Lab 4: Field Trip to Marmot Dam Site - Sandy River

Marmot Dam Site after 6 years of the removal

The Marmot Dam was built in 1913, and it was removed in 2007. Before that, the area was logging land. The dam was operated by PGE (Portland General Electric), and all the land around was also owned by PGE. Environmental agencies realized that the dam was obstructing the natural flow of salmon upstream, and they proposed to remove the dam to let the river flow free. Finally in 2007 the dam was removed, the land was acquired by the Western Rivers Conservancy, and then transferred to the federal government to be restored.

After almost 7 years since the removal, the area doesn't appear like it had a man made structure in the past. The Sandy River flows free, and there is no major obstruction of sediments. The land on the north of the river is recovering, trees have been planted in the land where there used to be a building. This building was also removed, the cement was broken and the soil was treated to avoid compaction, so the new seedlings can grow. To the south of the river, the forest looks intact. Mossy Western Redcedar and Big Leaf Maple trees cover the shaded ground where Sword Fern grows.

The reason why the dam was removed is because it was obstructing the flow of salmon upstream. The Sandy River offers primordial habitat for four species of salmon: Spring Chinook, Fall Chinook, Steelhead, and Coho. A salmon ladder was built for salmon to swim upstream, but this was not really effective. PGE realized that the expenses of removing the dam were significantly lower than the expenses of operating the dam.

Succession is taking place now. The soils where the building was are now more fertilized, and the young seedlings are growing, some of them are reaching about 5 feet tall. There are a few invasive weeds in the area like Vinca, Scotch Broom, and Himalaya Blackberry. The Sandy River Basin Watershed Council is working in removing the noxious weeds and restoring the site. They have an idea, they want the replanted area to grow, and when the trees are mature, they are going to build some facilities like campgrounds, a boat ramp and a kiosko, but vegetation succession has to be fully in place.

Steve Wise was explaining about the hydrology and geology of the Sandy River. The river originally serves as a channel for lahar coming from Mt Hood. The river has its source in the Sandy Glacier. The glacier erodes the rocks of the mountain and deposits huge amounts of sediments into the river (that is why it is called “Sandy”). The last lahar was about 300 years ago. The bottom of the river is very young; the rocks are composed mainly by basalt. After the removal of the dam, most of the sediments were washed downstream in just a couple of days after. These did not affect the wildlife of the river because of the natural sedimentation regime of the river and the low toxicity content. The sediments found today are from after the dam removal.

The forest is categorized as late succession according to Steve Wise. The average age of the trees is about 200 years, so the forest is not considered old growth. The dominant species is Western Redcedar (Thuja plicata) and Big Leaf Maple (Acer macrophyllum). The forest floor is composed of woody debris and a thick and soft layer of humus (dead needles and leaves). The canopy is very open, so there is plenty of sunlight reaching the forest floor. The understory dominant species are Sword Fern (Polystichum munitum) and Huckleberry. The branches of the trees are covered by thick layers of moss. Some species of fern form roots and soil on the branches of the Big Leaf Maple and create habitat, such as the Licorice fern (Polypodium glycyrrhiza). This is a peculiarity of this forest.