The chart below provides total annual fall Chinook redd counts by year from 1991 through 2018. The redd counts were collected from aerial surveys conducted by Idaho Power, the U.S. Fish & Wildlife Service and the Nez Perce Tribe for the Snake, Grande Ronde and Imnaha rivers during the noted spawning period.
|Year||Snake River||Grande Ronde River||Imnaha River||Clearwater River||Salmon River||Total|
Idaho Power’s Fall Chinook Program began in 1991 and is a voluntary operations plan for the Hells Canyon Complex.
The program provides a stable river flow level during the fall Chinook salmon spawning period, generally from mid-October to early December. The flow level provided during the fall Chinook salmon spawning period generally establishes what the minimum flow from Hells Canyon Dam will be once spawning is complete.
This minimum flow may be slightly lower than the stable flow provided during the spawning period. The minimum flow depends on the depth of the shallowest redds observed during the spawning period. The minimum flow level is the level that will protect the shallowest redd in the Snake River from being dewatered. Dewatering is when there is not enough water to sufficiently cover the redds.
Idaho Power maintains this minimum flow throughout the entire incubation and emergence period, which is approximately the end of May. The emergence period is when the fish swim up out of the redds. The stable flow level provided during the spawning period may vary from one year to the next. The flow level depends on the quantity of water available, which largely depends on the amount of snowpack within a given water year.
The U.S. Fish & Wildlife Service and Idaho Power started a cooperative research program to determine the quantity of spawning habitat and flow levels necessary to support sufficient spawning habitat for fall Chinook salmon. As part of this research program, fall Chinook salmon redd counts happen each year.
Redd counts monitor redd locations and establish the location and depth of the shallowest redd. Idaho Power and the U.S. Fish and Wildlife Service do weekly counts during the spawning period over the Snake River. In addition, the Nez Perce Tribe conducts aerial redd counts on the Clearwater, Grande Ronde, Imnaha and Salmon rivers. When the program began, aerial surveys were conducted by helicopter. Now, flights are done with drones that record video and still photography that enable careful analysis to obtain more accurate data while keeping biologists safely on the ground.
These survey flights account for the majority of the redds observed. However, in the Snake River, many redds cannot be observed from the air because they are in deep water. So, deep water redd searches using underwater videography occur during the spawning period as well as the aerial surveys. Redds as deep as 22 feet have been observed in the Snake River using underwater videography.