Current conversations, media and our own experiences point to fire seasons that are far from ordinary. However, from dendrochronology (the study of tree rings) and other data sources, analysis find that prior to Euro-colonization, multiple millions of acres burned in on average in California. California’s ‘worst’ year in recent history saw about 4.5 million acres burned… which when comparing to historic averages would be within the ‘normal’ range (prior to Euro-colonization). In fact, tree ring scars show that many areas burned as frequently as every 5-10 years!  Within the past century, our society along with forest managers have promoted and practiced a prohibition on abundant low-intensity fire, allowing unburned materials to build up in forests and woodlands that along with population increase has set the stage for the complicated relationship now experienced with wildfire.

Most of us who have lived any length of time in the rural west are stressed about wildfire through the summer and well into the fall.   We endure smoke, dramatic headlines, helicopters flying over, evacuations, and too many of us witness damage to places we hold dear, including our own properties.  Forests that have not yet been touched by fire are heavily loaded with dead wood, leaves, and duff ready to become an inferno at any moment. Where fires have burned there is often a heavy load of grasses, frequently mixed with the woody remnants of trees from the last fire. Everywhere we go, organizations involved with fire share dramatic photos of conflagrations consuming tall trees.  And then we see flashfloods over fresh burns like with the McKinney Fire dumping sediments into rivers so thickly that it kills fish.  It seems we are smothered in news of devastation from wildfire!

But let’s step back for a little perspective.  Wildfire is nothing new to the west. Even before the first people set foot on these lands, our forests burned frequently from lightning strikes. These forests evolved with wildfires.  As tribes developed, their people lived with wildfires, found prosperity from them, and learned to manage the land by intentionally setting fires.

This maintained relatively open forests and woodlands, and kept mountain meadows functioning as wetlands to feed headwater streams. Natural wildfires tend to become more intense as they go upslope.  Look to the Trinity Alps where most mountain tops remain open and rocky.  Many peaks have sufficient soil among the rocks to support trees, and some scattered trees growing near the top of Thompson Peak demonstrate that the Trinities have no true elevational tree line.  But trees do grow slowly on those peaks and high intensity fires have historically happened often enough to keep those peaks mostly bare. 

From dendrochronology (the study of tree rings) and other data sources, most analysis suggest that prior to Euro-colonization, multiple millions of acres burned in on average in California. Our ‘worst’ year in recent history saw about 4.5 million acres burned… which would be within the ‘normal’ range prior to Euro-colonization. Tree ring scars show that many areas burned as frequently as every 5-10 years!  But we stopped that abundant low-intensity fire, allowing unburned materials to build up in forests and woodlands, setting the stage for the conflagrations we now see.

Frequent low intensity fires keep those fuels cleared out forests and woodlands.  It also helped keep them from getting too dense, promoting the growth of large older deep-rooted trees while minimizing the number of young upstarts that dry out the surface soils.  By keeping pines and firs out of oak woodlands, these fires promoted habitat for deer and other wildlife.  For these reasons, tribes managed the lands in California with fire.

So one of the big questions of our time is… how do we get back to healthy systems that function well (and safely) with fire?

References and Further Reading

• Asarian, E. 2024. Water temperatures in the Klamath-Trinity Basin: flow, other key drivers, and climate change implications. Presentation on 2024-05-01, Science Symposium of the Trinity River Restoration Program. Riverbend Sciences, Arcata, California. Available: https://www.trrp.net/library/document?id=2647.
• Salmonid Federation Restoration. Fire and Fish Workshop and Information
• Scott L. Stephens, Robert E. Martin, Nicholas E. Clinton, Prehistoric fire area and emissions from California’s forests, woodlands, shrublands, and grasslands. Forest Ecology and Management. Volume 251, Issue 3. 2007. Science Direct.com
• Trinity River Restoration Program Featured Article: Sediment and Summer Thunderstorms. The River Riffle, July 2023.
• Gruell, George E. Fire in Sierra Nevada Forests: A Photographic Interpretation of Ecological Change Since 1849. Mountain Press Publishing Company, 2001
• Fire in California’s Ecosystems. (2018). United States: University of California Press.

The start of summer in Trinity County has been a hot one, with 100° plus degrees for 10 days straight in early July combined with another series of 100° degree days forecasted for the latter half of the month. As warm-bodied land dwellers, we cope with heat by seeking refuge: interior shelter, air conditioning, shade, and of course, water. Refreshing water comes in many forms – pools, sprinklers and creeks, lakes, and rivers – whatever is available to us! Cold-blooded salmon are not so different in this regard. Throughout their various life stages, they too seek water temperatures that provide opportunities for success.

Last August, in the River Riffle newsletter we published “A Brief Introduction to Thermal Ecology of the Trinity River”. The article describes the thermal ecology of local rivers which experience cold, wet winters and hot, arid summers, characteristics of the Mediterranean climate of our region. Salmonids thrive where thermal diversity is available because they can maximize growth and survival by seeking optimal temperatures. For example, a juvenile salmonid’s job is to eat, grow and survive. At this stage, juveniles move from higher velocity areas where they feed in relatively cold water before residing in slower warmer areas of pools to rest and digest. The warm, slow water helps them relax and digest. Moving to take advantage of temperature differences allows them to efficiently digest their food to gain weight and grow larger. Eat, rest, digest, and repeat. The more that young fish can take advantage of these diverse temperature and flow conditions the better they can digest food into growth, which in turn improves their chance of survival in the ocean.

The seasonal, daily, and spatial temperature environment in the Trinity River before dams were constructed differed considerably from what we see today. Year to year variability was driven by winter precipitation and snow accumulation. As temperatures rose in the spring, snowmelt provided a regenerative increase in flow and expansion of habitat. On the journey from melting snow through creeks and tributaries to the mainstem river, water was warmed by the longer, warmer days, often reaching optimal temperatures for growth. As the snowmelt receded and the river returned to summer low flow, water temperatures in large, deep pools would stratify into different layers. In both these ways, temperature variability was provided to not only juvenile salmonids but also native frogs, turtles, and aquatic insects. The diversity of temperatures within nooks and crannies, combined with different depths and sheltered areas of the river formed a robust underwater nursery for the aquatic wildlife below. The timing and magnitude of these transitions and the variability of temperature depended on rain and snow patterns months before, yet the pattern was predictable. Fish and wildlife evolved over millennia to take advantage of this cycle.

The presence and operation of Trinity and Lewiston dams have dramatically altered the temperature regime in the Trinity River. As the spring and summer days heat up, Trinity Reservoir water stratifies to form a large pool of cold water below a depth of about 6 feet. Above this depth, water temperatures are warmed by the sun. Although infrastructure on some dams allow water to be drawn selectively from throughout the water column, Trinity Dam can only draw from its deep cold-water pool.  Cold water that is released from Trinity Dam to Lewiston Reservoir warms as it flows towards Lewiston Dam, but releases to the river are still much colder throughout the summer than what the river experienced at Lewiston before the dams were in place.  While the pre-dam river and other undammed local rivers warm in the spring to provide ideal temperatures for fish growth, the cold-water releases from Lewiston Dam keep the mainstem Trinity River so cold that growth of native salmonids, frogs, and turtles is stunted.

As spring progresses into summer, migration can become limited to cooler times of day, as the adults rest in the cold bottoms of thermally stratified pools by day and resume their upstream migration when the entire river has cooled at night. Finally, when adult spring-run Chinook reach their over-summer holding habitat, they can spend weeks to several months in deep pools with slow water to conserve energy for spawning in the fall.

There are many objectives of the regulated flow releases to the Trinity River from Lewiston Dam.  Summer baseflow releases aim to provide favorable temperatures for migrating and holding spring Chinook who have lost access to habitat in the upper watershed above Trinity and Lewiston dams. This temperature mitigation is achieved by flow releases of 450 cubic feet per second from the end of the spring release until mid-October. A recent research paper published in the Hydrological Processes journal, The mechanics of diurnal thermal stratification in river pools: Implications for water management and species conservation (Buxton et al. 2022) explores the effects of summer flow management on the Trinity River. The research examines pre-dam flow records from 1911-1960 when summer flows averaged 177 cubic feet per second in the Trinity River in comparison to flows since 2000 that measure 2.5 times higher at 450 cfs. The increased flow and cold deep water drawn from Trinity Reservoir create summer temperatures that are around 18°F cooler (10°C) than pre-dam temperatures in summer at Lewiston. The post-dam Trinity River experiences more water and higher velocity, yet its habitat areas are remarkably smaller than the pre-dam environment which received less water and lower velocity.

Above, we mentioned pool stratification in Trinity Reservoir. The stratification occurs because the slow-moving water warms and becomes less dense, which causes it to buoy toward the surface. This stratification of temperatures occurs in river pools that are large and deep enough (typically greater than 9 feet in depth). Size is important because when flow through the pool slows to a very low speed it prevents the warm top water from mixing with the cold bottom water and allows for stratification. This thermal layering in stratified pools is important for salmonids because it provides both adult and juveniles the opportunity to place themselves in the right temperature at the right time of day to best improve their survival and/or growth.

A key component of the pool stratification research was the measurement and modeling of temperatures as well as flow velocities in correctly sized pools for holding spring Chinook both below and above the dams. Data were collected and compared from a control pool in the Trinity River upstream of Trinity Reservoir as well as below Lewiston Dam (Pear Tree Pool). The findings showed that the pool above the dams exhibited stratification, providing the range of beneficial temperatures that follow the annual pattern of sun exposure and summer flows that naturally occur in our region.

In contrast, stratification did not occur in the pool below the dam at Pear Tree. Flows were too fast, mixing all water into one layer of uniform temperature. While temperatures in the Pear Tree pool were suitable for holding adult spring Chinook, the higher velocities increased their energy expenditure, likely taxing their energy supply needed for building and protecting redds and ultimately laying and fertilizing eggs. Unfortunately, the pool also lacked temperature diversity for juvenile salmon. Juveniles were presented with uniform temperatures and higher velocities, also taxing their energy supply needed for digesting their food to put on weight. Not to mention losing the benefits of temperature diversity discussed above.

Temperature diversity combined with variable water depths and velocities, refuge from predators, and plentiful food are important factors determining a young fish’s ability to grow and survive. While we often think “cold water is best,” perhaps a more accurate statement would be “diverse water temperatures are essential”! While that may seem obvious, a little clarification can improve our understanding of the needs of fish and how we may better serve them. Rivers in our region support a multitude of aquatic and land-based animals, insects, birds, and amphibians. Providing habitat diversity – including variable water temperatures – are paramount to meeting the needs of our wildlife community, and Program scientists continue to learn about the complex interactions between temperature and ecosystem health, hoping to better inform management on the Trinity River.

Citations

1. Pinnix, W.D., S.P. Boyle, T. Wallin, T. Daley, and N.A. Som. 2022. Long-Term Analyses of Estimates of Abundance of Juvenile Chinook Salmon on The Trinity River, 1989-2018. U.S. Fish and Wildlife Service. Arcata Fish and Wildlife Office, Arcata Fisheries Technical Report Number TS 2022-40, Arcata, California. [link to download]
2. Buxton, T. H., Y. G. Lai, N. A. Som, E. Peterson, and B. Abban. 2022. The mechanics of diurnal thermal stratification in river pools: Implications for water management and species conservation. Hydrological Processes 36(11):e14749. DOI: 10.1002/hyp.14749. [Link to Download]

References

Asarian, J. E., K. De Juilio, S. Naman, D. Gaeuman, and T. Buxton. 2023. Synthesizing 87 years of scientific inquiry into Trinity River water temperatures. Report for the Trinity River [Link to download].

Naman, S., K. De Juilio, and K. Osborne. 2020. Juvenile salmonid temperature target recommendations. Memorandum to Ken Lindke, Fish Work Group Coordinator. Trinity River Restoration Program, Weaverville, California. [Link to download].

Abel, C., K. de Juilio, K. Lindke, S. Naman, and J. Alvarez. 2021. Shifting a portion of Trinity River spring releases from Lewiston Dam to the winter period: a flow management action to benefit juvenile salmonid habitat availability, growth, and outmigrant timing. White-paper for the Trinity River Restoration Program (TRRP). TRRP, Weaverville, California. [Link to download].

Many Trinity County residents are attuned to the annual water year forecasting prepared by the California Department of Water Resources, also known as the Bulletin 120 or B-120. Every year, the department gathers real time water accumulation information, snowpack data and uses modeling to forecast what to expect for the major snow bearing watersheds in California. The water bean counting starts October 1 (the nominal beginning of California’s wet season) with a final determination April 10 each year. The forecasts are broken up into several regions throughout California with the Trinity River at Lewiston Lake forecast filed under the North Coast Hydrologic Region.  The ultimate goal of the B-120 is to value expected amounts of water inflow to storage locations around the state. These data makes it possible for water managers to make local informed decisions about potential floods, the amount of water that can be released from reservoir systems, as well as what type of dry season residents and fire agencies could expect within their regions.

For the Trinity River Restoration Program, the April B-120 forecast determines the water year allocation for our yearly restoration flow releases, which were outlined in the 1999 Flow Study Evaluation and adopted in the 2000 Department of Interior – Record of Decision. These five water year types that determine the amount of water released to the river from year to year are categorized as Critically Dry, Dry, Normal, Wet and Extremely Wet. You can see the relative allocation for restoration purposes in the table below.

It is interesting to note that the State’s April B-120 has only overpredicted the water year type once, in 2008.  Currently the allocation for river restoration is the only conditioned amount of water released from Trinity & Lewiston Reservoir; where the Restoration Program’s yearly allocation is limited by water year type, the Central Valley Project can divert any amount in any water year type, usually diverting less in wetter years and more in drier years. Safety of dams releases and water releases to the Trinity River for ceremonial purposes or for Klamath River mitigation purposes are not part of the restoration release volume.

As mentioned above the State’s forecast uses a few different methods to determine how much water to expect as inflow into Trinity & Lewiston Reservoir. The most story-worthy data collection type are the on-the-ground, snow surveys which are conducted during a short window every February, March, April and May. The Trinity Alps snow surveys are led by two agencies: The U.S. Forest Service who motor in via snow Cat to several locations in the Trinity Alps Wilderness, and an expert group of backcountry cross country skiers led by The Watershed Research and Training Center. There are nearly a dozen survey courses established throughout the Trinity River watershed and these sites have been measured in exactly the same locations since the 1940s.

Each year The Watershed Center sends out a small team of between two and four backcountry skiers to travel through the Alps Wilderness and measure snowpack at three survey courses: Shimmy Lake, Red Rock Mountain, and Bear Basin. Once the team reaches a survey location, they drive a specialized aluminum tube tool called the Mt. Rose Sampler, into the snowpack until they hit ground. “It takes a few times to get used to doing it,” says Josh Smith who has been conducting surveys in the Alps since 2011, with the first full recorded season in 2012. The surveyors use the tool to measure the height of the snow, then carefully extract the tube from the snowpack and weigh the snow-filled tube using a handheld scale. These measurements allow the surveyors to calculate the Snow Water Equivalent in designated transects within the three courses for which their team is responsible for. The State uses the hand measurements from the snow survey teams to bolster additional data taken from unmanned sensors located across and just outside of the watershed. These data sources together feed into a model that predicts the volume of water that will flow into Trinity Reservoir that year.

That said, the Watershed Center is looking for local Trinity County residents who believe they have a sufficient mental and physical stamina to participate in this long-standing Trinity County tradition. “We get a lot of calls from people who think this is right for them,” Josh continues, “most people only come out once, and then they are done. It’s a real suffer-fest.”

If you’d like to learn more, please reach out to Josh Smith at the Watershed Training and Research Center by calling (530) 628-4206.

Not only is the hyporheic zone an area that supports great biodiversity, in a 2005 study this zone was also coined the “river’s liver” from findings that carbon and nitrogen cycling in the river was “controlled by the live sediments of the central river channel, which thus represent a “liver function” in the river’s metabolism.”[1] So, the hyporheic zone acts as a filtering mechanism for the river, and an area of rich biodiversity. But that’s not all!

The hyporheic zone provides a multitude of functions and it is critically important to the health of our waterways. Because the hyporheic zone acts as a filtration system through its porous sediments, it also promotes higher levels of dissolved oxygen through photosynthesis. Dissolved oxygen in waterways support anadromous fish species like chinook, steelhead and coho salmon by helping them to maintain a healthy respiratory function.

When Trinity River salmon return to spawn, they dig redds (or nests) to lay their eggs in. The female fish flaps her tail sideways into the river bed, digging down around 12″ to 14″ into the hyporheic zone. After the eggs are laid and fertilized, she covers them with rocks. These rocks protect the eggs and newly hatched alevin from predators. The eggs location in the hyporheic zone provides water flow that flushes metabolic wastes during egg development and provide dissolved oxygen for her embryos to breathe.

The presence of dissolved oxygen, protection from predators, and microvegetation for food makes the hyporheic zone a biologic hotspot for macroinvertebrates. While the insects nestle down in between rocks they feed on leaves, algae, and twigs. In a healthy hyporheic zone where flow and sediments are correctly combined the area supports an abundance of food for the macroinvertebrates which in turn supports an abundance of food for juvenile and adult salmon. And that’s not all! Due to the interaction with upwelling cooler ground water, hyporheic zones help to moderate stream temperatures during the lower flow summer and fall months.

The Significance of the Hyporheic in the Trinity River

The hyporheic zone covers the entire streambed and bank areas of a river bottom and the ability to perform its natural functions are influenced by many things. Some influences include whether the stream is straight or meanders, if there are any obstacles in the channel, such as log, boulder, or even the pier support for a bridge. Factors also include whether the stream has a single channel or multiple channels, and also how porous the streambed sediments are. For example, near Lewiston Dam where there are very few fine sediments, large amounts of the river’s flow go subsurface because they are conveyed in the hyporheic zone. This is an unnatural situation because the river requires all sized sediments, from sand and silt up to cobbles and boulders. So while semi-open pore spaces in the bed are desirable, if the pore spaces are fully open do to the lack of fines in the bed, salmon eggs will jiggle around in redds and die from abrasion as well as the speed of flow between rocks in the hyporheic zone will be too fast for microvegetation to grow and macroinvertebrates to live.

Thus, to combat a static river system, since 2000, the Trinity River Restoration Program has focused efforts around replenishing the critical building blocks of a river. This is achieved by gravel additions in the upper river, large wood placements along river banks and a yearly spring snowmelt hydrograph that is released from Lewiston Dam. Also, since the Trinity was heavily impacted by hydraulic and dredge mining the program allocates funds for watershed restoration and gives great attention to the diversification of channels and floodplains with mechanical rehabilitation.

These things combined help the dammed Trinity in healing itself, however, many gains are yet to be made with restoration and management. For example, Todd Buxton, PhD, is starting a three year study of the hyporheic zone in the Trinity River. The study will construct a mathematical model to simulate the flow rates and directions, temperatures, and dissolved oxygen in the hyporheic zone. At the same time, macroinvertebrates in the hyporheic zone will be measured where the study is being conducted at sites within the 40-mile restoration reach. This information will be used to develop a statistical model for predicting the density and species composition of macroinvertebrates based on the above characteristics in the hyporheic zone, since these aspects have a strong role in determining how many and what species of macroinvertebrates may be present. Application of the paired models will then enable scientists to better understand how surface flows and temperatures in the river can be better managed to promote macroinvertebrate populations and increase the availability of food for salmon.

Additionally, the program has made significant gains with increases in outmigrating salmon smolts since 2000, but these fish are not returning home at the rates that were observed before this year. Scientist have identified that the Program’s current management of flow timing could be a critical limiting factor for the fish of the Trinity River. If juvenile fish had more food available to them when they are emerging from their nests they may enter the ocean more robust. If elevated flows from the dam are timed with winter storms, then the Trinity River could add to the power of the tributary flows to increase mainstem water levels to prevent delta creation while simultaneously preventing sediments from smothering redds and other important living organisms within the hyporheic zone.

Understanding this unique area has been a challenge for river scientists because the hyporheic function is expansive. It is also unique to each river system and crosses over many scientific disciplines. The zone not only intrigues those interested in studying the microbiome of a river system, but also includes; ecologists, geomorphologists, hydrologists and environmental engineers, just to name a few. With each discipline and study within, scientists learn more and more about the fascinating world that exists beneath and alongside a river’s bed and how river restorationists can better understand to allow it to flourish.

References

Hyporheic Zone, Wikipedia

Lewandowski, J.; Arnon, S.; Banks, E.; Batelaan, O.; Betterle, A.; Broecker, T.; Coll, C.; Drummond, J.D.; Gaona Garcia, J.; Galloway, J.; et al. Is the Hyporheic Zone Relevant beyond the Scientific Community? Water 2019, 11, 2230. https://doi.org/10.3390/w11112230

The Significance of the Hyporheic Zone, Jana Hemphill, Deschutes Land Trust, 2021.

Citations

[1] Fischer, H.; Kloep, F.; Wilzcek, S.; Pusch, M.T. A river’s liver–microbial processes within the hyporheic zone of a large lowland river. Biogeochemistry 2005, 76, 349–371. [Google Scholar] [CrossRef]

When you go down to the river, it’s hard to ignore the assortment of sediment on the bed and banks – from sand and silt, to gravel, to larger cobbles, to the largest of boulders. Seeing rocks that contrast so strongly with the rough, jagged ones in the surrounding hills might beg the question – how did these get here, where did they come from, and how long ago did they arrive?

If a rock is rounded, more likely than not [1] it was transported by the river in a series of floods, originated from higher up in the watershed. Depending on the size these rocks may have arrived recently – perhaps as recently as the last flood. Large, rounded boulders that appear to be too large to have been rolled down the river on their own may have been in place since the last natural 100-year or 500-year flood and may remain there forever, or at least as long as Trinity and Lewiston dams are in place.

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[1] Gold miners washed much sediment into Trinity River valleys from ancient riverbeds created from tectonic lifting that are presently high up on mountain slopes.

A river’s function

Besides the ecological benefits that rivers provide us, they have two pivotal functions in nature – to move water and to move sediment from the mountains to the ocean (the process is illustrated below). As both water and sediment flow downstream, they interact with each other to create a mosaic of pools, riffles, runs, islands, meanders, bars, and all of the other physical features that draw people, plants, and animals to a river.

The Trinity Watershed is situated in a relatively young, steep, and highly erodible mountain range, and is therefore blessed with a plentiful supply of sediment. A healthy sediment supply is beneficial to fish, invertebrates, and floodplain vegetation. However, due to the placement of two dams in the Trinity River’s upper watershed an important element of restoration is giving gravel to the system below a dam since the river’s natural sediment supply is blocked. We know rivers below dams need a replenished sediment supply, however, a key question that geomorphologists continue to study is how much sediment, and what size distribution of sediment should be added? These factors are difficult to determine and are constantly being re-evaluated as part of our adaptive management program.

How we calculate amounts for placement

Gravel augmentations to the river are first determined using a defined sediment budget specific to the Trinity. Also, at some point below most dams, a river’s tributaries provide a sufficient supply of sediment to support the physical processes and biologic populations in the river. On the Trinity, that point is considered to be just below the junction of Indian Creek. In years past, during floods, the amount of sediment moving along the bottom of the river (called ‘bedload’) was directly measured with large strainers placed on the riverbed. Data obtained from the strainer monitoring station have indicated that minor floods may move 2,000-3,000 tons of bedload (the monitoring station that is nearest to Indian Creek is located just upstream of Douglas City campground). At this same location, larger floods may move 15,000-30,000 tons of bedload past the monitoring station. The Trinity Record of decision provided the program with a framework of how much sediment should be applied to the river below the dam with the expectation that geomorphologists study current conditions through time and then adapt management based on results found.

After many years of physically sampling bedloads, program scientists switched to a more efficient and safer technique called acoustical monitoring. This method uses underwater microphones to quantify the amount of sediment during floods by measuring the amount of noise generated by rocks rolling on the bed. The physical data collected from the past is then compared to the measured volume of noise and produces a calculated volume of sediment rolling past. Scientists then scale annual sediment augmentation projects to these measured amounts. Additionally, the riverbed is periodically surveyed below sediment augmentation sites to determine whether the effects of placements are positive or negative. If too much sediment is noted, this information is used to scale back future sediment augmentation plans. If you were to compare the original amounts of sediment proposed in the Trinity Record of Decision to what we add today, ROD volumes were 2-3 times higher!

Size matters

As for size, the high end of the size distribution of sediments is the grain diameter that salmon can move to construct redds, or nests in which they place their eggs for incubation. Salmon can spawn in gravels with a median diameter up to about 10% of their body length. This leads to gravel being placed in the river that was filtered through a 4-inch screen. The lower end of the size distribution of sediment for redd building is considered to be in the size range of small gravel, and so the sediment mixtures for placement in the river are also screened to remove sand and silt. If the sediment is too small, it just flushes all the way down the river during floods and doesn’t remain to provide any benefits. If the sediment is too large, it stays close to the augmentation site and causes the riverbed to become coarser there. The need for small to large gravels for placement makes considering the size distribution between these end points important. Too many small gravels make the riverbed overly mobile and easy to scour, which endangers salmon eggs incubating in the bed. However, a grain-size distribution that is skewed towards larger particles makes the riverbed too stable, so that salmon are unable to move the sediment when attempting to construct a redd. These considerations make the size class of sediment another subject of adaptive management, and over time TRRP has reduced the size of sediment that is added to the river. Studies have also pointed toward ways that coarse sediments (gravel and cobble) interact with fine sediment (sand and silt), and restoring a natural balance of these grain sizes is an objective of the sediment augmentation program at the TRRP.  In the future you may hear of sediment with a more natural size distribution (e.g., “bank run material”) being used in sediment augmentation projects.

Filling deep pools

When you observe a river during typical baseflows, pools are calm while riffles are noisy, turbulent and swift. From an above water view, its natural to think that sediments would settle from these active riffles to its calmer neighboring pools. During low flow, if you look underwater, the river only has the power to move finer sediments, like sand and silt. Conversely, coarse sediment, such as gravel and cobble move only when the hydrology of the river is powerful with high flow or flooding.

When rivers flood, we see something that river scientists call a “flow reversal”. Flow reversal is when deep pools transition into a high-energy environment where flow velocity is more vigorous than on riffles. In this instance water meets the pool (and its surrounding environment, like bedrock) with force and activates sediments of different sizes within the pool. These sediments are “scoured” from the pool and placed on the riffle below it typically expanding a pool’s depth and also building the riffle below. Next time during a high flow, check out the way a pool churns and take note to notice the way water interacts with the riffle that lye underneath. During high discharges, flows on riffles are comparatively slow because the surface is not as deep. This interaction causes the water to “feel” the bed and slow due to its rough texture. These interactions cause sediment to deposit on riffles and scour from pools during high flows. The size of sediments that move are directly correlated to the amount of water flowing down the river and these events are the force behind building the riffles and pools of the Trinity River.

TRRP sediment augmentation projects have sometimes been thought to contribute to the filling of deep pools in the river and there have been cases where pool depths have decreased in areas that the TRRP has worked to restore the river. However, TRRP studies have shown that this tends to occur where stream power decreases in the channel from lowering the elevation of adjacent floodplains and vegetation, which causes the flow to spread out instead of concentrate in the channel. In many areas of the Trinity River, lowering floodplains is necessary to reconnect them with the river during floods for the benefit of the fish, wildlife and plants that live there. This conundrum is another subject of adaptive management, and TRRP often avoids actions that would have a strong likelihood of affecting pool depths so that holding habitat for over-summering fish such as spring-run Chinook salmon remains available.  

The next time you visit the Trinity River, take a close look at the sediments that you see. Depending on the time of year, you may see salmon redds constructed of gravels.  You will also most likely find aquatic invertebrates and biofilm living on the gravel and cobbles surfaces. Dig into a sand and silt deposit along the channel margins and you might find juvenile lamprey wriggling around in these materials. You will certainly see how sediment forms the shape of the river. And hopefully you’ll come away with a greater appreciation of sediments that are the building blocks of the Trinity River!

Algae are an important component of a natural river ecosystem.  They photosynthesize, converting light into sugar for usable biological energy that forms the foundation of most foodwebs.  Cold, oligotrophic (nutrient-poor) rivers such as the Trinity are generally not perceived as having a lot of algae.  Back in 2018 a bloom of algae in the ‘restoration reach’ of the Trinity (roughly Lewiston to Helena) raised concerns; the bloom brought greater visibility of algae than we had seen in the Trinity in recent memory. 

For TRRP scientists, those concerns translated to numerous questions.  But questions that should be overlaid on the large body of literature from countless scientific studies done elsewhere.  That literature shows (repeatedly) a number of fundamental patterns: 

• A broad diversity of algae is natural in cold rivers like the Trinity.
• Young salmonids feed on small invertebrates that come both from the terrestrial environment and from within the river where algae is a primary food source. 
• In the river, algae grow primarily on the river bottom (it is “benthic”) while algae suspended in the water column (“planktonic”) is prevented from becoming thick by the swift outflow of the water. 
• Algae can be scoured from the river bottom during large floods, forming a reset for the benthic environment (what ecologists call a ‘disturbance’). 

Studies by Dr. Mary Power (U.C. Berkeley) and her associates, primarily in the nearby Eel River, had gone further with the studies of disturbance.  They found that while floods scour algae, they also scour some of the larger benthic invertebrates, particularly caddis fly larvae.  Caddis fly larvae consume large amounts of algae and build cases around themselves from wood and sand to prevent fish from eating them. Their studies found that floods are necessary for controlling caddis fly populations so that algae may rebuild, that chironomid larvae (nonbiting midges) then become more abundant, and then young salmon have more food available.

Prior to 2018, TRRP scientists had their hands full with issues higher up the food chain and thus more directly related to salmon production; algae had not been specifically studied within the Trinity restoration reach from an ecological point-of-view.  Questions arose… Was the bloom inappropriate for the Trinity? How much algae ‘ought’ the Trinity have?  Are the types of algae in the Trinity appropriate for producing chironomid larvae and thus fish food?  How does the abundance of algae in the Trinity correspond to natural flows or scheduled dam releases? 

In 2020 I began monitoring algae with a simple method to address these questions while not detracting from the ongoing focus on salmon.  I have been regularly measuring an index of algal abundance based on their visibility in underwater photography.  I have also been sampling the algae in winter and summer to identify the types of algae present.  A report from the first year of sampling is available at https://www.trrp.net/library/document/?id=2549. A second, multi-year report should be completed in 2024.

The algae in our focal reach of the Trinity River can be roughly placed into 3 groups: cyanobacteria (once known as ‘blue-green algae’), filamentous algae, and diatoms. 

Diatoms

Diatoms are amazing single-celled algae…  They form cell walls of silica (glass) with intricate patterns that look almost like spirograph art under a microscope, and many are actively mobile!  Some also are capable of capturing nitrogen, making them a particularly valuable food source for invertebrates, which then become a food source for young salmon.  These diatoms tend to have an orange color, so when aging Cladophora looks orange in the river, it actually is becoming covered with nutritious diatoms.  Diatoms also live directly on rocks, where they can build up to form a slippery surface.  When this “scum” of diatoms is removed from a river rock and placed under a microscope, not only is the beauty of those glass cell walls revealed, but also bazillions (OK – not a scientific term) of Chironomid larvae… fish food! 

While I lack specific data on the effect on invertebrates (and thus fish food), the pattern corresponded well with the studies by Dr. Power as well as observations from an ongoing invertebrate study being done in the Trinity. Indications are that Trinity and Lewiston dams, which prevent floods in winter, are preventing the ecological disturbance that builds algae communities to feed chironomid larvae for an optimal food supply as young salmon hatch and emerge from gravels.  Traditional ROD flows that don’t flood the river until April leaves an overly mature algae community (and invertebrates) in place while salmon fry emerge, then scours away the algae (the base of the food chain) when young salmon populations are peaking.  A shift in flow management to provide those scouring flows in winter should set the stage for more natural algal growth as well as optimal food supplies for emerging young salmon. 

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.