How Much Flow is Enough? The Effects of Altering Discharge on the Naches River, Washington
* How can we optimize water available for irrigation and meet habitat requirements of juvenile _Oncorhynchus mykiss_ ?
* Can we manage the Total Water Supply Available (TWSA) to achieve both of these objectives?
* Should the Wapatox Canal be improved?
Need and Benefit
Recently, the Reclamation's Yakima River Basin Watershed Enhancement Project purchased the Wapatox Power Plant Water Right (WPPWR). The WPPWR permitted diversions of up to 450 cubic feet per second (cfs) at the Wapatox Diversion Dam into the Wapatox Canal on mykiss (sea-going O. mykiss are steelhead and are listed as threatened) winter habitat (Mark Johnston, Yakama Indian Nation, personal communication). Now, all the water in the WPPWR is available to increase instream flows during the nonirrigation season (November through March).
In this study, we will derive the relationship between discharge (Q) and O. mykiss juvenile rearing habitat. We will acquire and use multispectral imagery to develop a Geographical Information System (GIS) (Whited et al. 2001, Bowen et al. 2002). This GIS can be validated and then used to calculate the area of habitat available at discharges (Q) from 250 cfs through 600 cfs. Once this relationship (Q/habitat) is known, Reclamation can strategically deliver that water necessary to meet O. mykiss juvenile life history needs and save the remaining water for other downstream uses.
The Q/habitat relationship that will be developed can also be used to strategically manage the TWSA. The TWSA is essentially the "one bucket" from which water is delivered to meet all the water demands for irrigation, fisheries, hydropower, etc. in the Yakima basin. Determining the point at which the benefits to the aquatic system stabilize as Q increases may help determine a target Q for the Wapatox Reach. The target Q could be less than the WPPWR and the difference in Q could be stored. If that difference between WPPWR and Q required for fish were only 5 cubic feet per second (cfs), then during the nonirrigation season alone the water saved could be 1, 488 acre-feet (AF). At $150/AF, the value of 1, 488 AF is $223,200/year. Since this research requires only $130,000/year, this study could provide $93,200 to Reclamation during the study duration. Thereafter, the $223,200 worth of water would be available every year.
This research will also provide insight into the need--or lack thereof--for Wapatox Canal improvements. During the irrigation season the Wapatox Diversion Dam and Canal are needed to deliver approximately 50 cfs for irrigation, and the canal was not designed to efficiently transport that amount of water. Approximately 200 cfs need to be diverted to effectively provide the 50 cfs for irrigation purposes. Modifications to the canal will likely incur significant financial costs.
* Are these costs necessary?
* Can we just divert 200 cfs, delivering 50 cfs to irrigation stakeholders?
* Or, is that 150 cfs essential to be left in the river to meet threatened O. mykiss life history requirements?
We intend to answer this final question by using the Q/habitat relationship in the Wapatox Reach. Then, river managers will know exactly how much water is enough to satisfy requirements of fish and determine the priority of Wapatox Canal improvements.
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