by Joshua Chenoweth
The historic dam removal on the Elwha River, the Elwha Ecosystem Restoration Project, is well underway. The Elwha Dam, the older and smaller dam (originally 105 feet), has already been removed and its reservoir, Lake Aldwell, has drained. As of July 1, 2012, seventy-five feet of Glines Canyon Dam, the larger of the two (originally 208 feet), has been removed. The reservoir formed by Glines Canyon Dam, Lake Mills, is more than halfway drained. As a result, we are witnessing the first full growing season in the newly exposed lakebeds!
Revegetation of the nearly 800 acres that will eventually be exposed is a primary goal of the Elwha Ecosystem Restoration Project. The Elwha Revegetation Project is a joint effort between Olympic National Park (ONP) and the Lower Elwha Klallam Tribe. Revegetation will be challenging, since the reservoirs have been accumulating inorganic sediments for nearly 100 years. Just under half of the sediments are fine-textured silts and clays. The fine sediments are 1-5 feet thick on the upper valley slopes of the former lakebed (Figure 1) and as much as 40 feet deep on the reservoir floor, the future valley bottom. Along the original shoreline the sediments are shallow, less than an inch at some sites. The only known plant life forms to colonize fine-textured surfaces are graminoids (rushes, sedges and grasses). The other half of the sediments are coarse textured (sands, gravels, and cobbles). Most of the coarse sediments are in Lake Mills and form the Lake Mills delta, which was nearly a mile long and 70 feet thick prior to the start of dam removal. The slow pace of dam removal is designed to allow the river to erode the delta forward into the reservoir, leaving 10-20 feet of sands and gravels covering the entire valley bottom, which is more than 2 miles long! This advancing delta will cover the existing layers of fine lake sediments that are 10-40 feet thick on the reservoir floor, creating new terraces 20-60 feet deep. Once dam removal is complete, the river will cut though the terraces and create a relatively wide floodplain on the original valley floor. However, Lake Mills inundated a wide valley, and remnant terraces will remain after dam removal is complete. We estimate that 30-40% of the valley bottom (130-175 acres) will be covered in deep terraces of lake sediments. Already terraces more than 20 feet high have formed during the early stages of dam removal (Figure 2). Thick terraces high above the water table and deep layers of silt and clay will be challenging surfaces for plant establishment.
In addition to the unique substrates, much of the landscape will be far from seed sources, meaning the seed rain density and diversity will be low. The revegetation program is designed to introduce genetically appropriate native species at sites far from seed sources and accelerate native forest development. Native forests are important to minimize invasive species establishment, repair ecosystem processes (i.e. hydrology) and provide suitable fish habitat along the river and its tributaries within the drained reservoirs. This can be achieved by seeding forbs and grasses, planting rooted native woody species, and enhancing natural revegetation by placing large woody debris onto thick terraces of sediment. We expect natural regeneration will be substantial within 150 feet of surrounding forests due to the close proximity to seed sources and shallow layers of sediment.
The revegetation effort began a decade ago when ONP started collecting and storing seed from the Elwha. Last fall, September 2011, dam removal began, and the receding reservoirs created our first opportunity to begin planting. Our approach was to plant as many different native species as possible to determine which plants might tolerate these unique conditions. We planted 30,626 native plants representing 47 species. Fourth Corner Nurseries produced 2,810 native woody plants from seed collected in the Elwha for the project. ONP produced approximately 22,000 container-grown plants at the Matt Albright Native Plant Center and Silvaseed Company produced 5,000 Douglas fir seedlings. Sites planted included a 23-foot high terrace of delta sediments (Figures 2 and 3), gently sloping valley walls covered in 4-5 feet of fine sediments, and a few ridgelines jetting out into the receding lake. We also seeded 1.25 acres in Lake Aldwell using eight different seed mixes of mostly grass species. Although it is a bit early to say how well this is all working out, there are some general observations about the natural and managed plant colonization worth reporting.
As predicted, sites close to established forests are regenerating well. In June of 2011, both reservoirs were permanently drawn down 18 feet to prepare for dam removal. As a result, these areas have been exposed for just over a year. What was unexpected was how the spring seed rain would combine with reservoir drawdown to create a ring of dense seedlings of willows and cottonwoods. The reservoirs acted as seed sinks, accruing willow
and cottonwood seed that drifted into the lakes via tributaries, the river, and the air. As the reservoirs were drawn down 18 feet over a period of ten days, the accumulation of saturated seed was deposited by waves onto the newly exposed slopes. Perfect conditions for germination and establishment (Figure 4)!
In addition to willows and cottonwoods, rushes (Juncus species) and sedges (Carex species) are prominently establishing on the fine sediments. Another prominent species is fringed willowherb (Epilobium ciliatum spp. ciliatum). Wet-loving species are dominant among the the first colonizers. Another prominent native species we are seeing is bigleaf maple (Acer macrophyllum) (Figure 5). The wind storms that occurred in November sent samaras out into the newly exposed reservoirs during drawdown on top of fresh snow. The result is a carpet of new seedlings at some sites. It will be interesting to see how many of these native species survive by the end of this summer when the sediments dry out for the first time. A few invasive species are also appearing in the reservoirs, such as reed canarygrass (Phalaris arundinacea), orchard grass (Dactylis glomerata), and herb Robert (Geranium robertianum). We are actively pulling and spraying invasive species with herbicides in an attempt to minimize their establishment.
At planted sites, we are already seeing some significant mortality of Douglas fir (Pseudotsuga menziesii). The rate of Douglas-fir mortality is particularly high in the 23-foot terrace of delta sediments. Species that seem to be tolerating the sandy terrace are Pacific madrona (Arbutus menziesii) (Figure 6) and Roemer’s fescue (Festuca roemeri).
Cottonwood (Populus balsamifera) also appears to be doing well when it has not been defoliated by deer and elk! Planted Douglas-fir in the fine sediments appears to be doing a bit better than the ones planted in the terrace (Figure 7), although we do expect the mortality rate to increase during the summer.
As of mid-June, the fine sediments were already forming a thick layer of crust at the surface. The crust will inhibit gas exchange and reduce the rate of water infiltration during the summer dry season. Water availability to plants will decline significantly. Native species such as Nootka rose (Rosa nutkana), oceanspray (Holodiscus discolor), thimbleberry (Rubus parviflorus) and snowberry (Symphoricarpos alba) have tolerated the fine sediments in plant trials and are doing well so far, and may prove to be some of the few woody plants capable of colonizing the fine sediments. The seeded sites are doing very well (Figure 8).
Blue wildrye (Elymus glaucus), slender hairgrass (Deschampsia elongata), common yarrow (Achillea millefolium), Sukdorf’s wormwood (Artemsia suksdorfii) and Oregon sunshine (Eriophyllum lanatum) have germinated and provided significant plant cover over deep deposits of fine sediment (Figure 9). Cracks in the sediments are already proving to be safe sites for seedlings (Figure 10), similar to safe sites that formed on Mount St Helens after the 1980 eruption.
Monitoring the Reservoirs and Future Plans
This summer we will be installing at least 60 plots designed to monitor the naturally regenerating and planted sites in the reservoirs. More will be added after Lake Mills is fully drained, which is expected to occur by May 2013. Data gathered from the plots, and empirical observations from walking the reservoirs will help us to adaptively manage the revegetation project for the next 6 years. The next planting season, which will begin in October and end in March 2013, we will be installing 70,000 plants and a few thousand pounds of seed! These numbers will increase to 100,000 and 3,000 pounds of seed for the 2013-2014 planting season. Planting will continue through the 2016-2017 planting season. By then we will know how well natural and managed revegetation is progressing in the unique conditions associated with the largest dam removal project ever attempted in the United States.