PORT ANGELES — The series of storms that drenched the North Olympic Peninsula earlier this month changed the course of the Elwha River, carried away entire sections of sediment islands and moved the river closer to its original channel in some places, said Andy Ritchie, Elwha restoration project hydrologist.
“Go and see it now. It’s a really exciting time to be watching the river. It won’t be the same next time you see it,” Ritchie said.
In a set of three storms that drenched the area from Dec. 9 through Dec. 11, about 1.5 million to 2 million cubic yards of sediment were removed from the former Lake Mills and Lake Aldwell lakebeds, and the river uprooted huge swaths of young trees, he said.
He said the amount of sediment is only an estimate based on the area of land that is missing, and a computer mapping program is expected to provide exact numbers later this week.
When the Elwha and Glines Canyon dams were removed, mapping showed about 34 million cubic yards were trapped behind the dam, starving nearby beaches and the river’s gravel bars.
Since the removal of the Elwha Dam in 2012 and leading up to the storms, more than 3 million cubic yards of sediment has been deposited at the mouth of the river, recreating beaches that existed before the dams were built in the early 20th Century.
Ritchie has been mapping the river and the sediments trapped behind the two dams, as well as their effect on the river downstream.
The river has been quiet for the last three years as flows remained low and lacked the power to transport large amounts of sediment.
The meager summer flows, which have dropped to only a few hundred cubic feet per second, settled into the deeper areas, many already lined with river cobbles, leaving the higher sediment terraces untouched.
“Ninety percent of the action happens during 1 percent of the flows,” Ritchie said.
The “three-year storm,” series that hit the North Olympic Peninsula raised the Elwha’s flow to 17,100 feet per second and kept it high for more than a day, which kept the sediment rolling, he said.
Ritchie said that rivers usually rise with each storm to scour exposed riverbanks then drop back to the established riverbed.
The series of three storms kept the river high enough to continue removing loose sediment much longer and enabled the river to make serious headway in clearing parts of the floodplain.
“Everyone was excited about this storm. It was the first chance to see what the river can do,” Ritchie said.
However, he said the trio of storms was minor compared to the five-year storms that swell the river to 21,000 cubic feet per second and 10-year storms that raise the river’s flow to 26,000 cubic feet per second.
Five and 10-year floods will make the biggest impacts, washing away even larger sections of sediment, he said.
In 2011, Lake Aldwell and Lake Mills were drained in preparation for the $325 million Elwha River restoration project to remove the Elwha and Glines Canyon dams.
The Elwha Dam was fully removed in 2012, and the last portions of the Glines Canyon Dam were demolished in August.
Salmon have already been seen swimming above both dam sites — giving them access to a total of 70 miles of fish habitat that had been closed off by the dams for 100 years.
The most obvious change after the storms was in the sediment terraces at the former Lake Aldwell.
Much of the former Lake Aldwell delta is simply gone, and the river, which has been flowing among large tree trunks on the east side of the river valley, is finding its way back to its original bed on the west side of the valley in an area previously buried in deep sediment, Ritchie said.
“The river has excavated the sediment and turned it back into a flood plain,” he said.
Ritchie said future floods will likely remove most of the remaining delta material, and the river will eventually return to the western side of the valley.
“It’s working its way down the original slope,” he said.
The section of delta that was partially washed away was already in use as a recreation area, and was featured in guided tours by U.S. Park Rangers from Olympic National Park who showcased the rapidly changing area where 12-foot saplings shaded the remnants of a long-sunken rowboat.
An infant forest of young trees, many of them naturally re-seeded in the area, and some trees planted by the program’s revegetation program, was also washed away in the flood.
Joshua Chenoweth, revegetation project director, spent more than a year on that delta attacking reed canary grass — a harmful invasive species that threatened to take over and prevent native species from moving in to the habitat.
Because of Chenoweth’s work in containing the invasive, the river washed natural ash and alder saplings downstream instead of picking up mats of reed canary grass and spreading the plant.
“My hat is off to him,” Ritchie said of Chenoweth efforts.
Ritchie said that it takes only an inch of the weed’s red root to establish a new plant in cracks and crevasses downstream.
If reed canary grass had taken hold at the site and later washed into the river by the floods, the weed would likely have spread and taken hold in multiple locations in the six-mile stretch between the delta and the Strait of Juan de Fuca, he said.
Visitors can hike through the new landscape left by the storms.
Trails through the former Lake Aldwell bed on the west side of the river can be accessed from the former boat ramp or from the dam site parking lot at Lower Dam Road, just off state Highway 112 at the north end of the former lake.
The east side of the river can be reached through a trail from a parking area off U.S. Highway 101 just east of the Elwha River bridge.
Dogs on leashes are permitted. No park admission pass is needed.
In the former Lake Mills, above the remnants of the Glines Canyon Dam, some of the sediment terraces are so high the river may never reach them, but sediment closer to the river’s usual levels is being steadily eroded, Ritchie said.
Currently, the river meanders around a narrow peninsula of sediment piled high just above the former dam site, but the tongue is eroding, and eventually the river’s path will remove it, he said.
With each storm, the water has eaten away at the southern portion of the terrace, narrowing it and causing the river to eat deeper into the sides of the valley.
Access to the Glines Canyon overlook, formerly the walkway over the Glines Canyon Dam spillways, can be reached via Whiskey Bend Road.
Park admission is required.
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Reporter Arwyn Rice can be reached at 360-452-2345, ext. 5070, or at arice@peninsuladailynews.com.
