Tongass engineers improve vehicle, fish travel

Tongass National Forest engineers recently worked with southeast area contractors to install two new bridges, one in Hoonah and one on Prince of Wales Island, replacing fish passage barrier culverts. Fish passage at stream and road intersections to ensure unimpeded fish migration is an important forest road consideration. 

Hoonah Bridge 130 foot long fish barrier culvert being removed

The Hoonah Forest Road 8502 fish barrier culvert was removed.

“We upgrade to bridges when the stream crossing needs to have more stream width than a culvert can afford,” said Leonard (Brad) Job, P.E., Engineering Staff Officer for the Tongass National Forest. “Bridges put the stream back into its natural bed.”

A 112-foot bridge with guard and approach rails replaced a fish passage barrier culvert on Hoonah Forest Road 8502 mile at marker 8.39, a route to Freshwater Bay.

In addition to the bridge installation, construction included a French drain to limit water seepage/ice formation in the road, and brushing away organic materials and removing rock knobs to increase visibility. The work took three weeks and cost around $900,000.

“The road is used by the community to access hunting, fishing, day-use picnicking, overnight camping, sightseeing, wildlife viewing, photography, and gathering forest products; as well as outfitter guide operations that conduct road based nature tours and guided fishing,” said Basia Trout, Hoonah District Ranger.

A similar but smaller project was completed on Prince of Wales Island. A 40-foot bridge was installed to replace a fish passage barrier culvert on Forest Road 3000 000 (Exchange Cove Road) at mile marker 84.181, northeast of Whale Pass. This project took three days to complete.

Between 1998 and 2019, the Tongass has re-installed, retrofitted or removed approximately 617 crossings that were not meeting passage standards in fish streams, and potentially impeding fish passage. Approximately 78 percent of the reinstallations were replaced with culverts, 20 percent were replaced with bridges, and two percent were retrofits or maintenance.

“Site conditions will dictate the appropriate replacement structure; and more importantly the effect the structure has on stream function and form,” said Heidi Lombard, lead fisheries biologist for aquatic organism passage on the Tongass.

Bridges tend to be more expensive than a stream simulated culvert; but they typically do not constrict the channel to as great of a degree, letting flood borne debris pass, allowing for vertical movement of the streambed, and permitting light throughout the crossing.

Hoonah bridge - completed bridge

The completed bridge on Hoonah Forest Road 8502 allows the stream to flow in its natural bed.

“Bridges are also more likely to facilitate passage of terrestrial species using the stream corridor for travel,” added Lombard.

Improperly located, installed, or maintained stream crossing structures can restrict fish migrations and adversely affect fish populations. The most common structure obstacles are excessive vertical barriers, debris blockages, and extreme water velocities that can inhibit fish passage, especially for smaller or juvenile fish.

Fish and aquatic resources on the Tongass National Forest offer subsistence, commercial and sport fisheries opportunities by providing spawning and rearing habitat for the majority of fish produced in Southeast Alaska. Upstream and downstream movement is essential for many fish species in the Tongass National Forest.

The Tongass National Forest has identified and surveyed 3,682 fish stream road crossings along approximately 5,000 miles of forest roads. Thirty-five percent of the Forest crossings are used by fish to migrate from the sea to freshwater spawning grounds, and 65 percent are resident fish streams.

Exchange Cove road and culvert

Exhange Cove road before bridge installation with culvert under road.

Showing new bridge with water running under it

Exchange Cove bridge allowing water to flow under the bridge.