Ensuring Water Delivery Reliability: Dam Operations, Discharge Regime, Hyporheic Flow, and Salmonid Egg Survival
* Can discharge from a reservoir be reduced after spawning is complete without harming egg survival?
If so, up to 14, 104 acre-feet of water can be liberated each year from Cle Elum River, Washington. Recent fieldwork showed that dam releases influence flow of water in the intragravel (i.e., hyporheic zone) region where salmonids lay their eggs. The field work showed that when discharge (Q) increases, upwelling in salmonid redds increases. Conversely, when Q decreases, upwelling in salmonid redds decreases. The questions remain:
* If upwelling decreases after spawning does this increase egg mortality?
* If upwelling decreases but not enough to influence egg survival, is it possible to reduce surface discharge and liberate water for future use?
Need and Benefit
The purpose of this research is to save water for irrigation delivery or ecosystem needs. Water will be saved through the information provided by this Science and Technology (S&T) Program research project: we will determine if a small increase in storage during the winter will harm spring chinook egg survival. If we find there is no harm to chinook egg survival, Reclamation may save up to 14,104 acre-feet of water. We intend to free up this water by studying two relationships. The first relationship is that of dam release to intragravel flow: in other words, does the Q released from Cle Elum Dam, Washington influence intragravel flow and how? Second, if intragravel flow changes with Q, is that change sufficient to alter chinook salmon egg survival in a significant manner?
How can we save 14,104 acre-feet of water per winter? In completed field work below Cle Elum Dam, we measured chinook salmon redd-intragravel flow at a Q of 205 cfs (10/18/00) and at 126 cfs (10/31/00). So, we now know the effect on the intragravel flow from reducing the discharge 79 cubic feet per second (cfs) after spawning is complete. If we could reduce Q by 79 cfs during the winter (in this case we use November 15 through February 15) for the incubation period (90 days), we could save 79 cfs for 90 days or 14, 104 acre-feet. The research we describe here is intended to determine if that reduction of 79 cfs would be detrimental to spring chinook eggs during this winter period.
Hyporheic flow data from October 2000, show that a reduction of 79 cfs influences intragravel flow. That influence can be measured in terms of downwelling. Downwelling is when the flow in the substrate where eggs are deposited is flowing downward from the surface (defined by a negative value). More downwelling could theoretically reduce egg survival. Our hyporheic flow data shows that when discharge increases after spawning there is less downwelling; when discharge decreases after spawning there is more downwelling.
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