The Search for a “Fish-Friendly” Tide Gate

I never thought that I’d learn so much about tide gates. I’m a biologist, so I know about fish identification, important measures of fish habitat, and how to catch fish in a scientific context. But why would I need to know anything about tide gates? Well, tide gates can have a big impact on the quality of habitat and the ability of fish to enter and exit these habitats. Tide gates along with levees (or “dikes”) are used in tidal areas of the Columbia River and other coastal rivers to help keep water out of the natural floodplain of the river. Tide gates let water out when the tide is receding and block water from entering as the tide comes back in. The floodplain can then be used for cropland, grazing or other development without the threat of seasonal or daily flooding.

Old-style, top-hinge, steel tide gates are difficult for fish to get past and don’t allow any exchange of water.

The Columbia River has experienced over 100 years of flood control. Through changes in river flow from dam regulation and the installation of levees and tide gates, the amount of shallow water habitat in the Columbia River has been greatly reduced. It’s estimated that over 60% of historical tidal freshwater areas of the lower Columbia River has been lost. Much of this lost habitat is behind levees and tide gates. These areas were where young salmon grew and hid from predators before heading to the ocean. Now, these young salmon have difficulty accessing feeding areas behind the tide gates and much of the habitat behind the gates is of poor quality. The low quality of the habitat is mostly due to the lack of water exchange between the river and the slough habitat behind the gates. This can cause higher water temperatures than salmon can tolerate. These higher temperatures can also be good for non-native fish species, like largemouth bass, that will prey on young salmon.

Chinook Salmon

Fortunately, restoration is taking place in parts of the Columbia River Estuary to recover some of this lost habitat. These restoration projects can take many forms. Sometimes it is possible to remove a levee or tide gate and allow the river to spill into its historic floodplain. Other times, flood protection must be maintained, so the options available for land managers are more limited. In these situations, new tide gate designs are being tested to determine if they allow better fish passage and water flow between the river and flood plain.

Julia Butler Hansen National Wildlife Refuge was created in 1972 for the protection and management of the endangered Columbian White-Tailed Deer. Land managed by the refuge includes uplands, riparian area (land along the water), wetlands and sloughs. Restoration to improve aquatic habitats, including the replacement of older tide gates with newly designed tide gates, has been occurring since 2007. Because the primary purpose of the Refuge is to protect White-Tailed Deer, removing levees to allow natural flooding of the refuge is not an option. Instead tide gates designed to allow improved fish passage and tidal water exchange have been installed with the hope that the aquatic habitat behind the levees improve and that fish have an easier time passing the gates.

My job is to determine if these new tide gates have improved fish passage (movement through the gate) and the aquatic habitat behind the gate. One way that we do this is to capture fish that enter through the new tide gates and compare the numbers and types to those that we caught when the old tide gates were in place. We also compare the numbers and species of fish captured to sloughs without tide gates to see how closely the sloughs with the new tide gates resemble sloughs without any tide gate at all. We also look at how temperature and other things to determine if the quality of the habitat has been improved by the new tide gates. In addition to all this, we also attempt to determine how long fish stay in these areas and how fast they grow.

Trapping fish entering through new tide gate.

Biologists collecting fish with a beach seine.

We will be collecting information on fish and these habitats through the spring of 2012. At that point we expect to have a pretty good idea of whether there is an improvement in fish access and habitat quality and, if so, how much of an improvement. If results are good, these types of tide gates will likely be used in other restoration projects where some level of flood control is needed but improvement for fish is desired.

Newer-style tide gates are designed to improve fish access and exchange of water.

Submitted by Jeff Johnson