In our last two issues we looked at possible Klamath River dam removal options under discussion, and what an actual dam removal could look like. Given the uncertainties of a blow-and-go removal, it’s likely that the four dams would be removed from the top down, in an orderly manner, if removal is ultimately approved. That still leaves the question of just what will happen to the 3.6 million tons of mud, sand and gravel that are estimated to have built up behind the dams. In terms of volume it’s estimated to be about 13.2 million cubic yards.
Two issues ago we reviewed some of what took place when two large dams were removed on the Elwha River, in Washington. Those two dams had locked up an estimated 27 million cubic yards of sediment, and when removed, 90% of that sediment quickly moved downstream and completely modified the lower river and nearshore ocean environment. If you missed that article and are interested in more detail, click here http://humboldtasa.com/category/rivers / to access the article on our website. Although the four Klamath dams are holding only about half as much sediment as the two Elwha dams, the bigger difference is that Iron Gate, as the lowest of the four dams under consideration, is 190 river miles (RM) from the ocean. The two Elwha River dams were located at about RM five and fifteen, so they were much closer to the river mouth.
Currently the river immediately downstream from Iron Gate reflects the fact that for the last century sediment which has washed downstream has not been replaced by sediment coming from upstream. From Iron Gate down river as far as Cottonwood Creek (RM 182) the Klamath is characterized by coarse cobble and cobble-boulder bars. The further downstream you travel, the more fine sediment is present, as it is replenished from tributaries. About 98% of the fine sediment in the river is currently deposited downstream from Cottonwood Creek. Thus the area which is most disturbed by the choked-off sediment flow is the reach from JC Boyle Reservoir downstream to about the Scott River. As far as the salmon are concerned, that stretch is either poor spawning grounds from the Scott River to Iron Gate, or simply inaccessible upstream from the dam. The lack of suitable spawning gravel below Iron Gate dam has been identified as one of the most serious limiting factors for salmon and steelhead spawning in the Klamath.
It’s estimated that 85% of the trapped sediment consists of fine-grained silts and clays which will move downstream quickly once the dams are removed and winter rains arrive. The remaining 15% is primarily coarser sand, pebbles and cobbles which will move more slowly. Within the currently buried riverbed reach, much of the silt movement will take place in the first two months. How fast everything starts moving downstream will depend on the nature of the winter rains the first few years after dam removal. The finest materials will mix with the water and be transported as suspended sediment the length of the river till deposited in the ocean. We can expect the Klamath to run dirtier than normal for at least two years following dam removal. Computer simulations estimate that in the first two years the stretch of river from Iron Gate downstream to about Willow Creek (Klamath tributary at RM 185) will experience about a one-foot rise in sediment level, with decreasing amounts of riverbed modification further downstream. Simulations run to estimate the situation fifty years out show things will probably stabilize with the riverbed about two to three feet higher in the Iron Gate to Cottonwood Creek reach, and about 12 inches higher in the first few miles downstream of Willow Creek. There would be measurable streambed effects downstream about as far as the Shasta River, but no significant effects below that point beyond dirty water.
The initial push of fine-grained sediment downstream could effectively bury some spawning areas, making them unusable by salmon. Furthermore, the heavy silt levels in the river will likely reduce the oxygen level, though just how much will again depend on the flow rate, which will depend on the rainfall pattern in the first couple of years. Coho do most of their spawning in the tributaries, so it’s unlikely this will affect them during the initial changes. Spring-run Chinook spawn primarily in the Shasta and Trinity rivers, and they don’t generally go upstream as far as Iron Gate. Whether they will move to utilize the newly opened territory without being physically placed there is unknown. Fall run Chinook are expected to quickly move past the current dam sites and use new spawning areas between where the dams currently stand. Summer steelhead, which are currently rare upstream of Seiad Creek (RM 131) will likely continue to move upstream once the water temperature stabilizes at a cooler level than currently develops during the summer. Steelhead generally spawn in the tributaries, using the Klamath as a migration corridor. Both summer and winter steelhead are expected to eventually move into ancestral territory well above where the dams now stand. If everything goes as envisioned, the first fish to pass the site of Iron Gate dam would do so in the fall of 2020.
The first salmon to encounter the new conditions would be the young spawned by the fall 2019 run. At hatching time there will be far more sand and silt in some spawning areas than normal, and there could be a substantial adverse impact on the survival rate of eggs laid in the section below Iron Gate. It’s possible that those fry hatching in the first eight miles downstream of the dam would have a low survival rate, and that could translate into reduced offshore fishing opportunities for the first couple of years afterwards. By about five years after dam removal, projections show substantially improved spawning substrate in that region than now exists. As mentioned in our prior article, projections show that once the initial river changes stabilize, future Klamath salmon and steelhead runs could increase as much as 81%.
As PacifiCorp and the many government agencies involved in this process announce new data or progress, we’ll try to keep everyone informed.