Arrival Timing at the Estuary

As the travel time foundations of the flow theorists have failed to appear in experimental testing, flow theorists retreat to their intuition that it is inherently better for salmon to get to the estuary earlier in the year. The primary basis for this belief is that before there were dams, it is believed that juvenile salmon got to the estuary faster—perhaps as much as a month earlier for juvenile spring and summer chinook.76 That is almost certainly an overestimate, but it seems likely that it takes juvenile salmon longer to get to the sea now.

The question of arrival timing is a difficult one, because estuary and ocean conditions vary from week to week and year to year. Salmon have evolved to adapt to these natural variations. It is probably true that hatchery and barging systems, in addition to the construction of the dams, have altered the timing when juveniles appear in the estuary, but it is also true that scientists have never measured whether the effect is positive, negative, or statistically insignificant.

Transportation gets juvenile salmon downstream faster than natural conditions, but this is never cited as an advantage of transportation. Indeed, studies of salmon which have been transported down below Bonneville Dam produce exactly the opposite result supposed by flow theorists: salmon transported later in the year have substantially higher survivals.

This could be a byproduct of hatchery management that dumps huge quantities of juvenile salmon into the river at the same time earlier in the year, so that there is not enough food for all the salmon and most perish. No one really knows. We do know that one of the most productive stocks of wild chinook salmon in the Columbia River Basin, the Lewis River, Washington stock, happens to migrate through the estuary two months later than all the other salmonid stocks.77

There have been huge changes in the estuary since the late 1800s. Scientists believe that the destruction of wetlands has caused a fundamental shift away from macroscopic plants (algae) toward microscopic plants (phytoplankton). This change in the food base is thought to favor shad over salmon. Shad eat the zooplankton that eat the phytoplankton; juvenile salmon prefer larger prey that eat algae.78 If that is true, that is yet another reason that removing dams won’t bring back salmon: we’d have to restore all the wetlands too. And if we took out the dams, we’d reduce the amount of food for the shad, since it appears that algae production in the reservoirs and the resulting “microdetritus input to the estuary is nearly equivalent to the macrodetritus cut off from the estuary by diking the wetlands”.79

From a long-term perspective, natural processes gradually turn estuarine marshes into swamps, and eventually dry ground, as plants trap sediment and cause the ground to rise.80 This natural process is much slower than human effects, but it too works against salmon, and could be yet another reason why historic peaks of salmon production can never again be attained—unless and until another ice dam breaks and scours out the Columbia River Gorge again.

76 U.S. Army Corps of Engineers, "Interim Status Report", at 2-4.

77 ISG, Return to the River 461.

78 C. Simenstad, L. Small & C. Intyre, Consumption Processes and Food Web Structure in the Columbia River Estuary, 25 Progressive Oceanography 271 (1990), cited in U.S. Army Corps of Engineers, "Interim Status Report", at 2-7 to 2-8.

79 ISG, Return to the River at 460.

80 ISG, Return to the River 458.

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