs only on non-ebb periods when the gates are open. Also, how wide a floodgate door is open likely relates to fish passage opportunity at floodgate sites. We calculated floodgate door openness results stratified by tidal stage and percentage of the channel width that is available for fish movement, and suggest the best indicators for upstream movement of fry-sized Chinook salmon opportunity at the floodgate site are: