We measure reservoir water levels by the elevation of the surface of the water above sea level. For example, Brownlee reservoir is 2,077 feet above sea level when full. We also provide the number of feet a reservoir is below full; “Brownlee reservoir is at elevation 2067, 10 feet below full.”
River flows are measured in cubic feet per second (cfs). The volume of one cfs is equal to a box that measures one foot on all sides. One cfs also equals 450 gallons per minute. If a stream is flowing at 10 cfs, 10 ‘boxes’ of water are going by every second, or 4,500 gallons per minute.
Idaho Power’s Stream Gaging group follows the standard stream gaging station numbering convention established by the U.S. Geological Survey (USGS). Each stream gage has a unique station number assigned to it based on the location of the gage. The numbering convention lists stations in order of downstream direction along a main stream.
Idaho Power’s water releases influence the Snake River. However, there are many other factors affecting the flow in the Snake River including spring flows, irrigation demand, flood control, snowpack, navigation and special flows for salmon, construction projects and local events.
Brownlee Reservoir is our only facility large enough to store water for more than a few hours. Stored water is stored energy. To meet our customers’ energy needs we would like to keep Brownlee full. Our generators there also are more efficient when the reservoirs are full. However, in the West there are many different demands on water. Electricity, recreation, irrigation, flood control, resident fish, endangered salmon, groundwater replenishment, wildlife and scenic values all have claims. Laws, regulations, and agreements obligate us to regulate flows to help meet these needs. For example, almost every spring, the U.S. Army Corps of Engineers requires us to release water so we have space to hold possible flood water from the spring run-off to protect the Portland, Ore. area.
In water terms, neither Oxbow nor Hells Canyon reservoirs are very big. The average outflow from Brownlee Dam can fill Oxbow Reservoir in a little more than six hours. Because Oxbow Dam is smaller than Brownlee, only about 25,000 cubic feet of water per second (cfs) can pass through its turbines. During the night we usually draft Oxbow Reservoir down to create space for water from the Brownlee Dam. In the morning when electricity demand rises and more water is passed through Brownlee Dam, we have room in Oxbow Reservoir for this increased volume of water. In this way, water level changes at Oxbow and Hells Canyon dams reduce the impact of flow changes on the river below Hells Canyon that can reach as much as 10,000 cfs a day.
As long as we can provide stable flows during their nesting seasons, the resident fish are generally okay. Keeping those flows stable for nesting is built into our operating plan. Our goal is to keep Brownlee Reservoir within the top 10 feet during the peak of the bass nesting season in June. Interestingly, our studies show that crappie are more adversely affected by high flows. It seems that they spend most of their time in the middle of the reservoir and are carried downstream by the force of the current in high water years.
Changing conditions sometimes make it impossible to know in advance. The failure of one of our coal-fired generating plants may cause us to increase our use of hydro generation. Sudden snow melts or heavy rains cause floods. Weather forecasts are not always accurate, and hot and cold spells cause customer demand to change drastically. Public safety or rescue operations may require a change in operations. Some of our power is purchased from other utilities, so changes in the wholesale electricity markets may also affect us. Occasionally we know that a change is going to happen, but we don’t know when. For example, flow changes for anadromous fish are usually called for when cooperating agencies see the need.
We strive to provide accurate and timely information about stream flows and Brownlee Reservoir’s elevation through this website and our toll-free recreation line. Always check our website, or call the toll-free number, 1-800-422-3143, before you leave on your trip to make sure that you, your equipment and your boat or vehicles are safe from possible stream flow or elevation changes.
Farewell Bend and Steck Park are at the tail waters head of the reservoir, almost 50 miles upstream from Brownlee Dam. When the reservoir level is below elevation 2,055 feet above sea level, the reservoir water doesn’t back up far enough to reach there. In that case, conditions there are dependent on flows in the river, not what is happening at the reservoir.
The choice of a flow for fall Chinook is a balancing act. Our goal is to provide the maximum amount of good spawning ground while ensuring the nests, or redds, remain submerged until the eggs hatch and the hatchlings leave the following May. This means our minimum flow from December through May has to stay high. If that flow is too high and we have a dry spring, there might not be enough water to keep the redds covered even if we were to drain Brownlee Reservoir. Biologists have found that 9,000 cfs is the optimum flow from Hells Canyon Dam to ensure the redds remain covered with water. Based on historical water records, 9,000 cfs is a flow we can typically provide through the following spring. However, from late October through early December when we would like to have a 9,000 cfs outflow from Hells Canyon, inflows are usually higher than 9,000. Since inflows are greater than outflows, we must reduce the water level at Brownlee Reservoir beforehand to create space to store the excess water.
Provisional means the status of the stream flow data is subject to revision upon review and correction by Idaho Power. Several reasons may cause provisional data to be inaccurate including:
- The backwater effect caused by the formation of ice
- Floating debris such as trees
- Beaver activity
- Other natural or unnatural occurrences
- Seasonal plant or algae growth
- Sediment deposition or erosion
- Mechanical problems with stage monitoring equipment
A water year is the 12-month period between Oct. 1 and Sept. 30. The water year is defined by the calendar year in which it ends. For example, the water year for 1998 is Oct. 1, 1997, through Sept. 30, 1998. However, the “irrigation year” for water is the 12-month period between Nov. 1 and Oct. 31.
Brownlee’s Three-Day Average Inflow may be used to determine Hells Canyon outflow for navigation requirements below Hells Canyon Dam depending on river conditions.