Climate change appears to be altering the flow characteristics of Puget Sound salmon streams, and the outcome could be an increased risk of extinction for chinook salmon, according to a new study.
I’ve long been interested in how new housing and commercial development brings more impervious surfaces, such as roads, driveways and roofs. The effect is to decrease the amount of water that infiltrates into the ground and to increase surface flows into streams.
Stormwater experts talk about how streams become “flashy,” as flows rise quickly when it rains then drop back to low levels, because less groundwater is available to filter into the streams.
The new study, reported in the journal “Global Change Biology,” suggests that something similar may be happening with climate change but for somewhat different reasons.
Climate models predict that rains in the Puget Sound region will become more intense, thus causing streams to rise rapidly even in areas where stormwater is not an issue. That seems to be among the recent findings by researchers with NOAA’s Northwest Fisheries Science Center and Washington Department of Fish and Wildlife:
“Over the last half century, river flows included in our analysis have become more variable — particularly in winter — and these changes are a stronger predictor of chinook population growth than changes in average winter flows or climate signals in the marine environment.
“While other impacts to this ecosystem, such as habitat degradation, may be hypothesized as responsible for these trends in flow variation, we found support for increasing flow variation in high-altitude rivers with relatively low human impacts.”
Joseph Anderson of WDFW, an author of the report, told me that chinook salmon, listed as threatened under the Endangered Species Act, may be particularly vulnerable to dramatic changes in streamflows. That’s because spawning chinook tend to show up before winter storms arrive — when the rivers at their lowest levels. The fish are forced to lay their eggs in a portion of the river that will undergo the most forceful flows once the rains begin to fall.
High flows can scour eggs out of the gravel and create serious problems for emerging fry, Joe said. Other factors may come into play, but the researchers found a strong correlation between the sudden variation in streamflows and salmon survival.
In the lower elevations, where development is focused, flow variability could result from both impervious surfaces on the land and more intense rainstorms. Efforts to infiltrate stormwater into the ground will become even more important as changes in climate bring more intense storms.
Stormwater management is an issue I’ve written about for years, including parts of last year’s series called “Taking the Pulse of Puget Sound.” See Kitsap Sun, July 16, 2014. Rain gardens, pervious pavement and infiltration ponds are all part of a growing strategy to increase groundwater while reducing the “flashiness” of streams.
Other strategies involve restoring rivers to a more natural condition by rebuilding side channels and flood plains to divert excess water when streams are running high.
According to the report’s findings, the variability of winter flows has increased for 16 of the 20 rivers studied, using data from the U.S. Geological Survey. The only rivers showing less variability were the Cedar, Duwamish, Upper Skagit and Nisqually.
The effect of this streamflow variability was shown to be a more critical factor for chinook survival and growth than peak, total or average streamflow. Also less of a factor were ocean conditions, such as the Pacific Decadal Oscillation and related ocean temperature.
Eric Ward, of Northwest Fisheries Science Center and lead author on the study, said many researchers have focused attention on how higher water temperatures will affect salmon as climate change progresses. High-temperature and drought conditions in California, for example, could damage the organs of salmon, such as their hearts.
Salmon swimming up the Columbia River and its tributaries could encounter dangerously warm waters as they move east into areas growing more arid. Some salmon species are more vulnerable to temperature, while streamflow may be more important for others. Coho salmon, for example, spend their first summer in freshwater, which makes extreme low levels a critical factor.
Eric told me that further studies are looking into how various conditions can affect each stage of a salmon’s life, conditions that vary by species. One goal is to build complex life-cycle models for threatened species, such as chinook and steelhead, to determine their needs under the more extreme conditions we can expect in the future.