Why Culverts Are Big

The short answer is because of requirements established by the regulatory agencies in interpreting the Endangered Species Act (ESA) as well as Oregon's migratory fish passage rules.

In 1998, the National Marine Fisheries Service listed the Lower Columbia River Steelhead Trout as a threatened species. This was followed in 1999 by an additional listing of the Columbia River Chum Salmon and the Lower Columbia River Chinook Salmon. A significant loss in habitat was one reason contributing to the decline in numbers that led to the listings. Development was considered to be a major contributor to this habitat loss, and the ESA required steps to improve conditions.

Beginning in the early 20th century, "correct" engineering design was to size culverts according to the required flow capacity. This led to the relatively small culverts we are accustomed to seeing. While these culverts usually pass storm waters with no problem, they are not very accommodating to migratory fish. As a result, public agencies owning roads that cross streams now design culverts and bridges with fish passage in mind. In fact, this issue now governs the size of culverts in almost all replacements.

What used to be a 24-inch culvert is now more typically a six-foot or larger diameter culvert. What was once a six-foot culvert can now be a 50-foot long bridge. In addition to the larger sizes, many fish and wildlife amenities are incorporated into the new designs. Culverts are filled a foot or two deep with native stream backfill to provide a simulated natural bottom to allow wildlife to travel under the road, rather than on top of it. Boulders are strategically placed to provide resting shadows, features that are beneficial to fish and are common in natural streams.

While the cost of a bridge is a significant increase over a culvert, the cost of a larger culvert project is not substantially higher. The depth of the culvert governs the cost of the project more than its size, and that would be the same whether it is large or small.

That's the long answer. For more information or if you have questions, please contact Janet Oatney at 503-846-7652 or Stephen Cruise at 503-846-7683.

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