Frequently Asked Questions (FAQ) on Rehabilitation
What is the difference between restoration and rehabilitation?
A range of precise definitions can be found. As used by TRRP, restoration is the goal of returning greater ecological function to the post-dam river through multiple management actions. Whereas rehabilitation is the action taken to repair channel problems caused by post-dam flows and accelerate development of ecosystem function.
Why are side channels important?
Juvenile salmonids use edge habitats along the river. Construction of side channels and alcoves is an efficient way to significantly increase these habitats. Additionally, adult salmonids have been found to frequently spawn in TRRP constructed side channels.
Why does TRRP remove vegetation?
Decades of low flows after the dams began operating allowed vegetation to grow dense on low flow banks of the river, fixing the banks in place, and inhibiting the geomorphic dynamics that are crucial for fish habitat in natural river systems. In the removal area we frequently excavate to lower the banks, creating floodplains for habitat during high flows. We save cottonwoods and large mature seed trees when possible. Removed trees are added back into the river for habitat.
Why does TRRP place wood at rehabilitation sites?
Wood, including both logs and smaller debris, is blocked by the dams. However, these materials provide habitat and nourishment for fish and encourage geomorphic dynamics such as scouring of pools behind logs and log jams. TRRP adds wood to make up for the deficit caused by the dams, providing habitat and encouraging river dynamics. Emphasis is given to larger wood (logs), particularly placed as log jam structures, due to the greater longevity and potential for initiating geomorphic processes.
Why does TRRP replant vegetation along the river?
Native plants create shade, shelter, and food for fish and wildlife. As flows rise terrestrial insects on native plants provide food for fish. As flows rise vegetation provides shelter for fish. Our revegetation is designed to encourage diverse riparian vegetation types on floodplains where appropriate for our restoration flows. We also use straw to encourage native plants and prevent erosion. Straw and seeding with native grasses discourages non native weeds.
Why does TRRP add gravel to the river?
Spawning gravels are blocked by the dams, leading to a deficit in the upper portions of the river. Additionally, dredge mining in many areas unnaturally reconfigured gravel distribution. TRRP rehabilitation and gravel augmentation is designed to increase the complexity of the river and change the shape of the channel. Commonly at rehabilitation sites, gravel removed from old dredger piles and cleaned is placed to form bars or meanders. We also create bars by adding gravel directly into a site or into the river during high flows.
How is this mechanical restoration different from the work done in the early 1990s? (The “feather edge” projects.)
The earlier projects are prototypes for the more ambitious program mandated by the ROD. While there were 9 prototype projects, an additional 47 projects were envisioned in the ROD, and on average each project will cover more river frontage than the original projects.