Mat-Su Borough looks to culverts to improve fish habitat


WASILLA, Alaska (KTUU) - Old culverts have created difficulties for fish in small and medium streams across Alaska, but improved infrastructure is being installed to improve fish habitat.

"In the stream what you'll see a lot of they'll put in the smallest culvert that they can get away with when they built the road because people just weren't thinking about fish," said Gillian O'Doherty, Fish Passage Improvement Program biologist with Alaska Department of Fish & Game. "So when a lot of these roads were built, they would just put in a four or six foot culvert or a series of them, and what that does it is kind of like taking a garden hose and putting your finger on the end."

In addition to increasing the flow creating challenges for juvenile or weaker swimming fish, the culverts also created a drop-off over time that required fish to jump into the culvert to move upstream.

Additionally, the culverts were bare metal, which froze earlier and thawed later than the adjoining portions of the stream.

"Which means that fish that are trying to reach overwintering grounds or are trying to out-migrate as smolt won't be able to go through that culvert because it's still blocked with ice," O'Doherty said.

In partnership with ADF&G and US Fish and Wildlife Service, the Mat-Su Borough has replaced around 115 culverts with new, wider culverts. The new design also is lined with rock and gravel, creating a continuous stream bed.

"The smaller systems people think 'oh they're really not that important,' but the overall mileage of the small systems in most areas is seven to eight to 12 times the amount of stream miles of bigger systems. And a lot of the over wintering areas, fish like to overwinter in areas with groundwater, they like overwinter near beaver ponds and wetlands and a lot of fish smolt out of those areas in the spring," O'Doherty said.

O'Doherty says while salmon are the primary interest driving the culvert improvement projects, other aquatic organisms will benefit.

"What we expect it to do is increase the overall resilience of the fisheries populations. It's very difficult in a small system like this to tie it to an increased production of adults, but we know now that several culverts have been replaced in Cottonwood Creek and there's plans to replace more. So fish really have unimpeded access to this entire habitat stretch. So if something bad happens in another creek or there's another impact, we have this population reserve which is really, really important up here," O'Doherty said.

In addition to the improvements for aquatic life, the updated culverts will also improve the infrastructure for road traffic.

"Before this project was done, there were to undersized culverts. They caused a restriction in flow that increased velocity through the pipe and in the winter time it caused ice problems where the ice would back up and come over the roads," Mat-Su Borough Public Works Project Manager Alex Senta said.

The wider design of the culverts allows debris to pass downstream without being caught at the culvert. They are designed to handle a 100-year flood.

"The other benefit when we replace culverts like this, it helps the infrastructure over the top of the road by protecting it from flooding and icing and some other things," Senta said. "Essentially, it will improve the safety of the road above it for anyone using the road."

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