| How and Why the Level of Flathead Lake Varies
 Flathead Lake is a natural freshwater lake that functions as a giant surge pond for the Flathead River, attenuating the flow of the river below the lake. The highest flow on record at the river gauging station at Polson was 82,100 cubic feet per second (cfs) on 28 & 29 May 1928, approximately half of the flow of the river at Columbia Falls during the flood of 1964. Flathead Lake's surface elevation is regulated by Kerr Dam (right), located on the Flathead River approximately 4.5 miles below the lake's outlet. Kerr dam was closed on 11 April 1938. Inflows to the lake are partially regulated by Hungry Horse Dam, located on the South Fork of the Flathead River near Hungry Horse. Hungry Horse Dam was closed on 21 September 1951.
Regulation has affected both the lake and the Flathead River below the lake.
Regulation has inverted the hydrograph during summer and fall (see our historic hydrographs page), and, the lake's mean annual elevation has increased by approximately eight feet.
Before Kerr Dam was closed, streamflow in the Flathead River below Polson began increasing in late April and peaked around the beginning of June. Summer, fall, and winter flows were low. After the dam was closed, flows in late fall and winter were increased, and spring flows were curtailed sharply. The change had a devastating effect on fisheries below the dam; investigating biologists called the reach a "biological desert." As part of the mitigation plan required by the renewed license, minimum flows were required (please see our new Minimum Flows page for more details and some very interesting hydrographs). North of Flathead Lake, the artifically high Flathead River backed up several miles, raising the level of the groundwater and the ire of farmers. Water years run from 1 October to 30 September, and are known by the year in which they end. Thus, the water year 1999 began on 1 October 1998 and ended on 30 September 1999. Daily streamflow measurements for the river at Polson began in 1907. Daily lake elevation measurements for the lake began in 1928 (end-of-month contents are available back to 1909). Daily streamflow measurements for the Flathead River above the lake date from 1928 for the mainstem, and for some earlier periods for the three forks. Enough streamflow and lake elevation data exist that hydrographs for the preregulation era can be constructed.
Operations are constrained by the license from the Federal Energy Regulatory Commission (FERC) and by various agreements. The lake normally is lowered to 2883 by 15 April. It normally is filled to 2890 by Memorial Day, and to 2893 by 15 June. When flooding is a risk, the spill-gates must be opened when the elevation reaches 2886 and not closed until the danger of exceeding 2893 passes. In practice, the lake is not held at exactly 2893 during the summer. From 1951 through 2000, the average summer elevation was 2892.75 (histogram).
On 25 June 1997, FERC issued an order approving a mitigation and management plan for Kerr Dam, thus changing the terms of the license. The mitigation plan required important changes in the operation of the dam. Below are the sections that are applicable to lake levels: The licensee shall operate the Kerr Project as a base-load facility, which precludes the load-following or peak power generation (releases and power generation are not changed within a day).
The licensee shall maintain releases at or above the following instantaneous minimum flow, in cubic feet per second (cfs) on a continuous basis as measured at the U.S. Geological Survey Polson gauge, provided that the passage of minimum flows from Flathead Lake downstream is not limited by the natural channel capacity immediately upstream of the Kerr Dam. The licensee shall manage the project in a manner that limits the occurrence of such channel capacity limitations.
Minimum Flow Requirements | Dates | Minimum Flows | August 1 to April 15 | Continuous at 3,200 cfs | April 16 to April 30 | Increased from 3,200 cfs to 5,000 cfs at 120 cfs per day | May 1 to May 15 | Increased from 5,000 cfs to 12,700 cfs at 510 cfs per day | May 16 to June 30 | Continuous at 12,700 cfs | July 1 to July 15 | Reduced from 12,700 cfs to 6,400 cfs at 420 cfs per day | July 16 to July 31 | Reduced from 6,400 cfs to 3,200 cfs at 200 cfs per day. |
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The minimum instream flows may be temporarily modified by operating emergencies beyond the control of the licensee, or fo short periods upon written approval from the Secretary of the Interior.
The licensee shall operate the Kerr Project in accordance with the following between-day restrictions on flow variations as measured at the U.S. Geological Survey Polson gage, escept as necessary to meet flood control requirements imposed by the U.S. Army Corps of Engineers under the Memorandum of Understanding with Montana Power Company (as amended in 1965), or as may be further amended by the U.S. Corps of Engineers subsequent to approval of the Secretary of the Interior.
Maximum Between-day Flow Changes | Mean Flow
(cfs, 24-hour average) | Maximum Change in Flow (cfs) | Less than 5,000 | 500 | Between 5,000 and 10,000 | 1,000 | Between 10,000 and 20,000 | 2,500 | Between 10,000 and 40,000 | 5,000 | Between 40,000 and 60,000 | 10,000 |
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The between-day restrictions on flow variations may be temporarily modified if required by operating emergencies beyond the control of the licensee, or for short periods upon prior written approval from the Secretary of the Interior.
The licensee shall operate the Kerr Project in accordance with the folloing hourly maximum allowable ramping rates, except as necessary to meet flood control requirements imposed by the U.S. Army Corps of Engineers under the Memorandum of Understanding with Montana Power Company (as amended in 1965), or as may be further amended by the U.S. Corps of Engineers subsequent to approval of the Secretary of the Interior.
Maximum Allowable Ramping Rates | Mean Flow
(cfs, 24-hour average) | Ramping Rate | Between 3,200 and 7,500 | 250 cfs/hour | 7,500 or greater | 1,000 cfs/hour |
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The hourly ramping rates may be temporarily modified if required by operating emergencies beyond the control of the licensee, or for short periods upon prior written approval from the Secretary of the Interior.
The Montana Power Company and the Confederated Salish and Kootenai Tribes (licensees) shall cooperatively develop and initiate a site-specific ramping rate study as part of the adaptive management planning process. The study shall incklude a two-year phase assessing effectiveness of improving physical habitat and the establishment of a biological baseline. Within six (6) months of floowing completeion of the first two years of the study, the licensees shall submit to the Secretary of Interior (Secretary) a report and recommendations, based on the results of the first two-year phase of the study, on proposed revisions to the existing ramping rates. To the extent the Secretary approves revisions to the existing ramping rates, the licensee shall operate under the revised ramping rates upon issyuanceof an order by the Commission amending the license as provided by the Secretary. The second phase of the study shall focus on the biological response to the existing or revised ramping rates. At the end of the ten years following the initiaton of the ramping rate study, the licensees shall subject to the Secretary a report and recommendations, based on the results of the study, on proposed revisions to the existing ramping rates. To the extent the Secretary approves the proposed revisions to the existing ramping rates, the licensees shall operate under the revised rampingrates upon the issuance of an order by the Commission amending the license as provide by the Secretary.
The licensees, in consultation with the U.S. Army Corps of Engineer[s], the Bureau of Reclamation, the Bureau of Indian Affairs, and the Montana Department of Environmental Quality, shall develop and implement a drought management plan for Flathead Lake. The drought management plan shall include, but not be limited to, provision for re-evaluation and adjustment of Flathead Lake flood control requirements and other provisions necessary to facilitate compliance with lower Flathead River minimum instream flow requirements designated by the Secretary of the Interior.
The licenssees shall consult with the U.S. Bureau of Rellamation on a weeky basis from April 20 to August 31 and on a biweekly basis the remainder of the year regarding the anticipated releases from Hungry Horse Reservoir. On the first Monday (workday) of each week or biweeky period, the licensees shall request notification from the Bureau of Reclamation of the Hungry Horse projected release(s) for the period of time commencing on that day and extending for the subsequent 21 days. The licensees shall, in a timely manner and within the maximum allowable changes ion flow discharge rates set for the Kirr Project, coordinate operations with the Hungry Horse Project releases. To facilitate the coordinated operations on Kerr and Hungry Horse, the licensees may participate in the Technical Management TReam process pursuant to the National Marine Fisheries Service Biological Opinion(s) on operation of the Fideral Columbia River Power System (or successor process) to the extent allowed by Federal law.
The licensees shall provide to the Secretary of the Interior annually oon or about May 1, but no later than May 10, an annual operational schedule to be supplemented on a monthly basis. The annual schedule shall include month-end estimates of water surface elevation at Flathead Lake and estimates of monthly discharges from Kerr Dam. The monthly supplement shall include a report of actual operations over the previous month and shall include daily water surface elevation at Flathead Lake and spill and releases at Kerr Dam. The annual schedule and monthly supplements can be based on hydrologic and operational information compliled by other cooperating agencies. The operational schedule shall take into account forecasted inflows to Flathead Lake, flood control and recreational requirements in Flathead Lake as well as minimum flow requirements at Kerr Dam.
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