Trinity River, CA: Restoration in transition on the largest tributary to the Klamath

Year: 2022
Presenter/s: Kyle De Juilio
Symposium Session: 2022 - 08 Beyond Dam Removal: Ecological Restoration of the Klamath Basin
Topics covered: adaptive management and monitoring, fish-salmon, flow augmentation, instream structure (culvert/bridge/dam), lessons learned, riparian, sediment transport, stream, and temperature


Watch the recorded presentation here:

The Trinity River is the largest and southernmost Tributary to the Klamath River, and in recent decades has produced roughly half of returning adult salmon to the Klamath Basin. Although the Trinity is home to the longest free-flowing river in California (South Fork Trinity River), and vast tracts of wilderness, it is heavily managed with a large hatchery, two large hydroelectric dams blocking fish passage to valuable habitat, and an inter-basin diversion carrying 50 to 90% of runoff from the upper watershed to the Central Valley Project and the Sacramento River since construction in the 1960s.
Major efforts to restore historically productive fisheries of the Trinity River have been underway for decades and have left managers with the responsibility of augmenting flows, fish, and sediment to the system, as well as re-imagining and constructing the river channel and valley floor to allow function and buoy productivity. As the Trinity River Restoration Program (TRRP) approaches 20 years of implementation it’s assessing restoration results, looking ahead, and proposing new strategies. Through compilation and synthesis of decades of data, conceptual models are being revised and important factors not previously investigated are being identified. This presentation will share some lessons learned and new directions. As it turns out you can have too little fine sediment, water temperatures can be too cold, and the streambed sediment conveyor belt has holes in it.

Growing evidence suggests that current environmental releases intended to restore form and function through allowing geomorphic change and riparian regeneration are causing unnatural cooling during the salmonid rearing period causing fish to be smaller at outmigration and delaying the development of invertebrates and amphibians. The TRRP is now pursuing changes to all its management actions through adaptive management and revised environmental permits, including an Environmental Assessment to redistribute flows to reflect a more natural pattern and “piggyback” on storm events to more efficiently accomplish geomorphic work, inundate habitat and improve temperatures for growth and outmigration of salmonids.

Prior to environmental flow releases, the streambed of the Trinity was overwhelmed by fines and starved of gravels. Scouring flows have restored a gravel bed river and resulted in a deficit of fines below the dam, which can impact spawning success and productivity, causing some to consider augmentation of fine grains. While gravel augmentation is a vested restoration action, gravel transport is not as orderly as originally envisioned. Rather than continual movement downstream there appear to be sources and sinks resulting in storage and flux, causing practitioners to develop site-specific intent for gravel additions.

Many of the prospective changes to management on the Trinity beg the questions: What do managers do when they accomplish an objective or perhaps go too far?; How do we adjust within existing constraints to address new objectives?; How do scientists combat effective but simplistic messaging from the 20th century with a more nuanced approach?, or in other words; How do we replace the black and white of Smokey Bear with the shades of Goldilocks?