Struggling salmon populations in south Snohomish County took a beating last week when normally tranquil streams such as North Creek and Little Bear Creek filled to overflowing, biologists said.
“Floods and high stream flows are part of the natural cycle,” said Ralph Svrjcek, water quality specialist with the state Department of Ecology.
But flooding fueled by fast-moving water rushing off parking lots, sidewalks and rooftops is not, he said.
Those flows likely washed out dozens of redds, the salmon nests where eggs laid earlier this fall were trying to grow into young fish, he said.
Near-record rain fell across Snohomish County on Dec. 3, especially on the south half. Trees, duff and soil normally would absorb much of that water. That can’t happen, however, when nearly half of a watershed is covered with impervious surfaces. Experts said that is the case with many south county streams.
“We’re dealing with more water at the ground level than we ever have before,” Svrjcek said.
Salmon redds often are buried under cobbles and rocks in faster-moving sections of streams, he said. That helps protect them from strong flows and also exposes them to water with high oxygen content. If the water moves too fast, however, even the bigger pieces of gravel get washed downstream.
“When the bigger rocks go, salmon eggs say, ‘I’m going with the flow, I’m gone,’ ” Svrjcek said.
Along with the pebbles and gravel, major amounts of sediment are washed into the water column, Svrjcek said. That silt can choke salmon redds.
Survey crews already were out assessing salmon redd damage as the streams recovered from last week’s storms, said Darren Friedel, a spokesman for the Washington Department of Fish and Wildlife.
Still, little is known about how the eggs fared. A better picture will emerge when any surviving young fish migrate out to Puget Sound this spring. The state will do migration surveys at that time, Friedel said.
The real impact won’t be known until the still-unhatched salmon return as adults to mate several years from now, he said.
“We do know these flood events can scour river channels, washing away salmon eggs and also affect salmon rearing in the streams,” Friedel said.
Newer and tougher rules regulating how much untreated storm water flows into creeks is helping to lessen the blow dealt by urban runoff, said Tim Walls, a senior planner for the county’s surface water management division.
He said it’s encouraging that stricter development regulations require more storm water runoff be contained at building sites, allowing it to percolate into the ground instead of flushing directly into storm drains and then into creeks.
Still, those retention ponds only capture some of the water that runs off of all the paved surfaces in a development during a flood, essentially slowing but not stopping floodwaters.
Low-impact development techniques are being promoted as well, but they’re often not required. Those include better on-site water retention and concrete structures that are designed to allow water to reach the ground below.
Walls cautioned that it will take time to see progress because the tougher rules are just now going into place. Also, salmon live away from the streams for most of their adult lives, and so it takes many years before their populations bounce back.
Some fish experts say still-tougher regulations are needed to truly protect fish populations in urban areas, especially the ones on the verge of dying out.
“If we want to have salmon in our urban streams, we really should be requiring new development to be constructed using low-impact design techniques,” said Tom Murdoch, executive director of the Everett-based Adopt-A-Stream Foundation.
He said builders should be required to build up to reduce the footprint of structures, to build using porous concrete, to incorporate green roofs and rain gardens and to design projects to contain all storm water runoff on their property. He said it’s also important to make changes to existing developments, where thousands of drains flush urban runoff right into streams.
Pilchuck Audubon Society member Bill Lider of Lynnwood said special rain-controlling pavement at his house, combined with small water retention ponds and vegetation, kept Monday’s heavy rain from flowing off his property.
“I recorded 4.15 inches of rain in 24 hours at our house and we had zero storm water runoff from our property,” Lider said.
Reporter Lukas Velush: 425-339-3449 or lvelush@heraldnet.com.
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