Water Bodies Affected by Discharge at Hydroelectric Facilities Idaho Power operates 17 hydroelectric projects along the mainstem Snake River from American Falls (river mile [RM] 715) to Hells Canyon (RM 247). Of the 17 projects, four operate on groundwater springs that discharge into the mainstem Snake River. Idaho Power also operates a hydroelectric facility on Cascade Reservoir on the North Fork of the Payette River. Most of the hydroelectric projects are operated as run-of-river (ROR) projects, where the mean inflow equals the mean outflow of the project on a daily basis. Most of the mainstem projects are used to follow load, meaning that during the day, discharge levels may fluctuate around the mean daily flow to meet peak power demands during the day.

Four of Idaho Power’s hydroelectric projects operate differently than ROR projects, including the Cascade (North Fork Payette), American Falls, Milner and Brownlee plants. Cascade and American Falls powerhouses are on United States (U.S.) Bureau of Reclamation (USBR) dams and reservoirs. Their primary purposes are for irrigation water supply and distribution for southern Idaho agricultural interests. Idaho Power generates power using water the USBR passes through the dam up to the capacity of the powerhouses. The Milner project operates with water available after large irrigation canals divert water from the Snake River during the irrigation period to supply southern Idaho agricultural interests. The Snake River is recharged to some degree by the large spring discharges downstream of Milner Dam. Brownlee Reservoir is a large storage reservoir and is the upper-most reservoir in the Hells Canyon Complex (HCC). Oxbow and Hells Canyon reservoirs, downstream of Brownlee Reservoir, help regulate Brownlee Reservoir outflow by managing discharge into the free- flowing river below Hells Canyon Dam. Brownlee Reservoir can store water and discharge less than inflow or be drafted to discharge greater than inflow on a larger time step than the daily operations associated with the ROR projects. Typical examples of the Brownlee Reservoir storage include using the reservoir for flood-control purposes as directed by the U.S. Army Corps of Engineers (USACE) or as part of special operations designed to protect fall Chinook salmon spawning and incubation downstream of the HCC. These operations require large seasonal draw-downs of Brownlee Reservoir.

In the ROR hydroelectric projects, the reservoirs have minimal influence on the quality of the discharge from the powerhouses in terms of dissolved oxygen (DO), water temperature, or other water-quality constituents. However, because of the large size of Brownlee Reservoir and the thermal stratification that establishes each year in the reservoir, Brownlee Reservoir discharge strongly influences the water quality in Oxbow and Hells Canyon reservoirs, as well as the Snake River immediately downstream of Hells Canyon Dam. Brownlee Reservoir has shifted the thermal regime of the Snake River such that warming of discharge waters is delayed in the spring and cooling is delayed in the fall relative to inflowing waters. In addition, maximum temperatures are cooler in the summer months and warmer in the winter months as a result of the thermal stratification in Brownlee Reservoir. More information on the effects of our hydroelectric facilities on water bodies can be found in the Biodiversity section.

Discharges at the hydroelectric facilities can also elevate levels of total dissolved gases (TDG). Elevated TDG levels are usually associated with the discharge being spilled through spill gates when river flows exceed the capacity of the powerhouse. Spilling the discharge can cause the water to trap atmospheric air under pressure and elevate the levels of TDG in the water. This can be especially harmful to aquatic life when bubbles of gas, mainly nitrogen, form in the tissues of aquatic organisms (fish and aquatic insects), creating serious trauma or death. Generally, when TDG levels exceed about 120 percent saturation, gas bubble trauma (GBT) symptoms begin. If animals can move to greater depths, the effects of elevated TDG can be compensated by increased pressure. The states of Idaho and Oregon have established a standard for discharges not to exceed 110 percent saturation. Idaho Power also participates in a supplemental hatchery program for fall Chinook salmon with the State of Washington and the Nez Perce Tribe. Idaho Power is responsible for rearing and releasing up to one million hatchery fall Chinook salmon as part of this supplementation effort. Fall Chinook salmon have rebounded significantly since 1991. In 1991, only 633 adult fall Chinook salmon were counted passing Lower Granite Dam (the next dam below Hells Canyon Dam — operated by the USACE) to spawn in areas below Hells Canyon Dam. In both 2014 and 2015, the number of returning adults approached 59,000 at Lower Granite Dam.

Idaho Power also regulates its discharge from the HCC to participate in a program designed to augment flows for migrating anadromous fish in the lower Snake River, which are influenced by dams constructed by the USACE. The development of large storage reservoirs in the Upper Snake basin, principally by the USBR, has altered the hydrograph such that, during spring months when anadromous fish are migrating to the ocean during the historic peak of the annual hydrograph, flows are reduced because upstream reservoirs are capturing and storing water. Flow augmentation is designed to increase flow during critical migration periods to assist fish passing through the lower Snake River reservoirs. Idaho Power provides up to 237,000 acre-feet of flow augmentation water from Brownlee Reservoir during periods thought to primarily benefit out-migrating fall Chinook salmon. This is being done through an interim agreement with the National Marine Fisheries Service until a new Federal Energy Regulatory Commission (FERC) license is received for the HCC.

Idaho Power also regulates the discharge from Brownlee Reservoir as mandated by the USACE as part of regional flood-control requirements for the Columbia River basin. Idaho Power drafts Brownlee Reservoir to elevations determined by the USACE during each year depending on the volume of the regional snow pack and flow forecasts. Once released from flood-control requirements, Idaho Power tries to refill Brownlee Reservoir as rapidly as possible to best accommodate power generation needs and access to recreational facilities. During the refill or drafting, Idaho Power also has operational guidelines to best protect warm-water fish spawning. This is an effort to maintain the quality of the fishery and recreational opportunities of Brownlee Reservoir.

National Pollution Discharge Elimination Standards (NPDES) permits are in place at the projects relative to the discharge of cooling water and oils and grease associated with the powerhouses at all projects where they are required. Idaho Power monitors and reports conditions relative to the permits. Other point discharges associated with Idaho Power facilities include the four Idaho Power -managed anadromous hatcheries. Three of the four facilities require NPDES permits based on the level of production associated with the facility. At the Rapid River, Pahsimeroi and Niagara Springs hatcheries, Idaho Power monitors total suspended solids (TSS) and total phosphorous (TP) levels associated with the effluent. Levels are monitored according to the terms of the NPDES permit. The TSS and TP levels in the hatchery discharges consistently comply with the levels identified in the NPDES permit to protect water quality in the receiving waters. At the Niagara Spring Hatchery, where a total maximum daily load (TMDL) is in place for the receiving water, Idaho Power improved its hatchery effluent treatment system prior to issuance and approval of the TMDL. Similar improvements to the hatchery waste treatment systems were made at Pahsimeroi as part of an overall facility renovation from 2006 to 2007.

Information is accurate as of December 31, 2017, unless otherwise noted.