Off-channel habitat created in a river’s flood plain – sloughs, beaver ponds, wetlands and side channels – can play an important role in salmonid production.
This habitat can improve growth and survival of young salmon, reduce the competition for food and space, and keep larger predators away.
Flood plains contain a diversity of habitats and have higher salmonid productivity than areas of continuous flow, according to a recent study, but the productivity of a side channel is determined by its connectedness to the main river channel.
Historically, off-channel habitat in floodplains made up to 84 percent of potential habitat for fish in the Columbia River basin. However, the availability of this type of habitat has been reduced by channelization, diking, water diversions, roads and other human activity, and that has led some to focus on stream restoration projects - dike removal and adding wood to streams - that reconnect off-channel areas, restore floodplain connectivity and recreate habitat.
“Fish use of side channels varied with the hydrological connectivity with the main stem river,” said Patrick J. Connolly, research fishery biologist with the U.S. Geological Survey. “If a side channel becomes disconnected, it may still have some real value to fish productivity and can contribute to life history diversity based on food, temperature, and habitat dynamics.”
Connolly and Kyle D. Martens, a fisheries biologist with the USGS Western Fisheries Research Center, Columbia River Research Laboratory, published their study, “Juvenile Anadromous Salmonid Production in Upper Columbia River Side Channels with Different Levels of Hydrological Connection,” in the April 30, 2014, Transactions of the American Fisheries Society (http://www.tandfonline.com/doi/abs/10.1080/00028487.2014.880740#.U5oi4_ldVqU).
The study looks at off-channel habitat and the impact on steelhead, coho salmon, and spring and summer Chinook salmon in Washington’s Methow River watershed.
Connolly said that the U.S. Bureau of Reclamation has teamed with multiple partners to conduct stream restoration for ESA-listed species throughout the western United States. In the Methow River watershed, the targets for stream restoration efforts have been for spring Chinook and summer steelhead. Most of the restoration projects have been to reconnect the river with its floodplain, “which serves to reawaken old side channels and to form new side channels through time,” he added. Most of these projects in off-channel areas have gone unstudied.
“The genesis of our work was funding from Reclamation to assess the fish response to these restoration actions,” Connolly said. “Our paper describes our pre-treatment findings, which focused on classification of side channels based on their hydrology and fish use of the classes of side channels identified.”
They studied the habitat and sampled fish in ten side channels in the upper Methow and Chewich rivers. All were low gradient, contained less than 25 percent of the water from the main channel during periods of low flow and were in unconstrained reaches. They were grouped by their level of connectivity to the main channel. The three categories studied were six side channels connected only during high spring flows (seasonally disconnected), two side channels that remained connected but only at the top or bottom of the side channel year round (partially connected), and two side channels that remain connected to the main stem of the river year round (connected).
According to the researchers, juvenile steelhead, juvenile Chinook and sculpin were the only species found in all types of channels. Coho were commonly found in seasonally disconnected side channels, but were mostly absent or found in low numbers in partially connected and connected side channels.
Steelhead and Chinook in seasonally disconnected channels were, respectively, 63 millimeters and 70 millimeters in size. They were 56 mm and 79 mm in connected side channels and 63 mm and 77 mm in partially connected size channels. Differences in length were found only among Chinook in the fall and that was found to be the case only in partially connected side channels.
The researchers also looked at smolt production rates in the three types of channels. Steelhead production rates were not significantly different among the channel types. Seasonally disconnected side channels were especially important for coho where they showed the highest production rates.
Generally, juvenile fish in deep pools survived better than in shallow pools less than 100 cm in depth in all channel types. For pools less than 100 cm in depth in seasonally disconnected side channels, the reasons for low survival are likely low dissolved oxygen, predation, lethal water temperatures, available food and lack of water.
Steelhead and coho favored total cover and multiple types of cover, while Chinook favored the presence of in-stream large wood debris, the report said.
All the side channels had few or no predatory fish, such as trout. The study concludes that the lack of these fish in the seasonally disconnected side channels could represent a refuge for the juvenile salmonids when compared to channels that maintain year round connection to the main river.
While the study did not consider adults, the authors believe that partially connected and connected side channels could provide additional spawning habitat that is not possible in seasonally disconnected side channels due to the timing of the disconnection.
So, what makes a good restoration project? Pools deeper than 100 mm improved the survival of juvenile salmonids in all types of side channels. Large wood or total cover is important to fish densities for steelhead, coho and Chinook salmon. Adding wood in addition to riparian restoration actions could provide temporary structures until natural sustainable processes are restored. Well placed wood will increase pool depth while providing cover for fish and likely increase survival.
Floodplain restoration will likely be more successful, the report concluded, “if it is focused on a diversity of habitat types because habitat types have complementary values and habitat use may be species-specific.”
Whatever restoration actions are taken, they should adhere to “process-based principles and should restore the drivers of the ecosystem function, not just the symptoms of the degradation,” according to the report.
“Habitat improvement in these seasonally disconnected side channels should be recognized as a worthy restoration strategy, especially when full connectivity of side channels may not be a feasible target (e.g., through lack of water availability) or when full connectivity may present too high a risk (e.g., flooding, stream capture, bank destabilization),” the report concludes.
“The other side of the coin is that: Disconnected side channels may not contribute to fish productivity on an annual basis, depending much on the flow year, and some lack the habitat (e.g., deep pools) to sustain any fish when disconnected,” Connolly said.
Columbia Basin Bulletin
The Columbia Basin Bulletin e-mail newsletter is produced by Intermountain Communications of Bend, Oregon and supported with Bonneville Power Administration fish and wildlife funds through the Northwest Power and Conservation Council's Columbia Basin Fish and Wildlife Program.
Articles republished by The Dalles Chronicle with permission.