The impact of tide gates is a general problem that has recently generated interest among the natural resource conservation community in the Pacific Northwest, particularly considering the recent interest in the importance of estuaries in the life cycle of salmonids. The effects of tide gates on estuarine function include the elimination of upland tidal flooding and they change the velocity, turbulence and pattern of freshwater discharge that fluctuates between water stagnation and flushing flows (Giannico, Souder 2004). There are six operational tide gates in Salt River that are used in conjunction with dams and levees to keep the tide from inundating low lying areas.
Tide gates create new physical, chemical and biological environments on both sides of a levee that may not be conducive to the native biota. Physical changes that commonly take place as a result of tide gates are an increase in water temperatures and a change in channel morphology. Chemical changes that commonly take place behind tide gates include an increase in nutrient concentration, an increase in turbidity, and reductions in dissolved oxygen and pH. (Giannico, Souder 2004).
The biological impact of tide gates that is relevant to the Salt River is the obstruction of fish migration. In the case of anadromous salmonids, tide gates prevent migration and impact the quality of their habitats (Giannico, Souder 2004). The duration that a tide gate is closed, the size of the opening and the velocity of water moving back and forth through the tide gate are all factors that influence the daily and annual migration of salmonids. Another biological impact of tide gates is change in the composition and abundance of aquatic plants. The slough channels become choked with submergent and emergent aquatic vegetation. On a site reconnaissance in July, 2004 Cuttoff Slough was filled with the submergent plant, eel grass, on the upstream and downstream sides of the tide gate.