Flow Variability in the Trinity River

Aaron Martin [Yurok Tribal Fisheries Department]

Imagine a winter storm brewing in the west, clouds accumulating over the mountain peaks dropping a dusting of snow and at lower elevations dripping down as rain. Waters accumulate and funnel toward low points in steep terrain running down stream paths catching sediment, leaves, and branches and delivering them into the tributaries of the Wild and Scenic Trinity River. Depending on the amount of precipitation or snow melt, creeks can daintily deliver cool mountain waters with smaller sediments while larger rain events can powerfully move tree logs and large rocks.

Between 1960 and 2022, when this wild pulse of storm-fed tributaries finally converged with the Trinity River, something peculiar happened; the mainstem did not match the same force of its smaller tributaries. The river’s flatlined flow left debris delivered by the tributaries stacked and settled at their mouths. In the wet winter of 2022-’23, the community witnessed Deadwood Creek deliver a plume of sediment from its wildfire-scarred upper reaches after a significant rain. The plume settled on a large group of salmon redds at the mouth of the creek, and the Trinity River, at its baseline winter flow of 300 cfs, couldn’t mobilize the sediments. The embryos developing in redds at the mouth of the Deadwood Creek likely died by suffocation.

As you are probably aware, the Trinity River has two dams which hold back waters from its upper watershed. In the absence of flow from those tributaries, the river then must rely on two hydrologic sources for river health: downstream tributaries and restoration flow actions released from the dam. Restoration flows have been managed by the Trinity River Restoration Program since 2004 and until quite recently, flow amounts during winter months were managed to maintain a consistent low flow (300 cfs) from October 15 through mid-April. Management was designed this way because the science of how flow influences the ecology of the river wasn’t as far along as the physical river sciences, and further, the water year type which establishes the volume of restoration flow is not determined by the California Department of Water Resources until April 15. Based upon that water year determination, program staff develop a hydrograph for its spring restoration release which intends to mimic an important ecological function of mountainous river systems – the snow melt flood.

As described in the first paragraph, the snow melt flood moves mountains, well the loose parts anyway. High flows during melt events benefit the river in form by moving sediments, logs and rocks which are critical in building habitat for salmonids. Logs slow upstream current, allowing migrating fish to rest and providing cover from predators. Rocks create oxygenation in riffles and provide nesting salmonids a place to dig their redds when spawning. Sediments, when settled and in healthy amounts, encourage proper algae and thus hatches of bugs, which feed fish on their path of migration.

With the snow melt restoration release the Trinity River began to heal from decades of dam-regulated flows. Data shows that the Program has been successfully sending more young fish to the ocean suggesting that habitat is increasing with more water in the system (Pinnix et al, 2022), but adult fish returns have not met expectations and scientists wanted to know why. In the mid-2010’s the Program began to explore the use of flows during winter months. A significant portion of rain events that trigger tributary flows happen from December to April and flows below the dam don’t match that pattern. Studies commenced on temperature, salmonid growth, food availability, habitat availability, redd scour, and geomorphic and hydrologic benefits of current and potential flow practices. The results from these studies became clear and culminated in the Trinity River Winter Flow Project report (Abel et al., 2022). The research led to a proposal of using some of the restoration flow volume earlier in the year by synchronizing a dam release with a winter storm between December 15– February 15 and then providing the river with an elevated base flow between February 15 and April 15; the balance of the restoration water volume would still be used to mimic spring snowmelt in the spring. The authors hypothesize that this change in flow management would more efficiently use the same amount of water to move rock in the river, while also increasing habitat and food availability for young salmon.

When you think about it, it makes sense. All the native species of the Trinity River evolved with winter storm flows, higher winter base flows, spring snow melt and dry hot summers. But for the past 56 years, water dispersal from the dam has been flipped and has largely disregarded vital variability in flow patterns outside of the spring and early summer. Key floodplain habitat that was once inundated for months during winter and provided an abundance of rearing areas and food production to Trinity River fish has been lost, unless flow patterns change. Catch data from downstream screw traps show that inundation of that rearing habitat does not occur until most juvenile salmonids are downstream of the restoration reach (Petros et al. 2017), meaning most juvenile salmon don’t have a chance to use all that habitat that has been created by restoration projects in the last 18 years, unless flow patterns change. We are also learning that water released from the dam can be so cold that it slows the growth of juvenile salmon and that the right temperatures and abundant food will translate into faster growth for salmonids (Lusardi et al. 2019).

Young fish need diverse habitats, appropriate temperatures and abundant food to thrive and survive as they travel down the river to the ocean. More natural flows, including winter floods, increased winter base flows, and spring snow melt better support a healthy ecosystem that many species depend upon. The system is a balancing act of physical processes like flow, seasonality, and a vast network of species that rely on each other to thrive.  The Trinity River Restoration Program is looking to give that power back to the ecosystem so that our cherished fish of the Trinity can be armed with bigger, healthier bodies when they meet the challenges of their harrowing migration to and from the ocean.

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