Studies analyze sediment release

One concern shared by dam removal opponents and proponents alike is associated with sediment transport and its effects on the life of the Klamath River, a concern which is addressed in a report titled “Effects of sediment release following dam removal on the aquatic biota of the Klamath River,” prepared by Stillwater Sciences for the State Coastal Conservancy.

The introduction states that dam removal would result in 1.3 to 2.9 million metric tons of fine sediment, including sand, silt and finer particles, being transported downstream.

The report states, “The primary impact on aquatic habitat from dam removal is predicted to result from the release of fine sediment when the reservoirs are drawn down in preparation for their eventual removal.”

The focus for the researchers was on the drawdown period, which they claim is when the most sediment would be released.

The effects on aquatic life and habitat are estimated based on simulations of sediment release scenarios, according to the release. The simulation, titled Dam Removal Express Assessment Models (DREAM–1), analyzed more than 70 different drawdown scenarios with multiple variables, including different drawndown dates and different drawdown rates.

The report on estimated effects is cited as an “analytical approach to meet multiple objectives.”

Those objectives listed in the report are: shortening the duration of high total suspended sediment (TSS) resulting from drawdown; avoiding secondary high spring TSS pulses resulting from reservoirs refilling during drawdown; avoiding high TSS levels during peak migration periods for anadromous salmonids (including Chinook and Coho salmon) and other aquatic resources; and maintaining the structural integrity of earthen embankment dams that are a part of the Klamath River project.

Based on predictions from the DREAM–1, researchers looked at potential population responses in the worst-case scenarios for Chinook salmon, Coho salmon, steelhead trout, coastal cutthroat trout, Pacific lamprey and green sturgeon. The report states that if it can be determined that fish populations can survive and recover under the worst case scenario, then any less detrimental scenario will likely increase survivability and recovery rates.

Researchers determined that for all fish types listed, those in the “mainstem” Klamath are expected to be impacted the most by mortality and non-lethal stress, although the report predicts a recovery in the long run.

Using a preferred drawdown scenario beginning in early November at a low rate that gradually rises, the report predicts that the Iron Gate drawdown would have an expected suspended sediment release of four to eight months, ranging from 50 parts per million (ppm) to 38,000 ppm. Parts per million can be defined as one gram of substance for every million grams of solution, in this case, one gram of sedimentary particulate matter for every million grams of river water.

Based on review of available scientific literature, according to the report, there have been a number of observed effects of suspended sediment on salmonids, including: avoidance of turbid waters by homing adults; avoidance or alarm reactions by juvenile salmonid; displacement of juveniles; reduced feeding and growth; physiological stress and respiratory impairment; damage to gills; reduced tolerance to disease and toxicants; reduced survival and direct mortality.

The researchers looked at laboratory studies that tested salmonid reactions to various TSS levels, determining that acute lethal effects seem to only occur if concentrations exceed 20,000 ppm.

“It appears that relatively short-term exposures to increases in TSS concentrations under 500-600 ppm would not likely result in substantial direct mortality to either juvenile or adult anadromous salmonids in the Klamath River,” the report states. If the duration is extended, however, “some direct mortality may be expected.”

The report goes on to say that exposures of 19 consecutive days to TSS concentrations of 90-270 ppm and higher have been reported as resulting in mortality in juvenile rainbow trout.

Other instances of mortality reported were large mortality rates in salmonid eggs at concentrations of less than 200 ppm and a correlation between larger particle sizes and higher mortality rates.

The report states that research indicates there are many naturally occurring high TSS events caused by rain and flooding. The researchers infer that since fish populations survive and recover from these natural events they will be able to recover from heightened TSS events caused by dam removal.

Ultimately, the report states that given the diverse conditions that are either helpful or harmful to the different species in the river, there are certain steps that can be taken to ensure the best conditions possible if dam removal is pursued.

The first suggestion is to protect “weak-year classes” of fish by choosing a year after good runs and during good ocean conditions.

The next suggestion is to manage annual harvest during the dam removal year, taking special consideration when setting take limits.