A Gated Community: How Flood Prevention Affects Fish Migration

When the state of Oregon was settled more than 150 years ago, its coastal areas were brackish marshes and wetlands prone to flooding. Draining these areas benefited pasture grazing in a state covered by lush timber forests. The delicate balance of the environment was poorly understood by early settlers, and the seemingly limitless abundance of natural resources provided few incentives for caution.
As a result, it has taken a century for the dots to be connected between timber, pastures, marshes, and one of the iconic animals of the Pacific Northwest: salmon.
In the early 2000s, in a favorable year, Oregon coastal salmon runs were between 11% and 19% of historical levels (defined as before 1850). In a bad year, of which there were many because of pollution, runs were between 3% and 6% of past highs. Other stocks along the Pacific Coast have similar, and sometimes worse numbers. While exact estimates vary, the EPA and ecologists state with certainty that some fish stocks have gone extinct and many more remain threatened.
Environmental changes have many causes that interact in complicated ways, but perhaps no single invention encapsulates Oregon’s history of environmental trade-offs and their effect on salmon better than the tide gate.
Tide gates, also known as flood boxes, have been used since the middle ages for flood protection and mosquito control. While low-lying areas can be protected from river overflows by berms, dikes or levees, they may still be affected by water flows from upstream sources. In these places, there needs to be a way for water to drain out into the river, but not flow back into the lowlands and flood. Tide gates, which are hinged flaps that cover drainage outlets, are rigged so ebbing water can open the gate and escape, but flow (incoming) tides shut the gate. And while tide gates have been effective at controlling lowland flooding, their wider implications are poorly understood.
“There isn’t a whole lot of science behind tide gates,” said Ryan McCormick, a fish pathogen screening engineer for the Oregon Department of Fish and Widlife.
The tension between competing land use needs is illustrated by a unique project on the Siuslaw River. An old drainage ditch running parallel to the Siuslaw River keeps surrounding real estate from being flooded. A 4-foot-wide pipe covered by a tide gate allows for outgoing water flow but prevents river water from inundating homeowners. The partly natural, partly man-made channel serves as a resting area for fish seeking a refuge from the high turbidity of the Siuslaw waters.
McCormick, who lead the project and instrument installation, said that the goal is to “understand how this particular tide gate creates hydraulics” that may or may not be good for fish. Instrumentation on both sides of the 6 pipe measures water depth, temperature, salinity, and levels of dissolved oxygen and pH. The units are set to take simultaneous measurements every 30 minutes. Inside the drainage pipe, a SonTek Argonaut unit measures water depth, velocity and temperature. Ron Nauman, of HydroScientific West, which supplied the equipment, said there is also a tilt sensor to measure the angle of gate opening. Read on for more.