2012: Biological and Physical Effects of “Fish-Friendly” Tide Gates
Correigh Greene, Jason Hall, Eric Beamer, Rich Henderson, and Bruce Brown
A number of restoration techniques exist to counter widespread estuary habitat and connectivity loss across the Pacific Northwest, ranging from dike breaching and removal to installation of “fish-friendly” or self-regulating tide gates (SRTs). However, the physical and biological effects of these techniques have not been rigorously examined. In this report, we focus on the effects of SRTs, and examine their effectiveness in two different ways. First, we used a spatially extensive design to compare three site types: SRTs, flap gates, and unimpeded reference sites. The study compared ten SRT sites located from the Columbia River estuary north to Samish Bay in northern Puget Sound, five traditional flap gate sites (designed to drain freshwater but prevent tidal inundation and saltwater intrusion), and five unimpeded reference sites. Second, we used a temporally extensive design at three SRT sites to determine changes in upstream cumulative densities of Chinook salmon across the rearing season, relative to downstream values, before and after SRTs were installed.
In the spatially extensive study, we studied physical metrics upstream and downstream of tide gates and at reference sites during three visits spanning the primary spring-summer fish rearing period. We also sampled fish and invertebrates above and below tide gates and at reference sites. We found that site type appeared to affect a number of physical metrics including connectedness, water elevation, and temperature, but the degree to which each of these site types affected these physical metrics varied. In addition, densities of Chinook salmon (Oncorhynchus tshawytscha) and estuary rearing fish species were much greater at reference sites compared to sites with either flap gates or SRTs. For other species, overall patterns did not strongly distinguish densities between reference sites and flap gate or SRT sites.
In the temporally extensive study, the upstream/downstream ratio of Chinook salmon cumulative density at all SRTs was higher than at a traditional flap gate. The cumulative density ratio at this site increased 6-fold after a passive flap gate was replaced with an SRT, indicating that SRTs can improve habitat use by salmon. However, cumulative density ratios decreased 7-fold when a passive and manually manipulated side-hinged gate was replaced with a SRT, and this measure at all three SRT sites was an eighth to a tenth that of reference channels.
Together, these findings indicate that SRTs vary substantially based on design and operation and consequently vary in performance, depending upon the metric of interest. For estuarine-dependent species in general and juvenile Chinook salmon in particular, SRTs support habitat use above gates much less than natural channels and a little better than traditional flap gates. For other anadromous salmon species that may spawn in creeks above tide gates, SRTs do not appear to strongly inhibit passage or juvenile rearing density. These findings suggest that estuary restoration with SRTs will have limited benefits for juvenile Chinook salmon and other estuarine-dependent species, but can result in some improvement in connectivity and rearing habitat quality compared to traditional flap gate designs. SRT designs and operation standards that maximize connectivity, and site selection criteria that focus on reconnection of large amounts of habitat may overcome some of the limitations of reduced habitat use associated with SRT installation. These potential reductions can successfully be evaluated by comparing the benefits of SRT installation with those of other estuary restoration techniques (e.g., dike breaching or setback).