Channel Rehabilitation FAQ

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.

What is the purpose of mechanically altering the river?

The river has been manipulated throughout its history through mining, logging, road development and the dams. The goal is to return natural function to the river by modifying flow, habitat and other areas that will provide, and support, habitat for fish and wildlife species found in natural river environments.

The current restoration strategy combines flow management and channel rehabilitation in a holistic manner to provide improvements in both the physical habitat and the ecology of the river.

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.

How are channel rehabilitation projects evaluated?

Fisheries biologists from multiple agencies, including the U.S. Fish and Wildlife Service, Yurok Tribal Fisheries Program and Hoopa Valley Tribe examine the change in juvenile rearing habitat a variety of flow levels before and after channel rehabilitation projects are constructed.

Boyce et al. (2018) found that that the TRRP’s channel rehabilitation projects have had a dramatic, immediate and almost completely positive short-term influence on the amount of rearing habitat at rehabilitation sites after construction. Channel rehabilitation projects have resulted in significant long-term increases in rearing habitat compared to pre-construction conditions at summer baseline flow (450 ft3/s).

Do channel rehabilitation projects influence adult salmon holding habitat and returns?

since 2010, several studies have been conducted to address the concerns over potential pool filling, which intensified in late 2010 when a perception became widespread among local fishermen that holes and runs were filling with sediment throughout the river (Gaeumann 2020).

Gauemann (2020) presents data quantifying depth changes over a six-year period in 105 pool locations distributed between Lewiston Dam and the North Fork Trinity River. Those data reveal that the depths of most pools remained essentially constant between 2011 and 2017. The study found that there is little evidence to suggest that pools are systematically filling or that gravel augmentation plays an important role in pool evolution. Data indicates that pools depth changes tended toward getting deeper.

Chinook salmon pre-spawn mortality rates have been variable across time, and are influenced by a myriad of factors, many of which are unrelated and out-of-the control of the TRRP and its rehabilitation projects.

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.




Current Project Sites

Channel Rehabilitation Sites 

Environmental Compliance