Diversion Impacts


The Natural System

The Trinity River’s native plants and animals are adapted to natural river flow patterns and habitat conditions. In the figure below, the annual life cycles of fall-run chinook salmon and two riparian plants, black cottonwood and narrowleaf willow, are overlaid on an example natural hydrograph (a hydrograph shows the average flow for each day for every day of the year). Salmon adaptation to the river’s hydrology is evident — late fall and early winter rainstorms (the peaks in October through December) help draw adult salmon up into the river; winter baseflows allow for spawning; and the spring snowmelt (April through June) assists juveniles in their journey to the ocean before the warm, low flows of summer.

The plants that live along the river are similarly adapted — winter rainstorms and spring snowmelt floods scour away dormant seedlings and saplings growing too close to the river’s edge. Seeds released in the spring and early summer sprout higher up on the bank when the water level is high; seedlings on these higher surfaces are able to grow during the slowly receding spring snowmelt, which has recharged the groundwater table. Understanding these adaptations helps us evaluate the impact of river management and focuses our restoration actions on critical factors. The linkages between natural processes and biota are integrated in the Trinity River Restoration Program recommendations.

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TRD Impacts on the Trinity River Channel

A river’s size and shape (“morphology”) is determined by the interaction of its flows and sediment (“fluvial processes”). Prior to the TRD, streamflows varied greatly between and within each year. In a given year, flows could range from as low as 100 cubic feet per second (cfs) during the summer up to over 100,000 cfs during rare rain-on-snow floods. After the TRD was complete, flows were held between 150 cfs and 300 cfs year-round except for occasional storm-response reservoir releases, the largest of which was 14,500 cfs in 1974. The natural flow variability and sediment supply created a complex and dynamic channel which was beneficial to salmon and steelhead. However, when the TRD was completed, sediment (cobble, gravel and sand) that had previously been supplied from the upper watershed was blocked from continuing down the river. The loss of sediment availability, and flow magnitude and variability, caused changes in the river channel downstream of Lewiston Dam. The most dramatic and obvious change was with riparian vegetation (the plants that live near the river). Under natural conditions, high winter and spring flows kept willows and alders from growing too close to the summer low flow channel (see photo below left). In the absence of high flows, these plants were able to sprout and become established in a thin band along the river’s edge (see photo below right). This dense vegetation traps sand, causing confining “riparian berms” to form. The berms, reaching up to 12 feet in height, act as levees to separate the river from its historic gravel/cobble bars and floodplains.

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Six years after TRD completion, encroachment of riparian vegetation on the low water edge was already severe at Gold Bar (six miles downstream of Lewiston Dam).



TRD Impacts to Salmon and Steelhead

The changes to the Trinity River altered the quantity and quality of habitat for salmon and steelhead. The habitat above the dams was entirely lost, and the remaining in-river habitat was severely degraded, especially from Lewiston Dam downstream to North Fork Trinity River. Salmon and steelhead in the Trinity River depended on its dynamic and alluvial nature (mobile and free to form its bed and banks) for high quality habitat. Natural fluvial processes, driven by high flows and adequate coarse sediment supply, created and maintained complex channel morphology which provided suitable conditions for adults, eggs, alevin, fry, juveniles and smolts.

After the dams, pools that were once deep and used by adult salmonids filled with sand. During summer, water temperatures reached lethal levels for juveniles and smolts, and gently sloping banks necessary for fry and juveniles were eliminated by riparian berms, and much of the spawning gravel was scoured away. Remaining spawning gravel was choked with sand. Our increasing scientific understanding of the importance of natural river form and process to salmon and steelhead populations reinforces a recovery strategy that restores those natural conditions.

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Coho salmon, a threatened species on the mainstem Trinity River, migrating upstream to its spawning grounds.

How much have the Trinity River fish stocks declined?

Prior to the dams, estimates of fish abundance were sporadic and imperfect. The best available estimates suggest that the number of fall-run chinook salmon returning to the river each year varied between 19,000 to 75,500. Post-TRD, the average returns were approximately 20% of those estimates. In 1984, Congress passed the Trinity River Basin Fish and Wildlife Management Act (PL 98-541). This Act directed the Secretary of the Interior to develop a management program to restore fish populations to near pre-dam conditions. At that time it was thought that a goal of 62,000 natural (non-hatchery) fall-run chinook salmon would maintain a healthy, sustainable population. The current Record of Decision does not specify numeric targets, but continues to strive for a naturally-spawning population at near pre-dam levels. Spring-run chinook salmon, coho salmon and steelhead have shown similar declines. Coho salmon is also listed as a threatened species pursuant to the Endangered Species Act.

Example of Salmon Habitat Changes

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The red areas in the figures above and above right highlight suitable fry rearing habitat. The pre-TRD channel (above) provided habitat over the wide range of flows expected during the winter months when fry are emerging from spawning beds. However, the riparian berm of the post-TRD channel (above right) confines the river, eliminates the gently sloping channel margins, and increases water velocities along the channel margins beyond that preferred by salmon and steelhead fry. The gentle slope of the natural gravel/cobble bars slows the water velocity along the edge of the river, creating rearing habitat for salmon and steelhead fry. Emerging fry hide among the clean cobbles and gravel where water velocity is low. As flows increase, the habitat migrates progressively higher up the channel margins, but never disappears. Yellow-legged frogs also use the gently sloping gravel and cobble bars to deposit eggs.